Brook rearrangement organolithiums

Organolithium 62, the intermediate in the Brook rearrangement of 61, must also be configurationally stable on the timescale of the rearrangement - though here the reaction is stereospecific with retention of stereochemistry (see section 8.2).33... 

Rearrangements ([1,2] and [2,3], except Brook rearrangements) of unstabilised organolithiums inversion (SE2inv)... 

As early as 1974 it was known that the Brook rearrangement of benzylic organolithiums such as 56 proceeded with inversion.32 Brook rearrangements of non-benzylic organolithiums proceed with retention (see also section 8.2). 

By contrast, the similar phosphate-phosphonate rearrangement proceeds with retention, whether it proceeds via a tertiary56 or a secondary57 organolithium 57 or 58, as does the related amide-ketone rearrangement of 59.58 These retentive rearrangements presumably involve C=0-Li or P=0-Li coordination not possible in the Brook rearrangement. 

Brook rearrangements may be carried out with either catalytic or stoichiometric base. With catalytic base, the reaction can be considered an equilibrium between 41 and 42. The strength of the Si-0 bond (about 500-520 kJ mol-1) compared with the Si-C bond (about 310-350 kJ mol-1) means that, provided the anion 33 forms reasonably rapidly (some degree of stabilisation is required), Brook rearrangement (alkoxide formation) is favoured over retro-Brook. Organolithiums 33 may be present as intermediates in the catalytic Brook rearrangement, but their reactivity cannot be exploited under these conditions. 

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. 

An alternative disconnection of the alkoxide requires the addition of a silyllithium reagent to an enone. Addition of stoichiometric base to the alcohol 51 produces an alkoxide 52, but no evidence of Brook rearrangement to generate 53 was found on protonation of the product. However, alkoxide 52 must exist in equilibrium with some of the organolithium 53, since alkylation with a soft electrophile (Mel) produced 54.41 The equilibrium concentration of the organolithium 53 is lessened in this case by the impossibility of O-Li coordination. 

The elegant application of the reaction between an a-suhbnyllithium reagent and acyl-silane to afford an enol silyl ether is descrihed in a S3mthetic route to limonoids. Brook rearrangement following the nucleophihc addition of the organolithium protects the oxy functionahty. 

Brook addressed this important mechanistic issue early, developing evidence for retention of configuration at silicon in the rearrangement. Based on the precedented assumption of inversion at asymmetric silicon in chloride displacement with either an organolithium or an alcohol, Brook and coworkers showed a stereochemical Walden cycle that implicated retention at silicon in the rearrangement step. Mechanistically, retention at silicon corresponds to a frontside-type displacement, consistent with the mechanism shown above. The displacement might implicate 10 as a pentacoordinate silicon intermediate rather than a transition state. 

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