Intersystem crossing sensitivity analysis

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... 

Furan-2-carbaldehyde has been much studied. A thorough analysis of the first two major electronic transitions has been carried out. Practical work is hampered by the resinification of the compound and by the presence of a trace impurity which gives rise to a long-lived pressure-independent component in the phosphorescence spectrum.23 The absence of n ->n excited emission and other facts implicate a very efficient double intersystem crossing.14 24 Whether or not sensitized by mercury, photodecomposition of the aldehyde gives much carbon monoxide, propyne, and allene. Small amounts of furan, carbon dioxide, and acetylene are also formed. 

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