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The formation of N-(2'-(diphenylphosphoryl)-[1,1′-biphenyl]-2-yl)-2,3,4,5,6-pentafluorobenzamide (3_eaaa) has been achieved through the palladium-catalyzed and Ag(I)-assisted C H functionalization of N-([1,1′-biphenyl]-2-yl)-2,3,4,5,6-pentafluorobenzamide (1_eaa) via phosphination and in the presence of diphenylphosphine oxide (2_a). The reaction was accompanied with small amount of carbazole derivative, (9H-carbazol-9-yl)(perfluorophenyl)methanone (4_eaa). Crystal structures of 3_eaaa and 4_eaa were both determined by X-ray crystal diffraction methods. Indeed, phosphination took place at the ortho-position of inter-annular ring of 1_eaa. Substituents with various electron-withdrawing/donating capacities on the amido-fragment of 1 were found to greatly affect the ratio distribution of products 3 and 4. By contrast, substituents on various locations of the biphenyl in 1 do not influence much on the outcome of the products distribution. A reaction mechanism is proposed to account for these experimental observations. Computational studies employing Density Functional Theory methods (DFT) on this proposed mechanism found that the electron density property of the palladium metal center in the intermediate II is critical to the formation of either 3 or 4. A relatively electron-poor palladium metal center favors the pathway of forming 3. On the contrary, the pathway of producing 4 will be prevailed while the palladium metal center is having fair amount of electron-density in the proposed intermediate II.

In the presence of norbornene, Pd(OAc)2 smoothly catalyzed the meta-C-H activation process of N-phenylbenzamide (4) and yielded a norbornene fused product, (1R,4S,4aR,8bS)-N-phenyl-1,2,3,4,4a,8b-hexahydro-1,4-methanobiphenylene-5-carboxamide (5a). The employment of Ag(OAc) is crucial to the success of this reaction. Norbornene related compounds, dicyclopentadiene and norbordiene, were used as the reagents and structurally similar compounds, 5b and 5c were formed although with much lower yields. Similar reaction was carried out for using N-benzylbenzamide (9) as starting reagent. The resulted norbornene fused product (1R,4S,4aR,8bS)-N-benzyl-1,2,3,4,4a,8b-hexahydro-1,4-methanobiphenylene-5-carboxamide (10) shows that the C-H activation indeed took place at the phenyl ring of acetophenone part rather than the benzyl side. Crystal structures of 5a, 5c and 10 were determined by X-ray diffraction methods. A reaction mechanism is proposed to account for the formation of 5a, which could be extended to describe the generation of other structurally related compounds as well.

Several multi-substituted 1,4-benzoquinone (BQ) derivatives, 4, 5, and 6, possessing C-N and C-P bonds were prepared through the assistance of silver-mediated C-H functionalization processes and characterized by spectroscopic methods. The crystal structures of some selected products were determined by X-ray diffraction methods as well. Reaction pathways are proposed to account for the formations of products from 4 to 5 then to 6. Three rather structurally distinctive compounds, 8b, 9, and 10, were obtained individually from the reactions of di-substituted compound 4aa under specified conditions. Crystal structures of these three compounds have shown that the scaffolding of each compound contains multiple fused rings. Attempts to reduce specifically the P O bond of the -P( O)Ph2 on a di-substituted 1,4-naphthoquinone N4aa to a P,N-bidentate hemilabile ligand resulted in an unexpected over-reduced compound 11. As revealed from the crystal structure of it, one of the ketone groups was completely reduced to CH2 and the other one was reduced to C-OH; yet, the targeted P O bond remains intact.

Transition metal-catalyzed ortho- C−H functionalization of amido-substituted 1,4-naphthoquinone, 3, in the presence of Pd(OAc)2 and tertiary amines led to the formations of 4 with newly generated pyrrole rings. Similar reactions were carried out with primary amines and yielded 5, having imidazole rings, as well as amination products 6. In the cases of reacting with secondary amines, the route for the formations of 4-like, rather than 5-type, products were prevailed. As revealed from the crystal structures of 4_Bu, 6_Bu, 6_Cy, and 4_Cy, cyclization processes indeed took place to form pyrrole and imidazole rings, respectively, which are fused to the former naphthoquinone framework. The crystal structure of 5_Cy shows that it is a product from amination reaction. Similar reaction was carried out using PCy3 as the phosphine source and an unexpected product 7 was obtained. The crystal structure of 7 exhibits that the moiety of PCy3 attaches to the framework of the former naphthoquinone and brings about a new P=C double bond. In addition, the amide fragment of 3 is hydrolyzed and reduced and forms C=N double bond. Here, distinct reactivity of amines from that of phosphines in this type of reactions have been manifested.

Several secondary phosphine oxides (SPOs, 2a-2d) as well as 2a-chelated palladium complex 2a_Pd were employed in Catellani type reaction for the synthesis of 1-methyl-9H-carbazole (4) from 2-iodotoluene and N-(2-bromophenyl)acetamide (3). The application of these ligands (2a-2d) and palladium complex (2a_Pd) in this reaction is beneficial in terms of increasing the yield of the major product, 4. An elaborated mechanism was proposed to account for the observation of 4 as well as some side products, 5, 6, 7, and 8, via C-C and C-N bonds formation processes.

Several new amino-type secondary phosphine oxide (SPO) pre-ligands (3a–3h) that contain P–N bonds were synthesized and characterized. SPOs 3a–3h can tautomerize to phosphinous acids (PA, 3a–3h) as genuine ligands. The formation of SPOs 3a–3h occurred first through nucleophilic attack on the imine carbon atom, then by the addition of RPCl2 (R = Ph, Cy, tBu, or iPr), and work-up under acidic conditions. The P–N bond in the newly prepared SPOs is evident from the crystal structures of SPOs 3d and 3h. Reactions of SPOs 3f, 3g, or 3h with Pd(COD)Cl2 (COD = cyclooctadiene) yielded palladium complexes 6f, 6g, or 6h. In these crystal structures, PAs 3f, 3g, and 3h act as didentate ligands through P and N donors. Intriguingly, the reaction of SPO 3f with Pd(COD)Cl2 also gave rise to palladium complexes 7fa, 7fb, and 8f. The crystal structures of 7fa and 7fb show that the palladium atom is chelated by PA 3f and coordinated by a phosphine-like ligand fragmented from SPO 3f through C–N bond dissociation. Finally, the syntheses of carbazole derivatives were pursued in Catellani reactions with SPOs 3g and 3h as pre-ligands. A mechanism is proposed to account for the catalytic reaction (see the Supporting Information). The optimized conditions for Suzuki reactions using selected SPO ligands are also reported.

