H Shifts

The tertiary-secondary 1,2-H shift O itlO is not rate-determining in the interconversion of 5 and 6, but may become so in a conformationally fixed system. It has been found for the interconversion of tertiary and secondary adamantyloxocarbonium ions that <10" sec at 70°C (Hogeveen and Roobeek, 1971a) as compared with k= 1-5 x 10 sec at 20°C for the reaction 5 6. The absence of interconversion between tertiary and secondary adamantyloxocarbonium ions is due to the circumstance that 1,2-H shifts do not occur in the tertiary adamantyl ion as a result of the effect of orbital orientation (Brouwer and Hogeveen, 1970 Schleyer etal., 1970). That the secondary adamantyloxocarbonium ion can lose CO is demonstrated by the reaction with isopropyl cation in SbFs—SO2CIF solution at 0°C with formation... 

The related [1,2]-H shift in the isomeric triplet 1-phenylethylidene (53) was also investigated. The triplet carbene, generated from irradiation of the corresponding diazo compound, was characterized in low temperature inert matrices by EPR, IR, and UV/VIS spectroscopy. In this case, the carbene was stable in Ar up to the temperature limits of the matrix (36 K). Irradiation, however, readily converted the carbene to styrene. 

A similar difficulty was uncovered in the related 1,2-H shift of another singlet... 

Singlet Carbene C-H Insertions Although [1,2]-H shifts are formally carbene C-H insertions, these rearrangements have different orbital symmetry aspects than those of intramolecular insertions. As described above, overwhelming evidence exists that triplet carbenes undergo abstraction-recombination reactions to... 

Since CHEC-II(1996) <1996CHEC-II(4)179>, when no examples were known, only one example of the reactivity of this type of system has appeared. Hence, the platinum(ll) 2,3-dihydro-l,2,4-oxadiazole complexes 127 undergo rupture of the N-O bond and a 1,2-H shift in the presence of hydrogen to give the ketoimines 128 (Equation 18) as stable white solids <2004JCD2741>. 

Allylic cations (180) were also generated by LFP of allenes (174) in TFE.86 Deuterium labels revealed that the cations 180 originate predominantly from vinylcarbenes (177), which are formed from 174 by way of a 1,2-H shift. Protonation at the central carbon of the photoexcited allenes87 is a minor reaction path with 174a,b,d. Vinylcarbenes are also known to arise in photolyses of cyclopropenes, 175 — 177.85bi88 However, LFP of 175 in protic media proved to be rather inefficient in generating allylic cations, presumably due to low quantum yields. 

Now the 2-butenes comprise only 73% of the products, while 1-butene, a minor thermolytic product, increases to 23%. It is at least conceivable that some of the 1-butene here derives from an excited state of diazirine 8, rather than exclusively from carbene 6.3,15 Computational studies indicate that the 1,2-H shifts of 6 leading to the 2-butenes are preferred to the 1,2-H shift that yields 1-butene by a AAG of 2.6-3.3 kcal/mol. This again suggests that the substantial yield of 1-butene obtained from the photolysis of 8 is not simply derived from carbene 6.16... 

Additional evidence for a second intermediate in supposed carbene reactions comes from numerous studies.17-29 In the earliest experimental approach, the carbene precursor, frequently a diazirine, was photolyzed in the presence of increasing quantities of an alkene, which trapped the carbene with the formation of a cyclopropane (5 in Scheme 1). If carbene 2 were the sole product-forming intermediate, as depicted in Scheme 1, then the ratio of its alkene addition product (5) to its 1,2-H shift rearrangement product (4) would vary linearly with alkene concentration Eq. 9. 

Photolysis of diazirine 14 gave >90% of vinyl chloride, but, in accord with the observations of Tomioka, Liu, and Bonneau,17-20 photolyses in the presence of increasing quantities of TME led to a strongly curved correlation of addn/rearr vs. [TME]. And, carbene 15 was trapped by TME with, at most, 66% efficiency, so that about a third of the diazirine was converted to vinyl chloride by a 1,2-H shift that seemed to bypass the carbene. 

Thus, excited diazirine 35-d6 less efficiently undergoes 1,2-D migration with nitrogen loss compared to the analogous 1,2-H shift process in excited 35 accordingly, excited 35-dt more frequently decays by fluorescence than does... 

H shift more readily occurs with loss of nitrogen.22 Excited diazirines decay by fluorescence, carbene formation, or 1,2-H(D) migration coupled with N2 loss. C-D bonds are stronger than C-H bonds, so that deuteration retards the latter pathway and therefore RIES, leading to an increase in both fluorescence and carbene formation from 35-d6 22... 

H shift. Indeed, the most recent calculations indicate no barrier to the collapse of a MeCCl/TME complex such a species is not located on the electronic energy surface.45... 

