From Silacyclopropanes

Seyferth and coworkers recently succeeded in synthesizing a variety of the long-sought substituted silacyclopropanes. They found that hex- 

However, the other silacyclopropanes synthesized are much more thermally stable than is hexamethylsilirane and do not serve as sources of SiMej at these low temperatures. 

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An organodilithiosilane 54 was unexpectedly obtained from silacyclopropane 53 by reacting it with an excess of Li, in dry oxygen-free tetrohydrofuran (THE), at room temperature for 30 min (Equation 11). There was no precedent for alkali metal-induced fragmentation of silirenes to dianionic silicon derivatives <1999JA10231>. 

The insertion products from silacyclopropanes are valuable for stereoselective synfhesis of 1,3-diols [55]. In particular, oxasilacyclopentanes 21 a and 21 b, derived from trans- and cis-187, can be converted into a variety of functionalized 1,3-diols by highly diastereoselective alkylation using silyl enolates (Section 10.2.1.1, Scheme 10.17) and allylsilanes, and subsequent oxidation of the silicon-carbon bond (Scheme 10.254). 

The coordination compound 76 was stable enough for isolation and recording of its NMR spectra, from which a rigid silacyclopropane structure could be deduced. The mechanism of complex formation has also been investigated in detail by matrix techniques. 

Novel thermal and metal-catalyzed di-tert-butylsilylene 161 transfer reactions have been reported by Woerpel < / /.308-312 The transfer reactions required the inital preparation of cyclohexene-derived silacyclopropanes 169-171, which has been achieved by trapping of di-fert-butylsilylenoid, generated from /-Bu2SiCl2 and lithium, with cyclohexenes (Scheme 26).305 It is noteworthy that these reactions occur with remarkably high diastereoselectivities when 2-substituted cyclohexenes are used. The silacyclopropanation of 169 with functionalized cyclopentenes under thermal conditions (115°C) has provided /razy-silacyclopropanes, such as 172, with diastereoselectivities up to 96 4, whereas no silacyclopropanes were obtained from the direct reaction of the same cyclopentenes with /-Bu2SiCl2 in the presence of lithium (Scheme 26).308... 

In 1964, silacyclopropanes were suggested as intermediates in the dehalogenation of chloromethylsilanes using Na/K vapor. The ultimate product, dimethylvinylsilane, also results from dimethylsilylene and ethylene, providing further support to the silirane as intermediate (Scheme 1) (64JA1442). 

However, the only evidence that the intermediate is a silacyclopropane comes from the photolytic generation of phenylmethylsilylene in the presence of 2,3-dimethylbuta-1,3-diene. Methanol gives (8), indicating a 1,4-addition to a vinylsilacyclopropane (Scheme 19) (75JOM(86)C23). Also the addition of dimethylsilylene to 1,3-cyclohexadiene gives 7-silanor-bornene (9) and 3,3-dimethyl-3-sila-l,4,6-heptatriene (10), both of which can be formed... 

Silylene extrusion from siliranes in the presence of alkynes, notably bis(trimethyl-siiyl)acetylene, gives the silirene (35) in good yield (Scheme 41) (76JA6382). Compound (35) is more stable thermally than hexamethylsilirane and shows 2 Si NMR absorptions for the ring atom at 5 = 106.2 p.p.m., some 50 p.p.m. downfieid from those of silacyclopropanes, and about 100 p.p.m. downfieid from normal cyclic and acyclic tetraalkylsilanes. Notable reactions include alcoholysis and the insertion of aldehydes and ketones, dimethylsilylene... 

Attempts to generate the parent silacyclopropane from 2-ethyl-1,1,1-trimethyldisilane in an analogous manner failed69 (for an example of a matrix-isolated silacyclopropane cf 92, Section IV.B). 

Ando and coworkers found that the silylene Dip2Si 382 (Dip = 2,6-di-/-propylphenyl), formed on photolysis of the trisilane 382 in the presence of Cgo, gave an adduct assigned the structure of the silacyclopropane 384 which evidently arose from addition of the silylene across the C=C between two six-membered rings of the fullerene198. A segment of the structure of 384 is shown in equation 47. 

