Silver-catalyzed silylene transfer

Department of Chemistry, University of Illinois, Chicago, Illinois 

2 Reactivity and Attributes of Metal Silylenoids and Silylmetal Complexes 

3 Transition Metal Silylenoid Complex-Catalyzed Hydrosilation Reactions 

4 Transition Metal Silylenoid-Catalyzed Atom Transfer Reactions 

4 Silver-Mediated Transfer of Di-tert-Butylsilylene to Olefins 

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Silver-catalyzed silylene transfer to disubstituted olefins was also possible (Scheme 7.10).11,74 The transformation was stereospecific c/s-2-butene afforded cw-dimethylsilacyclopropane 66a and tftmv-2-butene generated tram-dimethylsila-... 

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). 

Scheme 7.17. Kinetic studies on silver-catalyzed silylene transfer.

To elucidate the mechanism of silver-catalyzed silylene transfer, kinetic studies were performed by Woerpel and coworkers (Scheme 7.17).83 The reaction of cyclohexene silacyclopropane 58 and styrene in the presence of 5mol% of (Ph3P)2AgOTf was followed using 1H NMR spectroscopy. The kinetic order in cyclohexene silacyclopropane 58 was determined to be 1. In contrast to the rate acceleration observed with increasing the concentration of 58, inhibition of the rate of the reaction was observed when styrene, cyclohexene, or triphenylphosphine concentrations were increased. Saturation kinetic behavior in catalyst concentration was observed. Activation parameters were determined to be A// = 30(1) kcal/mol and A A 31(7) eu (entropy units). Similar activation parameters were observed in... 

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. 

While exploring the scope of silver-catalyzed silylene transfer to olefins, Woerpel and Calad observed that the electrophilic silver silylenoid species reacted preferentially with the enolizable ester group to produce vinylsilane 129 instead of a... 

The synthetic utility of this cascade reaction was underscored by the facile transformation of crotyl tiglate into 1,3-diol 137 (Scheme 7.39).82 Silver-catalyzed silylene transfer to crotyl tiglate produced silalactone 136 nearly quantitatively as a 97 3 mixture of diastereomers. Diol 137 was generated from silalactone after reduction of the lactone moiety with lithium aluminum hydride followed by oxidation of the C-Si bond 65>66 81... 

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. 

The silyl group-transfer reaction, or the transfer of a silylene or a silylenoid intermediate to an unsaturated C—C bond, is analogous to nitrene and carbene transfers (136). Fewer methods were developed for the silylenoid transfer this is likely due to the difficulty of handling the substrates and products (137). Woerpl and co-workers (138) discovered several silver-catalyzed silylene-transfer reactions, which greatly enriched silylene-transfer chemistry and its applications. 

Figure 31. Silver-catalyzed silylene transfer to unsaturated C—C bonds.

A series of silacyclopropene derivatives, obtained by silver-catalyzed silylene transference to alkynes, presented Si NMR signals ranging from —48.7 to —63.6 ppm <20050M6212>. The silacyclopropane derivative 35, showed an Si NMR signal at —140.7 ppm, while for its analogous silacyclopropene derivative 32, appeared at —152.3 ppm <2003OM2233>. 

Cyclohexene silacyclopropane 47 undergoes silver-catalyzed silylene transfer, acting as an efficient method for silacyclopropane 48 synthesis (Equation 5) <2004JOC4007>. Kinetic studies of the transfer reaction suggested a possible mechanism for silver-mediated silylene transfer with a kinetic order of one for 47 <2004JA9993>. 

Silver phosphonate was also used as catalyst for silylene transfer to 1-heteroatom-substituted alkynes, resulting in alkoxy-, amino-, and alkylthiolate-substituted silacyclopropenes 61. Silver-catalyzed silylene transfer to 1-halo- and 1-sulfonyl-substituted alkynes led to insertion of silylene into the carbon-heteroatom bond. Attempts of Cu-catalyzed... 

Silver-catalyzed silylene transfer was developed as a mild and efficient method for silacyclopropene 48 synthesis. Employing low temperatures for the transfer improved diastereoselectivity and substrate compatibility compared to other methods of synthesis (Equation 5) <2004JOC4007>. 

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