New methods for the stereoselective synthesis of olefins: Double bond formation via 1,1-bimetallics, a new hydroboration agent, and functionalized alkenyllithiums.

The formation of carbon-carbon double bonds plays an important role in synthetic organic chemistry. Numerous methods exist for the synthesis of olefins, each possessing its own advantages, disadvantages,and varying degrees of selectivity. Because of these differing properties, the search continues for general methods for the formation of double bonds. Herein are described four new reactions for the synthesis of functionalized olefins with high stereoselectivities. The first two methods are based on 1,1-organobimetallic species, the third is a new hydroboration method, and the last is the substitution of existing olefins. The reaction of allylzinc halides with vinylic Grignard reagents results in the formation of a zinc, magnesium 1,1-bimetallic compound. This then reacts with alkylidenemalonates under mild conditions to form the corresponding olefins with cis/trans ratios approaching 100:0. The hydrozirconation of alkenylzinc reagents allows the generation of a different type of 1,1-bimetallic species. The zinc, zirconium reagent formed, after reaction with aldehydes, also leads to functionalized alkenes, with almost complete trans selectivity. The third method involves the use of a new hydroborating agent, pinacolborane. This mild, selective reducing agent allows the addition of a hydrogen-boron bond across an alkyne to form the corresponding olefinin good yields, even in the presence of other functional groups. The formation of alkenyllithium intermediates which contain functional groups is also described. These can be generated by a lithium-iodine exchange reaction on a vinylic iodide. After transmetallation to the zinc compound, the reaction with activated electrophiles is possible, leading to functionalized olefins with excellent E/Z selectivities. These four methods should find wide applications in synthesis. In each case, excellent cis/trans selectivities are obtained while maintaining a good tolerance of other functional groups present in the substrates.PhDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103152/1/9303833.pdfDescription of 9303833.pdf : Restricted to UM users only