Reductive dehalogenation alkynes

Tetranuclear alkyne clusters involving cobalt can adopt a variety of geometries. Reductive dehalogenation of ( Bu)(PhC=C)PCI by [Co2(CO)8] affords 39 in which a chain of four cobalt centers is supported by the functionalized alkyne, as shown in Eq, (15).146-147... 

The other major dehalogenation pathway involves elimination of two halogens, leaving behind a pair of electrons that usually goes to form a carbon-carbon double bond. Where the pathway involves halogens on adjacent carbons, it is known as vicinal dehalogenation or reductive -elimination. The major pathway for reductive transformation of lindane involves vicinal dehalogenation, which can proceed by steps all the way to benzene (28). Recently, data has shown that this pathway not only can convert alkanes to alkenes, but can produce alkynes from dihaloalkenes (29). 

The usefulness of nucleophile-promoted Iminium Ion-alkyne cyclizations derives from the ready availability of alkynylamines and the subsequent transformations of the cyclization products made possible because of their vinylic functionality (e.g., equations 1 and 2). Equation 2 illustrates use of this chemistry to elaborate an exocyclic tetrasubstituted double bond with complete stereooontrol. Net "reductive iminium ion alkyne cyclizations can be accomplished by dehalogenation of vinyl halide cyclization products. The conversion illustrated in equation 3 is a key step in an efficient, practical synthesis of the cardiotonic frog alkaloid pumiliotoxin A. ... 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

Since 2-chloropyridines are readily dehalogenated by reduction, for example, by H2/Raney-N1, the above sequence allows the conversion of 2-pyridones into pyridines. (2) 2-Pyridones may serve as diene components in (4 + 2)-cycloadditions. The Diels-Alder reaction with activated alkenes or alkynes leads to derivatives of 2-azabicyclo[2.2.2]octa-5,7-dien-3-one 13 by the addition of acetylenedicarboxylate to l,4,6-trimethyl-2-pyridone (14), as shown below [145] ... 

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