Silylene transfer compounds

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

Silylene transfer to a,j3-unsaturated carbonyl compounds has been shown to provide a stereoselective method for the synthesis of compounds possessing quaternary carbon stereocenters (Scheme 36).61... 

The reaction of benzo[A]-l,3-diazasilole 85 with lithium alkyls yields the insertion product 112. It was suggested that the initial step of this reaction is the formation of the donor-acceptor complex 113 (Scheme 10) <2002JOM272, 2002JOM150>. The tetracoordinated silicon compounds 112 might have synthetic potential as silylene transfer reagents. 

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... 

Silver-catalyzed silylene transfer was reported as a general method for the synthesis of silaziridines <07OL3773>. Despite the sensitivity of silaziridines to water and air, these intriguing compounds could be isolated in good yield. 

Compound 1 did not react with unstimned internal olefins such as tetramethylethylene, /ra s-3-hexene, tram-stilbene, cyclooctene, cyclohexene, or cyclopentene. But imposing strain to the olefinic moiety resulted in a clean silylene transfer to the double bond Norbomene formed with 1 the tricyclic silacyclopropane 6. Whereas 2 did not add to the double bond of 7, methylene cyclopropane 8 could be transformed into spiro[2.2]pentane 9 by reaction with 1. Addition of 2 to bicyclopropylidene allowed the convenient synthesis of dispiro[2.0.2.1]heptane 10 in a quantitative manner. 

Coordinated silylene ligands have been invoked or suggested as intermediates in a number of chemical processes, including Rochow s Direct Process94, catalytic redistribution of silanes39 and various silylene-transfer reactions95-99. Evidence for such species is primarily circumstantial, and M=Si double bonds have never been detected in these systems. Presently there is no conclusive evidence for the involvement of silylene coordination compounds in any transition-metal-mediated silylene-transfer reactions. Some reactions that may involve silylene ligands are discussed below. 

Terminal transition-metal silylene complexes have proven to be elusive synthetic targets, but it seems likely that future investigations will uncover viable routes to stable M=Si double-bonded species. Such compounds are of interest as model systems for investigating the reactivity discussed in the previous section, and for providing new synthetic intermediates for silylene transfer reactions. Recent ab initio SCF MO calculations predict that (CO)5Cr=SiH(OH) should be a relatively stable molecule, but that it may be difficult to isolate due to its susceptibility to nucleophilic attack at the silicon atom. The Cr=Si bond dissociation energy was calculated to be 29.6 kcal mol-1, compared to the analogous Cr=C bond dissociation energy of 44.4 kcal mol-1107. 

After a decade of research the basic principles in the chemistry of decamethylsilicocene (1) seem to be understood. This compound shows the reactivity of a nucleophilic silylene due to the fact that the Tt-bonded pentamethylcyclopentadienyl ligands are easily transferred to a-bonded substituents during the reaction. The steric requirements of these substituents permit reactions with bulky substrates. The migratory aptitude and the leaving-group character of the pentamethylcyclopentadienyl groups... 

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. 

To test this hypothesis, several silylene/silyl transfer experiments were carried out with the compounds 1 and 2. Here we report the cross experiments with alkyl(chloro)germanes, transfer experiments with chloro- and diphosphanes, and trapping reactions with three phosphaalkenes. 

This concluding statement might be transferred without any restrictions to the history of the development of the silylene chemistry, the sila analogs of the carbenes CR2. Again, there are main milestones describing the efforts in this area first, silylenes were discussed as highly reactive intermediates in gas phase reactions of suitable silyl precursors, then followed by the spectroscopic characterization in the gas phase or in low temperature matrices. In a third period, these compounds were kinetically and/or thermodynamically stabilized and characterized in conden d phase and in the solid state, even by single crystal X-ray analysis. 

Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review. Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc and the photoinduced electron-transfer reactions involving organosilicon compounds. ... 

The redistribution reactions mentioned above indicate that the opening of a coordination site not only promotes oxidative addition and possibly a-bond metathesis reactions, but — difG ndy from carbon compounds — also induces migrations of silicon substituents with the concomitant formation of silylene complex intermediates. It was pointed out by Tilley and co-workers that the transfer of a silyl substituent from silicon to platinum is easier in three- than in four-coordinated... 

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