Silacyclopropane - An Equilibrium

Institut fltr Organische Chemie Georg-August-Universitat Gottingen Tammannstr. 2, D-37077 Gottingen, Germany 

Summary New silacyclopropanes were synthesized quantitatively under mild thermal conditions by reaction of olefins with cyclotrisilane (cyclo-(Ar2Si)3, Ar = Me2NCH2QH4) 1, which transfers all of its three silylene subunits to terminal and strained internal olefins. Thermolysis of silacyclopropanes 3a und 3b indicated these compounds to be in a thermal equilibrium with cyclotrisilane 1 and die corresponding olefin. Silaindane 13 was synthesized by reaction of 1 with styrene via initially formed 2-phenyl-1-silacyclopropane 3d. Reaction of 1 with conjugated dienes such as 2,3-dimethyl-l,3-butadiene, 1,3-cyclohexadiene or anthracene resulted in the formation of the expected 1,4-cycloaddition products in high yield. 

Spectroscopically pure silacyclopropane 3a was formed quantitatively, when 1 was heated at 40 °C for 12 h with an excess of 1-pentene in CgDj. Especially the characteristic Si NMR shift values [6] (3a 8 = -76.6 ppm) proved the proposed cyclic structure. Isolation of analytically pure 3a from the reaction mixture was impossible due to its inherent thermal instability. Heating a concentrated solution of 3a in for 5.5 h at 40 °C resulted in a 1 1 3 mixture of 3a, 1, and 1-pentene. An analogous partial retro reaction to 1 and an olefin was also shown by silacyclopropane 3b, which was similarly synthesized by reaction of 1 with excess 1-hexene. 

The reaction is initiated by extrusion of a silylene 2 from 3a or 3b, thus paralleling the well known equilibrium between hexamethylsilacyclopropane and dimethylsilylene [7]. In the absence of a silylene trapping reagent [8] dimerization of 2 to disilene 5 takes place. Addition of a third silylene to the Si=Si double bond eventually yields cyclotrisilane 1 [9]. The reversibility of the cyclotrisilane formation from 3a and 3b provides evidence, that the reverse reaction of 1 with olefins includes free silylenes 2 as reactive species as well. 

It is tempting to assume, that the facile formation of silylene 2 from cyclotrisilane 1 is due to the effective stabilization of 2 by intramolecular coordination of the dimethylamino group to the silicon centre [10], which should lower the activation energy of a dissociation process from 1 to 2. Reaction of 1 with benzylvinylether resulted in a complex reaction mixture, from which 12 % of vinylsilane 4 was isolated 4 is presumably formed by rearrangement of the unstable oxy-substituted silacyclopropane 3c. 

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