Palladium complexes Cope rearrangement

Palladium(O) complexes also catalyse the low-temperature 3-aza-Cope rearrangement of jV-allylenamines in the presence of trifluoroacetic acid as co-catalyst62 (Scheme 15). 

The palladium-catalyzed decarboxylative coupling of allyl 2-(benzo[c(jthiazol-2-yl)acetates 118 provides a facile approach to 2-(but-3-enyl)benzo[c(jthiazoles 122 <07JA4138>. The reaction is initiated by nucleophilic attack of Pd(0) on the allyl ester to give Pd-7t-allyl complex 119, which undergoes nucleophilic attack at the less substituted allylic carbon from the benzothiazole nitrogen to produce 120. Decarboxylative dearomatization leads to intermediate 121, and a subsequent aza-Cope rearrangement driven by rearomatization affords the final product 122 and accounts for the unusual regioselectivity. This appears to be the first report of a tandem allylation/aza-Cope reaction driven by decarboxylative dearomatization/ rearomatization. 

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 Cope rearrangement takes place with hydrocarbon substrates. Nickel(O) and palladium(II) species were first demonstrated to show opposite effects in the rearrangement, the nickel(O) catalyst being responsible for ring expansion (equation 28) and the palladium(n) complex for ring contraction (equation 29). The reaction shown in equation (29) is catalyzed less effectively by H2[PtCl4]. 

The bis(acetonitrile)palladium dichloride catalyzed rearrangement of R,E)-4 results in a 7 3 product distribution of 5 and 6. but a virtually complete 1.4-chirality transfer is observed (S 98 % ec)994. A possible mechanism for this type of reaction is that the palladium dichloride diene complex undergoes oxidative addition of the allylic C—C bonds to form a bis( /3-al-lyl)palladium(IV) intermediate 7 or 8, which can then reductively eliminate to give a palladium ) dichloride complex of the Cope product994 (for another mechanism see p 3572). 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C— N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 -I- 2 -I- 2] and [4 + 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds. " The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(ll) intermediates. The carbopalladation reactions of aUenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

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