Several di-substituted diimines (3a–3f) and heteroatom-substituted unsaturated secondary phosphine oxides (HASPO, 6a–6f) were prepared and characterized. Compounds 6a–6f are regarded as pre-ligands because of their ability of tautomerization to heteroatom-substituted phosphinous acid (HAPA, 7a–7f). An unexpected 3e-coordinated palladium dibromide 8e was observed from the reaction of compound 6e with PdBr2. Molecular structures of pre-ligands 6a, 6c, and 6e, as well as palladium complexes 8e were determined by single crystal X-ray diffraction methods. When pre-ligand 6a was applied to palladium-catalyzed Suzuki-Miyaura cross-coupling reactions, satisfactory yields were obtained. Density functional theory were employed to examine the electronic properties of HASPO 6a–6f pre-ligands, their corresponding 1,3-di-N-substituted tautomers 7a–7f, and the saturated counterpart 7as of 7a. Compound 7a is the most effective and genuine ligand in Suzuki-Miyaura reaction that is confirmed by its higher-lying lone-pair (LP) molecular orbital (HOMO-1). The LP orbital of 7c–7f is lower-lying HOMO-5. For each 7c–7f, two conformational rotamers with minute energy difference were located. Hirshfeld charge and population analysis of 7c–7f were also calculated in order to comprehend the electronic properties for these two rotamers for each HAPAs. Besides, the steric effect of HAPAs was estimated in terms of the Percent Buried Volume (%Vbur). This model has shown that 7a has similar steric property to that of PCy3, which is an effective ligand in Suzuki-coupling reactions.

A series of cyclohexane-1,2-diamine (3a–3d) and benzene-1,2-diamine derivatives (3e–3h) were pre- pared. Followed by hydrolysis, the reaction of 3a–3c with PCl3 successfully led to the formation of cor- responding metastable saturated heteroatom-substituted secondary phosphine oxides (HASPO 4a–4c), a tautomer of the saturated heteroatom-substituted phosphinous acid (HAPA). Whereas ambient-stable diamine-coordinated palladium complexes were obtained, HAPA-coordinated palladium complexes were not successfully synthesized. The molecular structures of HASPO 4c, Pd(OAc)2(3a), PdBr2(3b) and Pd(OAc)2(3c) and [Cu(NO3)(3d)+][NO3−] were determined by single-crystal X-ray diffraction method. Catalysis of in-situ Suzuki-Miyaura cross-coupling reactions for aryl bromides and phenylboronic acid using diamine 3a as ancillary ligand showed that the optimized reaction condition at 60 °C is the combination of 2 mmol % 3a/3.0 mmol KOH/1.0 mL 1,4-dioxane/1 mmol % Pd(OAc)2. Moreover, moderate reactivity was observed when using aryl chlorides as substrates (supporting infor- mation). When diamine 3d was employed in Heck reaction, good tolerance of functional groups of aryl bromides were observed while using 4-bromoanisole and styrene as substrates. The optimized condi- tion for Heck reaction at 100 °C is 3 mmol % 3d/3.0 mmol CsF/1.0 mL toluene/3 mmol % Pd(OAc)2. In general, cyclohexane-1,2-diamine derivatives exhibited better catalytic properties than those of benzene-1,2-diamines.

Recently, cyclic heteroatom-substituted secondary phosphine oxides (cyclic HASPOs: (X^X)PH( O), X = NR or O atom) have been applied to various catalytic reactions as preligands and with notable success. Similar to secondary phosphine oxides (SPOs: R2PH( O)), cyclic HASPOs are conventionally regarded as air- and moisture-stable compounds. Nevertheless, as reported herein, cyclic diaminophosphine oxides (cyclic N–P–N type HASPO: (RN^NR)PH( O), where (RN^NR) represents 1,2-diamino group) are susceptible to hydrolysis. The hydrolysis process of two P–N bonds in cyclic diaminophosphine oxides, 1a and 1b, has been thoroughly investigated by 31P NMR experiments. Density functional theory (DFT) methods have also been employed for elucidating the reaction mechanisms of hydrolysis for diaminophosphine oxides, 1c and 1b, respectively. The results pointed to a stepwise-concerted (SN2I@P4–SN2@P4) mechanism for consecutively double hydrolysis of 1c. 1c has been employed as a model for 1a and other diaminophosphine oxides with smaller substituents on nitrogen atoms. In contrast, a concerted-stepwise (SN2@P4–SN2I@P4) mechanism for 1b, bearing two bulky –tBu groups, was validated. Accordingly, kinetic stability which brought about by two bulky substituents indeed prevents 1b from being hydrolyzed. The results from 31P NMR experiments also showed that hydrolysis of 1a proceeded faster in acidic solution than in neutral condition. Unexpectedly, 1a exhibited unusual resistance toward hydrolysis in alkaline solution. This implies that the addition base is crucial to cyclic diaminophosphine oxides against hydrolysis in catalytic reaction in the presence of trace amount of water. Moreover, the effect of Neighboring Group Participation (N.G.P.) is presumed to account for the notable stability of preligand 1a in basic condition.

The direct and selective activation of C–H bonds for the synthesis of N-heterocycles (C–N bonds) is an important area in modern synthetic chemistry due to its wide application in both in academics and industry. These compounds are frequently found as backbones in bioactive molecules and fine chemicals and are often used as organocatalysts or as auxiliary ligands in transition metal complexes assisted catalyses. Various kinds of palladium-containing complexes as active catalysts in the C(sp 3 )-H or C(sp 2 )-H bond activation have attracted increasing attention because of their accessibility and catalytic efficiencies. This review summarizes the recent developments in this area which employs Pd 0 –Pd IV as versatile catalyst precursors to the syntheses of N-heterocycles. The N-heterocycles, including five- and six-membered rings, are described separately. Furthermore, some of the asymmetric C–H bond activation and mechanistic aspects of these catalytic reactions are discussed.

