Stereoselectivity of 1,2-hydrogen shifts in carbene rearrangements

Isolation of a diazoalkane intermediate in the photic Bamford-Stevens reaction

“…When we compare the ratio of E olefin 11 to Z olefin 12 from the thermal and photic Bamford-Stevens reactions, we note lower stereoselectivity in the latter case. We established that the photoproducts are stable to our reactions' conditions, and so the difference is real.…”

“…Control experiments established that these two olefins were stable to our irradiation conditions. When we used sodium hydride in place of sodium methoxide to generate the salt 13, our yield of hydrocarbon mixture after Vycor-filtered irradiation was quantitative, and the ratios of the components were alkanes (11%), (E)-spiroalkene 11 (73%), (Z)-spiroalkene 12 (14%), and three unidentified products (total 2%).…”

4-tert-Butyl-2,2-dimethylcyclohexylidene. A surprising lack of stereoselectivity in a 1,2-hydrogen shift to an alkylcarbene

“…ratio in brexan-5-ylidene 1 was 138 (eq l).1 Examination of models indicates that 1 is sufficiently distorted so that the exo H is much closer to alignment with the empty p orbital on the carbene center than is the endo H. Thus the migratory ratio greatly favoring exo-H migration might be interpreted as an affirmation of a number of theoretical predictions which state that the hydrogen which migrates is that which aligns with the empty p orbital (eq 2, path a rather than b is favored).2 Our (1) more recent work indicated, however, that cautious interpretations of the elegant experimental work of Nickon and his coworkers were in order, since, in an apparently unbiased3 bicyclo[2.2.1 [carbene 2 (eq 3), the exo-H/endo-H preference was 13 (at 190 °C).4 Thus it might be that factors other than stereoelectronic control are operative in bicyclo[2.2.1 [carbene systems.…”

Substituent effects and the role of negative hyperconjugation in siloxycarbene rearrangements

“…It is widely believed that hyperconjugation dictates which rearrangements predominate when multiple rearrangements of singlet carbenes are possible. For example, Nickon et al, 20 Freeman et al 21 and Kyba et al 22 found that 1,2-H shifts in 2-norbornylidenes occur preferentially from the exo rather than the endo position. Similarly, Press and Shechter 23 and Kyba et al 24,25 reported that 1,2-H shifts in cyclohexylidenes occur with a noticeable preference for axial migration over equatorial migration.…”