Search

A cheap and easily recoverable heterogeneous catalyst could efficiently trigger C–P bond formation by selective addition of phosphites to the carbonyl group of aldehydes and ketones. The reactions afford the desired products in good to excellent yields and do not require the presence of a solvent. Furthermore, nearly stoichiometric amounts of reagents could be used as the hydrotalcite catalyst does not cause the condensation of the carbonyl derivatives, even when aliphatic aldehydes are employed.

Blue LED-induced reaction between aryldiazonium salts and arenes to biaryls proceeds with some interesting regioselective features depending on the substitution patterns of both reaction partners as well as the use of additives (e.g., K2CO3).

Palladium catalyzed cross-coupling reactions are one of the most widely used class of transformation as shown by the Nobel prize awarded in 2010 to Heck, Negishi, and Suzuki. Computational chemistry has a long-standing partnership with organometallics catalysis, especially with palladium. But even in a largely explored field, novelties can emerge from interplay between experiments and theory. Recent advances grounded on computational chemistry have shown that cooperative effect can explain reactivities; that despite the large number of well-known Pd(0)/Pd(II) catalytic cycle, Pd(IV) is also a realistic intermediate in some cases; that noncovalent interactions can regulate selectivities. So, despite its wide use and recognition, palladium complexes are still full of surprises! WIREs Comput Mol Sci 2013, 3:529–541. doi: 10.1002/wcms.1137 For further resources related to this article, please visit the WIREs website.

Joint palladium/norbornene organometallic catalysis allows for straightforward access to dibenzo[c,e]azepines. These synthetically challenging polycyclic frameworks form in one pot via a three-component coupling of an aryl iodide, a bromobenzylamine, and an olefin. A key, atroposelective aryl–aryl coupling from chelated Pd(IV) intermediates dictates the outcome of the cascade. DFT modeling sheds light on the complex mechanism that allows the complete diasteroselectivity to be observed.

Photochemical reaction of diaryl disulfides with N-acyl cyanamides and iPr2NEt proceeds with 1,3-migration of the acyl group and provides a new approach to isothioureas.

The cyanamide moiety provides many opportunities as a building block for the synthesis of nitrogen-containing heterocycles. This review discusses the reported methods for the preparation of alkyl- and N-acylcyanamides and illustrates their reactivity vis-a-vis nucleophilic additions, cycloadditions, radical chemistry and coordination chemistry. 1 Introduction 2 Biological Relevance of the Cyanamide Moiety 3 Access to Cyanamides 3.1 Cyanation of Amines 3.2 Palladium Catalysis 3.3 Deoxygenation of Isocyanates 3.4 Desulfurization of Thioureas 4 Reactivity of Cyanamides 4.1 Nucleophilic Additions 4.2 Cycloadditions 4.3 Additions to Carbonyl Groups 4.4 Chlorinations and Oxidations with tert-Butyl-N-chloro­cyanamide 4.5 Cyanamides as Radical Partners 5 Coordination Chemistry of Cyanamides 6 Conclusion

Following a current trend of chemical research, photochemical investigations are moving from molecular to supramolecular species. Some of the results obtained by the authors with supramolecular species containing metal complexes are briefly reviewed, with particular emphasis on (i) cage-type complexes, (ii) host-guest systems, (iii) metal catenates, and (iv) oligonuclear metal complexes.

The chiral synthons 1 ( a – d ) were submitted to boron-mediated asymmetric aldol condensation with acetaldehyde and benzaldehyde providing, in high diastereomeric excess (>95%), R,R -configured aldols 2 ( a – f ) which are useful intermediates to enantiomerically pure β-hydroxy-α-amino acids.

Selectively substituted biaryls containing a ketone unit are obtained under mild conditions in satisfactory yields by a simple reaction of ortho-substituted aryl iodides and ketones in the presence of palladium and norbornene as catalysts.

Two highly enantio- and diastereoselective one-pot procedures for the synthesis of epoxy alcohols with up to three contiguous stereocenters are reported. Route one involves asymmetric addition of an alkylzinc reagent to an enal followed by diastereoselective epoxidation. Route two entails asymmetric vinylation of an aldehyde with divinylzinc reagents and subsequent diastereoselective epoxidation. The oxidant for the epoxidation is generated by exposure of the allylic alkoxide intermediate and the remaining organozinc reagent to dioxygen. Upon addition of catalytic titanium tetraisopropoxide, the directed epoxidation yields the epoxy alcohols with good to excellent yields.

