Cycloaddition Reactions of Aldehydes to Tetramesityldisilene and Tetramesitylgermasilene:  Evidence for a Biradical Intermediate

Abstract

The mechanism of the addition of aldehydes to disilenes and germasilenes was investigated by examining the reaction between trans-(2-phenylcyclopropyl)carboxaldehyde and tetramesityldisilene or tetramesitylgermasilene. [Dimesityl(2-cis-4-trans-1-oxa-5-phenylpentadienyl)silyl]dimesitylsilane (1) and 2,2,3,3-tetramesityl-4-phenyloxa-2,3-disilacyclohept-6-ene (2) were formed in the reaction between the aldehyde and the disilene, and the analogous [dimesityl(2-cis-4-trans-1-oxa-5-phenylpentadienyl)silyl]dimesitylgermane (4) and 2,2,3,3-tetramesityl-4-phenyloxa-2,3-silagermacyclohept-6-ene (5) were formed in reaction with the germasilene. It is proposed that compounds 1, 2, 4, and 5 are the result of either disproportionation or ring closure of an intermediate biradical derived from initial bond formation between the oxygen atom of the aldehyde and the doubly bonded silicon atom of the dimetallene.

Mechanistic Studies of the Addition of Carbonyl Compounds to Tetramesityldigermene

“…7 We have used molecular probes to investigate the mechanism of the addition of carbonyl compounds to both disilenes and germasilenes. 8,9 The probes, trans-2-phenylcyclopropane carbaldehyde (1) and trans,trans-2-methoxy-3-phenylcyclopropane carbaldehyde (2), contain the same (alkoxy)phenylcyclopropylcarbinyl framework found in the hypersensitive probes developed by Newcomb. 10,11 The cyclopropane carbaldehydes are able to discriminate between the development of a radical or a cationic center at the aldehydic carbon by following different, subsequent reaction pathways.…”

“…10,11 The cyclopropane carbaldehydes are able to discriminate between the development of a radical or a cationic center at the aldehydic carbon by following different, subsequent reaction pathways. 8,9 The oxy-substituted phenylcyclopropylcarbinyl radical has been shown to undergo rapid ring-opening toward the phenyl substituent, whereas the analogous oxonium ion does not rearrange. However, in the event that the cation does undergo ring-opening, it has been shown that cleavage toward the methoxy group in the (alkoxy)phenyl-substituted cyclopropane ring will occur.…”

“…In the addition of aldehydes 1 and 2 to tetramesityldisilene and -germasilene, only ring-opened products were isolated. 8,9 The difference in the structure of the adducts signals a significant change in the mechanism of the addition of carbonyl compounds to digermenes in comparison to disilenes and germasilenes. The formation of 2,3-digermoxetanes 3 and 4 is consistent with two plausible mechanisms: the concerted addition of the carbonyl moiety to the Ge-Ge double bond or the formation of a zwitterionic intermediate.…”

“…There is a dramatic difference in the behavior of the disilene and the germasilene, 8 Finally, a qualitative comparison of the rates of the addition of aldehydes 1 and 2 to the three dimetallenes was possible by monitoring the disappearance of the characteristic yellow color of the dimetallene. The disilene reacted instantly upon addition of the aldehyde in the cold (∼ -60°C), the germasilene required warming to room temperature before the characteristic color of the germasilene had completely faded, and the digermene required heating for complete reaction.…”

REACTIONS OF CYCLOTRIGERMANE WITH p-BENZOQUINONES: OLIGOMERS AND A NOVEL 1,3-CYCLOADDUCT

“…The 05606,062 and 0 1 GeiGe 2 angles are decreased respectively to 87.66(8)° and 91.09(6)°, whereas ΟδίΟ^βΣ and C 51 C 5 6Gei angles are increased to 120.5(2) and 117.8 (2). The bond distance C51-C56 (1.381(3) A) is close to that of the normal C=C double bond 5 , and the corresponding angles C^Cs-iCse (122.8(3)°) and CsiCseGe·, (117.8(2)°) are also very near to the values expected for an sp -hybridized C atom.…”