Cycloaddition of carbonyl compounds and alkynes to (di)silenes and (di)germenes: reactivity and mechanism.

The chemistry of silenes, germenes, disilenes and digermenes has been extensively studied over the last 35 years; however, not much is known about the mechanisms of many of the fundamental reactions of this class of compounds, including cycloadditions. This review describes the current understanding of the reaction pathways for the cycloaddition of carbonyl compounds and alkynes to (di)tetrelenes. The question of whether or not the cycloaddition reactions of (di)tetrelenes follow the Woodward-Hoffman rules, established for alkenes and alkynes, is addressed.

Addition of cyclopropyl alkynes to a Brook silene: definitive evidence for a biradical intermediate.

The addition of three newly developed mechanistic probes, (trans-2-phenylcyclopropyl)ethyne, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1a-c, to a Brook silene, 2-tert-butyl-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silene, 10, was examined. When alkyne 1a was added to silene 10 products derived from a formal ene reaction were obtained. When alkynes 1b-c were added to silene 10, in addition to the typical silacyclobutenes, a variety of silacycloheptenes were obtained in which the cyclopropyl ring had clearly opened.

Cyclopropyl alkynes as mechanistic probes to distinguish between vinyl radical and ionic intermediates.

[reaction: see text] The reactions of (trans-2-phenylcyclopropyl)ethyne, 1a, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, 1b, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1c, with either aqueous sulfuric acid or tris(trimethylsilyl)silane (or tributyltin hydride) and AIBN have been investigated. Protonation and addition of the silyl (or stannyl) radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively. Under both reaction conditions, 1a yielded products derived from ring opening toward the phenyl substituent.

Determination of the rate constant for ring opening of an alpha-cyclopropylvinyl radical.

The rate constant for ring opening of the 1-(trans-2-phenylcyclopropyl)ethen-1-yl radical, 4, generated by photolysis of the corresponding vinyl iodide 2, is reported. The value of the rate constant was determined by the tin hydride method and was found to be (1.6+/-0.