Cyclization, radicals diradical mechanism

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

The most likely multistep mechanism of this type is shown in the lower part of Figure 15.17. It is a two-step mechanism where the diastereomeric diradicals F and G are the two intermediates that allow for rotation about the configuration-determining C—C bond. Each of the two radical centers is part of a well-stabilized allyl radical (cf. Section 1.2.1). Biradicals F and G cyclize without diastereocontrol to deliver the [4+2]-cycloadducts biradical F forms a mixture of l 2trans,cis-[D]2-C and 12trans,trans [D]2-C, since a rotation about the C2—C3 bond is possible but not necessary. For the same reason, biradical G forms a mixture of 1 2cis,cis-[D]2-C and 12cis,trans [D]2-C. 

These examples again have some mechanistic implications in that they appear to rule out cyclization via 5n2 displacement of the halide by a samarium ketyl. However, one cannot distinguish between a mechanism based on allylsamarium addition to the carbonyl versus an electron transfer mechanism as outlined for the alkyl hdide substrates above. Both mechanisms allow for isomerization of the double bond (via 1,3-allylic transposition in the case of an allylmetallic, or configurational instability in an allylic radical in a diradical coupling mechanism) and also provide reasonable routes for generation of butadiene. Further mechanistic work is clearly required in order to provide a more detailed understanding of all of these intramolecular Barbier-type reactions. 

The accumulated spectroscopic evidence clearly points to the participation of both 828 and 82a in the SET-photoinduced degenerate methylenecyclopropane rearrangement of d -lla. When combined with the chemical results, it is possible to propose a cation radical cleavage (CRCL)-diradical cyclization (DRCY) mechanism for this process (Scheme 24). Cation radicals < 2-773 and undergo... 

Bergman proposed that the reaction mechanism of the cyclization under thermal conditions (200 °C) involved the initial generation of a 1,4-benzenediyl diradical species known as para-benzyne (2). Bergman reported that when the reaction was carried out in a hydrocarbon solvent, such as 2,6,10,14-tetramethylpentadecane, benzene was formed as the final product. This suggests that the hydrocarbon solvent (RH) acts as a hydrogen atom donor to quench the diradical intermediate 2. This result hints at the radical nature of the mechanism operative in the Bergman cyclization. 

The reaction mechanism has tJso been investigated by DFT calculations, lending support for the stepwise mechanism [111, 112]. The first key intermediate is the diruthenium nitrenoid species (S = 1/2). In the transition state of the C-H bond scission, the coordinated nitrenoid interacts with the allylic H atom, polarizing the C-H bond. Subsequent H atom transfer to the N atom generates a diradical species comprising a diruthenium amide tethered to an allylic radical (S = 3/2). Of interest, the unpaired electrons are primarily localized at the two Ru centers (and the allylic fragment), with litde spin density (0.14) at the N atom. Finally, radical rebound forms the cyclized amine. 

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