Palladium-catalyzed cyclization of 1-iodo-substituted 1,4-, 1,5-, and 1,6-dienes as well as of 5-iodo-1,5-dienes in the presence of carbon monoxide

A series of omega-alkene-containing alkenyl iodides were prepared via (a) various allyl-, homoallyl-, and higher omega-alkenylmetalation reactions of alkynes involving Zn, Al-Zr, Zn-Zr, and Cu, (b) trans-hydroalumination and Cu-catalyzed trans-carbomagnesiation of propargyl alcohols, and (c) Zr-promoted alkyne-alkene coupling. Various factors affecting three cyclic acylpalladation (Types I-III Ac-Pd) and three cyclic carbopalladation (Types I-III C-Pd) processes of the omega-alkene-containing alkenyl iodides under the influence of CO and Pd-phosphine catalysts have been delineated. In the presence of methanol or other alcohols at relatively high pressures (30-100 atm) of CO (Conditions IV), Type II Ac-Pd products containing five-membered ketones, such as 22, 26-28, 30, and 32, can be obtained generally in high yields. The order of rates of various carbonylative cyclization reactions producing five-membered rings is as follows: lactonization > Type II Ac-Pd reaction > C-enolate trapping with malonate anion. The preparation of six-membered ketones via the Type II Ac-Pd process is less satisfactory. Attempts to prepare seven-membered ketones failed, and no attempts were made to obtain small ring ketones. In the absence of an external nucleophile (Conditions I and II), alpha-alkylidenecyclopentanones can be obtained in high yields via the Type I Ac-Pd process in cases where the omega-alkenyl group is 1,2-disubstituted. Terminal vinyl-containing 1,4-pentadienyl iodides can give Type I Ac-Pd products, e.g., 31, 33, 35, 56-58, and 65, in moderate to good yields only with the stoichiometric amount of a Pd-phosphine complex. In sharp contrast, 1,5-hexadienyl iodides give predominantly Type III Ac-Pd products, e.g., 5, 60, 63, and 64, in moderate yields under comparable conditions. The Type III Ac-Pd products can be cleanly converted to the corresponding Type II Ac-Pd products via alcoholysis. At 1 atm of CO in the presence of an alcohol (Conditions III), noncarbonylative cyclic carbopalladation process (Types I-III C-Pd) can be observed along with premature esterification. With terminal vinyl-containing alkenyl iodides, the cyclic Heck reaction (Type I C-Pd) is the dominant path. In cases where the omega-alkenyl group is 1,1-disubstituted, however, the Type II C-Pd process can be observed selectively using either i-PrOH as an external nucleophile or a mixture consisting of H2O, MeOH, and DMF (1:10:20). Collectively, the three Ac-Pd processes and the Type II C-Pd process in conjunction with novel and efficient methods for the preparation of the required omega-alkene-containing alkenyl iodides provide a potentially useful methodology for the preparation of five- and six-membered-ring compounds.