Carbene
A rare exception are the persistent carbenes,[5] which have extensive application in modern organometallic chemistry.
Α-elimination typically occurs when strong bases act on acidic protons with no good vicinal leaving groups.
A geminal dihalide exposed to organolithiums can undergo metal-halogen exchange and then eliminate a lithium salt: Zinc metal abstracts halogens similarly in the Simmons–Smith reaction.
[7] Mercuric and organomercury halides (except fluorides) can stably store a wide variety carbenes as the α-halomercury adduct until a mild thermolysis.
Diazirines and epoxides photolyze with a tremendous release in ring strain to carbenes, the former to inert nitrogen gas.
Photolysis, heat, or transition metal catalysts (typically rhodium and copper) decompose diazoalkanes to a carbene and gaseous nitrogen; such are the Bamford–Stevens reaction and Wolff rearrangement.
As with metallocarbenes, some reactions of diazoalkanes that formally proceed via carbenes may instead form a [3+2] cycloadduct intermediate that extrudes nitrogen.
Singlet carbenes have a single lone pair, typically form from diazo decompositions, and adopt an sp2 orbital structure.
9-Fluorenylidene has been shown to be a rapidly equilibrating mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference, although extensive electron delocalization into the rings complicates any conclusions drawn from diaryl carbenes.
[10] Lewis-basic nitrogen, oxygen, sulphur, or halide substituents bonded to the divalent carbon can delocalize an electron pair into an empty p orbital to stabilize the singlet state.
[citation needed] Singlet carbenes are typically electrophilic,[4] unless they have a filled p orbital, in which case they can react as Lewis bases.
[13] Carbenes add to double bonds to form cyclopropanes,[14] and, in the presence of a copper catalyst, to alkynes to give cyclopropenes.
Alkyl carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.
Both inter- and intra-molecular insertions admit asymmetric induction from a chiral metal catalyst.
Tetrafluoroethylene is generated via the intermediacy of difluorocarbene:[22] The insertion of carbenes into C–H bonds has been exploited widely, e.g. the functionalization of polymeric materials[23] and electro-curing of adhesives.
