Reactivity of a Disilyne RSi≡SiR (R = SiⁱPr[CH(SiMe₃)₂]₂) toward π-Bonds:  Stereospecific Addition and a New Route to an Isolable 1,2-Disilabenzene

Abstract

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...

Neutral Diboron Analogues of Archetypal Aromatic Species by Spontaneous Cycloaddition

“…1). [12] As regards heavier group 13 alkene analogues, Tokitoh and co-workers described the addition of alkynes to a dialumene-benzene adduct leading to the formation of 1,2-dialuminacyclobut-3-enes, IV ( Fig. 1), [13] while Power and co-workers synthesized 1,4-digallacyclohexa-2,5-diene V by double cycloaddition of phenylacetylene to a digallene (Fig.…”

Reaction of a Stable Digermyne with Acetylenes: Synthesis of a 1,2-Digermabenzene and a 1,4-Digermabarrelene

“…The stereoselective [2+2] cycloaddition of the stable disilyne R Si Si¸SiR Si (7; R Si : Si[CH(SiMe 3 ) 2 ](i-Pr)) with cis-and trans-2-butenes to generate disilenes 8 was reported by Sekiguchi and coworkers. 11 Recently, we reported the reaction of stable disilyne 1 and digermyne 2a with ethylene to furnish cyclic products 9 and 10.…”

“…11,14,15 In the case of the carbon analogues, the trimerization of acetylene to afford a sixmembered cyclic π-conjugated system (i.e., an aromatic ring system) usually requires an appropriate transition-metal catalyst and severe conditions, as for example in the well-known Reppe reaction. In contrast, a cyclic six-membered π-conjugated aromatic ring system containing two heavier group 14 elements can be generated under ambient conditions in the absence of any transition-metal catalysts from the reaction of two molecules of acetylene and one heavier dimetallyne (vide supra).…”

“…So far, treatment of disilynes or digermynes with acetylene remains the only synthetic method to generate 1,2-disila-or 1,2-digermabenzenes. 11,14,15 Such transformations are not only of great interest due to their unique reactivity, but the resulting heavy aromatic systems that include two heavy atoms are also highly interesting with respect to structural aspects, such as bond-alternation, planarity of the aromatic ring, and other features that may be affected by the double-bond character of the E=E bonds.…”

“…11,21 Similarly, the reaction of 2b with tolan in C 6 D 6 at room temperature resulted in the formation of 1,2-digerma-3,4-diphenylcyclobutadiene 18 as a stable, red, crystalline compound. The structural parameters of 18, determined by single-crystal X-ray diffraction, are similar to those of 17, Scheme 3.…”