Diynes protonolysis

The reaction of nBu2ZrCp2 with 2 equivalents of PhC CPh provides the novel bicyclic gem-dizirconium complex 140 [236] (Scheme 7.42). Protonolysis of complex 140 with 3 n HC1 gives bibenzyl in 88% yield, while its deuterolysis with D20 provides tetradeuterio-bibenzyl 141 with 92 % deuterium incorporation. The dual path nature (142 versus 140) of the reaction of Cp2Zr with alkynes is an important factor in designing Zr-promoted cyclizations of alkynes, enynes, and diynes. 

Conjugated cis,cis-d ieney. Dihydroboration of a conjugated diyne, for example dodcca-5,7-diyne (1), with dicyclohexylborane followed by protonolysis of the organo-borane intermediate with acetic acid leads to a conjugated di.cu-diene (rw.t is-dodeca-... 

Conjugated diynes can be reduced to cis enynes by the well-known hydroboration-protonolysis sequence . cw-5-Dodecen-7-yne (215) is obtained from the diyne 213 in 76% yield as shown in equation (23). The orientation of addition of the disiamyl-borane was established by using CH3CO2D in the second step. Addition of a second mole of disiamylborane to 214 is very slow, but reduction to the cis,cis diene can be accomplished by using dicyclohexylborane instead . 

A different course of events is followed by diynes in the presence of a ruthenium catalyst and water (Scheme 11.88). 2i The mechanism is believed to involve the formation of a ruthenacyclopentadiene 11.268, which is attacked by water at the less-hindered position. Break-up of the ruthenacycle and protonolysis of the carbon-ruthenium bond delivers the product 11.267. 

Selective hydroboration of symmetrical conjugated diynes with dialkyl-boranes provides a route to acetylenic ketones in yields >70%. Protonolysis of the intermediate boranes also gives the corresponding c/5-ene-ynes in high yield (Scheme 2). ... 

Variation of the electronic and steric nature of the ruthenium catalysts allow the complementary carboxylative cyclization of 1,6-diynes (Scheme 36) [150]. Lee and coworkers described how a variety of carboxylic acids condense with 1,6-terminal diynes in the presence of catalytic amounts of [Ru(p-cymene)Cl2l2. P(4-F-C6H5)3 and 4-dimethylaminopyridine to give cyclohexylidene enol carboxylates with exclusive ( )-selectivity. The proposed mechanism involves the initial formation of a ruthenium vinylidene species I followed by intramolecular cyclization induced by the nucleophilic attack of the carboxylate anion to afford a vinylmthenium species II. Final protonolysis furnished the product and turns the catalyst over. 

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