Arenes dehydrohalogenation

In 1980 Sonogashira reported a convenient synthesis of ethynylarenes - the Pd-catalyzed cross-coupfing of bromo- or iodoarenes with trimethylsilylacetylene followed by protiodesilylation in basic solution [15]. Prior to this discovery, formation of terminal acetylenes required manipulation of a preformed, two-carbon side chain via methods that include halogenation/dehydrohalogenation of vinyl- and acetylarenes, dehalogenation of /1,/1-dihaloalkenes, and the Vils-meier procedure [ 14]. With the ready availability of trialkylsilylacetylenes, the two-step Sonogashira sequence has become the cornerstone reaction for the construction of virtually all ethynylated arenes used in PAM and PDM synthesis (vide infra). 

The hydrogenolysis of arylpalladium(II) species is a widely used reaction for the dehydrohalogenation of arenes. In combination with the Catellani reaction sequence, hydrogenolysis of a complex of type 36 leads to a meto-substituted arene product. In Catellani s stoichiometric investigations of the orf/io-alkylation [43] and orf/to-arylation [88, 89] of the PNP dimer, H2 or NaBH4 were used for reduction of the resultant o,o -disubstituted arylpalladium(II) species (Scheme 38). 

The Group VIB arene-metal tricarbonyl complexes are effective homogeneous catalysts for the dehydrohalogenation of aliphatic alkyl halides to create an olefin function [17]. Often mixtures of several isomers occur in these reactions with terminal alkenes being formed as the major product (Scheme 5). This suggests that the proportion of products depends on their relative rate of formation (kinetic control). 

In addition to the cross-coupling reactions of alkyl halides or sulfonates with organometallic nucleophiles, the former can also directly undergo C-C bond formation with nonactivated C-H bonds of alkenes or arenes in a Heck-type reaction. Iron-catalyzed intramolecular dehydrohalogenation of 2-iodoethanal alkenyl acetals with phenyl-magnesium bromide in the presence of a catalytic amount of iron(II) chloride provides tetrahydrofliran derivatives in moderate yields (Scheme 4-240). Allyl 2-halophenyl ethers provide dihydrobenzofiiran derivatives under these conditions. ... 

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