Laser flash photolysis study of substituent effects on the rate of 1, 2-H migration in a series of benzylchlorocarbenes

Laser flash photolysis of para-substituted 3-chloro-3-benzyldiazirines in isooctane produces the corresponding carbenes, which react with pyridine to form ylides or undergo 1,2-H migration to form the (Z)- and (E)-beta-chlorostyrenes. The rate for the 1,2-H migration is determined by plotting the pseudo-first-order rate constants for the growth of the ylide vs [pyridine] and extrapolating to zero pyridine concentration. In the case of (p-chlorobenzyl)chlorocarbene, the carbene decay can be monitored directly at 310 nm, whereas in [p-(trifluoromethyl)benzyl]chlorocarbene, the 285-nm trace may bc analyzed as the sum of the absorption of the carbene and of the product beta-chlorostyrenes. A Hammett plot of the logarithms of the rate constants for 1,2-H migration vs sigma(p) values gave a rho-value of -1.0, consistent with a hydride-like 1,2-H shift to the carbene center. The rate constants for the cyclopropanation of carbenes with tetramethylethylene (TME) were determined. In particular, the cyclopropanation of (p-chlorobenzyl)chlorocarbene with TME gave a negative activation energy of -4.7 kcal mol-1.PT: J; CR: BONNEAU R, 1989, J AM CHEM SOC, V111, P5973 BONNEAU R, 1989, J PHYS CHEM-US, V93, P4802 CARMICHAEL I, 1986, J PHYS CHEM REF DATA, V15, P1 DOYLE MP, 1987, J ORG CHEM, V52, P1619 EVANSECK JD, 1990, J PHYS CHEM-US, V94, P5518 GOULD IR, 1985, TETRAHEDRON, V41, P1987 GRAHAM WH, 1965, J AM CHEM SOC, V87, P4396 HO GJ, 1989, J AM CHEM SOC, V111, P6875 HOUK KN, 1984, J AM CHEM SOC, V106, P4291 JACKSON JE, 1989, J AM CHEM SOC, V111, P6874 JONES M, 1980, REACTIVE INTERMEDIAT, V2 JONES WM, 1980, REARRANGEMENTS GROUN KIRMSE W, 1971, CARBENE CHEM LAVILLA JA, 1989, J AM CHEM SOC, V111, P6877 LIU MTH, 1908, J ORG CHEM, V52, P4223 LIU MTH, 1985, J CHEM SOC CHEM COMM, P982 LIU MTH, 1987, CHEM DIAZIRINES, CH5 LIU MTH, 1989, J AM CHEM SOC, V111, P6873 LIU MTH, 1989, J CHEM SOC CHEM COMM, V12 LIU MTH, 1990, J AM CHEM SOC, V112, P3915 LIU MTH, 1992, J PHOTOCH PHOTOBIO A, V63, P115 MORGAN S, 1991, J AM CHEM SOC, V113, P2782 MOSS RA, 1986, J AM CHEM SOC, V108, P7028 MOSS RA, 1990, J AM CHEM SOC, V112, P5642 SCHAEFER HF, 1979, ACCOUNTS CHEM RES, V12, P288; NR: 25; TC: 27; J9: J AMER CHEM SOC; PG: 4; GA: HT801Source type: Electronic(1