Singlet state arylnitrenes

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

They found that the triplet state is the ground state for all arylnitrenes studied. The lowest singlet state of nitrene 49 and its derivatives 56 to 58 was predicted to be the open-shell singlet. ° 3 The singlet-triplet energy gap AEgj was calculated to be 8.4 kcal/mol for 49 at the AMI level, and both... 

When aryl azides are heated above 130°C in inert aromatic or aliphatic solvents nitrogen is evolved and an arylnitrene generated that reacts with the solvent, dimerizes or, if suitably substituted, cyclizes. Thermal decomposition of aryl azides generates arylnitrenes in the singlet state which usually collapse rapidly to the triplet ground state before reaction unless certain structural features are present in the molecule. 

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

The assignment of the transient absorption spectrum of Fig. 2 to PN is supported by the similarity of its spectrum to that of the longer-lived per-fluorinated singlet arylnitrenes. " The decay of this transient absorption is accompanied by the formation of cyclic ketenimine K. Furthermore, the electronic absorption spectrum of PN in the A2 state calculated at the CASPT2 level is in good agreement with the transient spectrum (Fig. 2). 

When, thermolysis of an aryl azide leads to an arylnitrene, the latter is generated in the singlet (182) state, which can be in equilibrium with the triplet (183). Electron spin resonance experiments have... 

Insertion by arylnitrenes into aliphatic C-H bonds, which is usually an unfavored process, is now generally thought to involve the triplet state. Two obvious mechanisms are conceivable. Classical insertion of an arylnitrene into a C-H bond should involve a singlet species ... 

The photochemistry of phenyl azide and its simple derivatives have received the most attention in the literature. The results of early studies were summarized in a number of reviews. " Over the last decade, modem time-resolved spectroscopic techniques and high level ab initio calculations have been successfully applied and reveal the detailed description of aryl azide photochemistry. This progress was analyzed in recent reviews. Femtosecond time resolved methods have been recently employed to study the primary photophysical and photochemical processes upon excitation of aryl azides. The precise details by which aryl azide excited states decompose to produce singlet arylnitrenes and how rapidly the seminal nitrenes lose heat to solvent and undergo unimolecular transformations were detailed. As a result of the application of modem experimental and theoretical techniques, phenylnitrene (PhN) - the primary intermediate of phenyl azide photolysis, is now one of the best characterized of all known organic nitrenes. " 5 "-2° - ... 

In the ultrafast LEP study of 80, the formation of two intermediates have been observed in acetonitrile at ambient temperature. One of the species, the S2 excited state of 80 (> ax 350 nm) has a lifetime within the instrument response (300 fs) due to its rapid convertion to the dissociative Si state. The second intermediate, singlet nitrene 82 ( ax 420 nm) has the shortest lifetime of any singlet arylnitrenes observed to date -1.8ps (k = 5.6 X 10 s ). Assuming that the pre-exponential factor for cyclization of 81 is 10 s" the activation energy to cyclization could be estimated as 1.7kcal/mol. 

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