H-migration

The exclusive and quantitative formation of oxepins upon Pd-catalyzed decomposition of 4-diazomethyl-4-methyl-4 //-pyrans (Entry 11) contrasts with the results of the CuCl-promoted reaction which affords a 2 1 mixture of oxepin (by 1,2-C migration) and 4-methylene-4//-pyran (by 1,2-H migration) under otherwise identical conditions 381J. When the methyl group at C-4 of the diazo precursor is replaced by H, the metal-catalyzed route to thiepins is no longer viable Pd- or Cu(I)-catalyzed decomposition of 4-diazomethyl-4//-thiopyrans invariably leads to 4-methylene-4H-thiopyrans 378 (Entry 10). Only the proton-catalyzed decomposition of these diazo compounds affords the desired thiepin, albeit in low yield 378). 

A more recent study of 36 in perfluorohexane lent support to this conclusion.73 Firstly, the experimental barrier to 1,2-H migration in 36 was much smaller than predicted by ab initio calculations (2.56 vs. 7.4 kcal/mol), whereas the measured a for the rearrangement of 36-dg compared well with the computed 5.6 kcal/mol. Secondly, the observed activation parameters revealed large KIEs (E° — ) = 3.18 kcal/mol, and AD/AH = 158. These results support the... 

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

The 10-fold disadvantage of the latter 1,2-H migration can be attributed to destabilization of the corresponding transition state by the inductively electron-withdrawing CF3 bystander.14... 

The authors proposed mechanism for dimerization involves initial formation of metal vinylidene complex 9 via 1,2-H-migration. A second molecule of arylacetylene acts as a dienophUe in a formal [4 + 2] Diels-Alder cycloaddition with 9. A subsequent... 

The intermediacy of a platinum vinylidene in Yamamoto s reaction was supported by the results of isotopic labeling studies. D FT calculations were used to further probe the proposed reaction mechanism. In contrast to the prevailing model of alkyne/ vinylidene interconversion for Rh(I)-catalysts, direct Ca Cp 1,2-H-migration is implicated in the formation of vinylidene 130. Direct C—H insertion via a single... 

Intramolecular nucleophilic attack on the a-position of the vinylidene complex 148 affords a zwitterionic species (149). Next, a formal 1,3-H-shift must occur before release of the observed product (146). Based on the results of an isotope labeling study, this process is believed to occur via two discrete 1,2-H-migration events (149—>150—>147). Interestingly, no back-migration of heteroatom substituents is observed [50]. 

The solvent effect on the regioselectivity of the AuCls-catalyzed cycloisomerization of a bromoallenyl ketone was evaluated by density functional theory calculations. Upon the generation of the gold carbenoid intermediate from cyclization of the aUene precursor, the tetrahydrofiiran solvent can act as a proton shuttle to assist the 1,2-H migration to afford the 2-bromofuran product (14S2149). 

Tilley and co-workers (86) proposed a new mechanism (called the Glaser-Tilley mechanism), which involves activation of two (sp ) Si—H bonds of the silane substrate, direct addition of (sp ) Si—H to alkene and finally the 1,2-H-migration and reductive elimination step (Scheme 12). Theoretical study by Beddie and Hall (87) showed that the highest energy transition state in this novel mechanism is more than 8 kcal/mol lower in energy than the highest energy transition states in the Chalk-Harrod and modified Chalk-Harrod mechanisms. 

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