Tetrahedron p. 1973 - 1983 (1981)
Update date:2022-08-03
Topics: Mechanism Isoprene Regioselective Experimental Nickel Cyclodimerization
van Leeuwen
Roobeek
A study of various Ni-ligand catalysed oligomerizations of isoprene has shown that with π-acidic P ligands the selectivity to cyclodimers amount to 97%. A new type of ligand is introduced, viz. fluoroalkyl phosphites having π-acceptor properties comparable to those of, e.g. PCl3. With tris(hexafluoroisopropyl) phosphite the main product is 1,4-dimethy1-4-vinylcyclohexene. A detailed explanation based on a two-step mechanism is given. As to the first step, for a series of ligands having similar steric properties the changes in product distribution as a function of the electronic ligand parameter are explained in terms of a gradual change in HOMO-LUMO interactions between Ni and the olefins, with strong π-acidic ligands promoting the head-to-head coupling of the isoprene molecules. The second step, involving reductive elimination of a cyclodimer from the metal, shows an increasing selectivity towards substituted cyclohexenes for the head-to-head and tail-to-tail intermediates with increasing π-acidity of the ligand. The qualitative orbital treatment presented as an explanation is also applicable to reactions found for other metallacyclopentanes.
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Doi:10.1016/S0040-4020(01)92443-3
(1981)Doi:10.1016/S0040-4039(01)92909-0
(1981)Doi:10.1016/j.tet.2005.08.007
(2005)Doi:10.1021/ja00291a032
(1985)Doi:10.1016/j.tet.2004.06.141
(2004)Doi:10.1016/S0040-4039(01)92926-0
(1981)