New pre-ligands of 1-aryl-1H-imidazol-2-yl substituted secondary phosphine oxides (HRP( O)-Im:SPO-Im) were prepared and characterized by spectroscopic means. The SPO-Im pre-ligands are able to tautomerize to their corresponding genuine ligands, the 1-aryl-1H-imidazol-2-yl substituted phosphinous acids ((HO)RP-Im:PA–Im) while the condition permitted. Further reactions of selected SPO-Im pre-ligands toward Pd(COD)Cl2 or PdBr yielded bis-(PA–Im)-coordinated cis-palladium dichloride or dibromide complexes, respectively. These cis-palladium dihalide complexes are all having an intramolecular hydrogen bond between two coordinated PA–Im ligands (R(Im(H)+)P–O−⋯HO–PRIm) by transferring a proton from one of the two P–OH groups to its free nitrogen site of imidazolyl substituents; and, therefore, imposes the formation of zwitterionic cis-form palladium complexes. Three crystal structures of these square–planar zwitterionic palladium complexes were determined by the X-ray diffraction methods. Additionally, two novel NHC carbene coordinated palladium complexes were obtained from the crystal-growing process of the above-mentioned zwitterionic cis-palladium dihalide complexes. The crystal structures of two NHC carbene coordinated palladium complexes were also successfully determined. Density-functional theory (DFT) calculations were carried out to substantiate the feasibility of the proposed reaction pathways for the formation of NHC carbene coordinated palladium complexes. Finally, applying selected SPO-Im pre-ligands to Heck reactions with pre-optimized reaction conditions exhibited satisfactory performances.

The reactions of indole ( 1 ) with iodotoluene in the presence of norbornene or dicyclopentadiene led to the formation of indole derivative ( 2a ). It is through the C–H activation at ortho -position of indole by Catellani-type reactions. Besides, several palladium complexes ( 4a , 4b , 5a , 5b , 6a , and 6b ), containing either single or double norbornene or dicyclopentadiene fragments via insertion into the Pd–C(aryl) bond of the formerly formed palladium-containing intermediate, were obtained and characterized by spectroscopic means as well as by X-ray diffraction methods. The ratios of 2a to 5a could be altered by employing different palladium sources. Triple insertion of norbornene or dicyclopentadiene into the Pd–C(aryl) bond was not observed.

A new route to the direct preparation of H-phosphinate esters has been explored. The ring-opening reaction of cyclic ether (tetrahydrofuran or tetrahydropyrane) was carried out with magnesium halide in the presence of phosphine halide (PRCl2 or PCl3). The process is straightforward and all the reagents are relatively cheap and readily available. Magnesium halide-mediated THF ring-opening (SN2@C) and the subsequent SN2@P elementary reactions that giving rise to the intermediate of haloalkyl phosphinates have been discussed based on our experimental findings (Path I: SN2@C−+SN2@P). Another possible route, the direct SN2 between THF (nucleophile) and phosphine halide (electrophile) that followed by THF ring opening by halide dissociated from phosphine halide (Path II: SN2@P−+SN2@C), was also proposed. However, path II is the least likely reaction path because neutral THF is not a good nucleophile. H-phosphinate esters could be readily available in the subsequent hydrolysis process. Considering the ionic bond strength in magnesium halides and the nucleophilicity of halides dissociated from MgX2 in protic solvents like water, MgBr2 is recommended for ring-opening reactions of cyclic ethers.

Preligands of quinolyl-substituted secondary phosphine oxides (SPOs, 2a–d) were prepared and characterized. The unique palladium complex 3, having a distorted-square-pyramidal structure, was obtained from the reaction of 2 equiv of 2c with Pd(COD)Cl2 or [Pd(μ2-Cl)(η3-allyl)]2. In the crystal structure of 3, an apical chloride ligand and a supramolecular tetradentate ligand composed of a deprotonated 2c′ and a neutral 2c′ were resolved (2c′: PA form of 2c). Intriguingly, the gas-phase optimized geometry of 3 converged to a distorted-square-planar structure, which was predicted by density functional calculations. The solid-state distorted-square-pyramidal structure of 3 can only be explained with the consideration of environmental effects (i.e., the electrostatic interactions between the surrounding molecules). As also evidenced by 31P NMR experiments performed in different deuterated solvents, the crystal structure of 3 is retained in solution. In the crystal structure of 3, a long Pd–Cl bond was analyzed ...

An efficient, mild, and simple protocol is presented for the oxidation of aldehydes and diols to carboxylic acids utilizing 70% aq TBHP as oxidant and t-BuOK as additive. The oxidation of aldehydes could be achieved by two methods under aqueous medium. Excellent yields of products were obtained in short reaction times. Notably, the products were isolated by simple filtration technique and do not involve chromatographic separation. These reactions may prove to be valuable alternatives to traditional metal-mediated oxidations. Oxidation does not require any transition metals or organic solvents in reaction, making this protocol green.

Several new indolyl secondary phosphine oxides (SPOs), C8H6N–P( O)(H)(tBu) (4a′), MeC8H5N–P( O)(H)(tBu) (4b′), and PhC8H5N–P( O)(H)(tBu) (4c′), were prepared from the reactions of corresponding 1H-indoles with PtBuCl2 and hydrolysis followed. Further reaction of 4a′, a tautomeric form of C8H6N–P(OH)(tBu) (4a), with Pd(COD)Cl2 yielded a di-palladium complex, [(4a)(4a-H+)Pd(μ-Cl)]2 (6a). Crystal structures of PhC8H5N–P(Cl)(tBu) (3c), (4c′) and (6a) were determined by single-crystal X-ray diffraction methods. The stability of this type of SPO is presumably caused by the indolyl scaffold. In addition, SPOs are air- and moisture-stable preligands and are advantageous for the purpose of storage. In this work, these SPO ligands were employed in the Suzuki–Miyaura cross-coupling reactions as catalytic precursors. This catalytic condition has been tolerated to a variety of functional groups and the coupling reactions underwent efficiently with the arylbromides and arylchlorides, respectively.

