The reaction of 2,2,3,3-tetrabromo-1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasilane with four equivalents of potassium graphite (KC8) in tetrahydrofuran produces 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyl-2-tetrasilyne, a stable compound with a silicon-silicon triple bond, which can be isolated as emerald green crystals stable up to 100 degrees C in the absence of air. The SiSi triple-bond length (and its estimated standard deviation) is 2.0622(9) angstroms, which shows half the magnitude of the bond shortening of alkynes compared with that of alkenes. Unlike alkynes, the substituents at the SiSi group are not arranged in a linear fashion, but are trans-bent with a bond angle of 137.44(4) degrees.

The one-electron oxidation reaction of tris[di-tert-butyl(methyl)silyl]silyl and -germyl anions with dichlorogermylene-dioxane complex results in the formation of stable tris[di-tert-butyl(methyl)silyl]silyl and -germyl radicals 1 and 2, representing the first isolable radical species of heavier Group 14 elements lacking stabilization by conjugation with pi-bonds. The crystal structures of both silyl and germyl radicals 1 and 2 showed a completely planar geometry around the radical centers. The ESR spectra of 1 and 2 showed strong signals with characteristic satellites due to the coupling with the 29Si and 73Ge nuclei. The small values of the hyperfine coupling constants a(29Si) and a(73Ge) clearly indicate the pi-character of both radicals, corresponding to a planar geometry and sp2 hybridization of the radical centers. Both 1 and 2 easily undergo halogenation reactions with carbon tetrachloride, 1,2-dibromoethane, and benzyl bromide to form the corresponding halosilanes and halogermanes.

Recently, numerous multiple bond species containing heavier group 14 elements have been isolated and characterized. 1 In particular, considerable interest has been focused on the nature of alkene analogues of silicon 2 because of their unusual structures and bonding since the isolation of a stable tetramesityldisilene by West and co-workers. 3 In many cases, the π bond of the disilenes has displayed an increased reactivity toward many reagents, compared with that of alkenes and alkynes, because of the relatively small HOMO-LUMO gap and its biradical character. 4 For example, the π bond of disilenes is known to undergo smooth [2 + 2] cycloadditions toward alkenes and alkynes to give the disilacyclobutane and disilacyclobutene derivatives, respectively. 1,2 On the other hand, much less is known about the π bond nature of disilynes with a silicon-silicon triple bond, which has two distinct π bonds (π in and π out ), 5,6 although a few papers have described the reactivity of alkyne analogues. 7 Now, a comparison of the chemical behavior of heavier group 14 element alkyne analogues with that of alkynes is of special interest. To understand the nature of the π bond of a silicon-silicon triple bond, we have examined the reaction of disilyne 1 5a with alkenes and alkynes. In this paper, we present the results of the stereospecific cycloadditions of 1 to 2-butenes, 8 together with those of the cycloaddition of 1 to phenylacetylene to give an isolable 1,2-disilabenzene derivative. Furthermore, we report theoretical studies on the mechanism of the reactions, showing that these reactions involve in the initial step [1 + 2] cycloaddition (the interaction between the LUMO (π in *) of 1 and the HOMO of 2-butenes or acetylene), instead of a direct [2 + 2] cycloaddition.When a hexane solution of disilyne 1 was treated with an excess of cis-2-butene at room temperature, cis-3,4-dimethyl-1,2-disilacyclobutene 2a was obtained as the sole product in 89% yield (Scheme 1). 9 This reaction proceeded cleanly and was complete within 30 min. On the other hand, the reaction of 1 with trans-2-butene under the same conditions produced trans-3,4-dimethyl-1,2-disilacyclobutene 2b as yellow crystals in 85% yield. 9 In contrast to the reaction with cis-2-butene, it took 1 day to complete this reaction. Most importantly, both reactions proceeded stereospecifically, as was determined by NMR spectra and X-ray analysis (for 2b). 9 Although an excess of 2-butenes was used in both reactions, the subsequent cycloaddition reaction was not observed.To gain a mechanistic insight, we have performed theoretical calculations of the reaction of disilyne 1 with 2-butenes. Figure 1 shows the energy profile along the reaction path calculated at the B3LYP/[Si, 6-311+G(2df); C and H, 6-31G(d)]//B3LYP/3-21G* level. 10 The interaction between the in-plane LUMO (π in *) of 1 and the HOMO of 2-butene, resulting in [1 + 2] cycloaddition, is the first step in both reactions to produce the silacyclopropylsilylene intermediate (Int1). 11 The alternative interaction between...

The highly twisted tetrakis(di-tert-butylmethylsilyl)disilene 4 was prepared and reacted with (t)BuLi in THF, producing disilene anion radical 5 upon one-electron reduction. The anion radical 5 was isolated in the form of its lithium salt as extremely air- and moisture-sensitive red crystals. The molecular structure of 5 was established by X-ray crystallography, which showed a nearly orthogonal structure (twisting angle of 88 degrees ) along the central Si-Si bond, with a length of 2.341(5) A, which is 3.6% elongated relative to that of 4. The interesting feature of 5 is that one of the central Si atoms has radical character, whereas the other Si atom has silyl anion character. An electron spin resonance (ESR) study of the hyperfine coupling constants of the (29)Si nuclei indicates that rapid spin exchange occurs between these central Si atoms on the ESR time scale.