Rearrangement allylsilanes

Diethyl[dimethyl(phenyl)sUyl aluminum. ( HjlCHOiSiAUC H,), (1). The aluminum reagent is prepared in situ by addition of (C HO AICl to QHdCHdiSiLi in THF at 0°. The reagent converts allylic phosphates into allylsilanes. Rearrangement products can be formed from unsymmetrical allylic phosphatc>... 

Similai selectivity is observed in the synthesis of allylsilanes where X = CF3SO3 and Y = Si(CH3) 3 (304). Alkenyl- and alkynylborates containing a leaving group in the y-position rearrange to aUyhc and aUenic boranes, respectively (305). 

Claisen and Carroll rearrangements of hydroxyalkenylsilanes provide an asymmetric synthesis of allylsilanes from optically active secondary alcohols39,40. 

The assembly of the benzazepane framework 2-338 (e. g., found in cephalotaxine [187]) starting from 2-336 has been achieved by the group of Schinzer [188] utilizing a combination of a Beckmann rearrangement to give the iminiumion 2-337 and an allylsilane cydization (Scheme 2.79). 

Scheme 2.79. Synthesis of benzazepanes via a Beckmann rearrangement/allylsilane-cyclization sequence.

Knolker and coworkers also used a domino [3+2] cycloaddition for the clever formation of a bridged tetracyclic compound 4-172, starting from a cyclopentanone 4-168 and containing two exocydic double bonds in the a-positions (Scheme 4.36) [57]. The reaction of 4-168 with an excess of allylsilane 4-169 in the presence of the Lewis acid TiCLj led to the spiro compound 4-170 in a syn fashion. It follows a Wag-ner-Meerwein rearrangement to give a tertiary carbocation 4-171, which acts as an electrophile in an electrophilic aromatic substitution process. The final step is the... 

Retro-Brook rearrangement is relatively common and enjoys widespread application in many facets of organic synthesis. For example, vinylsilane synthesis from aUyloxysUane (eqnation 103)"°, allylsilane synthesis (eqnation 104)"°, and lithium eno-late formation from silyl enol ether (equation 105)" were reported. 

The [1,4]-rearrangement of the allyloxysilane system is a potentially useful method for allylsilane synthesis (equation 106). However, in general, a [1,2]-rearrangement may occur predominantly in this system (see Section n.F.l). To this end, the author s group recently found that the rearrangement of 171 in the presence of excess amount of HMPA provides the [l,4]-retro-Brook rearrangement product 172 in excellent yield (equation 107). ... 

Organoaluminum-promoted Beckmann rearrangement/methylation of cyclohexanone oxime mesylate, followed by allylation of ketimine 40a and Mannich cyclization of the intermediate iminium-allylsilane, provides piperidine 40b possessing cxo-unsaturation (08BKC1669). 

A tandem Beckmann rearrangement/allylsilane was used to produce the optically active cephalotaxine framework 482 (equation 208). Asymmetry could be induced as a result of stereochemical effects. 

When 32 is photolyzed in the presence of methyl allyl ether (62),50 51 the generated silylene 13 initially coordinates to the ether oxygen to give 63, w ich subsequently rearranges to allylsilane 64 (Scheme 17), a reaction... 

The TiCU-induced three-component coupling reaction of an a-haloacylsilane, allylsilane and another carbonyl compound gives 48 in good yield. A silyl enol ether intermediate is suggested (equation 31)82. The reaction of a cyclopropyl ketone with allylsilane yields a mixture of skeletal rearranged products83. 

Claisen-Ireland rearrangement of 239 followed by DIBALH reduction gives a mixture of syn and anti isomers 240 (equation 194)34-347. In a similar manner, orthoester Claisen rearrangements of 241 give stereoselectively the corresponding chiral allylsilanes 242 (equation 195)348. 

Ally lie alcohols from allylsilanes. The adducts of benzeneselenenyl chloride to ullylsilanes on treatment with SnCl2 or Florisil undergo dechlorosilylation and rearrangement to give the less substituted allyl selenide. When oxidized, the allyl selenidcs are converted into the allylic alcohol in which the hydroxyl group occupies (lie more substituted site (6, 338). 

Previously, Ireland-Claisen ester-enolate rearrangement of the corresponding a-propionyloxy-allylsilane led to model system 5.44 Therefore, elaboration to 4 via rearrangement of 15 was pursued. To complete our retrosynthetic analysis, a plausible route to 15 was devised, involving straightforward homologation of 2P,3a-disubstituted cyclohexanone 17 to cyclohexene-carboxaldehyde 16, which in turn undergoes silylanion addition and subsequent acylation (Eq. 8). 

The Claisen reaction can be catalyzed by palladium 328 thus, the [3,3]-sigmatropic rearrangement of a bulky allylsilane derivative 12 proceeds with high selectivity (Scheme 26.11).329-331... 

Allylsilanes generated from y-hydroxyvinylsilanes and enol ethers via a Claisen rearrangement have been used to synthesize cyclopenten-4-ols (Figure Si6.2). 

Benzyl and allylsilanes 87 and 89 can be formed by reductive lithiation and [l,4]-retro-Brook rearrangement of benzyl and allylsulfides 86 and 88 containing silyl ethers.63 The reaction, by virtue of the intermediacy of a configurationally unstable benzyl or allyllithium, is stereoselective but not stereospecific. In the latter case, intramolecularity assures the regioselective formation of 89. 

---