Carbopalladation of alkynes

Termination of cyclic carbopalladation of alkynes via caibonylative lactamization can be achieved more satisfactorily with alkenyl or aryl halides containing an oo-caiboxamido or co-sulfonamido group than with those containing an 0)-amino group. The method appears to be satisfactory for the preparation of certain piperidines (e.g., 102) <96T(52)11529>. 

Because of their high reactivity and their low steric demand, alkynes are highly versatile partners. The resulting vinylmetal species are also important reactive entities. Accordingly, the intermolecular carbopalladation of alkynes has attracted the interest of organic chemists for years.42-45... 

Cacchi S, Fabrizi G (2002) Carbopalladation of Alkynes Followed by Trapping with Nucleophilic Reagents. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1335... 

Alkynes are more reactive than alkenes in carbopalladation. Facile insertion of internal alkynes to some Pd—C bonds (carbopalladation of alkynes) generates the alkenylpalladiums 2 and 7 by mainly selective syn addition of organopalladium species 1 and 6 to alkynes. Formally the species 2 can be generated by oxidative addition of appropriately substituted alkenyl halides 3 to Pd(0). 

Whereas alkene insertion is followed by facile dehydropalladation whenever there is a /S-hydrogen to afford alkenes and Pd(0) catalytic species, the alkyne insertion produces the thermally stable alkenylpalladium species 2 and 7, which can not be terminated by themselves and further transformations are required in order to terminate the reactions and to regenerate Pd(0) species for catalytic recycling. In other words, it is generally believed that the reaction of generated alkenylpalladium species 2 and 7 can not be terminated, because the f-R elimination (formation of alkynes or allenes) even in the presence of a /1-hydrogen is not possible. Therefore the carbopalladation of alkynes is a living process, in which alkynes play a role of relay to pass the ability of carbon-carbon bond formation to other reactants. 

IV.2.5 Carbopalladation of Alkynes Followed by Trapping with Nucleophilic Reagents... 

Over the last fifteen years or so the domino process involving the intermolecular and intramolecular carbopalladation of alkynes followed by trapping of the resultant... 

In contrast to the diverse insertion chemistry of vinylpalladium intermediates discussed in Sects. IV.3 and IV.5, the reactions of vinylpalladium complexes with electrophiles had not been reported until recently. Although a single report on the annulation of the o-mer-curio benzaldehyde with diphenylacetylene into the corresponding indenols and inde-nones catalytic in palladium and stoichiometric in copper had been communicated in 1992, the more synthetically useful protocol for the catalytic version of this type of transformation remained unknown until 1999. In this section the intermolecular carbopalladation of alkynes with aryl halides followed by the intramolecular trapping of the formed vinylpalladium species with ketones, aldehydes, and nitriles will be discussed. 

B. CARBOPALLADATION OF ALKYNES FOLLOWED BY TRAPPING WITH KETONES. 

Cascade intermolecular carbopalladations of alkynes followed by intramolecular trapping with electrophiles represent not only a novel type of organic transformation involving vinylpalladium intermediates, but also provide synthetically useful routes toward differently substituted indenols, indanones, indenones, and naphthy-lamines." Although these protocols are restricted to the intramolecular trapping with electrophiles, a wide application of this methodology toward the synthesis of various types of complex carbocychc compounds may be anticipated in the near future. 

IV.3 Palladium-Catalyzed Tandem and Cascade Carbopalladation of Alkynes and 1,1-Disubstituted Alkenes... 

B. Pd-CATALYZED TANDEM AND CASCADE CARBOPALLADATION OF ALKYNES TERMINATED BY CARBONYLATION... 

The catalytic cyclic carbopalladation of alkynes can proceed to produce five- and six-membered rings at 1.0-1.1 atm of CO without premature incorporation of CO, and in situ regeneration of Pd-phosphine catalysts can be achieved in high yields by termination of the cyclic carbopalladation via deferred carbonylative esterification and lactonization, as suggested by the results shown in Scheme 1. One key to observing the formation of the desired products in high yields is to employ 1.0-1.0 atm of CO. Some representative examples are summarized in Scheme 3. 

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