A series of cyclohexane-1,2-diamine (3a–3d) and benzene-1,2-diamine derivatives (3e–3h) were pre- pared. Followed by hydrolysis, the reaction of 3a–3c with PCl3 successfully led to the formation of cor- responding metastable saturated heteroatom-substituted secondary phosphine oxides (HASPO 4a–4c), a tautomer of the saturated heteroatom-substituted phosphinous acid (HAPA). Whereas ambient-stable diamine-coordinated palladium complexes were obtained, HAPA-coordinated palladium complexes were not successfully synthesized. The molecular structures of HASPO 4c, Pd(OAc)2(3a), PdBr2(3b) and Pd(OAc)2(3c) and [Cu(NO3)(3d)+][NO3−] were determined by single-crystal X-ray diffraction method. Catalysis of in-situ Suzuki-Miyaura cross-coupling reactions for aryl bromides and phenylboronic acid using diamine 3a as ancillary ligand showed that the optimized reaction condition at 60 °C is the combination of 2 mmol % 3a/3.0 mmol KOH/1.0 mL 1,4-dioxane/1 mmol % Pd(OAc)2. Moreover, moderate reactivity was observed when using aryl chlorides as substrates (supporting infor- mation). When diamine 3d was employed in Heck reaction, good tolerance of functional groups of aryl bromides were observed while using 4-bromoanisole and styrene as substrates. The optimized condi- tion for Heck reaction at 100 °C is 3 mmol % 3d/3.0 mmol CsF/1.0 mL toluene/3 mmol % Pd(OAc)2. In general, cyclohexane-1,2-diamine derivatives exhibited better catalytic properties than those of benzene-1,2-diamines.

This review describes the development in the use of secondary phosphine oxides ligands in transition-metal-catalyzed oxidative coupling reactions to the formation of C C, C N and C S bonds. Also the developments in the recent literature are highlighted in the areas of cycloaddition, coupling of acyl halide with organometallic reagents, oxidations and other reactions. The secondary phosphine oxides are known to exist in the tautomeric forms and are configurationally stable; this property enables a variety of useful transformations for accessing optically active organophosphorus compounds which are useful in the formation of C C bonds and asymmetric hydrogenation reactions. Finally, the mechanistic aspect employing DFT technique in these oxidative coupling reactions is described in this article.

Several adiabatic potential energy surfaces (PES) were employed for probing the processes of the formation of the cobaltacycles (η5-C5H5)(PMe3-κP)Co(1,4-C4(PPh2)2R2-1κC,4κC′) (3P) and their convers...

Two air- and moisture-stable secondary phosphine oxides (SPOs), namely, 2-(tert-butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole (3a, R = H; 3b, R = OMe) were prepared and characterized. A tautomeric equilibrium exists between the secondary phosphine oxide 3a (or 3b) and its corresponding phosphinous acid 3a′ (or 3b′). The reaction of phosphinous acid 3a′ (or 3b′) with Pd(COD)Cl2 (or PdBr2) yielded the corresponding monopalladium complex bis(2-(tert-butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole)PdX2 (5a, R = H, X = Cl; 5b, R = OMe, X = Br) in cis-form as well as dipalladium complex [(μ2-2-(tert-butylhydrophosphoryl)-1-(2-R-phenyl)-1H-imidazole-N,P)Pd(I)X]2 (6a, R = H, X = Cl; 6b, R = OMe, X = Br). The phosphinous acids 3a′ and 3b′ act as P,N-chelating ligands in the formation of 6a and 6b, which feature direct Pd(I)−Pd(I) bonds. Good to excellent reactivities were observed by applying these ligand-chelated palladium complexes in Suzuki−Miyaura coupling reactions.

An unexpected cobalt-containing zwitterionic complex, [{(μ-PPh 2 CH 2 PPh 2 )CO 2 (CO) 4 }{μ,η-(R1)PC≡CCH 2 (R 2 )}] (8; R1 = Ph 2 Bz; R2 = 3-CoBr 3 -1-benzimidazole), has been obtained during an attempt to modify the imidazole moiety in the alkyne-bridging dicobalt complex [{(μ-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 )-{μ,η-Ph 2 PC≡CCH 2 (IM)}] (5; IM = benzimidazole) to a carbene-type substituent by reaction with benzyl bromide. As revealed in the crystal structure of 8, it is clear that the benzyl bromide was nucleophilically attacked by the phosphane moiety of 5 with the result the addition of the benzyl moiety. This phosphane site in 8 is in a tetrahedral environment and presumably cationic in nature. In addition, an unusual anionic species CoBr 3 - was found to have attached itself to the free nitrogen site of the imidazole moiety. Thus 8 can be regarded as an unusual metal-containing zwitterionic complex. When the phosphane site of 5 was replaced by a non-reactive substituent, for example, [{(μ-PPh 2 CH 2 PPh 2 )CO 2 (CO) 4 }(μ,η-RC≡CCH 2 (IM)}] (5': R = H; 5_O: R = Ph 2 (O)P), nucleophilic attack took place exclusively on the imidazole moiety and led to the formation of [{(μ-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η-RC≡CCH 2 (IM')} + ]Br - [9': R = H; 9_O: R = Ph 2 (O)P; IM' = 3-benzyl-1H-benzimidazole]. Complex 9' could act as a carbene precursor, which was demonstrated by the formation of the bis-9'-coordinated palladium complex [trans-({(μ-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 }{μ,η-HC≡CCH 2 (IM')}) 2 PdBr 2 ] (11) on treatment with [Pd(cod)Cl 2 ]. Fair-to-good efficiencies were observed in Heck reactions employing either isolated 11 or in situ 9'/PdCl 2 (2:1) as the catalytic precursor.

