Carbonyl compounds carbopalladation

For the Heck reaction as discussed in Section III.2.1 the final position of the olefi-nic double bond of the products must not necessarily be the same as in the starting materials (for example Schemes 8, 9, and 10 of Section III.2.1) [1], The selectivity is often driven by stereochemical requirements, because the /1-hydrogen elimination step which forms the double bond proceeds exclusively in a syn manner (if a trans /3-hydrogen is eliminated, one should suspect major deviations from the general mechanism of the Heck reaction, for example electrophilic substitution instead of carbopalladation). An impressive example of a double bond migration is depicted in Scheme 1 - instead of olefins the coupling reaction of iodobenzene 1 with the olefmic alcohol 2 results in the isomeric aldehydes 3 and 4 as final products [2], Reactions of this type have emerged as valuable tools for the synthesis of carbonyl compounds and also as crucial steps in domino processes. 

A systematic investigation of cyclic acylpalladation of haloenes, haloynes, and related electrophiles conducted since 1983 [30] has led to the development of three types of cyclic acylpalladation processes (Types I—III Ac-Pd) and Pd-catalyzed carbonylation-induced ketene [2 + 2] cycloaddition (Sect. 3.1). Collectively, these cyclic acylpalladation and related reactions have provided a number of new and attractive routes to cyclic compounds. Significantly, they nicely complement and supplement the non-carbonylative cyclic carbopalladation reactions. Thus, they have become integral and indispensable parts of the carbopalladation-based cyclization methodology. 

The mechanism of the cyclization reactions in Scheme 15 is yet unclearl °n i either an unusual 5-endo-trig carbopalladation takes place as key step or a cyclopalladationi i followed by reductive elimination. More typical examples of the latter type of process are discussed in Sect. D. Whatever the mechanism, the yields achieved are good to excellent in every case. Moreover, this type of starting material is easy to build up, even in situ from aniline derivatives and suitable carbonyl compounds (Scheme... 

Although attempts to use allenes in the cycUc carbopalladation-carbonylative termination process have not been successful, various types of dienes, oUgoenes, enynes, and other rr-compounds may prove to be worth exploring. 

---