Stereochemistry Brook rearrangement

Interestingly, the stereochemical outcomes of these reactions depend on the substituents. With aryl substituents at the carbon atom, an inversion of stereochemistry was observed (equation 28)78, while with alkyl groups at the carbon atom, the reverse Brook rearrangement occurs with retention of the stereochemistry (equation 29)80. 

Remarkably, 57, the trimethylsilyl analogue of 55, appears to racemise much more slowly. At -50 °C, it undergoes a Brook rearrangement (section 8.2) to give 59 with 99% inversion of stereochemistry. If the anion 58 is held at -40 °C for 20 h, or 25 °C for 13 min, the product is formed with about 10% racemisation, indicating the rearrangement is reversible and that... 

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... 

Equation 126 involves a syn p elimination of a / -silylalkoxide as in the Peterson reaction equation 127 involves a Brook rearrangement of an a-silylalkoxide with elimination of a /Meaving group. The evidence presented concluded that 127 predominated and that 127 proceeds via an anti stereochemistry for the elimination. The... 

Addition of ketone enolates to acyl silanes, followed by Brook rearrangement and closure of the resulting carbanion onto the pendant ketone carbonyl leads to 1,2-cyclopropanediol products such as 67. In the presence of an appropriate electrophile and using a sodium amide base, 67 serves as a homoenolate equivalent, generating y siloxy- hydroxyketone 68 as a mixture of diastereomers whose stereochemistry was not determined. ... 

We now propose that the proton transfer process could be occurring via an intramolecular 4-centered transition state between the newly formed silyl ketal (Figure 10). A similar 4-centered transition state has been previously proposed by Sommer and Fujimoto to explain retention of the stereochemistry at an asymmetric silicon center during the exchange of alkoxide groups (Scheme 37).34 Brook and co-workers also proposed a 4-centered transition state for the thermal rearrangement of (3-ketosilanes (Scheme 38).35... 

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