The reaction of (η5-C5H4iPr)Co(PPh3)2 with PhC CPPh2 furnished two isomeric cyclobutadiene-substituted Cp′CoCb diphosphines, [(η5-C5H4iPr)Co(η4-1,2-(PPh2)2C4Ph2)] (5-cis) and [(η5-C5H4iPr)Co(η4-1,3-(PPh2)2C4Ph2)] (5-trans). Further reaction of 5-cis with one molar equivalent of Pd(COD)Cl2 gave palladium complex [(η5-C5H4iPr)Co(η4-1,2-(PPh2)2C4Ph2)-PdCl2] (6) in good yield. Both of the molecular structures of 5-cis and 6 were determined by single-crystal X-ray diffraction methods. Unexpectedly, the palladium complex 6 was found to be more efficient than the combination of the commonly used Buchwald’s ligand, biphenyl-2-yl-di-tert-butyl-phosphane, with Pd(OAc)2 as the catalytic precursor in the Suzuki–Miyaura reaction between ferroceneboronic acid and 4-bromoaldehyde. The X-ray structural analysis and DFT study of several palladium complexes containing sandwich-type diphosphine chelating ligands revealed that the variations in bite angles are much larger than those in bite distances. The more energetically favorable conformation in the Pd(II) complexes is the one with bite angle close to 90°.

Several new ferrocenyl secondary phosphine oxides, (η5-C5H4-P(═O)(Ph)(H))2Fe (7aB) and (η5-C5H4-P(═O)(R)(H))(η5-C5H5)Fe (7M, a: R = Ph; c: R = Cy), were prepared and characterized by spectroscopic means. Further reaction of Pd(COD)Cl2 with 2 molar equiv of (η5-C5H4-P(═O)(R)(H))2Fe (7bM, b: R = tBu) led to the formation of the palladium dimer [(7b′M)(7b′M-H+)Pd(μ-Cl)]2, 8. In addition, the reaction of Pd(OAc)2 with 1 molar equiv of 7bM yielded a bis-7bM-coordinated palladium acetate, [(7b′M)(7b′M-H+)Pd(OAc)], 9. The molecular structures of 7aB, 7aM, 7bM, 8, and 9 were determined by single-crystal X-ray diffraction methods. A tautomeric equilibrium between 7aB and its isomeric form (η5-C5H4-P(OH)(Ph))2Fe (7a′B) indeed took place. This was also true for the case of 7aM and its isomeric form (η5-C5H4-P(OH)(Ph))(η5-C5H5)Fe (7a′M). Palladium-catalyzed Suzuki−Miyaura reactions employing 7aB and 7bM as ligands gave satisfactory results.

N,N,O-Tridentate ligands 1–9 were prepared from the condensation of amines with nine aromatic aldehydes or ketones. These ligands are thermally stable and neither air- nor moisture-sensitive. Combination of either 2-methoxy-6-[(pyridine-2-ylmethylimino)-methyl]-phenol, 1 or 2-(benzothiazol-2-yl-hydrazonomethyl)-4,6-di-tert-butyl-phenol, 6 with Pd(OAc)2 furnished an excellent catalyst precursor for the Suzuki–Miyaura cross-coupling of various aryl bromides with arylboronic acids. The effects of varying solvents, bases, and ligand/palladium ratios on the performance of the coupling reaction were investigated. The molecular structures of both free ligand 1 and its palladium acetate complex 10 were determined by single-crystal X-ray diffraction methods. The DFT studies revealed that the catalytic performance of palladium complexes involving this type of a ligand may differ greatly upon a small variation in its structure.

A cobalt-containing P,N-ligand, alkyne-bridged dicobalt phosphine [(μ-PPh2CH2PPh2)Co2(CO)4(μ,η-Me2NCH2C CP(Cy)2)] (4c), was prepared from the reaction of Me2NCH2C CPR2 (2c) with a dppm-bridged dicobalt complex [Co2(CO)6(μ-P,P-PPh2CH2PPh2)] (3). Further reaction of 4c with Pd(COD)Cl2 gave a 4c-chelated palladium dichloride complex (μ-PPh2CH2PPh2)Co2(CO)4(μ,η-Cy2PC CCH2NMe2)PdCl2 (5c). Compound 5c was characterized by spectroscopic methods and crystal structure of 5c was determined by single-crystal X-ray diffraction techniques. It shows that 4c is an authentic cobalt-containing P,N-bidentate ligand. Amination and Suzuki reactions employing either a 4c/palladium salt system or an isolated 5c as the catalytic precursor led to satisfactory results.

Two cobalt-containing bulky monodentate phosphines {[(μ-PPh2CH2PPh2)Co2(CO)4][(μ,η-(tBu)2PC CAr]} (4cm: Ar = 3-CF3C6H4; 4cmm: Ar = 3,5-(CF3)2C6H3) were prepared from the reaction of Co2(CO)6(μ-PPh2CH2PPh2) (3) with each corresponding alkynes (tBu)2PC CAr. Both compounds were converted to their oxidized forms {[(μ-PPh2CH2PPh2)Co2(CO)4][(μ,η-(tBu)2P( O)C CAr]} (4cmO: Ar = 3-CF3C6H3; 4cmmO: Ar = 3,5-(CF3)2C6H3) in the presence of oxide. Further reactions of 4cm and 4cmm with Pd(OAc)2 gave palladium complexes {[(μ-PPh2CH2PPh2)Co2(CO)4][(μ,η-(tBu)2PC C(Ar)-κC1)]Pd(μ-OAc)} 5cm (Ar = 3-CF3C6H3) and 5cmm (Ar = 3,5-(CF3)2C6H2), respectively. By contrast, reactions of 4cm and 4cmm with Pd(COD)Cl2 gave products, [{μ-P,P-PPh2CH2PPh2}Co2(CO)3(μ-CO){μ,η-(tBu)2PC CAr}]-PdCl2] 8cm and 8cmm, respectively, with unique bonding modes. Several crystallines of [(4cm)2Pd3(μ-Cl)(μ-CO)2)(μ-Cl)]2 (9) were obtained along with crystallines of 8cm during the crystallization process. The crystal structures of all three compounds, 4cmmO, 8cmm and 9 were determined by single-crystal X-ray diffraction methods. Fair to excellent efficiencies were observed for employing 4cmm/palladium salt as catalytic precursor in amination as well as in Suzuki coupling reactions.

