Silatropic migration

Chromium-based reagent systems, for pinacol coupling, 11, 63 Chromium carbenes, in ene-yne metathesis, 11, 272 Chromium carbonyl compounds with bridging hydrides, 5, 206 computational studies and spectroscopy, 5, 203 experimentally determined structures, 5, 204 nitro and nitroso compounds, 5, 205 silatropic migrations, 5, 249 with very weakly bonded ligands, 5, 205 Chromium carbonyl hydrides, preparation and characteristics,... 

The class of phosphaalkenes with isolated P=C double bonds was first synthes ized by Becker.33 His synthetic strategy starting from trimethylsilylphosphines and acyl chlorides is still the most versatile (Protocol 3). The principle is based on the easily achievable, 1,3-silatropic migration of a silyl group bonded to phosphorus to a doubly bonded element such as nitrogen, oxygen, or sulfur. The process is favoured energetically by the construction of the P=C double bond with concomitant formation of a very stable silicon-element bond. 

Alkyl and acylsilanes Tetraalkylsilanes are inert to the carbonyl coupling reaction conditions, as shown by the syntheses of 43 and 44 [61, 62]. Despite their dose similarity to sterically hindered ketones, McMurry reactions of acylsilanes are limited to the preparation of the 1,2-disilylated stilbenes 45 (R = H, Br or OMe) silyl enol ethers are also obtained as by-products, resulting from silatropic migration [63], No such coupling of aliphatic acylsilanes has yet been reported. 

Facile migration of silyl groups in 5-silylcyclopentadienes is usually referred to as fractional 1,2-silyl rearrangement or 1,5-suprafacial silatropic rearrangement (equation 39). The fundamentals of this migration have been well discussed in BB1. Several examples of this type of migration have also appeared in recent papers95-105. 

Another well-established process available for the preparation of PC double bonds is related to the easy migration of phosphorus-bonded silyl functions toward an a-positioned, doubly bonded element such as N, O, or S [Eq. (4)]. By this silatropic movement the double bond is... 

The silatropic process combined with the addition reaction can be realized with quite a number of compounds, including carbodiimides [Eq. (6a)], isocyanates, carbon disulfide [Eq. (6b)], or carbon dioxide [Eq. (6c)], which exhibit cumulated hetero double bonds. The first step, insertion between the P-Si bond, is followed by a silyl migration (32-35). 

Komatsu et al. have developed unique methods for the generation of 1,3-dipoles from organosilanes (Scheme 10.222). Linder thermal conditions, N-(a-silylbenzyl) imines and -amides are converted, via 1,2- or 1,4-silatropic shift of the silyl group, into azomefhine ylides (153 from the amide) which react with dipolarophiles [578]. Similar thermal 1,4-silyl migrations of a-silylnitrosamines and S-a-silylben-zyl thioesters provide convenient routes to azomethine imines 154 [579] and fhio-carbonyl ylides 155 [580], respectively. 

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