Date of Award
Doctor of Philosophy (PhD)
The properties of model transition metal complexes in the skeletal rearrangements of some 1,4-dienes catalyzed by a mixture of dichlorobis (tri-n-butylphosphine) nickel(II) and diisobutylaluminum chloride have been examined. These rearrangements are exemplified by the skeletal isomerizations of cis-1,4-hexadiene, the type I rearrangment (equation 1) and 3-methyl-1,4-pentadiene, the type II rearrangement (equation 2). The active catalyst species in the type I rearrangement has been limited to the nickel component since coordinately unsaturated ethylenebis (tri-o-tolylphosphite)-nickel(0) on combination with HCl skeletally rearranges cis-1,4-hexadiene to the type I product, trans-2-methyl-1,3-pentadiene. This catalyst combination also produces the double bond isomerization products trans, cis- and cis, cis-2,4-hexadienes with the latter being the major 2,4-hexadiene isomer. The type I skeletal transformation was further substantiated by the formation of (1) 2-methyl-1,3-butadiene from 1,4-pentadiene, (2) 2,3-dimethyl-1,4-pentadiene from 3,3-dimethyl-1,4-pentadiene and (3) 1,1-dideuterio-2,3-dimethyl-1,3-butadiene from 1,1-dideuterio-2-methyl-1,4-pentadiene.
These results are entirely consistent with the 2,1-addition of nickel hydride to the least substituted terminal double bond of the 1,4-diene prior to the rearrangement. The fact that both cis- and trans-2-methylvinylcyclopropanes were isomerized to the predominant product trans-2-methyl-1,3-pentadiene suggests that the rearrangement takes place through a cyclopropylcarbinylnickel intermediate.
The type I rearrangement was not observed if (1) the nickel hydride precursor was coordinately saturated, (2) the internal double bond of the 1,4-diene has the trans configuration or (3) the 2,1- addition of metal hydride is prohibited by the bulky methyl substituents in 2,3-dimethyl-1,4-pentadiene.
The results obtained with the (R3P)2PdCl2/R2AlCl catalyst combination suggests that the type I skeletal rearrangement was not limited to only the nickel system. They also indicate that the metal hydride need not be generated by the Beta-elimination of the elements of metal hydride from an alkylnickel species initially formed from the catalyst combination of (R3P)2NiCl2 and R2AlCl.
The aluminum component apparently plays an important role in the type II rearrangements since the double bond isomerization products were the only products detected when 3-methyl-1,4-pentadiene was treated with the catalyst derived from ethylenebis (tri-o-tolylphosphite)nickel(0) and HCl. This catalyst combination, within its life-time, did not produce the type II product from 3,3-dimethyl-1,4-pentadiene and 2,3-dimethyl-1,4-pentadiene or remove the deuterium label from C-1 in 1,1-dideuterio-2-methyl-1,4-pentadiene.
Model compounds of the type 1 and 2 exhibited pronounced downfield shifts in their pmr spectra for several of the alkenylaryl protons. This phenomenon is best explained by the paramagnetic anisotropy associeated with the metal ion. This explanation was substantiated by the observations that: (1) those protons which are deshielded occupy an apical site above the metal ion, (2) a transition in the visible region for the nickel compounds of 1 varied identically with the spectrochemical series and (3) the largest downfield shifts are found for those protons which, because of conformation preferences, are forced to occupy positions at an apical site of the metal.
Stauffer, Richard D., "Properties of Model Transition Metal Complexes in the 1,4-Diene Rearrangement" (1972). Theses and Dissertations. 3400.