Cyclization-carbonylation palladium catalysis

Aryl-, alkenyl- and alkynylpalladium species readily undergo carbonylation reactions because carbon monoxide as a loosely bonded ligand can reversibly insert into any palladium-carbon bond [110]. Thus, 2-allyl-l-iodocyclopentene (148), under palladium catalysis, reacts with carbon monoxide in two modes, depending on the excess of carbon monoxide and the catalyst cocktail (Scheme 3-39) [110a]. With a slight excess (1.1 atm of CO) in the presence of [Pd(PPh3)4] in tetrahydrofuran, 148 cyclized with one CO insertion to yield 3-methylenebicyclo[3.3.0]oct-l(5)-en-2-one (152), and under 40 atm of CO with [Pd(PPh ,)2Cl2] in benzene/acetonitrile/methanol, methyl 2- 3 -(2 -oxobicyclo[3.3.0]oct-1 (5 )-enyl) acetate 149 after two CO insertions (Scheme 3-39). 

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

The reactions described in this section are not unique to ruthenium catalysis. These transformations can also be achieved using a palladium or a nickel catalyst. Since carbonylative cyclizations leading to cyclic carbonyl compounds are useful transformations in organic synthesis, these reactions are included in this section. 

The stereochemistry of the metallo-ene cyclization step plays a crucial role in the accessibility of subsequent reaction steps. Thus, only the civ-isomer of the cyclization product can undergo further carbonylative cyclization to form the bicyclic products, while the fram-isorner can Only undergo linear carboxylation 65. Product distribution also depends on the catalyst used. Thus, for selective carbonylative bicyclizations, nickel catalysis, especially Ni(cod)2/1,4-bis(diphenylphos-phino)butane, are more effective than palladium systems, such as Pd(dba)2/triphenylphosphane55. 

Gabriele et al. reported a sequential homobimetallic palladium-catalyzed cascade reaction for the synthesis of benzofuran derivatives 207 [80] (Scheme 6.57). Sequential homobimetallic catalysis is different in catalytic cycles promoted by the same metal but in different oxidation states. After oxidative addition and oxopalladation, 5-exo-dig cyclization occurs, followed by carbonylation, affording the intermediate 206. The allylic alcohol is reduced by H—Pd—I through formation of a ji-aUyl complex and the elimination of water. Finally, the product observed is afforded... 

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