Several cobalt-containing P,N-ligands, alkyne-bridged dicobalt phosphines [(μ-PPh2CH2PPh2)Co2(CO)4(μ,η-Me2NCH2C CPR2)] (4a: R=tBu; 4b: R=Ph; 4c: R=Cy), were prepared from the reactions of corresponding alkynylphosphines Me2NCH2C CPR2 (2a: R=tBu; 2b: R=Ph; 2c: R=Cy) with a dppm-bridged dicobalt complex [Co2(CO)6(μ-P,P-PPh2CH2PPh2)] 3. A unique palladium complex ion pair [(μ-PPh2CH2PPh2)Co2(CO)4(μ,η-Me2NCH2C CP (tBu)2)Pd(η3-C3H5)]+[(η3-C3H5)PdCl2]− 7a was obtained from the reaction of 4a with [(η3-C3H5)PdCl]2. Compounds 4a, 4b, and 4c are authentic cobalt-containing P,N-bidentate ligands and can be used for ligation of palladium from various sources such as Pd(OAc)2 or [(η3-C3H5)PdCl]2. Satisfactory efficiencies were observed for the amination reactions of aryl bromides with morpholine employing either a 4a-chelated palladium complex formed in situ or pre-formed 7a as the catalytic precursor.

Several cobalt-containing bulky monodentate phosphines (μ-PPh2CH2PPh2)Co2(CO)4(μ,η-(tBu)2PC C(C6H4R)) (4a: R=H; 4b: R=p-F; 4cp: R=p-CF3; 4cm: R=m-CF3; 4d: R=p-OMe) were prepared from the reactions of (tBu)2PC C(R–C6H4) (2a: R=H; 2b: R=p-F; 2cp: R=p-CF3; 2cm: R=m-CF3; 2d: R=p-OMe) with Co2(CO)6(μ-PPh2CH2PPh2) 3. Further reactions of 4a, 4b, 4cp, 4cm, and 4d with Pd(OAc)2 yielded unique palladium complexes (μ-PPh2CH2PPh2)Co2(CO)4(μ,η-(tBu)2PC C(C6H3R)-κC1)Pd(μ-OAc) (5a: R=H; 5b: R=p-F; 5cp: R=p-CF3; 5cm: R=m-CF3; 5d: R=p-OMe, respectively). The strong electron-withdrawing substituents, –F and –CF3, assist the ortho-metalation process during the formation of 5b, 5cp, and 5cm. The more positively charged palladium center in 5b (or 5cp, 5cm) enhances the probability for PhB(OH)3− to attack the metal center and the rate of reduction thereafter. DFT studies on the charges of these palladium centers support this assumption. The enhancement of the reaction rates of the Suzuki–Miyaura cross-coupling reactions using 5b, 5cp, and 5cm is thereby attributed to this effect.

A new route to the direct preparation of H-phosphinate esters has been explored. The ring-opening reaction of cyclic ether (tetrahydrofuran or tetrahydropyrane) was carried out with magnesium halide in the presence of phosphine halide (PRCl2 or PCl3). The process is straightforward and all the reagents are relatively cheap and readily available. Magnesium halide-mediated THF ring-opening (SN2@C) and the subsequent SN2@P elementary reactions that giving rise to the intermediate of haloalkyl phosphinates have been discussed based on our experimental findings (Path I: SN2@C−+SN2@P). Another possible route, the direct SN2 between THF (nucleophile) and phosphine halide (electrophile) that followed by THF ring opening by halide dissociated from phosphine halide (Path II: SN2@P−+SN2@C), was also proposed. However, path II is the least likely reaction path because neutral THF is not a good nucleophile. H-phosphinate esters could be readily available in the subsequent hydrolysis process. Considering the ionic bond strength in magnesium halides and the nucleophilicity of halides dissociated from MgX2 in protic solvents like water, MgBr2 is recommended for ring-opening reactions of cyclic ethers.

Two phosphinites 2,6-OMe, 4-Me, 1-OPR2C6H2 (5a: R=Ph; 5b: R=tBu) were prepared in good yields. Two methoxy groups are placed on the 2,6 positions of the phosphinites deliberately thereby to avoid a plausible orthometallation process while coordinating toward palladium metal. Further reaction of 5a with PdCl2 in the ratio of 2:1 and 1:1 gave 5a ligated palladium complexes {(5a)PdCl(μ-Cl)}2 7a and {(5a)2PdCl2} 8a, respectively. As revealed from their crystal structures, the former is a dimeric complex with anticipated molecular arrangement; nevertheless, the latter is a monomeric complex with unexpected, energetically unfavorable cis form. By contrast, only dimeric form was observed from the reaction of 5b with PdCl2. It is believed that the bulky substituents, tBu, on 5b prevent the formation of a monomeric complex in cis form. Fair to good efficiencies were observed for the Suzuki–Miyaura cross-coupling reactions employed in situ-prepared 5/Pd(OAc)2 as the catalytic precursor.

A series of cobalt-containing alcohols and diols were prepared and characterized. Intramolecular hydrogen-bonding was observed for the cobalt-containing diols [Co 2 (CO) 6 (μ-η-(HO)R 1 R 2 CC CCR 1 R 2 (OH)] ( 1 : R 1 = CH 3 , R 2 = C 2 H 5 ; 2 : R 1 = CH 3 , R 2 = C 3 H 7 ), [Co 2 (CO) 6 (μ-η-(HO)Ph 2 CC CCPh 2 (OH)] ( 3 ) and [(μ-PPh 2 CH 2 PPh 2 )Co 2 (CO) 4 (μ-η-(HO)Ph 2 CC CCPh 2 (OH)] ( 4 ). Potentially all the four compounds could serve as chelating O,O-ligands. In principle, it is possible for compounds [(μ-PPh 2 NHPPh 2 )Co 2 (CO) 4 (μ-η-HC CCPh 2 OH)] ( 5b ), [Co 2 (CO) 6 (μ-η-HC CC 2 H 4 OH] ( 6 ) and [Co 2 (CO) 6 (μ-η-HC CC 3 H 6 OH)] ( 7 ) in their syn -conformations to behave as chelating O,N-ligands. To the best of our knowledge, compounds 5b , 6 and 7 are the first reported examples of PPh 2 NHPPh 2 -bridged dicobalt complexes.