Scheme 7.12. In situ functionalization of silacyclopropanes derived from olefins.

Insight into the mechanism of silver-catalyzed silylene transfer from cyclohexene silacyclopropane to an olefin was obtained using bistriphenylphosphine silver triflate as a catalyst.83 Woerpel and coworkers chose to employ ancillary ligands on silver to address the both the poor solubility of silver triflate as well as its propensity to decompose to afford a silver(0) mirror or precipitate. The addition of triphenylpho-sphine, however, attenuated the reactivity of the silver catalyst. For example, the reaction temperature needed to be raised from —27°C to 10°C to obtain a moderate rate of reaction (Scheme 7.15). 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

Scheme 7.20. Potential catalytic cycle for silver-mediated di-tert-butylsilylene transfer from cyclohexene silacyclopropane 58 to styrene.

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

Transition metal complexes were known to facilitate the addition of silylene to acetylenes from a variety of different sources.60,61,90,91 These conditions, however, often required heating, and the initially formed silacyclopropene often incorporated a second molecule of the acetylene to afford a silole.92,93 With their discovery of low-temperature silver-mediated di-ferf-butylsilylene transfer conditions from cyclohexene silacyclopropane 58 to olefins, Woerpel and coworkers set out to investigate the... 

Woerpel and Clark identified silver phosphate as the optimal catalyst to promote di-ferf-butylsilylene transfer from cyclohexene silacyclopropane to a variety of substituted alkynes (Scheme 7.25).95 While this silver salt exhibited attenuated reactivity as compared to silver triflate or silver trifluoroacetate, it exhibited greater functional group tolerance. Both di- and monosubstituted silacyclopropenes were easily accessed. Terminal alkynes are traditionally difficult substrates for silylene transfer and typically insert a second molecule of the starting acetylene.61,90 93 Consequently, the discovery of silver-mediated silylene transfer represents a significant advance as it enables further manipulation of monosubstituted silacyclopropenes. For enyne substrates, silylene transfer the alkynyl group was solely observed. The chemoselectivity of the formation of 99f was attributed to ring strain as theoretical calculations suggest that silacyclopropenes are less strained than silacyclopropanes.96 97... 

Toward this end, Woerpel and Nevarez examined the possibility of di-tert-butylsilylene transfer from cyclohexene silacyclopropane 58 to imine 169a (Scheme 7.48).123 Thermolysis produced a mixture of silaaziridine 170a and an imine-dimer byproduct (171). The results by Brook and coworkers suggested that if the temperature of silylene transfer were lowered, isolation of 170a without formation of byproduct 171 would be possible. As anticipated, exposure of cyclohexene silacyclopropane 58 to imine 169a in the presence of substoichiometric amounts of silver triflate produced only 170a. This silaazridine could be purified by bulb-to-bulb distillation to afford the product in 80% yield. Copper salts required... 

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 silacyclopropanes 21 and 22 by irradiation with a high-pressure mercury lamp proceeds simultaneously by two different routes, one leading to the formation of a 1-alkenyl substituted silane via a 1,2-hydrogen shift which has never been observed in the photolysis of the silacyclopropanes produced from methylphenylsilylene with olefins, and the other involving the usual 1,3-hydrogen shift. The photochemical reaction of 21 is shown in Eqs. (27) and (28) as a typical example. 

In this case, no product arising from the reaction of the silicon-carbon double-bonded intermediate with methanol can be observed at all. However, on prolonged irradiation of the solution two products, 1,1-dimethyl-2,3-benzo-5-trimethylsilyl-l-silacycIopentene (48) and 1-methoxy-dimethylsilyl-l-trimethylsilyl-2-phenylethane are obtained in 17 and 7% yield, in addition to the (Z)- and (E)-isomers (15 and 12% yield). The formation of the latter compound can best be understood by the transient formation of a silacyclopropane followed by reaction with methanol (98). The mechanism for the production of 48 in the prolonged irradiation of PhCH=CHSiMe2SiMe3 is not fully understood but is tentatively given in Scheme 16. 

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