Diradical species rearrangements with

A nitrene generated from the reaction of A-aminophthalimide (101) and PhI(OAc)2 was key to the metal-free ring expansion of alkylidenecyclopropanes (102) and an alkylidenecyclobutane.85 The authors propose two plausible mechanisms for these ring-expansion reactions either an aziridine is formed which undergoes facile rearrangement to form the final 2,2-disubstituted cyclobutylidene hydrazine product (103), or reaction of the alkylidenecyclopropane with the nitrene generates an ionic or diradical species which rearranges. 

TTie nonracemic secondary ethers (58) and (60) rearrange with over 90% retention of stereochemistry at the migrating center (equations 16 and 17). A lithium-bridged diradical species (63 Scheme 2) is postulated to account for the observed diastereoselectivity. Recombination of the radical pair must occur more rapidly than inversion of the radical center, judging from the high degree of retention observed with (58) and (60). A two-directional application of this rearrangement was used to prepare the syn-skipped polyols (66) and (68 Scheme 3). 

A [1,3]- or [1,5]-H shift is formally required for the rearrangement of 162 to benzene (Scheme 6.40). Quantum-chemical calculations predict that the hydrogen atom migrates in two steps, that is, in consecutive [1,2]-H shifts, with the species 177 being the intermediate, which has to be described either as a diradical [117] or a car-bene [116, 117]. The experimental activation enthalpy for the conversion of 176 into benzene [112] was correctly simulated by the energy of the transition state separat-... 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

In contrast to di-radicals such as 121 (Scheme 19.33), di-radicals originated by the Myers-Saito rearrangement are known to have an ionic character in protic solvents and this nature is thought to reduce their DNA damaging ability. However, it has been shown by Shibuya that the presence of an electron withdrawing group in the benzylic position produces a dehydrotoluene species with an enhanced diradical character. 

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

The di-TT-methane rearrangement of 1,4-pentadiene has been modeled using CAS-SCF/4-31G calculations. The results indicate that a singlet 1,3-diradical is the key intermediate. This species can be reached from the excited state via a Cl that involves vinyl migration. It consists of a vinyl group associated with the central carbon of an allylic system and is similar in structure to the Cl involved in alkene sigmatropic rearrangements (see p. 1093). The structure of the Cl is also consistent with the... 

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