Dihalodiazirines and the photochemical generation of dihalocarbenes: new light on old problems

Abstract

The Graham oxidation of amidines to halodiazirines, coupled with the diazirine exchange reaction, makes available many new diazirine precursors of carbenes. Nitration of the phenoxy moieties of phenoxychloro‐ or phenoxyfluorodiazirine converts them to leaving groups, ultimately permitting the preparations of CCl2, CClF, and CF2. Absolute rate constants and activation parameters are measured for the additions of these carbenes to several alkenes. The comparative contributions of ΔH‡ and ΔS‡ to ΔG‡ are assessed for these reactions, as are the dependences of ΔH‡ and ΔS‡ on the structures and reactivities of the carbenes and alkenes. Copyright © 2009 John Wiley & Sons, Ltd.

Fluorohalomethylsulfonium Salts as a Fluorohalocarbene Source

“…Only unactivated alkene 8a reacted leaving electron poor alkene–vinylsulfone ( 10 ) completely unaffected. This strongly supports dihalocarbene reactivity due to its electrophilic nature . On the contrary, monofluoromethyl sulfonium salt 12 gave only fluorinated cyclopropane 13 originating from the electron poor sulfone 10 .…”

“…This strongly supports dihalocarbene reactivity due to its electrophilic nature. 22 On the contrary, monofluoromethyl sulfonium salt 12 23 gave only fluorinated cyclopropane 13 23c originating from the electron poor sulfone 10. Due to the nucleophilic character of the ylide species, the preferred reaction partner was a good Michael acceptor 10 leaving vinylbiphenyl (8a) completely unreacted.…”

Recent Advances in Monofluorinated Carbenes, Carbenoids, Ylides, and Related Species

“…The stability increases when moving upward in the halogen group (F > Cl > Br > I) [13] because of the increased electronegativity and more efficient 2p-2p orbital overlap for fluorine compared to heavier halogens. [14] Significant efforts were made for determining structural and thermodynamic properties of fluorinated carbenes. Squires et al determined the monofluoromethylene heat of formation ΔH f,298 (CHF) = 34.2 � 3.0 kcal/mol.…”

“…Increasing the number of fluorine atoms at the carbene center increased the stability of CF 2 > CFCl > CCl 2 but decreased the reaction rate with alkenes. [14] Historically, fluorohalocarbenes were generated using halofluoromethanes 9 . However, these are not environmentally friendly reagents because of their potential ozone-depleting properties and limited accessibility, which have facilitated the development of various alternative reagents.…”

“…In 2019, Vittorio Pace and Renzo Luisi developed the synthesis of α-fluoroepoxides and aziridines (Scheme 8). [28] This strategy is based on the deprotonation of fluoroiodomethane to give fluoroiodomethyllithium (14) and is followed by cyclization with ketone 15 to deliver fluorinated epoxide 16 in low to excellent yields. However, when imine 17 was used, carbenoid 14 addition product 18 formed in high yield.…”