Polyazacrown ethers containing one, two, and four appended naphthyl or anthracenyl units have been synthesized. The absorption spectra and photophysical properties of the novel compounds have been investigated in CH2Cl2 solution. Acid titration of the amine nitrogens of the polyazacrown ethers causes strong changes in the absorption and fluorescence spectra. In the unprotonated compounds of the naphthalene family, the naphthalene-type emission is completely quenched and a weak, unstructured, and broad fluorescence band with maximum at 440 nm is observed. Upon addition of trifluoroacetic acid, the absorption maximum is displaced to the red by a few nanometers, and a revival of the strong naphthalene-type emission at 340 nm is observed. These results are accounted for by the change in the nature of the lowest excited state (CT to π-π*) upon protonation. In the compounds of the anthracene family, the deprotonated forms of the species containing the polyazacrown groups show an absorption around 300 nm and a long wavelength tail at lower energies, neither of which are present in the 9-[(methylamino)methyl]anthracene. In all cases a weak, anthracene-type fluorescence is present, increasing in intensity on protonation. Furthermore, the emission spectra of the compounds with two or four anthracene moieties show an excitation-dependent, broad emission band at lower energies, which almost disappears upon protonation. This behaviour can be accounted for by the quenching of the anthracene-type emission caused by lower lying charge-transfer levels and, in the case of the compounds containing two and four anthracene moieties, of conformers where an anthracene-anthracene interaction is present. In all cases, each equivalent of added acid causes protonation of one equivalent of crown nitrogen. However, in the azacrown with four appended naphthyl units the revival of the naphthalene-type fluorescence does not parallel the number of added protons. This shows that the higher energy π-π* levels of the protonated units are quenched by the lower energy CT levels of the units involving the not yet protonated crown nitrogens.

We report the results concerning the absorption spectra, luminescence spectra and luminescence lifetimes of the complexes [(η 5 -Me 5 C 5 )Ir(bpy)Cl]Cl ( 1 ), [(η 5 -Me 5 C 5 )Ir(phen)Cl]Cl ( 2 ), [(η 5 -Me 5 C 5 )Ir(bpy)H]BPh 4 ( 3 ) and [(η 5 -Me 5 C 5 )Ir(phen)H]BPh 4 ( 4 ) (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline and η 5 -Me 5 C 5 = pentamethylcyclopentadienyl = Cp*). For all complexes examined the low energy absorption band is assigned to metal-to-ligand charge-transfer transitions involving the diimine ligands. For the hydrido complexes 2 and 4 the luminescence observed both at 77 K and at room temperature is assigned to metal-to-ligand charge-transfer transitions involving the diimine ligands. The chloride complexes 1 and 3 were not found to emit.

The Co(sep)/sup 3 +/ complex (sep = sepulchrate = 1,3,6,8,10,13,16,19-octaazabicyclo(6.6.6)eicosane) in aqueous solution forms ion pairs with the I/sup -/ and Br/sup -/ anions, which exhibit a relatively intense charge-transfer absorption. In deoxygenated aqueous solution no net reaction is obtained upon continuous light excitation in the ion-pair charge-transfer bands, but formation of transient species is observed in flash photolysis experiments. For the Co(sep)/sup 3 +/-I/sup -/ system, I/sub 2//sup -/ ions are formed that decay in the 50-..mu..s time scale to give I/sub 3//sup -/. The latter species disappears in a time scale of seconds, leading the system back to the preexcitation conditions. For the Co(sep)/sup 3 +/-Br/sup -/ system, only formation of the Br/sub 2//sup -/ transient is observed, followed by regeneration of the preexcitation conditions in the 20-..mu..s time scale. In order to elucidate the kinetic aspects of the transient formation and disappearance, pulse radiolysis experiments on the Co(sep)/sup 3 +/-I/sup -/ and Co(sep)/sup 3 +/-Br/sup -/ systems have been carried out. The rate constants of the reactions of Co(sep)/sup 2 +/ with I/sub 2//sup -/, I/sub 3//sup -/, and Br/sub 2//sup -/ have been measured, and a complete picture of the redox processes that follow flash photolysismore » and pulse radiolysis excitations has been obtained. 5 figures.« less