Triplet Carbene Intramolecular Reactions

Singlet Carbene C-H Insertions Although [1,2]-H shifts are formally carbene C-H insertions, these rearrangements have different orbital symmetry aspects than those of intramolecular insertions. As described above, overwhelming evidence exists that triplet carbenes undergo abstraction-recombination reactions to... 

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. 

Analysis of the product distributions arising from both sensitized and non-sensitized irradiation of 2-allyloxyphenyldiazo species (8) showed that the C—H insertion product and much of the cyclopropanation arise from the triplet carbene.16 For the singlet carbene, intermolecular 0—H insertion with methanol is about 50 tunes faster than intramolecular addition to the double bond, hi this system, intramolecular reactions and intersystem crossing of the triplet carbene proceed at similar rates, hi the closely related indanyl system (9), the smaller RCR angle stabilizes the singlet state relative to the triplet and the intramolecular reactivity is dominated by the singlet state.17... 

Carbenes are highly reactive and undergo insertion into a-bonds, cycloaddition reactions, dimerization, complex formation and intramolecular reactions. The singlet carbene, which often acts as an electrophile, gives different products than the triplet carbene, which behaves as a radical. Despite their very different nature, they manage to produce the same product in some reactions. 

The reactions of triplet carbenes result from a stepwise addition involving the biradical intermediate shown in Scheme 10.11 B, which can undergo C-C bond rotation. Normally, radical combination is a very fast reaction, and closure to a cyclopropane is further facilitated because it is intramolecular. The reason that the rate of closure is slower than normal in this case is that an electron spin flip is required prior to closure (Eq. 10.56). The addition of a triplet carbene to an alkene first gives a triplet biradical that cannot cyclize. Instead, a spin flip must first occur, followed by radical combination to complete the ring closure. The stereochemistry of the alkene is typically not completely lost in the product, which indicates that the spin inversion and bond rotation rates must be comparable. 

Carbenes can react either (1) intramolecularly or (2) intermolecularly. The basic reactions are addition and insertion. Triplet carbenes can also react via an abstraction mechanism. As typical examples, the intra- and intermolecular reactions with a double bond are shown in Schemes 4 and 5, respectively. 

The rather short lifetime (a few nanoseconds) of the triplet excited carbene makes extensive studies of intermolecular processes difficult. However, the excited-state lifetime (60 ns) of triplet dimesitylcarbene (19c) is exceptionally large, probably because of decreased efficiency of both intermolecular and intramolecular deactivation pathways. Intermolecular rate constants for the reaction with CCI4, O2 and 1,4-cyclohexadiene have been determined. 

In these C-H insertion reactions, the similarity with cyclopropane formation by intramolecular cycloadditions to alkenes is clear, and the mechanisms mirror one another quite closely. As with the cyclopropanation reactions, the path of the reaction differs according to whether the carbene is a singlet or triplet. Singlet carbenes can insert in a concerted manner, with the orbitals overlapping constructively provided the carbene approaches side-on. 

The transient ketocarbene is also able to effect both intramolecular and intermolecular H-abstraction. The fact that a triplet state of the carbene is often implicated follows from observations that sensitized photolyses frequently enhance the abstraction pathway. Abstraction and radical combination results in the formal insertion of the carbene into the C—H (or S—H, N—H, etc.) bond. Insertions may also occur into C—C, C—O, C—S, C—Hal, etc. bonds the mechanisms are often not known, but a singlet state of the ketocarbene is probably involved in many cases. Reactions at soft basic centers (e.g. —S or —Br ) generally proceed by the preliminary formation of an ylide. The multiplicity of the possible pathways is illustrated in Scheme 7, equation (14) (photolysis in dioxane affords only 27, 57% and 28, 43%), and Scheme 8. ... 

Triplet sensitization of two 1,2-diaryIdiazoethanes in methanol solution gave rise to stilbenes, by intramolecular 1,2-H shifts, and ethers, by reaction of the corresponding carbenes with methanol.This method of generating the intermediate carbenes bypasses the singlet excited diazo compounds, and thus eliminates the possibility that the products arise by direct reaction of these excited species. The results go some way towards establishing that 1,2-H and 1,2-C shifts can compete with alcohol trapping of a spin-equilibrated carbene. 

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