1,1-Ethylborationof Diethynyl(dimethyl)silane: Formation of a Silole Followed by [4+2]Cycloadditions to 7-Silanorbornadienes and 7-Silanorbornenes

Abstract

Diethynyl(dimethyl)silane, Me2Si(C≡C-H)2 2, reacts with triethylborane, Et3B 1, by 1,1- ethylboration to give a silole, 3-diethylboryl-4-ethyl-1,1-dimethyl-sila-2,4-cyclopentadiene 3, as a short-lived intermediate which reacts further with 2 or dimerises via [4+2]cycloadditions to the 7-silanorbornadiene 4 or the 7-silanorbornene derivatives 5, respectively. Since the silole 3 was not detected by NMR, the analogous reactions were studied using 1-alkynyl(ethynyl)- dimethylsilanes, Me2Si(C≡C-H)C≡C-R 6 [R = Bu (a), iPent (b), tBu (c)]. In these cases, 1,1-ethylboration gave the siloles 7a,b,c and 7'a,b, now detected in the reaction solutions, and also 7-silanorbornadienes as [4+2]cycloaddition products. Major products were identified, and the progress of the reactions was monitored by NMR spectroscopy (1H, 11B, 13C and 29Si NMR).

Metallacyclopentadienes and related heterocycles via 1,1‐organoboration of alkyn‐1‐ylmetal compounds

“…The polyynes with terminal hydrogen (60,61) react with triethylborane in boiling toluene. However, complex mixtures are formed in both cases [77,78]; this can be understood considering the results of the 1,1-ethylboration of diethynyl(dimethyl)silane [59].…”

“…This was shown by the 1,1-ethylboration of Me 2 Si(C C H)C C R 1 (R 1 = Bu, Pent i , Bu t ), where the siloles 38 and 39 (as mixtures of isomers) can be detected prior to [4+2]cycloadditions with the starting alkyne. This [4+2]cycloaddition plays a minor role for R 1 = Bu t , and in this case the isomer 39 is formed almost exclusively, since the reactivity of the Si C C Bu t unit toward the initial intermolecular 1,1-ethylboration is much lower than that of the Si C C H unit [59].…”

Activation of Silicon- and Tin-Carbon Bonds. 1,1-Carboboration of Alkynylsilanes and -stannanes

“…The siloles with R 1 = H (Scheme 4) are not stable at ambient temperature. They undergo fast [4+2] cycloadditions to give 7-silanorbornadiene and 7-silanorbornene derivatives (Wrackmeyer and Süß, 2002). In any case, this is a highly convenient silole synthesis (Wrackmeyer, 2006, Wrackmeyer andTok, 2008;Khan et al, 2009a;Khan et al, 2009b;Khan et al, 2009c).…”

1,1‐Carboboration through Activation of Silicon–Carbon and Tin–Carbon Bonds

“…The siloles with R 1 = H (Scheme ) are not stable at ambient temperature. They undergo fast [4+2] cycloadditions to give 7‐silanorbornadiene and 7‐silanorbornene derivatives 23,56. Nonetheless, this is a highly convenient silole synthesis,57,58 through which siloles are formed in essentially quantitative yields.…”