The second of our principal concerns is the contribution of quantum mechanical tunneling (QMT) to singlet carbene 1,2-H shifts and related reactions. There is strong evidence that QMT is important in the low temperature matrix reactions of (e.g.) t-buty 1 chlorocarbene (18)58 and benzylchlorocarbene (10a).59... 

An unusually negative entropy of activation (—16.1 e.u.) was also observed for the 1,2-H shift.60... 

Three possibilities were considered to account for the curved Arrhenius plots and unusual KIEs (a) the 1,2-H shift might feature a variational transition state due to the low activation energy (4.9 kcal/mol60) and quite negative activation entropy (b) MeCCl could react by two or more competing pathways, each with a different activation energy (e.g., 1,2-H shift and azine formation by reaction with the diazirine precursor) (c) QMT could occur.60 The first possibility was discounted because calculations by Storer and Houk indicated that the 1,2-H shift was adequately described by conventional transition state theory.63 Option (b) was excluded because the Arrhenius curvature persisted after correction of the 1,2-H shift rate constants for the formation of minor side products (azine).60... 

Goodman concluded that QMT dominated the 1,2-H shifts of MeCCl or MeCBr at low temperatures. At higher temperatures, the classical (activated) mechanism with a = 4.9 kcal/mol, became important, but even at 298 K, QMT was considered to account for >85% of the reaction.60 Theoretical studies supported the incursion of QMT in these 1,2-H rearrangements, accounting for both the KIE and entropic peculiarities.63 However, calculations implied that the classical mechanism overtook QMT at quite low temperatures (>200 K), so that the 1,2-H shift would be very largely classical at ambient temperature.63 It was also deemed possible that QMT could be important in 1,2-C shifts.63... 

The studies of MeCCl refocused attention on benzylchlorocarbene (10a). Earlier studies of 10a, over a temperature range of 0-31°C, afforded linear Arrhenius correlations for the 1,2-H shift, with Ea = 4.5-4.8 kcal/mol and log A 11.2 s-1.36 Additionally, LFP studies of p-CF3 and p-Cl substituted benzylchlorocarbenes (lOf and lOg) in isooctane over a temperature range of —3 to 47°C gave linear Arrhenius correlations with a (4.9 and 4.5 kcal/mol) and log A (10.9 s-1) values comparable to those found for parent carbene 10a.64... 

In the more polar solvent, chloroform, linear Arrhenius correlations were observed for the rearrangements of both 10a and 10b from —55 to 60°C. Arrhenius parameters were a = 3.6 kcal/mol and log A = 10.4 s-1 for 10a, and Ea = 4.05 kcal/mol and log A = 10.3 s 1 for 10b.66 These results accord with the idea that polar solvents stabilize the polar 1,2-H shift, hydride-like transition state (51), accelerating this reaction at the expense of potential competitors.4,22... 

Of course carbene C-H insertion reactions are well known absolute kinetics have been reported for the insertions of ArCCl into isooctane, cyclohexane, and n-hexane,67 and of PhCCl into Si-H, Sn-H, and C-H bonds.68 More recently, detailed studies have appeared of PhCCl insertions into a variety of substrates bearing tertiary C-H bonds, especially adamantane derivatives.69 Nevertheless, because QMT is considered important in the low temperature solution reactions of MeCCl,60,63 and is almost certainly involved in the cryogenic matrix reactions of benzylchlorocarbene,59 its possible intervention in the low temperature solution reactions of the latter is a real possibility. We are therefore faced with two alternative explanations for the Arrhenius curvature exhibited by benzylchlorocarbene in solution at temperatures < 0°C either other classical reactions (besides 1,2-H shift) become competitive (e.g., solvent insertion, azine formation), or QMT becomes significant.7,59,66... 

The possible intervention of classical, competitive reactions in the low temperature solution chemistry of benzylchlorocarbene (10a) requires careful investigation. There are reasons to suspect azine (48) formation Goodman reported minor yields of azine in analogous MeCCl experiments,60 and Liu et al. found 40% of 48 in the photolysis of neat diazirine 9a.65 Perhaps azine formation is also significant at low temperature in hydrocarbon solvents. If so, the intervention of bimolecular azine formation, in competition with the unimolecular carbene 1,2-H shift, could lead to a nonlinear temperature dependence for the disappearance of 10a. Arrhenius curvature could then be explained without invoking QMT. 

The significant incursion of intermolecular products implies that the kinetic data previously obtained for the disappearance of 10a at low temperatures66 is biased and should not be used in Arrhenius treatments of the 1,2-H shift reaction. Therefore, the curved Arrhenius correlations do not require a QMT rationalization. 

---