Insertion, Abstraction, and Rearrangement Reactions of Carbenes

We have already briefly discussed (Sect. 8.1) reactions other than cyclopropanation of alkenes that are typical for carbenes, namely insertion into C-H and O-H bonds, abstraction of H-atoms, and rearrangements. Except for ether formation (insertion into the OH bond) and rearrangement of diazocarbonyl compounds (Sect. 8.6) they are of little interest in organic synthesis. 

The relationship between the reactivities of singlet and triplet carbenes in some of these processes has, however, attracted the interest of many investigators. Chemical evidence from such studies led to the conclusion that the singlet state can be intercepted due to its higher energy and that it displays a different chemistry from that of the triplet, since Skell and Woodworth developed their rule in the late 1950 s and indicated that it may also be applied to carbene reactions other than cyclopropanation. 

In Section 8.1 the observation of Doering et al. (1956a) was discussed, in which Doering demonstrated the extremely low (almost nonexistent) selectivity of methylene insertion in the photolysis of diazomethane in pentane. Is this a reaction of the singlet or triplet An answer to this question was obviously impossible in 1956. 

More informative is a process described by Roth (1972). The direct and the sensitized (benzophenone) photolysis of diazomethane in toluene yields ethylbenzene, but a CIDNP effect can only be observed from the sensitized reaction. This result leads to the conclusion that the direct photolysis is a concerted CH insertion of a highly reactive species, whereas the sensitized reaction takes place by a hydrogen abstraction, followed by a radical addition (8-24). It is, therefore, likely that it is the singlet that reacts in the first reaction, but the triplet in the second process. 

Moss and Joyce (1978) demonstrated in an elegant labeling experiment that the triplet abstracts an H-atom before the new CC bond is formed Triplet fluorenylidene ( Fl) abstracts an allylic H-atom at 77 K from 2-methylprop-l-ene, labeled with at the CH2 group (8-25). In the radical pair 8.38 both termini of the allylic radical have an even chance (except for the isotope effect) to 

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Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

Phenyl azides (azidoarenes), introduced by Knowles and co-workers,[8 9] are the most abundantly used class of photophores. Examples include 4-azidophenylalanine (1) and 4-azido-3-nitrophenylalanine (4) (Scheme 1). Irradiation (<300 nm) of phenyl azide (13) generates nitrene 14, electrophilic in nature, which prefers insertion into O—H and N—H bonds over C—H bonds. Nitrenes are considerably less reactive and, therefore, more selective than carbenes. Nevertheless, due to their short life span (0.1-1 ms) they react indiscriminately with virtually any amino add residue in the target protein.1101 Intramolecular rearrangements do not compete effectively with intermolecular proton abstraction and insertion reactions (Scheme 4). 

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