Silacyclopropanation, silver

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... 

Silver is often used as a halophile. In the context of six-electron species, the role of silver atoms in carbene, nitrene, and silylene transfer reactions, including aziridination, CH insertion, ring expansion, and silacyclopropanation, has been reviewed.9... 

Exploration of the reactivity of cyclohexene silacyclopropane led Woerpel and coworkers to discover that the inclusion of metal salts enabled silylene transfer to monosubstituted olefins at reduced temperatures (Table 7.1).11,74 A dramatic reduction in the temperature of transfer was observed when cyclohexene silacyclopropane was exposed to copper, silver, or gold salts. Silver salts were particularly effective at decomposing 58 (entries 6-11). The use of substoichiometric quantities of silver triflate enabled ra-hexene silacyclopropane 61 to be formed quantitatively at —27°C (entry 6). The identity of the counterion did affect the reactivity of the silver salt. In general, better conversions were observed when noncoordinating anions were employed. While the reactivity differences could be attributed to the solubility of the silver salt in toluene, spectroscopic experiments suggested that the anion played a larger role in stabilizing the silylenoid intermediate. 

One drawback of this methodology was the sensitivity of the product silacyclopropane to the reaction conditions, which prevented isolation of the product. Woerpel and coworkers were able to overcome this debilitating limitation by changing the anion of silver catalyst to trifluoroacetate and reducing the catalyst load to <1 mol% (Scheme 7.11).74 The combination of these two modifications enabled the isolation of the silacyclopropane product after careful Schlenk filtration. Isolation of the silacyclopropane allowed for either zinc-64,75 or copper-mediated functionalization of the silacyclopropane to produce 1,2- or 1,3-disubstituted oxasilacyclopentanes.63... 

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 relative reactivities of various silacyclopropanes were compared to gain insight into the reversibility of the reaction (Scheme 7.16).83 Woerpel and coworkers observed that the reactivity of the silacyclopropane toward the silver catalyst depended on the ring size, and that cyclohexene silacyclopropane was the most reactive. Notably, benzyl-substituted silacyclopropane 63d did not react when exposed to the silver complex. This lack of reactivity was interpreted as evidence in support of the irreversibility of silylene transfer to monosubstituted olefins. 

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... 

A catalytically active silylsilver intermediate was observed using low-temperature 29Si NMR and IR spectroscopy (Scheme 7.18).83 Exposure of cyclohexene silacyclopropane 58 to the silver complex produced cyclohexene as well as a new species, which exhibited two doublets at 97 ppm (./Agsi = 260 and 225 Hz) in the 29 Si 1H NMR spectmm. The 29 Si H NMR spectra of this species are consistent with a Lewis base-stabilized metal silylenoid. Tilley and coworkers have reported that (Et3P)2Pt(H)-Si(Sf-Bu)2(OTf) appears at 52 ppm,39 and Jiitzi et al. observed... 

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.

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... 

Woerpel and Calad tested for the formation of the silacarbonyl ylide by interrogating the behavior of the electrophilic silver silylenoid intermediate 115 toward a,(3-unsaturated carbonyl compounds (Scheme 7.37).82 They hypothesized that formation of silacarbonyl ylide 131 might trigger a 6jt-electrocyclization to form oxasilacyclopentene 132. As anticipated, exposure of cyclohexene silacyclopropane 58 to substoichiometric amounts of silver trifluoroacetate in the presence of a,(3-unsaturated carbonyl compounds 130 produced oxasilacyclopentenes 132. The reaction tolerated a substitution at the a and/or (3 position and was general for both esters and ketones. 

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-catalyzed di-tert-butylsilylene transfer to imines proved general (Scheme 7.49).123 Exposure of alkyl- or arylimines with IV-benzyl or iV-aryl groups to cyclohexene silacyclopropane 58 in the presence of 1 mol% of silver triflate produced silaaziridines 170. Even ketimine 169f was tolerated as a substrate. 

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... 

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>. 

---