Sigmatropic rearrangements, palladium -catalyzed

Many metals catalyze sigmatropic rearrangements including several that likely function by it-activation such as mercury, palladium, platinum, and, most recently. 

This series of rearrangements includes the dithia-Claisen rearrangement mentioned above (Section IV.E.l) as well as the palladium-catalyzed [3,3]-sigmatropic isomeriza-tions of allyl methyl N-aryldithiocarbonimidates 627 (refluxing dioxane, 20 h, 62-90%) (equation 275)376 and a Pd11-catalyzed tandem [2,3]-sigmatropic shift, followed by 1,3-dipolar cycloaddition which takes place at equilibrium between O-allyl ethers of oximes 628 and the corresponding N-allyl nitrones 629 (equation 276)377. 

The [3,3]-sigmatropic rearrangement of a 1,5-hexanediene is known as the Cope rearrangement and usually proceeds through a chair transition state. Generally, a large substituent at C-3 (or C-A) prefers to adopt an equatorial-like confirmation.303 304 As the reaction is concerted, chirality at C-3 (or C-4) is transferred to the new chiral center at C-l (or C-6). The reaction can be catalyzed by transition metals.305 The use of a palladium catalyst allows for the reaction to be conducted at room temperature instead of extremely high temperatures (Scheme 26. lO).306-307... 

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... 

The Cope- and Claisen rearrangements are also catalyzed by square-planar transition metal complexes. Palladium complexes have been responsible for spectacular increases in the rates of Cope rearrangements by factors of up to 10 . Certain other sigmatropic rearrangements are also catalyzed by transition metal complexes. 

The palladium(II)-catalyzed [3,3] sigmatropic rearrangement of the (4.S ,5.S )-diacetate, derived from tartaric acid (entry ll)21 produced only the conjugated (2/ ,7R)-diacetate the other possible conjugated (25,35)-diacetoxy-4,6-diene derivative was not detected. 

V-Silylated vinylcopper reagents are acylated with acid chlorides followed by palladium-catalyzed 1,5-sigmatropic rearrangement to form silyloxy dienes in moderate yield (equation 57)." ... 

Partial reduction of the two ester groups in 4 with diisobutylaluminum hydride in toluene-hexane at —78 °C provides in situ a dialuminate which, as an aldehyde equivalent, reacts under Wittig—Homer conditions to provide good yields of the diolefin 24. Subsequent conversion of 24 to (4i ,5i )-l,8-(bisbenzyloxy)-2( ),6(E)-octadien-4,5-diol (25) provides the essential framework for a palladium(II)-catalyzed [3,3]-sigmatropic rearrangement to 2S,1S) 2,7-(bisacetoxy)-l,8-(bisbenzyloxy)-3(j, 5( )-octadiene (26) (Scheme 6). The orginal chirality is completely translated into the dissymmetric 3,5-octadiene framework, which has C2 chirality [18]. 

There are also several indole syntheses which are based on the use of arylhydroxyamines and which involve a [3,3]-sigmatropic rearrangement similar to that involved in Fischer cyclization. Substituted JV-acetylarylhydroxyamines can be vinylated by palladium-catalyzed addition-elimination with vinyl acetate. Under mild heating, rearrangement and cyclization occurs (Scheme 61) <84HCA1647, 88HCA344>. 

Palladium-catalyzed cyclizations of bromodialkenyl ethers, in which the key step is vinylic substitution of bromine, result in low to moderate yields of tetrahydrooxepines and methyleneoxepanes <83JOC3894>. y,5-Unsaturated seven-membered lactones are prepared in 50-60% yields from 2-phenylthio-3-butenyldiazomalonates <90CL613> on treatment of the latter with rhodium acetate the reaction involves a [2,3]-sigmatropic rearrangement of the generated cyclic allylsulfonium ylides. 

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