The olefinic selectivity of dibromocarbene

The Use of Flow Chemistry for Two-Phase Dibromocyclopropanation of Alkenes

“…The only exception was the monosubstituted alkene, 1-heptene, where moderate yields were observed, even when the amount of bromoform was increased from 1.5 equivalents to 2 equivalents relative to the alkene ( Table 2, Entries 8-9). This is in accord with the wellknown observations that the rate of reaction will increase with increased alkene substitution [38,39].…”

The correlation of carbenic reactivity

“…We next reinvestigated the classic generation of CBr2 from bromoform and potassium ZerZ-butoxide.5•6 There appeared to be minimal carbenoid involvement, and wcBr2 (crown ether value) was determined to be 0.65. 27 Extension of the correlation toward less selective carbenes led us to BrCCOOC2H5 and HCCOOC2Fl5, for which relative reactivities had been determined, but not at 25 °C. 28 We therefore redetermined these data at 25 °C, employing the usual competitive technique.6•7 The carbenes were generated Journal of the American Chemical Society / 99:12 / June by photolyses of the respective diazo compounds ( >300 nm); insertion products were destroyed by ozonolysis;29 and the cyclopropane products were assayed with capillary GC (calibrated, flame ionization detector, electronic integrator).…”

“…Here, the determination of mean appeared quite straightforward: we observed klctrd/kln 4.0, and the normal log-log correlation gave wcbi-2 = 0.65, with correlation significant at the 99.9% confidence level. 27 Nevertheless, it is now clear that greater steric hindrance opposes CBr2 as compared to CCb additions: <5cBr2/<5cci2 =1.11 for correlations of the relative reactivities of these carbenes toward RCH=CH2 with Es, the steric substituent constants of the alkyl groups. 27 If one arbitrarily enhances mcBr2obsd by 11%,39 the relative steric demand of CBr2, one obtains /ß 2 = 0.72, closer to wcBr2calcd = 0.82.…”

“…27 Nevertheless, it is now clear that greater steric hindrance opposes CBr2 as compared to CCb additions: <5cBr2/<5cci2 =1.11 for correlations of the relative reactivities of these carbenes toward RCH=CH2 with Es, the steric substituent constants of the alkyl groups. 27 If one arbitrarily enhances mcBr2obsd by 11%,39 the relative steric demand of CBr2, one obtains /ß 2 = 0.72, closer to wcBr2calcd = 0.82. 40 In principle, steric hindrance corrections could be determined, relative to CCI2, for all the carbenes in Table III, and these could be explicitly included in eq 2.…”