Carbon dioxide (CO 2 ) is a nontoxic, abundant, and renewable carbon source. [1] The utilization of this environmentally friendly raw material in carbon-carbon bond-forming reactions is one of the most important challenges in homogeneous transition metal catalysis. [2] To date, two types of catalytic transformations using CO 2 with C À C bond formation have been investigated intensively: a) the substitution of Ar-Y with CO 2 (Scheme 1 a) and b) the hydrocarboxylation of C-C unsaturates (Scheme 1 b). The former reactions involve the carboxylations of organozinc [3a,b] and organoborane [3c-f] compounds, C À H bonds, [3g-j] and bromoarenes [3k] (Scheme 1 a). Recently, we reported the Ni-catalyzed carboxylation of chloroarenes with CO 2 in the presence of Mn powder under ambient conditions. [4a] As for the hydrocarboxylation (Scheme 1b), the reactions of dienes, [5a,b] alkenes, [5c] and alkynes [5d] have been reported. However, in all these cases, highly reactive and pyrophoric Et 3 Al [5a,b] or Et 2 Zn [5a,c,d] must be used as a hydride source. We recently reported the hydrocarboxylation of alkynes, by employing stable and easyto-handle hydrosilanes as the hydride source. [4b] Besides these reactions, catalytic heterocarboxylation, in which the heteroatom functionality and CO 2 are simultaneously and catalytically incorporated into unsaturated substrates, is extremely useful, since the reaction will provide a valuable synthetic route employing CO 2 for the formation of highly functionalized carboxylic acid derivatives. For the silacarboxylation of alkynes, the only precedent is the stoichiometric reaction of 1-hexyne (one example) reported by Fleming et al., who carried out the reaction of 1-hexyne with a stoichiometric amount of (Me 2 PhSi) 2 CuLi·LiCN followed by trapping of the resulting Cu species with CO 2

A regiodivergent silacarboxylation of allenes under a CO2 atmosphere with PhMe2Si-B(pin) as a silicon source in the presence of a copper catalyst at 70 °C has been developed. The regioselectivity of the reaction is successfully reversed by the proper choice of ligand; carboxylated vinylsilanes are obtained with rac-Me-DuPhos as the ligand, whereas the use of PCy3 affords carboxylated allylsilanes. Thus, two different carboxylated silanes can be selectively and regiodivergently synthesized from a single allene substrate.

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.

The boraformylation of allenes with B (pin) 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 PhMe Si-B(pin) as the silicon source.