Donor–Acceptor Adducts of a 1,3‐Disila‐2‐oxyallyl Zwitterion

Abstract

In the presence of a Lewis acid or base, cyclotrisilene cSi3Tip4 (Tip = 2,4,6-triisopropylphenyl) reacts with CO to form stable adducts of a 1,3-disila-2-oxyallyl zwitterion. Coordination of an N-heterocyclic carbene (NHC) to one silicon center results in a localized Si=C bond, whereas B(C6F5)3 bonds to the carbonyl moiety leading to the delocalization of the positive charge over the oxyallyl moiety. Computational analysis of the electronic structure of the Lewis acid adduct reveals a considerable interaction between the two silicon atoms in 1,3-position and thus aromaticity akin to the homocyclopropenium cation.

Functional Disilenes in Synthesis

“…The oxyallyls 145 a , b were also trapped by quenching with water, trifluoromethanesulfonate (TMSOTf) or methanol to yield the stable tetracyclotrisilene compounds 147 a – c (Scheme ). Alternatively, the ambiphilic character of intermediate 145 a can be exploited for its stabilization by either a Lewis acid or a base: in the presence of Et 2 O⋅B(C 6 F 5 ) 3 at 25 °C, the stable 1,3‐disila‐2‐oxaallyl‐borane adduct 148 is obtained in 62 % yield (Scheme ) . As expected the Lewis acid coordinates to the oxygen atom of the carbonyl moiety.…”

“…The Lewis acidity of the silicon atoms adjacent to the carbonyl unit in 145 a was proven by trapping with an NHC. The exposure of a mixture of 2 and NHC 125 (which is known to exist in equilibrium with the base‐stabilized disilenyl silylene 128 ; see Scheme ) to carbon monoxide gave compound 149 in 64 % yield as a red precipitate (Scheme ) …”

Reductive Cleavage of Carbon Monoxide by a Disilenide

“…Stable carbenes with a suitable HOMO–LUMO gap readily react with CO to form either oxyallyls or ketenes, but do not seem to result in reductive coupling 13. In collaboration with the Sekiguchi group, we recently discovered the direct carbonylation of cyclotrisilenes to produce highly functionalized cyclic silenes in the absence of a catalyst,14 presumably via a highly reactive oxyallyl intermediate 15. In neither of these cases, the CO bond was fully cleaved.…”

Reductive Cleavage of Carbon Monoxide by a Disilenide

“…[13] In collaboration with the Sekiguchi group,w er ecently discovered the direct carbonylation of cyclotrisilenes to produce highly functionalized cyclic silenes in the absence of ac atalyst, [14] presumably via ah ighly reactive oxyallyl intermediate. [15] In neither of these cases,t he COb ond was fully cleaved. Arare case of full CO cleavage has been reported for the frustrated Lewis pair, B(C 6 F 5 ) 3 and P(tBu) 3 ,which reacts with CO/H 2 mixtures under ambient conditions to incorporate aC H 2 group into aB À C aryl bond in the final product.…”