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This account summarizes the Cu-catalyzed borylative functionalization of unsaturated hydrocarbons that we have developed over the past decade. First, we focus on the regioselective hydroboration of unsymmetrical internal alkynes and 1,2-dienes. Second, the borylative allyl-allyl coupling using 1,2-dienes, B2pin2, and allyl phosphate is presented. Third, we introduce boro-acylation and boroformylation of 1,2-dienes using an acid anhydride and formate, respectively, as the electrophile. Lastly, we describe the synthesis of 2-boryl-1,3-butadienes and cyclic allylborates. These reactions can offer a versatile method for synthesizing a broad range of useful compounds, and also provide a detailed understanding of the reaction mechanism, which could lead to the development of more efficient and selective catalysts.

The sila-acylation of allenes is performed in the presence of a copper catalyst using PhMe2Si-B(pin) and esters as the silyl and acyl sources, respectively. β-Silyl-β,γ-unsaturated ketones are obtained regioselectively in good to high yields. The sila-imination of allenes is also achieved using nitriles as electrophiles. Exposure of the reaction mixture to air results in the production of cyclic silyl peroxides.

We have succeeded in the synthesis of various tetrasilatetrathia[8]circulenes with alkyl and aryl groups on the silicon atoms. We also disclosed the effect of phosphine ligands on palladium-catalyzed silylation of tetraiodotetrathienylene and rhodium-catalyzed intramolecular silylation of tetrasilyltetrathienylenes. Experimental and theoretical analysis revealed the effect of the substituents on the silicon atoms on their electronic property.

A series of pyridine ligands bearing poly(ethylene glycol) (PEG) chains at the para position was synthesized and characterized by NMR and ESI‐HRMS analysis. 1H NMR analysis showed that pyridines coordinate to Pd(OAc)2 even when a long PEG chain is attached to the pyridine ring. In the Pd‐catalyzed oxidation of alcohols, the pyridines bearing longer PEG chains were found to be efficient ligands.

Zinc-catalyzed synthesis of acylsilanes using carboxylic acids and a silylborane has been achieved in the presence of pivalic anhydride. Various carboxylic acids were converted to the corresponding acylsilanes. The in situ formation of mixed anhydrides was essential in the present reaction.

The regioselective sila-acylation and silaformylation of 1,3-dienes was achieved over a copper catalyst using a silylborane as a silyl source. β,γ-Unsaturated ketones with a (dimethylphenylsilyl)methyl moiety at the α-position were obtained using esters, while β,γ-unsaturated aldehydes were obtained using formate esters.

A highly regioselective and stereoselective difunctionalization reaction of 1,3-diene with amine and disilane to form C-N and C-Si bonds via a one-step Pd/Cu/O2 system is disclosed. The difunctionalization reaction affords allylic silanes, including the allylic amine moiety, in up to 92 % yield in the absence of any acid, base, or external ligand. The developed synthetic methodology can be scaled to 100 g in high yield with high Z-selectivity, which demonstrates the feasibility of the reaction for industrial applications.

Carbon dioxide (CO2) is one of the most important materials as renewable chemical feedstock. In this review, the Co- and Rh-catalyzed transformation of CO2 via carbon–carbon bond-forming reactions is summarized. Combinations of metals (cobalt or rhodium), substrates, and reducing agents realize efficient carboxylation reactions using CO2. The carboxylation of propargyl acetates and alkenyl triflates using cobalt complexes as well as the cobalt-catalyzed reductive carboxylation of α,β-unsaturated nitriles and carboxyamides in the presence of Et2Zn proceed. A Co complex has been demonstrated to act as an efficient catalyst in the carboxylation of allylic C(sp3)–H bonds. Employing zinc as the reductant, carboxyzincation and the four-component coupling reaction between alkyne, acrylates, CO2, and zinc occur efficiently. Rh complexes also catalyze the carboxylation of arylboronic esters, C(sp2)–H carboxylation of aromatic compounds, and hydrocarboxylation of styrene derivatives. The Rh-catalyzed [2 + 2 + 2] cycloaddition of diynes and CO2 proceeds to afford pyrones.

Herein, copper-catalyzed borylative and silylative transformations of allenes using borylcopper or silylcopper as the active catalytic species are described. First, the synthesis and characterization of borylcopper and silylcopper complexes are briefly introduced. Next, the borylative transformations of allenes are summarized including hydro­boration, carboboration, and borylative allylation of carbonyl compounds. We next deal with the silylative transformations of allenes such as hydrosilylation, carbosilylation, and silylative allylation of carbonyl compounds.1 Introduction2 Synthesis of Borylcopper and Silylcopper Complexes3 Borylative Transformations4 Silylative Transformations5 Conclusions and Future Outlook

Carboxylic acids 1a and 1b bearing three poly(ethylene glycol) (PEG) chains and carboxylic acid 1c bearing one PEG chain were designed and synthesized. The carboxylic acids 1a–c were fully characterized by NMR and ESI-HRMS analyses. In a Pd-catalyzed aerobic oxidation of 1-phenylethanol, 1a and 1b worked well as carboxylate ligands.

