Palladium-Catalyzed Formal [4+2] Cycloaddtion of o-Xylylenes with Olefins

Link: http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/asap/abs/ja070012l.html

From Prof. Ryoichi Kuwano's group at Kyushu University, Japan

A report on formal Diels-Alder reaction of compound of type 1 with alpha,beta-unsaturated carbonyl compounds and derivatives which is catalyzed by Pd complex recently appeared in JACS ASAP.

The reaction was proposed to proceed either through intermediate A (path a) or B (path b) (Scheme 1).

In the initial reaction conditions screening using compound 1a and 2a to obtain 3a, it was found that the reaction proceeded best using Lewis basic solvent (DMSO and DMF), and diphosphine ligand (dppe) (Table 1).

When 1a was heated under the optimal reaction conditions without 2a, only trace of 4 was detected but 5 was not observed, indicating that the 0-xylylene was not formed.

The scope of reaction was explored with several trans-disubstituted and geminal-disubstitued alkenes. The reaction was found to be stereospecific in trans-alkenes. The reaction also worked with acrylonitrile (entry 5) and styrene (entry 6) (Table 2). The reaction failed to proceed in the cases of strongly electron-deficient olefins, such as maleate, and fumarate, as well as cyclic olefins.

When substrates with substituents in the aryl rings (1b-e) were used, regioisomeric mixtures of products were obtained. These results indicated that most likely intermediate A (palldacycle) was involved in the cycloaddition process with alkenes because if it were intermediate B, products would have been afforded with perfect regioselectivity (through 1,4-addition of the anion to the olefin, followed by displacement of Pd with the nascent enolate).