Oxasiletanides

The spirocompounds 34 (M = Ti or Zr) have been prepared . Studies of the thermolysis of pentacoordinate 1,2-oxasiletanides 35, potential intermediates in both the Peterson reaction and the homo-Brook rearrangement of p-hydroxyalkylsilanes with bases, in the presence of a proton source afforded the olefin, RCH=C(CF3)2 and/or the alcohol, (CF3)2CHOH <99CL1139>. 

One interesting example of pentacoordinate silicon is a 1,2-oxasiletanide, reported by Okazaki and coworkers61, which can be regarded as the intermediate of the Peterson reaction. Thus, treatment of a /1-hydroxysilane (28) with butyllithum afforded oxasiletanide... 

Oxasiletanide 161a has been synthesized and its structure determined by X-ray crystallographic analysis199. On slight heating a similar oxasiletanide 161b was observed to decompose to the alkene and lithium silanoxide, as shown in equation 129. 

In contrast, the base-catalyzed elimination may proceed via a 1,3-shift of the silyl group after deprotonation, or with the formation of a pentacoordinate 1,2-oxasiletanide that subsequently undergoes cycloreversion ... 

Formation of a pentacoordinate 1,2-oxastannetanide by a base-induced rearrangement of a bis(/3-hydroxyalkyl)-stannane has also been reported <1997PS513>. Synthesis of pentacoordinate anionic oxasiletanides, oxagermeta-nides, and oxastannetanides has been reviewed (Scheme 17) <19980M367, 2000JOM256, 2002CSR195>. 

Intermediacy of pentacoordinate oxasiletanides in the Peterson olefination reaction has been discussed <2002CSR195, B-2004MI18>. Products of thermolysis of 1,2-oxasiletanides indicate that they are the intermediates of both Peterson and homo-Brook reactions <1999CL1139, 2003BCJ471>. 

The exact pathway of the Peterson reaction is still not clear despite the intensive research effort. Most of the mechanistic studies suggest that both the stepwise and concerted pathways are feasible under basic conditions. In the concerted pathway a pentacoordinate 1,2-oxasiletanide is formed. The stepwise pathway is expected when chelation control operates in the reaction. The driving force is the formation of a very strong Si-0 bond. Under acidic conditions the 3-hydroxysilane undergoes an E2 elimination to afford the other alkene isomer. 

Kawashima, T., Naganuma, K., Okazaki, R. Generation and Decomposition of a Pentacoordinate Spirobis[1,2-oxasiletanide]. Organometallics 1998,17, 367-372. 

On the other hand, elimination under basic conditions should proceed in a syn manner. Two possible pathways have been postulated for the elimination of a silyl-oxide moiety after deprotonation of a hydroxy group with an equimolar amount of base (Scheme 2.4). One is the stepwise 1,3-migration of a silyl group from carbon to oxygen, followed by elimination of a trimethylsilyloxide moiety. The other involves the formation of a pentacoordinate 1,2-oxasiletanide 2, which is in equilibrium with the j8-silylalkoxide anion 1, and extrusion of the trimethylsilyloxide moiety therefrom. The reaction mechanism has not yet been entirely elucidated. 

In contrast to the stepwise addition mechanism, there is a report in which a concerted mechanism is proposed for the addition process, involving a 1,2-oxasiletanide intermediate. Peterson reaction of bis(trimethylsilyl)methyllithium with benzaldehyde gives a mixture of almost equimolar amounts of the E- and Z-isomers of styrylsUane, whereas Peterson eUmination of the corresponding oxidosilane 14 generated by other methods affords the ( )-styrylsilane with very high isomeric purity (Schemes 2.12 and 2.13) [36). The above results indicate that yS-oxidosilane 14 is not formed as a major intermediate and that the Peterson reaction involves nearly simultaneous formation of C-C and Si-O bonds to give a 1,2-oxasiletanide intermediate 13 directly. 

The protodesilylation should take place via a postulated 1,2-oxasiletanide transition state, which leads to an alkene in the absence of any proton source. Protonation of the isolated pentacoordinate 1,2-oxasiletanides 7b actually gives the desi-lylated alcohol 206 after hydrolysis in competition with the Peterson reaction (Scheme 2.131) [25, 28] (see Section 2.2.1.2). The decisive factors with regard to the... 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

A subsequent basic elimination can proceed via two pathways (i) deprotonation of the hydroxyl followed by a silyl 1,3-shift 20, (ii) deprotonation followed by formation of a penta-coordinate 1,2-oxasiletanide 21 which then collapses to give the alkene product, 23. 

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