Methanol reaction with singlet carbene

The reaction of singlet carbenes with alcohols has been studied by PAC and the results have led to some interesting mechanistic implications 41,42 The rates, kr, and heats of reaction, A HT, of the singlet carbenes with methanol vary with substitution (Fig. 3).53... 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

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... 

Photodissociation of a diazo compound produced a carbene for the initial intermediate species. The lifetime of the formyl carbene produced by the laser irradiation of formyldiazomethane was investigated [111] (Scheme 3). Two different rise parts [fast and slow (900 ps)] were observed in the TG signal and the slower component is assigned to the lifetime of the singlet formylcarbene. In the presence of methanol, the singlet carbene reacts with methanol to produce the inserted product. The rate constant of the insertion reaction was determined as k = 4.3 x 109 M 1 s. The observed rate constant was consistent with that obtained from a flash-photolysis experiment. 

The time-resolved method was applied to another photodissociation reaction [124], Upon the photoexcitation of diazo compounds (Fig. 16), nitrogen is dissociated to yield the singlet carbenes. In alcoholic solvents such as methanol, the O—H bond is inserted into the carbene part quickly (within 40 ps for diphenylcarbene), and an ether is formed. The AH and AV values for the ether formation from DPDM in methanol were measured by the time-resolved method. A similar TG signal to the DPCP case was observed after the excitation of DPDM in methanol. The signal rises within 50 ns after the excitation and decays monotonously. The time profile of the signal was found to be expressed well with a tri-exponential function... 

If intramolecular reactions cannot occur, the carbanion-carbocation character of a singlet carbene can lead to reactions that occur through ionic species. Singlet carbenes react with methanol by nucleophilic abstraction of a proton, resulting in a carbocation, which subsequently reacts with the alcohol to produce an ether. For... 

The quantum yield of the photolysis of diphenyldiazomethane is 0.25, 0.78 and 2, in xylene , methanol , and in the absence of solvent at 5461 A. The primary process undoubtedly leads to the formation of singlet diphenyl-carbene " but spin inversion to the ground triplet is so rapid that only the latter reactions have been observed . The variation of (f) with the solvent conditions has been rationalized on the basis of the competing reactions... 

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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