Nickel-catalyzed carboxylation of aryl and vinyl chlorides employing carbon dioxide has been developed. The reactions proceeded under a CO(2) pressure of 1 atm at room temperature in the presence of nickel catalysts and Mn powder as a reducing agent. Various aryl chlorides could be converted to the corresponding carboxylic acid in good to high yields. Furthermore, vinyl chlorides were successfully carboxylated with CO(2). Mechanistic study suggests that Ni(I) species is involved in the catalytic cycle.

Carbon dioxide (CO 2 ) is a readily available and renewable chemical feedstock, although thermodynamic considerations limit its widespread use in chemical reactions.[1] For effective utilization of CO 2 , transition-metal catalysts are required. [2] Useful transformations of CO 2 such as 1) cycloaddition via a metallacycle [3] and 2) carboxylation of organozinc and organoboron compounds [4] have been reported to date. Besides these reactions, the hydrocarboxylation [5] of C-C multiple bonds using CO 2 is also very promising. The first example of hydrocarboxylation using CO 2 was achieved using a nickelcatalyzed electrochemical reaction with alkynes,[5a] 1,3-diynes,[5b] and 1,3-enynes [5c] as substrates. Later, in supercritical CO 2 , palladium-catalyzed hydrocarboxylation of terminal alkenes was reported.[5d,e] As for more efficient hydrocarboxylations, recent nickel-catalyzed reaction of styrenes [5f] and palladium-catalyzed reaction of allenes [5g] were reported with either ZnEt 2 [5f,g] or AlEt 3[5g] as reducing agents. These reactions are very useful, but such strong and extremely airsensitive reducing agents were indispensable in the reactions. Herein we report the copper-catalyzed hydrocarboxylation of alkynes using CO 2 (balloon). [6,7] The use of mild and easy-tohandle hydrosilane [8] as a reducing agent realizes highly efficient hydrocarboxylation of alkynes to afford a,b-unsaturated carboxylic acids (2; Scheme 1).The hydrocarboxylation of diphenylacetylene (1 a) with CO 2 (balloon) was carried out using HSi(OEt) 3 as a reducing agent in 1,4-dioxane (Table 1). The yield of (E)-2,3-diphenyl-2-propenoic acid (2 a) was determined by GC methods after derivatization [9] to the corresponding methyl ester 2 aMe. By employing [IPrCuCl] + tBuONa (Table 1, entry 1) or [IMesCuCl] + tBuONa (Table 1, entry 2) as a catalyst, 2 aMe was obtained in only trace amounts and 49 % yield, respectively. In the latter case, a considerable amount (19 % yield) of cis-stilbene (3 a) was observed as a by-product. When [IPrCuF] [10] was used as a catalyst, 2 aMe was obtained in 41 % yield while reducing the formation of 3 a to 3 % (Table 1, entry 3). The new complex [IMesCuF] was synthesized from [IMesCuCl] similar to the synthesis of [IPrCuF], and its structure was confirmed by X-ray crystallography (Scheme 2).[9] As a result, [IMesCuF] was a much more Scheme 1. Hydrocarboxylation of alkynes using CO 2 and hydrosilanes.

Recent carbon-carbon bond forming reactions of carbon dioxide with alkenes, alkynes, dienes, aryl zinc compounds, aryl boronic esters, aryl halides, and arenes having acidic C-H bonds are reviewed in which transition-metal catalysts play an important role.

Taking control of boron: A highly regio‐ and stereoselective copper‐catalyzed hydroboration of unsymmetrical internal alkynes has been developed. The regioselectivity was successfully controlled by the choice of catalytic species (copper hydride or boryl copper; see scheme).

The highly selective copper-catalyzed hydroboration of allenes has been developed. Allylboranes and alkenylboranes were selectively prepared by the judicious choice of catalytic species (copper hydride and boryl copper). Furthermore, two types of alkenylboranes could be selectively synthesized by the choice of an appropriate ligand. Mechanistic studies confirmed that the protonation of a (Z)-σ-allyl copper species, which was isolated and structurally characterized by single-crystal X-ray diffraction, was a key step in these reactions. Besides allenes, this method is also applicable to the selective hydroboration of 1,3-diene derivatives to afford allylboranes and homoallylboranes.