Anionic H shift

By contrast, in the [1,2] anionic H shift, the dominant FMO interaction is between the Is orbital of the one-atom component and t/q of the two-atom component. For there to be positive overlap between orbitals where bond-making and bond-breaking take place, the two-atom component must react antarafacially. Thus, for this shift to be a thermally allowed process, the H atom must have partial bonds to the top and bottom faces of the C2 unit simultaneously. Because this arrangement is geometrically impossible, [1,2] anionic H shifts are thermally disallowed reactions. 

The subsequent steps are a sequence of base-induced H-shifts to give the anionic species 5, from which loss of nitrogen (N2) leads to a carbanionic species 6. The latter is then protonated by the solvent to yield hydrocarbon 3 as the final product ... 

By aromatization of the five-membered ring with Li the helicene 62 is obtained. In this compound the ring current in ring B is restored, but the distance between the terminal rings is enlarged compared to hexahelicene. By an accurate analysis of the shifts and comparison with the 8-values of the fluorenyl-anion the shifts of all protons except that of H(16) could be ascribed to the effects of i, ii and iii. The large downfield value of H(16) must be ascribed to the neighbourhood of the Li+-ion in the contact ionpair. 

A laser flash photolysis study of phenylacyloxycarbene (75) allowed the measurement of the rate of 1,2-acyl shift of between 105 and 106 s 1 in pentane at room temperature.82 The activation parameters were in good agreement with calculations. High-level calculations supported a carbanion-like attack by the carbene lone pah on the carbonyl (76) whereas the effects of substituents on the rate suggested an acyl anion-like transition state (77). The electron-donating, stabilizing effect of OAc slows the 1,2-C and 1,2-H shifts in alkylacetoxycarbenes (78),83 allowing 1,2-acetyl shifts to compete. 

The reaction of vinylic phenyliodium salts (57) with cyanide anions could be mistaken for a simple substitution reaction.59 However, the presence of both allylic (58) and vinylic (59) nitrile products suggests a more complex picture. Deuterium labelling experiments show that the allylic product is formed via the Michael addition of cyanide to the vinylic iodonium salt, followed by elimination of iodobenzene and a 1,2-hydrogen shift in the 2-cyanocycloalkylidene intermediate (60). H-shift occurs from the methylene carbon in preference to the methine carbon. The effects of substitution and different nucleophiles were examined. 

It is a [1,5]-sigmatropic rearrangement. The figure T in the square brackets shows that the same atom is at one end of the new o bond as was at one end of the old c bond. One atom has moved in a 1,5 manner and these are often called [1,5]-sigmatropic shifts. This is often abbreviated to [1,5]H shift to show which atom is moving. This particular example is important because sadly it prohibits a most attractive idea. The cyclopentadiene anion is.very stable (Chapter 8) and can easily be alkylated. The sequence of alkylation and Diels-Alder reaction looks very good. 

In 2006, Scheidt and coworkers [44] reported the first enantioselective direct nucleophilic addition ofthe silylated thiazolium salt 148, a precursor of the equivalent acyl anion, to nitroalkene 149 in the presence of tetramethylammonium fluoride (TMAF) and stoichiometric amounts of quinine-based thiourea 81b, producing the chiral [3-nitroketone 150 in 67% yield and with 74% ee (Scheme 9.51). The acyl anion equivalent 152 can be generated by the desilylation of 148 with TMAF, followed by the 1,2-H shift of the resulting alkoxide 151. The observed asymmetric induction indicates that there is a strong interaction between the thiourea and the nitroalkene during the carbonyl anion addition step. 

H-shift (Reaction (3.15)) was considered feasible based on (i) the analogy to the well-known, solvent-assisted 1,2-H-shift within alkoxy radicals [19] and (ii) the exo-thermicity based on the homolytic bond dissociation energies (BDEs) of the N-H (406 kj mol ) and the C-H bond (363 kj mol ) (representative values for the Gly anion [47]). However, both pulse radiolysis and y-radiolysis experiments concluded that the 1,2-H-shift in aminyl and amidyl radicals derived from amino acids and peptides must be rather slow (kis 1.2 x 10 s ) [37, 40]. 

Some evidence for a reverse 1,2-H-shift was also obtained. In their studies on the radical-induced decarboxylation of Gly anion, Bonifacic et al. observed a proton-catalyzed decarboxylation of H2N-C"H-C02, which may proceed via Reactions... 

Receptor 33 also incorporates a secondary Lewis acid anion binding site. This molecule is the zinc metallated ansdogue of 17 with the cobal-tocenium reporter groups replaced with ferrocenes [27]. The freebase precursor to 33 in dichloromethane solution shows no significant anion induced shifts in the H NMR signals of the amide protons, whereas the metalloporphyrin binds bromide (JC = 6200M ), nitrate (K = 2300M i)... 

However, no ring opening products resulting from anions 5a,b are detected. Based on the higher basicity of the secondary anions obviously, a 1,5-H-shift giving the more stable a-silyl-substituted carbanions 7a,b (a-effect) takes place (Scheme 3). 

The less hindered needle-like anions 7a,b again ate allowed to attack sUacyclobutane la,b. In accordance to the following reaction scheme the formation of two different anions, the primary anions 8a,b and the secondary anions 9a,b is plausible (Scheme 4). As discussed for the formation of 7a,b, the secondary anions 9a,b rearrange to the doubly a-silyl-stabilized anions 10a,b by subsequent 1,5-H-shift (Scheme 5). The carbanions 10a,b are stable at -78 °C and, as an example, 10b is detected by NMR spectroscopic methods. Due to steric reasons, no indication of a further attack of the anions 10a,b to sUacyclobutane la,b is observed. 

---