Cycloaddition reactions silylenes

Some cycloaddition reactions of 4 are summarized in Scheme 1. This shows that silylene 4 undergoes reactions with nitriles [14], phosphaalkynes [15], silyl azides [16], diazabutadienes [17], 2,2 -bipyridyl and its derivatives [18, 19], a-ketoimines [19], and pyridine-2-aldimines [19]. 

The reactions of silylenes with 1,3-dienes giving the corresponding 3-silolenes are typical of the cycloaddition reactions of silylenes. The mechanism of these reactions has been investigated in detail, " " and it has been proposed that the reactions of silylenes with 1,3-dienes proceed via initial [1 + 2] addition followed by the isomerization of the resulting 2-vinylsiliranes to the corresponding 3-silolenes. However, the observation and isolation of the intermediary 2-vinylsiliranes has been limited to only a few examples because vinylsiliranes readily isomerize... 

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. 

The first step of the retro-reaction involves loss of silylene 79, which could be trapped with 1-pentyne to give the known silirene 81 (equation 125). In the absence of a trapping agent, 79 recondenses to 77, probably by first dimerizing to the disilene Ar2Si=SiAr2 followed by 2 +1 cycloaddition to give 77 (equation 126). From the principle of microscopic reversibility, the fact that silylene is formed in the retro-reaction leads to the conclusion that 79 must also be an intermediate in the cycloaddition reaction. 

The cycloaddition reaction of singlet GeX2 (X = F, Cl) with formaldehyde was studied employing the HF/6-311+G theory.106 The reaction proceeds in two steps barrierless formation of an intermediate complex followed by rate-determining isomerization to form the product. The results were compared with those from other cycloadditions of germylenes and silylenes. 

In a [2+1] cycloaddition reaction the disilacyclopropane 143 is formed from silylene 85 and the ephemeral silene 144 (Equation 11) <2004OM2848>. 

The silylene 153, which exists in an equilibrium with the corresponding dimer in solution, underwent a [3+1] cycloaddition reaction with mesitonitrile oxide to afford l-oxa-2-aza-4-silete 154 (Scheme 59). The latter compound is unstable and its presence in the reaction mixture was established only by means of H NMR spectroscopy <2006JOM(691)1341>. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

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... 

Reactions with Radicals, Carbenes, Nitrenes, and Silylenes. 2.8 Cycloaddition Reactions 3 Reactivity of Nonconjugated Rings... 

Although the currently available results still do not provide a uniform scheme, they do clearly indicate that silylenes bearing bulky substituents such as 2, and also dimesitylsilylene [14], xmdergo [2+1]-cycloadditions to the double bonds of 1,3-dienes rather than [4+l]-cycloadditions. In contrast, the behavior of disilenes towards the C=C double bonds of alkenes and conjugated dienes is still not clear. While additions of the stable tetraaryldisilenes to such double bond systems are still unknown [1], the marginally stable disilene 3 is able, in individual cases, to take part in both [2+2]- and [4+2]-cycloaddition reactions. 

Cycloaddition Reactions of Dimethylaminomethyl-silylene with Dienes and Heterodienes... 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored. ... 

Summary Monoaminosilylenes and bis(diethylamino)silylene are formed by thermolysis of amino-substituted disilanes and characterized by trapping with dienes. In the case of MeSiNMe2, cycloaddition reactions were extended to 1,4-diheterodienes, unsaturated ketones and imines allowing an easy synthetic access to functionally substituted unsaturated silicon heterocycles. The syntheses of an isolable, unsymmetric diaminosilylene and of related germylenes and stannylenes are described. The results are presented and discussed in relation to theoretical work on stabilization of donor-substituted silylenes and recent work on isolable diaminosilylenes done by or in cooperation with other groups. 

Scheme 14.3 Cycloaddition reactions of silylenes as exemplified by the silylene 46.

Trichlorosilane also serves as a silylene precursor in the presence of phosphonium salt as a catalyst to undergo cycloaddition reactions with 1,3-dienes and alkynes (Scheme... 

Ferrocene-containing complexes are finding ever-increasing role in materials science. Heterobimetallic complexes of ferrocenes with silylene ligands are a new class of complexes. The heterobimetallic Co(l) complex displayed reasonable catalytic activity for [2 + 2 + 2] cycloaddition reactions of phenylacetylene substituted benzenes and pyridines (11). 

The chemistry of stable silylenes has been the subject of considerable research since the first isolation of silicocene by Jutzi in the 80s (1). More recently, silylenes stabilized by coordination of a donating ligand on the silicon(II) atom have attracted much attention due to their high reactivity strongly related to the nature of ligands, and we have demonstrated that phosphine-stabilized silylenes display unique properties. Of particular interest, the phosphine-stabilized silicon(II)-hydride 5 immediately reacts with olefins and alkynes via [2-1-1] cycloaddition reactions. In the case of diphenylacetylene, the corresponding pentacoordinate... 

A fair number of other synthetically useful ways to form SiASi bonds exists. An incomplete list comprises electrochemical methods, dehydrocoupUng of hydrosilanes, cycloaddition reactions, rearrangement reactions, silylene insertion, and radical recombination. Most of these will be shown or described in the following sections outlining the formation and reactivity of different types of oligosilanes. 

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