Cyanide, Brook rearrangement

Isomerizations, Under PTC conditions (presence of tetrahexylammonium bromide) 1,3-diaryIpropynes can be isomerized to diarylallenes by KOH at room temperature.A Brook rearrangement of acylsilanes occurs upon addition of cyanide ion. ... 

Scheme 6,18 Formation of Q-silylcyanohydrins by cyanide ion-initiated Brook rearrangement under phase-transfer catalytic conditions.

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. ... 

Johnson s group developed a catalytic asymmetric cyanation/1,2-Brook rearrangement/C-acylation of acylsilanes with cyanoformates (Scheme 19.14). In the presence of (i ,/ )-(salen)Al 19, the corresponding cyanohydrin trimethylsilyl ethers of a-keto esters were obtained in moderate to good enantioselectivities (61-82% enantiomeric excess). Access to chiral (silyloxy)nitrile anions is facilitated by metal cyanide-promoted Brook rearrangement reaction of acylsilanes. 

The observation that acylsilanes such as 17 could directly provide intermediate 18 via cyanide addition and [l,2]-Brook rearrangement has led to an improved process that is catalytic in cyanide. The reaction produces O-silylated aryl-aryl, aryl-alkyl, and alkyl-aryl acyloin products in moderate to high yield.Further work by Johnson and co-workers established the superiority of lanthanum tricyanide for this process, which allows selective access to even alkyl-alkyl acyloin products. ... 

DeglTnnocenti and co-workers established in 1987 that acylsilanes could react with activated olefins in a cyanide-catalyzed sila-Stetter reaction, and recent work by Scheldt and co-workers has established the utility of thiazolylidine catalysts in this Stetter variation." The reaction is presumed to occur via a catalyst-initiated [l,2]-Brook rearrangement, which serves to provide the silylated acylanion donor intermediate 23. The use of excess 2-propanol in the reaction facilitates silyl transfer and catalyst turnover. Compared to the classical Stetter approach, this variation provides comparable yields of 1,4-diketone products from aromatic (20) and aliphatic (21) acylanion precursors. 

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