A new category of secondary phosphine oxides (SPOs) (5a–5j) with/without benzo-fused five-membered heterocyclic substituents were prepared. These new compounds are air- and moisture-stable ligands and have the advantage of long-term storage. Some of the ligands as well as ligand coordinated palladium complexes (6f′ and 6f′′) and platinum complexes (7b_trans & 7i_trans) were prepared and their structures were determined using single crystal X-ray diffraction methods. The crystal structure of 6f′ revealed the formation of diamond shape di-palladium complexes with a Pd2Cl2 core. As for the structures of 7b_trans & 7i_trans, the processes for the generation of the trans-form of the bis-phosphine ligand coordinated platinum complexes are shown. These SPOs exhibit notable efficiencies in palladium-catalyzed Suzuki–Miyaura reactions. Moreover, organic compounds (9k and 10c) with unexpected conformations were obtained from Heck-type Catellani reactions. Reaction pathways are proposed to accommodate the probable routes for the formation of all organic products.

A palladium dimer with a cobalt-containing phosphine ligand, {(μ-PPh2CH2PPh2)Co2(CO)4(μ,η-(tBu)2PC CC6H4-κC1)Pd(μ-Cl)}2 (3), was prepared from the reaction of its monomer precursor, (μ-PPh2CH2PPh2)Co2(CO)4(μ,η-(tBu)2PC CC6H4-κC1)Pd(μ-OAc) (2), with LiCl. The crystal structure of 3, determined by X-ray diffraction methods, revealed a doubly chloride-bridged palladium dimeric conformation. Suzuki coupling reactions of bromobenzene with phenylboronic acid were carried out catalytically using these two novel palladium complexes 2 and 3 as catalyst precursors. Factors such as the molar ratio of substrate/catalyst, reaction temperature, base and solvent that might affect the catalytic efficiencies were investigated. As a general rule, the performance is much better by employing 3 than 2 as the catalyst precursor.

A cobalt-containing monodentate phosphine [(μ 2 -PPh 2 CH 2 PPh 2 -κ 2 P)Co 2 (CO) 4 ][μ 2 -η 2 -PhC≡CP(i-Pr) 2 ] 2f, was prepared from the reaction of (μ 2 -PPh 2 CH 2 PPh 2 -K 2 P)Co 2 (CO) 6 1 with PhC≡CP(i-Pr) 2 . It was accompanied by an oxidized compound, [(μ 2 -PPh 2 CH 2 PPh 2 -K 2 P)Co 2 (CO) 4 ][μ 2 -η 2 -PhC=CP(=O)(i-Pr 2 )] 2fo during the chromatographic process. Further reaction of 2f with Mo(CO) 6 resulted in the formation of a 2f-ligated molybdenum complex 4, [(μ 2 -PPh 2 CH 2 PPh 2 -κ 2 P)Co 2 (CO) 4 ][μ 2 -η 2 -PhC≡CP(i-Pr 2 )-κP]-Mo(CO) 5 .

Two adiabatic potential-energy surfaces are employed for probing the processes of [Co 2 (CO) 8 ]-mediated C 6 H 5 N=NC 6 H 5 formation from NH 2 C 6 H 5 . Elementary steps, including oxidative addition of the coordinated amine proton to the cobalt center, reductive elimination of H 2 , CO association, and the coupling process of the diamino fragments, are modeled and examined by using DFT methods at the B3LYP/631LAN level. The formation of C 6 H 5 N=NC 6 H 5 from NH 2 C 6 H 5 through reductive coupling is a thermodynamically unfavorable process. Three hydride-migration processes, from the proton of N-H to the cobalt center, are established as the most energy-demanding steps. The activation energies (ΔG*) are calculated as 49.4, 55.4, and 33.3 kcal mol - 1 , respectively, for the proposed reaction route 1. These large activation energies might be reduced slightly by purposely adding small protic molecules, such as H 2 O, or by changing the active metal from Co to a heavier metal, such as Rh or Ir. An alternative pathway, route 2, is also proposed, in which transition states with four-membered rings are formed. By this route, severe strain caused by the formation of three-membered rings during the hydride-migration processes in route 1 can be avoided. Route 2 is established as the more energy-feasible reaction pathway.

A novel palladium complex 4, cis-dichloride(l,2-bis(diphenylphosphino)vinyl-P,P',C)palladium(II)-(bis(diphenylphosphino)methane-P,P')cobaltacarbonyl, was obtained and characterized from the treatment of [(μ-Ph 2 PCH 2 PPh 2 )Co 2 (CO) 4 ][(Ph 2 PC≡CPPh 2 )-PdCl 2 ] 3 with hydrochloric acid. The framework of 4 can be regarded as a grouping of two metal-containing fragments: -Co(CO) 2 (dppm) and PdCl 2 (μ-P,P-Ph 2 PCH= C(-)PPh 2 ).

