In memory of Xinwei Ma
Activation of carbon dioxide and converting it into useful chemical feedstock have attracted much attention owing to the fact that CO 2 is abundant, inexpensive, nontoxic, and environmentally benign. [1] However, the challenges still to be overcome are its lack of thermodynamic and kinetic stability. For the reaction of allylic tin species, [2] aryl boronates, and limited examples of 1-alkenyl boronates, [3] aryl or alkyl zinc substrates [4,5] can react with carbon dioxide, usually under palladium, nickel, copper, or rhodium catalysis [Scheme 1, Eq. ( 1)]. In addition, a stoichiometric amount of Ni or Ti reagents have been used to mediate the reaction of CO 2 with alkene, [6] diene, [7] alkyne, [8] or allene [9] substrates to form fivemembered metallaoxacyclic intermediates A, which may undergo further reactions to afford carboxylation products [Scheme 1, Eq. ( 2)]. There are very limited reports on the catalytic reactions of alkyne [10] or allene [11] substrates involving A-type intermediate using 20 mol % of [Ni(cod) 2 ] and 10 equivalents of 1,8-diazabicyclo[5.4.0]undec-7-ene.
Furthermore, there are a few reports on substituted alkynes that can undergo a stereoselective titanium-or rhodium-catalyzed syn-hydrozincation [12,13] or rhodium-or nickel-catalyzed carbozincation [13,14] reaction [Scheme 1, Eq. ( 3)]. With this notion in mind, we envisioned that organozinc reagents generated in situ from hydro-or carbozincation of unsaturated hydrocarbon species may react with CO 2 to afford the corresponding carboxylic acids in a convenient manner [Scheme 1, Eq. ( 3)]. Rovis and co-workers reported the hydrozincation/carboxylation of styrenes with [Ni(acac) 2 ] (10 mol %; acac = acetylacetonate) and Cs 2 CO 3 (20 mol %). [15] Takaya and Iwasawa reported such a hydrocarboxylation of allene with 1-2.5 mol % of a silyl pincer-type palladium complex. [16] However, it should be noted that both reports involve the reaction of very reactive allylic or benzylic metallic species with CO 2 . So far, there are no such reports on alkyne substrates; the challenge here would be the lower reactivity of the 1-alkenylic zinc generated in situ towards CO 2 [17] and the regioselectivity of the alkynes. Herein, we report the concise highly regio-and stereoselective three-component nickel-catalyzed (1-3 mol %) synhydrocarboxylation of alkynes [11,12] with diethyl zinc and the subsequent efficient reaction with carbon dioxide mediated by CsF to afford stereodefined and synthetically useful 2alkenoic acids. This reaction has been applied to the highly regio-and stereoselective synthesis of 3-alkylideneoxindole [18] and a-alkylidene-g-butyrolactam. [19] Initially, diphenylacetylene (1 a) was treated with CO 2 in the presence of 10 mol % of [Ni(cod) 2 ], 20 mol % of PCy 3 , and 3 equivalents of ZnEt 2 . Pleasingly, 10 % of the expected synhydrocarboxylation product, that is, (E)-2,3-diphenylacrylic acid 2 a, was formed together with 25 % of the hydrolysis product 4 a (Table 1, entry 1). Various bases were then screened with no obvious improvement (Table 1, entries 2-4). Then we tested the effect of inorganic salts such as ZnBr 2 , LiCl, KF, and CsF as the ligand [3a,b, 20] (Table 1, entries 5-8). We observed that when 3 equivalents of CsF were used, the hydrocarboxylation product 2 a was formed in 59 % yield together with 11 % of the ethylcarboxylation product, that is, Scheme 1. Previous work and our concept for CO 2 activation.