Silylenes with alkenes

Further reactions of the silylene r-Bu2Sit produced by photolysis of hexa-r-butylcyclotrisilane with alkenes and cycloalkenes have recently been described213. 

The formation of isopropyldichlorosilane can be explained by the reaction of elemental silicon with, hydrogen chloride and isopropyl chloride formed by the hydrochlorination of propylene. As an alternative, it is possible that a silicon-carbon bond is formed by the reaction of a silylene intermediate, generated on the surface of silicon metal powders, with alkene... 

Solely the photolysis of 1 with the heterocyclic, five-membered ring compounds fliran, thiophene and selenophene furnished other products instead of the respective siliranes (see below). The reactions of tetra-t-butyldisilene 3 with alkenes gave rise to a more complex palette of products. In the cases of cyclopentene and cyclohexene, no reactions with the alkene were observed. Instead, compounds 24, 25, 27, and 28 were formed these results can be rationalized in terms of a dimerization of the disilene with / elimination of isobutene 26 and subsequent reactions of the disilene and the silylene with 26 [7, 8],... 

A preparative method for the generation of di-/-butylsilylene via metal promoted a-elimination of dihalosilanes has been developed. The silylene could be trapped with alkenes to give silacyclo-propanes in 65-70% yield (Scheme 35) <64AG(E)226,88AG(E)1355>. 

Silicon and germanium also exhibit carbene-like chemistry. A review comparing carbene and silylene chemistry was published. All the examples of germylene chemistry are from theoretical studies of their structure and their interaction with alkenes. A practical demonstration of silylene chemistry was reported. Ab initio calculations have been performed on the addition of silylenes to alkenes and they shed new light on the presence of intermediates, the structure of the transition state and the effect of substituents. ... 

Reactions of silylenes with alkynes present an alternative approach for the silylation of alkynes without the aid of transition-metal catalysts (4). The bis-silylation reaction has been accomplished in a stereospecific manner via a 1,4-silyl migration with the easily available NHC-stabilized silylaminosilylene Ar SiMe3)N(Cl)Si- hPr) (Ar = 2,6-iPr2C6H3, hPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) under metal-free conditions (5), representing the first successful approach for the selective bis-silylation of alkynes with a donor-supported silylene as the silylation reagent. Furthermore, the alcoholysis of these bis-silylated alkenes gave trimethoxylsilyl-substituted alkenes. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

The silacyclopropanation of acyclic and cyclic alkenes with 169, catalyzed by AgOTf, occur at room temperature or even below to yield new cyclosilapropanes 173-177. In the case of chiral /3-pinene, the silacyclopropanation occurs enantioselectively (dr > 95 5) (Scheme 26).312 Mechanistic studies have been undertaken, which suggest that silyl silver complexes play an important role in the catalytic cycle of the silylene transfer.310... 

A key report investigated a variety of substrates in their reaction with silicon in an effort to find evidence for silylene intermediates during the silicon direct process reaction. When silicon, copper and methanol were reacted as described above but in the presence of alkenes, alkyldimethoxysilanes and (MeO SiH were formed95-97. The use of allyl propyl ether instead of alkenes gave allyldimethoxysilane, with 38% selectivity. These results and the reaction of silicon with MeCl in the presence of butadiene to give silacyclopent-3-enes indicates intermediate formation of silylenes. 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

Siliranes are also formed by the reaction of the cyclotrisilane [2-(Me2NCH2)C6H4]6Si3 with terminal and strained internal olefins under mild thermal conditions. The products obtained from the thermolysis of the siliranes thus prepared suggest a thermal equilibrium of the silirane with the cyclotrisilane and the corresponding alkene. This observation provides evidence for an equilibrium between the silylene and the cyclotrisilane and, moreover, proves that free silylenes are involved in the silylene transfer reaction48. 

The above theoretical analysis for a variety of dimer structures of silylenes requires inevitably a definition of disilenes different from that of alkenes, molecules with carbon carbon double bonds. Geometry around a typical C=C double bond is planar and the double bond length (134 pm) is shorter than the corresponding single bond (154 pm). BDE of ethylene to two methylenes is ca. 170 kcal mol-1 which is 1.9 times larger than for the C C single bond (90 kcal mol-1 for H3C-CH3) the BDE of ethylene really almost doubles the BDE for ethane ... 

The stable silylenes 83-85 do not react with conventional C=C double bonds however, diazasilole 83 is an efficient catalyst for the polymerization of alkenes, terminal alkynes, and 1,3-butadienes <2000ACR704, 2002USP028920, 2004JOM4165>. The stable bisaminosilylene 85 reacts with the activated double bond in 177-phosphirenes 134. The heterobicyclobutane 135 is however only a transient species and after addition of a second silylene 85 phosphasiletes 136 were isolated. Use of more sterically demanding substituted phosphirenes hampered the attack of the second silylene and the phosphasiletes 137 and 138, which are valence isomers of bicyclobutane 135, were obtained (Scheme 14) <2004AGE3474>. 

Silver compounds are versatile catalysts for various cycloaddition reactions, including [2 + 1]-, [2 + 2]-, [3 + 2]-, and [4 + 2]-cycloadditions. An example for the silver-catalyzed formation of three-membered rings by [2+ l]-cycloaddi-tion is the silacyclopropanation reaction of mono- and disubstituted alkenes by silylene transfer from the cyclohexene silacyclopropane 432 that was reported recently by Woerpel et /.355,355a (Scheme 127). The reaction tolerates a number of functionalities in the substrate (OBn, OSiR3, BuTlC, etc.,) and is stereospecific with regard to the cisjtrans... 

The photolysis of tris(trimethylsilyl)phenylsilane results in formation of trimethylsilylphenylsilylene in high yield, together with a small amount of a silicon-carbon double-bonded intermediate, which will be described in detail later. This silylene has a high reactivity toward unsaturated organic substrates such as alkenes and alkynes (44). 

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