Macrocyclic dinuclear complexes have been gaining popularity in the design of homogeneous catalysts. Herein, we report the design of such a complex featuring catalytically active sites fixed inside the ring and its synthesis using a cross-coupling reaction.

The boraformylation of allenes with B2 (pin)2 and a formate ester as boron and formyl source, respectively, proceeds in the presence of a copper catalyst. The reaction selectively affords the corresponding β-boryl β,γ-unsaturated aldehydes in good to high yields. Furthermore, the silaformylation of allenes was achieved with a formate ester and PhMe2 Si-B(pin) as the silicon source.

Various carboxylic acids with different substitution patterns were used as ligands in intra- and intermolecular Pd-catalyzed C–H activation reactions to investigate steric effect of the ligands. Bulky carboxylic acids suppress deactivation of Pd catalysts.

This review summarizes transformations using copper hydride (Cu-H), boryl copper (Cu-B) or silyl copper (Cu-Si) as active catalyst species. Semihydrogenation, hydroboration, and hydrocarboxylation have been developed using Cu-H as active catalyst species. Preferential reduction of sterically hindered ketones is accompanied by Cu-H species bearing a bowl-shaped phosphine as a crucial ligand. Employing Cu-B species, hydroboration of alkynes and allenes, synthesis of 2-boryl-1,3-butadiene, and borylative allyl-allyl coupling are established. Silacarboxylation of alkynes, regiodivergent silacarboxylation of allenes, and silylative allylation of ketones involve Cu-Si species. The key to these selective transformations is regioselective addition of Cu-H, Cu-B, or Cu-Si species to carbon-carbon multiple bonds of alkynes and allenes to afford the corresponding alkenylcopper or allylcopper intermediates.

Cobalt-catalyzed carboxyzincation reactions employing carbon dioxide and zinc metal powder are developed. By using alkynes as substrates, regio- and stereodefined (Z)-β-zincated acrylates are provided. The corresponding alkenylzinc moiety can be converted to various substituents, affording multisubstituted acrylic acids. Furthermore, by adding electron-deficient alkene to the reaction system, the four-component coupling reactions of alkyne, alkene, CO2, and the Zn atom proceed via carboxyzincation.

We describe a new concept for rotaxane synthesis through intramolecular slippage using π-conjugated molecules as rigid axles linked with organic soluble and flexible permethylated α-cyclodextrins (PM α-CDs) as macrocycles. Through hydrophilic-hydrophobic interactions and flipping of PM α-CDs, successful quantitative conversion into rotaxanes was achieved without covalent bond formation. The rotaxanes had high activation barrier for their de-threading, so that they were kinetically isolated and derivatized even under conditions unfavorable for maintaining the rotaxane structures. (1) H NMR spectroscopy experiments clearly revealed that the restricted motion of the linked macrocycle with the rigid axle made it possible to control the kinetic stability by adjusting the length of the rigid axle in the precursor structure rather than the steric bulkiness of the stopper unit.

In this study, π-conjugated molecular wires strapped by cyclic π-conjugated side chains were efficiently synthesized by the integration of dehydrobenzo[20]annulene units by intramolecular Glaser-type cyclization under high dilution conditions.

The formal hydroacylation reaction of allenes has been developed employing carboxylic anhydrides as acyl sources and hydrosilanes as reducing reagents in the presence of a commercially available palladium complex as a catalyst. The reaction affords α,β-unsaturated ketones regio- and stereoselectively. The similar catalyst system is also effective for the reduction of carboxylic anhydrides to the corresponding aldehydes employing hydrosilanes.

A highly selective carbon-carbon bond-forming transformation of carbon dioxide (CO2) to alcohol oxidation level has been disclosed. By employing a copper/bisphosphine catalyst system and hydrosilanes as mild and easy-to-handle reducing agents, various allenes can be reacted with CO2 to regioselectively obtain homoallylic alcohols. Esters and other reducible functionalities on the allenes remain intact during the reaction, whereas CO2 is reduced to alcohol oxidation level.

