Brook rearrangement-intramolecular

Scheme 6.20 Lithium phosphite-initiated Brook rearrangement/intramolecular Michael additions.

Nucleophilic addition to an acylsilane followed by a 1,2-Brook rearrangement and final trapping of the resulting carbanion in either an acylation or intramolecular... 

These reactions are thought to proceed by initial formation of the lithio propargylic alcohol adduct, which undergoes a reversible Brook rearrangement (Eq. 9.14). The resulting propargyllithium species can equilibrate with the allenyl isomer and subsequent reaction with the alkyl iodide electrophile takes place at the allenic site. An intramolecular version of this alkylation reaction leads to cyclic allenylidene products (Eq. 9.15). 

Brook and retro-Brook rearrangements are generally considered to proceed intramolecularly via pentacoordinate silicon-containing intermediates such as 34 and 35.29-32 The... 

Phenyllithium (and lithiophosphites) also bring with them sufficient stabilisation of the organolithium for Brook rearrangement to occur readily.40 With acylsilane 49, intramolecular Michael addition leads to cyclic structures 50. 

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. 

The key step of the mechanism is the solvent-controlled [1,4]-Brook-rearrangement, which proceeds through an intermediate having a pentacoordinate-silicon atom. This rearrangement does not take place until HMPA is added to the solvent. A similar solvent effect has been observed by K. Oshima, K. Utimoto and co-workers.The rearrangement was found to be completely intramolecular based on the results of a crossover experiment by A.B. Smith et.al. ... 

Acylsilanes also function as intramolecular radical acceptors, but, interestingly, a radical-Brook rearrangement takes place to give the silylated cycloalkanols (Scheme 4-20) [40]. 

Explored initially by Reich,cyanide addition to acyl silanes can produce Brook rearrangement, leading to an additional cascade of steps in appropriately designed systems. For example, Takeda, using a crown ether as a phase transfer agent but under non-aqueous conditions, showed that cyanide addition to acyl silanes and subsequent Brook rearrangement led, in the case of y bromo acyl silane 59 to intramolecular alkylation and formation of cyclopropanone cyanohydrin derivative 60. ... 

The final focal point of silicon chemistry to be mentioned in this introduction deals with the continuing evolution of the Brook rearrangement. At its inception, the Brook rearrangement involved the intramolecular C O silicon migration of a-oxidosilane 99 to an a-silyloxy anion 101 via the intermediacy of pentavalent siliconate 100 (eq 23). ... 

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