Carbopalladation alkyne termination

As a typical intermolecular carbopalladation and termination, hydroarylation of alkynes are carried out extensively in the presence of HCO2H as a hydride source. Formation of regioisomers is observed in the reaction of asymmetric alkynes, and ratios depend on the nature of the substituents. High regioselectivity was observed in the reaction of the tertiary propargylic alcohol 14 to give 15 as a major product [5]. The (Z)-2-arylcinnamates 17, rather than 3-arylcinnamate 18, was obtained by the hydroarylation of methyl phenylpropiolate (16) [6]. 3-Substituled quinoline 21 was prepared by the regioselective hydroarylation of 19, followed by treatment of 20 with an acid without isolation [6]. 

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

Scheme 2.33 Isocoumarins by carbopalladation of internal alkynes terminated by esters [74].

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>. 

Negishi E, Coperet C (2002) Palladium-Catalyzed Tandem and Cascade Carbopalladation of Alkynes and 1,1-Disubstituted Alkenes Terminated by Carbonylative Reactions. In Negishi E, de Meijere A (eds) Handbook of Organopalladium Chemistry for Organic Synthesis. Wiley, New York, p 1431... 

In more complex reaction cascades an additional alkyne-insertion step can occur. Thus starting with intramolecular carbopalladation of a vinyl iodide to a carbon-carbon triple bond, followed by two intramolecular alkene-insertion steps and termination with dehydropalladation, a palladium-catalyzed synthesis of l-(5 -methylbicyclo[3.1.0]hex-T-yl)-5,5-bis(carboethoxy)cyclo-hexadiene (52) starting from l-iodo-4,4-bis(carboethoxy)-ll-methyldodeca-l,ll-dien-6-yne (51) is achieved. ... 

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. 

The stereo-defined benzylidenecyclopentane 132 was obtained by trans addition of a phenyl group and a carbanion to the terminal alkyne 131. The cyclization proceeds via trans carbopalladation [43]. As a related reaction, stereo-defined 3-arylidenetetrahydrofuran 134 was prepared by the reaction of iodobenzene, propar-gyl alcohol, and Michael acceptor 133. The reaction is understood in terms of Michael addition of propargyl alcohol to 133, followed by carbopalladation of the triple bond as shown by 135 [44]. 

The fate of this Tj -aUcyne-organopalladium intermediate has been found to depend on a number of reaction variables. For example, in the presence of a nucleophile close to the carbon-carbon triple bond, it can enter the carbopalladation-f f oxypalladation,- " or aminopalladation-f t reductive elimination domino reaction path or, when the reaction is carried out with a terminal alkyne, it can produce coupling products. ... 

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