I. Direct observation of o-xylylene (o-quinodimethane) in solution, Dimerization kinetics of some o-quinodimethanes; II. Kinetic and product studies on the dimerization of [alpha]-methyl-substituted o-quinodimethanes; III. Direct observation of 1,2-napthoquinodimethane and 9,10-phenanthroquinodimethane, Dimerization kinetics of resonance-stabilized o-quinodimethanes
Fluoride ion induced 1,4-elimination from o (trimethylsilyl)- methyl benzyl trimethylammonium iodide (7) has been used to generate o-xylylene (1) in solution. o-Xylylene (1) was observed directly in solution by UV-visible spectroscopy. Using stopped-flow UV-visible spectroscopy it was determined that the dimerization of 1 follows second-order kinetics with a rate constant of 9.94 (+OR-) 0.32 x 10('3) L mol('-1) sec('-1) at 25(DEGREES)C in CH(,3)CN. Two ring-alkylated derivatives of 1, 4-t-butyl-1,2-xylylene (28) and 3,4-(1,1,3,3-tetramethyltrimethyl- ene)-1,2-xylylene (29) were generated and observed by UV-visible spectroscopy. The rate constant for the dimerization of 28 was 3.90 (+OR-) 0.15 x 10('3) L mol('-1 )s('-1) at 25(DEGREES)C and that for 29 was 5.59 (+OR-) 0.21 x 10('3) L mol('-1) s('-1) at 25(DEGREES)C. The similar rate constants observed for the dimerization of 1, 28, and 29 led to the conclusion that when undergoing dimerization the o-quinodimethane monomers prefer a non-endo approach;The dimerization kinetics of 5-ethylidene-6-methylene-1,3-cyclohexadiene (9) and 5,6-bis(ethylidene)-1,3-cyclohexadiene (10) were studied by stopped-flow UV-visible spectroscopy. At 25(DEGREES)C in CH(,3)CN k(,9)/k(,1) = 0.59 and k(,10)/k(,1) = 0.016. The dimerization of 9 and 10 led predominantly to one stereoisomer in each case. The similar rate constants for the dimerization of o-xylylene (1) and 9, and the much slower rate constant observed for the dimerization of 10 provided evidence for a stepwise mechanism for the dimerization of benzenoid o-quinomethanes;The fluoride ion induced 1,4-elimination methodology used in the generation of o-xylylene (1) was utilized in the formation and observation of 1,2-napthoquinodimethane (2) and 9,10-phenanthro- quinodimethane (3). o-Quinodimethane (2) was observed in solution by UV-visible spectroscopy while 3 was observed by ('1)H NMR spectroscopy. Both of these species were considerably more stable;than 1, with k(,1)/k(,2) = 163 and k(,1)/k(,3) = 590,000 at 25(DEGREES)C. Both 2 and 3 dimerized following second-order kinetics; *DOE Report IS-T-1294. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.