Aminations of aryl bromides by morpholine have been investigated using palladium complexes as catalyst precursors modified by the cobalt-containing phosphane ligand [(μ-PPh2CH2PPh2)Co2(CO)4][μ,η-PhC≡CP(tBu)2] (1). Reactions were carried out under various conditions with satisfactory results. The components of the optimised reaction conditions consist of 1.0 mmol of aryl bromide, 1.2 equiv. of morpholine, 1.4 equiv. of NaOtBu, 1 mL of toluene and 1 mol-% of a 1:1 mixture of 1 and Pd(OAc)2. In contrast, related aminations using [(μ-PPh2CH2PPh2)Co2(CO)4][μ,η-pyC≡CP(tBu)2] (2), [(μ-PPh2CH2PPh2)Co2(CO)4][{μ,η-PhC≡CP(cy)2}] (3) and [(μ-PPh2CH2PPh2)Co2(CO)4][μ,η-PhC≡CP(iPr)2] (4) as phosphane ligands do not exhibit comparable efficiency. A novel palladium complex 5, i.e. Pd(OAc) chelated by deprotonated 1 was obtained from the reaction of 1 with Pd(OAc)2. As revealed by the crystal structure, an orthometallation process occurs between the phenyl ring of 1 and the palladium ion. Presumably, the process is accompanied by release of one equivalent of acetic acid. The coordinated acetate ligand binds to the palladium ion in a bidentate chelating mode. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The preparation and characterization of a novel dicobalt-containing monophosphine ligand, 4a, is presented. Palladium-catalyzed Suzuki−Miyuara reactions employing 4a/Pd(OAc)2 were pursued. The optimized reaction conditions were found to start with 1 molar equiv of arylhalides, 1.5-fold of phenylbronic acid, 3-fold of KF in 1 mL of THF, and 1 mol % of 4a/Pd(OAc)2 as catalytic precursor. The 31P NMR studies reveal moderate reductive capacity of 4a toward Pd(OAc)2. The unique bonding mode of 4a toward Pd ensures that the ratio of 4a/Pd is equal to 1:1. Two plausible active species, I and [I-OAc]-, were proposed as the catalytically active species in the Suzuki−Miyaura cross-coupling reaction. The validity of this assumption was examined by 31P NMR spectra and density functional theory (DFT) means. In addition, we have demonstrated theoretically that the dicobalt moiety of 4a acts as an effective auxiliary in stabilizing the Pd(0) center during catalytic reaction.

Palladium-catalyzed Suzuki cross-coupling reactions employing Schiff-bases as ligands toward a series of substituted arylbromides and boronic acids were pursued. In the presence of a N,O-bidentate ligand, 2-[1-(2,4,6-trimethyl-phenylimino)-ethyl]-phenol 5 , the catalytic reactions could be carried out efficiently at room temperature with a wide array of arylbromides, even with electronically deactivated arenes. A deprotonated 5 , 5′ , chelated palladium acetate complex, [ 5′ Pd(II)(OAc)(solv)] 8 , was proposed as a precursor of a genuine catalytically active species.

Biphasic Suzuki-coupling reactions of aryl and benzyl bromides employing a cobalt-containing phosphine ligand chelated palladium complex 2 were carried out in various reaction conditions. Comparisons of the catalytic efficiencies in the presence/absence of a phase-transfer agent, TBAB, were presented. In addition, the effects of altering solvents, temperatures, catalysts, and substrates on the reactions were monitored and reported. Better yields were commonly observed while a phase-transfer agent TBAB was participated in the reactions. The factor of reaction time is more crucial than that of temperature in short reaction hour experiments. Obviously, an induction period for the reduction of Pd(II) to Pd(0) active species is needed for this type of reaction.

A bulky, dicobalt complexed, mono-dentate phosphine, [(μ-PPh2CH2PPh2)Co2(CO)4](μ,η-PhC CPCy2) (4), was prepared from the reactions of the bis(diphenylphosphino)methylene (dppm)-bridged dicobalt complex Co2(CO)6(μ-Ph2PCH2PPh2) (2) with PhC CPCy2 (3). Combination of 4 and Pd(OAc)2 (1:1) gave an active catalyst for the palladium-catalyzed Suzuki coupling of aryl bromides with phenylboronic acid; the catalytic reactions can be performed even under low catalyst loadings (0.1–0.001 mol% 4/Pd(OAc)2). Compound 4 has been proved to be an authentic and effective mono-dentate phosphine ligand. Crucial factors such as 4/Pd(OAc)2 ratio, base being used, solvent volume, temperature, and electronic variation of the aryl bromides in reactions were also investigated and results are reported.

Treatments of a bis(diphenylphosphino)methylene (dppm) bridged dicobalt complex, Co2(CO)6(dppm) (4), with propargylamine and 4-ethynylaniline at 25 °C for 24 h gave [(dppm)Co2(CO)4(μ-HC CCH2NH2)] (5) and [(dppm)Co2(CO)4(μ-HC CC6H4NH2)] (6), respectively. Interestingly, only alkynyl amines bridged dicobalt complexes were obtained rather than the previously observed coupling products. The results are in acceptance with the proposed mechanism which describes the formation of the coupling products {[Co2(CO)6(μ-HC C–)]–CH2NH}2C O (1) and {[Co2(CO)6(μ-HC C–)]–C6H4N }2 (2) from the reaction of Co2(CO)8 with propargylamine and 4-ethynylaniline, respectively. Similar results were attained for the reactions of 4 with propioamide and 1-ethynylcyclohexylamine at 25 °C for 24 h which yielded [(dppm)Co2(CO)4(μ-HC CC( O)NH2)] (7) and [(dppm)Co2(CO)4(μ-HC CC6H10NH2)] (8), respectively. Reaction of 1-ethynylcyclohexylamine with one molar equivalent of Co2(CO)8 in THF at 25 °C for 15 min gave an alkyne bridged dicobalt complex, [Co2(CO)6(μ-HC CC6H10NH2)] (9). Direct treatment of 3-ethynlaniline with one molar equivalent of Co2(CO)8 in THF at 25 °C for 1 h gave an alkyne bridged dicobalt complex, [Co2(CO)6(μ-HC CC6H4NH2)] (11) and an azobenzene derivative, {[Co2(CO)6(μ-HC C)]C6H4N }2 (10). Further treatments of 8, 9, and 11 with one molar equivalent of Sanger’s reagent, 2,4-dinitrofluorobenzene, in THF at25 °C for 48 h gave [(dppm)Co2(CO)4(μ-HC CC6H10NHC6H3(NO2)2)] (13), [Co2(CO)6(μ-HC CC6H10NHC6H3(NO2)2)] (14), and [Co2(CO)6(μ-HC CC6H4NHC6H3(NO2)2)] (15), respectively.