Reductive Dimerization of Alkynes

The extent and types of interfering reactions during hydroalumination of alkynes were examined in Section 3.11.2.2 (equations 18-24). To a lesser degree two of these reactions are also encountered in A1—H additions to alkenes and cycloalkenes. Analogous to the reductive dimerization of alkynes (equation 23) is the dimerization of a-alkenes by Bu 2AlH, which can be conducted catalytically in hydride to give high yields of dimer (equation 30). ... 

Reductive dimerization of alkenes and alkynes can be achieved with various catalysts in the presence of dihydrogen or other hydrogen donors, such as tetrahydroborates or / -H-eliminat-ing aluminum alkyls. 

Reductive dimerization of two alkyne moieties with dihydrogen forming 1,3-dienes (together with linear and cyclic trienes and tetraenes) is catalyzed by [Cointernal alkynes, such as diphenylcthync and 2-butyne. c i.s-addition products are formed. 

Recent studies on the allylation of alkynes with bis (7r-allyl) nickel have revealed that the Ni(0) generated in this process causes the trimeri-zation and, more importantly, the reductive dimerization of a portion of the alkyne (8). A deuterolytic work-up led to the terminally di-deuter-ated diene (5), supporting the presence of a nickelole precursor (4) (Scheme 1). The further interaction of 4 with 1, either in a Diels-Alder fashion (6) or by alkyne insertion in a C-Ni bond (7), could lead to the cyclic trimer 8 after extrusion of Ni(0), thereby accounting for the trimerizing action of Ni(0) on alkynes. This detection of dimer 5 then provided impetus for the synthesis of the unknown nickelole system to learn if its properties would accord with this proposed reaction scheme. Therefore, E,E-l,4-dilithio-l,2,3,4-tetraphenyl-l,3-butadiene (9) was treated with bis (triphenylphosphine) nickel (II) chloride or l,2-bis(di-phenylphosphino ethane)nickel(II) chloride to form the nickelole 10 (9) (Scheme 2). The nickelole reacted with dimethyl acetylenedicarboxylate to yield 11 and with CO to produce 12. Finally, in keeping with the hypothesis offered in Scheme 1, 10a did act as a trimerizing catalyst toward diphenylacetylene (13) to yield 14. 

Transmetalation from Zr to Cu is a highly beneficial process as it combines the ease of preparation of organozirconocenes from alkenes and alkynes with the wide scope of organocopper reagents in organic synthesis. Schwartz and coworkers were first to demonstrate transmetalation to Cu in their report on the reductive dimerization of alkenylzirconocenes [94]. Virgili et al. used this transformation to prepare dialkoxy-1,3-butadienes [95]. The copper-mediated coupling of alkenylzirconocene 85 and phenethynyl bromide was reported to yield 86 [96] (Scheme 19). The latter two reactions can also be mediated by ox-ovanadium complexes [97]. 

In another type of alkyne, dimerization is the reductive coupling in which two molecules of alkyne, the same or different, give a 1,3-diene ... 

Suginome et al. discovered that the use of Ni(acac)2 and DIBAL-H as a reductant results in the double insertion of alkynes 280 across the B-Si bond in silaborane 281 to afford silaborated butadienes 282-284 (acac = acetylaceto-nate). NMR experiments revealed the stereoselective formation of head-to-head dimerization product 282 as the... 

This activation process can be assumed to be the initial step in the formation of dinuclear copper(II) acetylide complexes, as first proposed by Bohlmann and coworkers 40 years ago (Scheme 6) [10f]. Deprotonated alkyne units 11 (or the corresponding JT-complexes 10) generated therein, stepwise displace the negatively charged counter ions of copper(II) salt dimers (12). The dinuclear copper(II) acet-ylide complex which finally results (14) collapses to the coupled product under reductive elimination of copper(I). The existence of higher-order copper acetylide... 

The molybdenum dimer 547 can be converted, by alkylation, to a cation (548), which can be transformed into an anion (549) by two-electron chemical reduction. Treatment of 547 with excess methyllithium leads, after work-up in air, to the bis (/z-methylthiolate) 550, which can also be prepared by addition of methyllithium to 548. The addition of vinylmag-nesium bromide to 548 yields 551. The anion reacts with alkynes, activated olefines, and an allene to form 552, 553, and 554, respectively (305). 

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