N-Heterocyclic carbenes bearing poly(ethylene glycol) chains of different lengths have been designed and employed as ligands in palladium-catalyzed coupling reactions. The catalyst system having longer chains was found to be highly efficient for Suzuki-Miyaura coupling and borylation reactions employing aryl chlorides under mild reaction conditions.

We synthesized symmetrically insulated oligo(para-phenylene) and oligothiophene with a pseudo-linked [3]rotaxane structure by full rotation of glucopyranose units via a flipping (tumbling) mechanism in the π-conjugated guest having two permethylated β-cyclodextrin units at both ends. We also succeeded in the synthesis of an organic-soluble fixed [3]rotaxane by a cross-coupling or complexation reaction of thus formed pseudo linked [3]rotaxane. Oligo(para-phenylene), oligothiophene, and porphyrin derivatives were used as π-conjugated guests with stopper groups.

The hydroallylation of allenes was developed by employing a hydrosilane and allyl chlorides in the presence of a copper catalyst. The reaction provided (E)-1,5-dienes mainly in good to high yields.

We successfully synthesized a hetero face-to-face porphyrin array composed of ZnTPP and RuTPP(DABCO)2 (TPP: 5, 10, 15, 20-tetraphenylporphyrin, DABCO: 1,4-diazabi-cyclo[2.2.2]octane) in 2:1 molar ratio. A cyclic Zn porphyrin dimer (ZnCP) was also used as the host molecule for the Ru porphyrin. In the latter, the Ru-DABCO bonding in RuTPP(DABCO)2 was stabilized by the host-guest complexation. Reaction progress kinetic analysis of the ligand substitution reaction of RuTPP(DABCO)2 and that in ZnCP revealed the stabilization mechanism of the Ru-DABCO bonding. Photoinduced electron transfer (PET) from the Zn porphyrin to the Ru porphyrin was observed in the porphyrin array. The host-guest stabilization of unstable complex for construction of a donor-acceptor-donor structure is expected to be a new method for an artificial photosynthesis.

Triarylalkene-capped conjugated rotaxanes were synthesized via a palladium-catalyzed tandem Sonogashira/hydroaryl reaction between aryl halides and terminal alkynes bearing two permethylated α-cyclodextrins (PM α-CDs) with high regioselectivity because of the insulation effect of the PM α-CDs. Moreover, sequential Sonogashira coupling and hydroarylation reactions using different aryl substrates afforded a regio- and stereoselective trisubstituted alkene as a single product. This new class of rotaxane-forming reactions can be used to increase the diversity of rotaxane skeletons, and thereby the material functionalities of rotaxanes.

The development of new tuning methods for π-conjugated insulated molecular wires with strictly defined axle lengths as well as positions and degrees of macrocycle coverage would provide unprecedented insight into insulation effects in functionalized materials. Herein, iterative reactions of oligo(phenylene ethynylene) (OPE) linked with permethylated α-cyclodextrins were carried out to fabricate insulated molecular wires with a defined length and insulation in desired areas. Insulated OPEs were elongated in a stepwise manner by performing sequential coupling/deprotection reactions. The insulated areas on the OPE units in each expansion step were selectively controlled by means of programmed solvent conditions (high/low polarity). Moreover, a completely insulated OPE (up to a linked [11]rotaxane) with high structural regularity and high covering ratio was synthesized by appropriate tuning of the Pd catalyst and an extension unit bearing a traceless capping unit based on a tert-butyldimethylsilyl group. This strategy may guide the development of the selective synthesis of fully insulated, partially insulated, and uninsulated molecular wires with well-defined lengths and covered/uncovered areas.

The nickel-catalyzed double carboxylation of internal alkynes employing carbon dioxide (CO2) has been developed. The reactions proceed under CO2 (1 atm) at room temperature in the presence of a nickel catalyst, Zn powder as a reducing reagent, and MgBr2 as an indispensable additive. Various internal alkynes could be converted to the corresponding maleic anhydrides in good to high yields. DFT calculations disclosed the indispensable role of MgBr2 in the second CO2 insertion.

Borylative allyl-allyl coupling using allenes, bis(pinacolato)diboron, and allyl phosphates has been developed in the presence of a copper catalyst bearing an N-heterocyclic carbene ligand. The reaction affords boryl-substituted 1, 5-diene derivatives in good to high yields with high regioselectivity and Z selectivity.

Iron-catalyzed reduction of acid chlorides to the corresponding aldehydes with a hydrosilane as a reducing agent has been developed. A simple mixture of a commercially available iron oxide (FeO) and tris(2,4,6-trimethoxyphenyl)phosphine (TMPP) as a catalyst realized the reduction of acid chlorides to the corresponding aldehydes under mild reaction conditions.

