Rearrangements in the excited states RIES

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

With the intent to selectively derivatize CyD oligosaccharides,80 Abelt enlisted nitrogenous carbene precursors as labile guests.81 The involvement of supramolecular carbenes could be inferred based on intramolecular products stemming from 1,2-H shifts. Of course, 3//-diazirines that possess a-C-H bonds, like 3-methyl-3-phenyl-3//-diazirine (6) (Scheme 2), are susceptible to rearrangements in the excited state (RIES) that mimic the results of carbene 1,2-H shifts,82 e.g., 6 —>10 (Scheme 2). 

Recently, diazirine rearrangement in the excited state (RIES) that mimics the result of cyclopropylcarbene fragmentation has been postulated. For examples, see (a) Ref. 128d,e (b) Thamattoor, D.M., Jones Jr., M., Pan, W. and Shevlin, P.B. (1996). Tetrahedron Lett. 37, 8333-8336... 

Rearrangement in the excited state (RIES) of diazirine 45 might be the actual route to enyne 47 because MeOH, which is relatively reactive and in excess, is expected to trap carbene 46 completely... 

This concerns essentially the photoactivated diazirine 56 which may react diieclly by a rearrangement in the excited state (RIES) or ring-open to the labile diazo compound 61. Under slightly acidic conditions, protonation of the carbene to carbenium ion 63 and the intermediacy of diazonium ion 62 should also be considered. 

In light of the above results, the involvement of another intermediate is required but the question of its identity remains. A number of intermediates have been proposed, including a carbene-olefin complex (COC), an excited carbene, a diazo compound, an excited-state diazirine, and a biradical. These proposed intermediates are potentially involved in two types of mechanisms the COC mechanism and the rearrangement in the excited-state (RIES) mechanism. 

Clearly, rearrangements do occur in the excited states of diazirine and diazo carbene precursors. Kinetic studies of carbenic rearrangements need to consider the possible intervention of RIES when absolute rate constants are partitioned between competitive rearrangement pathways on the basis of product distributions.28... 

With the non-nitrogenous precursor 32, a linear plot of 31/30 versus [TME] was obtained. The photochemistry of 32 is simple. The photochemistry of 19 we believe involves a Rearrangement in Excited State (RIES) mechanism via biradical 33. [38]... 

If Scheme 2 accurately represented the PhCH2CCl chemistry, curvature in the addn/rearr vs. [alkene] correlation would persist when the carbene was generated from 37. The absence of curvature in this case counts against Scheme 2 (and the CAC mechanism), but accords with the RIES mechanism, Scheme 3. Elimination of the diazirine precursor eliminates the diazirine excited state. From 37, both cyclopropane formation and 1,2-H rearrangement proceed from a single (carbene) intermediate, and addn/rearr vs. [alkene] is linear.25... 

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