Intermolecular reactions dehydropalladation

Hosokawa, T., Murahashi, S.-l. Other intermolecular oxypalladation-dehydropalladation reactions. Handbook of Organopalladium... 

Intermolecular, enantioselective Heck reactions require a cyclic olefin as substrate, since syn carbopal-ladation of a cyclic olefin results in a geometrically defined a-alkyl-palladium compound. By necessity, the subsequent syn dehydropalladation must take place away from the newly formed chiral centre, thereby affording a chiral product. 

Grigg and Xu have developed a variety of so-called queuing cascades involving allenes. The intra-intermolecular carbopalladation sequence of the <9-iodo-A-methyl-A -(methylallyl)aniline 142 and 1,1-dimethylallene 143 with subsequent / -dehydropalladation leads to the 1,3-dienyl-substituted indole derivative 144, which is immediately trapped by an added dienophile (e.g., A-methylmaleimide) in a Diels-Alder reaction to yield 145 (Scheme 37)7 ... 

An example of the use of an intermolecular carbopalladation in complex molecule synthesis is the preparation of a PAF (platelet activating factor) antagonist (Scheme 11). In the key step, an intermolecular Heck reaction of 2-naphthyl triflate with 2,3-dihydrofuran 71 yields 2-naphthyl-2,3-dihydrofuran 72 in 52% yield with excellent enantioselectivity. The reaction presumably occurs via the cationic manifold and the alkene is isomerized by a hy-dropalladation/dehydropalladation reaction. The minor product 2,5-dihydrofuran 73 is obtained in 26% yield with modest enantioselectivity favoring the opposite absolute configuration at the key center. Critical to the reaction is the use of the sterically demanding and highly basic proton sponge [l,8-bis(dimethylamino)naphthalene] as the base. It is... 

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