This review summarizes the transformation of alkynes using a copper hydride (Cu–H) or boryl copper (Cu–B) species as the active species. The semihydrogenation, hydrocarboxylation, and hydroboration of alkynes have been developed using Cu–H as the active species. The hydroboration, diboration, carboboration and boracarboxylation of alkynes involve the Cu–B species. These transformations afforded multi-substituted alkenes that are potentially good intermediates in organic synthesis. The key to the regioselective transformation is the addition of the Cu–H or Cu–B species to an alkyne, to afford the corresponding alkenylcopper intermediate regioselectively.

Palladium-catalyzed formal arylacylation of allenes using acid chlorides and arylboronic acids has been achieved. The reaction afforded the corresponding α, β-unsaturated ketones regio- and stereoselectively.

The copper-catalyzed silylative allylation of ketones with allenes and silylboranes via an in situ generated β-silylallylcopper intermediate has been developed. Various ketones and allenes were con...

Something solid to build on: 2-Boryl 1,3-butadienes with various substitution patterns were formed in good to high yields in a copper-catalyzed borylation of α-alkoxy allenes with bis(pinacolato)diboron (see scheme; Bn=benzyl, pin=pinacolate, L is an N-heterocyclic carbene ligand). The products were found to be useful intermediates for the synthesis of cyclic vinyl boranes, α,β-unsaturated ketones, and functionalized multisubstituted dienes.

Reaction and work-up conditions for the regioselective transformation of various 1-substituted, 1,1- and 1,3-disubstituted allenes with carbon dioxide to homoallyl alcohol derivatives are developed which tolerate numerous functional groups.

An efficient synthesis of aldehydes from acid chlorides with hydrosilanes as a reducing agent in the presence of a palladium catalyst has been achieved. A simple mixture of commercially available Pd(dba)2 and Mes3P as a catalyst realized the reduction of various acid chlorides including aliphatic acid chlorides and α,β-­unsaturated acid chlorides to the corresponding aldehydes in good to high yields under mild reaction conditions.

A copper catalyst bearing a suitable Xantphos derivative or NHC ligand was found to be highly efficient for the selective semihydrogenation of non-polar unsaturated compounds using a mixture of a silane and an alcohol as reducing agent. The catalytic system was useful for the selective semihydrogenation of internal alkynes to (Z)-alkenes with suppression of overreduction to the corresponding alkanes. Furthermore, semihydrogenations of terminal alkyne, 1,2-diene, 1,3-diene, 1,3-enyne and 1,3-diyne systems were also achieved selectively.

A highly efficient cobalt-catalyzed reductive carboxylation reaction of alkenyl trifluoromethanesulfonates (triflates) has been developed. By employing Mn powder as a reducing reagent under 1 atm pressure of CO2 at room temperature, diverse alkenyl triflates can be converted to the corresponding α,β-unsaturated carboxylic acids. Moreover, the carboxylation of sterically hindered aryl triflates proceeds smoothly in the presence of a nickel or cobalt catalyst.

Magnesium chloride (MgCl2), a stable and cheap salt, works as an active catalyst for the synthesis of cyclic carbonates from epoxides and atmospheric-pressure carbon dioxide (CO2). Various epoxides are converted to the corresponding cyclic carbonates in good to high yields.

A highly efficient hydroesterification of alkynes employing aryl formates has been developed without the use of external carbon monoxide and at ambient pressure. The reaction in the presence of a palladium-xantphos catalyst system selectively affords α,β-unsaturated esters in good to high yields. Use of an aryl formate is crucial and alkyl formates did not react at all. The hydroesterification of norbornene and terminal alkenes also readily proceeded under similar reaction conditions. A mechanistic study showed that conversion of aryl formates to carbon monoxide and phenol derivatives occurred in the hydroesterification. Xantphos is highly effective as a ligand both in the conversion of aryl formates and the hydroesterification reactions.

Transition-metal-catalyzed transformation to produce various kinds of carbonyl compounds is one of the most powerful synthetic methods. Among them, transition-metal-catalyzed addition reactions of carbonyl compounds to carbon-carbon multiple bonds such as alkynes are environmentally friendly because all atoms of the substrates are retained in the products. Although the addition of aldehydes to alkynes, hydroacylation, has been intensively studied, the addition of acid chlorides, formamides, and formates has not been fully developed. One plausible explanation for it is the occurrence of competing decarbonylation reactions. In addition, relatively lower reactivity of formamides or formates limits the application as substrates in these addition reactions. In this paper, we overview recent results on the transition-metal-catalyzed addition reactions of carbonyl functionalities to alkynes.