Effect of coordination with rhodium on the selectivity of the mass spectral fragmentation of 4,4-disubstituted 2,5-cyclohexadien-1-ones

A comparative study was carried out on the electron impact fragmentation of 4,4-disubstituted 1-oxo-2,5-cyclohexadienes and their rhodium acetylacetonate complexes. The coordination of the diene ligands with rhodium leads to an increase in the selectivity of the decomposition of the molecular ions, which occurs exclusively with loss of the most stable radical located at the geminal unit of the hydrocarbon ..pi..-ligand and leads to (4-methyl-1-oxocyclohexadienyl)acetylacetonatorhodium cations. 15-, 16-, 17-, and 18-Electron complexes containing 4,4-dialkyl-1-oxo-2,5-cyclohexadiene (L) or 4-methyl-1-oxocyclohexadienyl ligands are obtained in the reaction of (acacRh x (CO)/sub n/)/sup +/ and ((C/sub 5/H/sub 5/)Rh(CO)/sub n/)/sup +/ ions (n = 0-2) with 4,4-disubstituted 1-oxo-2,5-cyclohexadienes in the gas phase. The formation of (acacRh(CO)L)/sup +/ and ((C/sub 5/H/sub 5/)Rh(CO)L)/sup +/ ions indicated the reduced ..pi..-donor capacity of 1-oxo-2,5-cyclohexadienes relative to 1-alkylidene-2,5-cyclohexadienes in reactions with rhodium-containing cations.