Copper‐Catalyzed Silacarboxylation of Internal Alkynes by Employing Carbon Dioxide and Silylboranes

Abstract

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

Transition Metal-Catalyzed Carboxylation of Organic Substrates with Carbon Dioxide

“…In the case of unsymmetrical alkynes bearing alkyl groups (Me, Et) adjacent to phenyl moieties, the boryl group ends up regioselectively at the alkyl-substituted carbon atom. This is presumably due to sterics, as also shown by Tsuji in the silacarboxylation reaction [95]. Notably, the terminal alkyne phenylacetylene also gave the cyclic boralactone (and not phenyl propiolic acid) in very high regioselectivity -in accordance with the strongly differing substituent sizes.…”

“…In 2012, Tsuji presented the catalytic silacarboxylation of internal alkynes (Fig. 20) [95]. The reaction of 1-phenyl-1-propyne with CO 2 and Me 2 PhSi-B(pin) as silicon source led to the formation of a silalactone in the presence of a strong base (12 mol% NaOtBu) and copper(I) chloride precatalysts (2.5-3 mol%) bearing electron-rich and bulky phosphine ligands such as P(tBu) 3 and PCy 3 or the NHC ligand IMes.…”

Transition Metal-catalyzed Fixation of Carbon Dioxide <i>via</i> Carbon–carbon Bond Formation

“…Previously, boracarboxylation of alkynes was achieved using Cu catalysts and bis(pinacolato)diboron as the boron source 62) . We reported the first catalytic silacarboxylation of internal alkynes employing CO2 and silylborane in the presence of a Cu catalyst (Scheme 20) 63) . The reaction of 1-phenyl-1-propyne (11a) was carried out using readily available Me2PhSi-B(pin) as the silicon source in the presence of a Cu catalyst in octane at 100 under CO2 at 1 atm pressure.…”

Making C–C Bonds from Carbon Dioxide via Transition-Metal Catalysis

“…Alkynes, allenes, and butadienes exhibit enhanced reactivity in comparison to simple alkenes and can undergo a range of reactions that incorporate carbon dioxide, including reductions [87][88][89][90], carbon additions [91,92], heteroatom additions [93,94], and double carboxylations (Fig. 17) [95].…”

“…17) [95]. From a synthetic standpoint, both sila- [93] and boracarboxylation [94] are useful strategies for CO 2 introduction, since the vinylmetal product generated can be further elaborated under standard transition metal catalyzed cross-coupling conditions with a suitable electrophile. Iwasawa's hydrocarboxylation of allenes catalyzed by Pd(II) pincer complex 3 bears the distinction of being a rare example of constructing carboxylic acids bearing a-quaternary carbons [87].…”