Hydride shifts structure

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

Draw a Lewis structure for the carbocation which would result from a 1,2-hydride shift in the more stable (initially-formed) cation. Is this carbocation more stable than the initially-formed ion What would be the product of bromide addition to this cation Is this the observed product ... 

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

A number of the purported syntheses of dibenzo[f>,/][l,4]diazocines in the literature have to be revised and the structure of the products corrected.1 Instead of the expected 1,4-diazocine derivatives 1, isoindolobenzimidazoles 2 are actually isolated, the formation of which requires a hydride shift after transannular attack of one imine group onto the other. 

Stable solutions at temperatures below — 150°C in SbFs—SO2 and FSO3H— SbFs—SO2, where the structure is static and hydride shifts are absent. Studies by H and NMR, as well as by laser Raman spectra and X-ray... 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

How does structure determine organic reactivity, 35, 67 Hydrated electrons, reactions of, with organic compounds, 7,115 Hydration, reversible, of carbonyl compounds, 4, 1 Hydride shifts and transfers, 24, 57... 

Figure 3. Calculatedfree energy barrier for 1,2-hydride shift of cyclohept-1-enyl cation and the optimized structures (MP2/6-31 G(d)).l2b...

Treatment of benzaldehydes with ethyl diazoacetate and a catalytic quantity of the iron Lewis acid [ -CpFe(CO)2(THF)]+BF4 yields the expected homologated ketone (80). However, the major product in most cases is the aryl-shifted structure (81a), predominantly as its enol tautomer, 3-hydroxy-2-arylacrylic acid (81b). This novel reaction occurs via a 1,2-aryl shift. Although the mechanism has not been fully characterized, there is evidence for loss of THF to give a vacancy for the aldehyde to bind to the iron, followed by diazoacetate attachment. The product balance is then determined by the ratio of 1,2-aryl to -hydride shift, with the former favoured by electron-donating substituents on the aryl ring. An alternative mechanism involving epoxide intermediates was ruled out by a control experiment. 

The proton nmr behaviour of the norbornyl ion provides a wealth of information which, however, dso appears to be of limited value in the structural problem. The cation has been observed by H-nmr in many solutions containing SbFs as well as in GaBr3—SO2 Qensen and Beck, 1966). At —80° in SbFs—SO2 it exhibits 3 peaks, 5T86 (area 6), 2 82 (area 1) and 5 01 (area 4). The assignments and experimental coupling constants are shown in Figure 5 (Olah et al., 1970). The 5-01 p.p.m. peak indicates equivalence of the four protons, which is caused by rapid 6,1,2-hydride shifts. 

In summary, the temperature dependence of the proton nmr spectra does not provide a sufficient basis to decide upon the norbornyl cation structure. The slow 3,2-hydride shift might be rationalized by either a non-classical or classical interpretation, but both the activation energies and the pre-exponential factors might be influenced by solvation. If this is the case the arguments based on the pmr observations become still less persuasive. 

Fig. 2.2.S.3 Structure of rhamnose isomerase, and proposed hydride-shift mechanism for the ketol isomerization.

The combination of steps 1 and 2 corresponds to a 1,2 hydride shift (R = H) or a Wagner-Meerwein rearrangement (R = alkyl). The intermediate bridged nonclassical structure... 

Wanless and Kennedy (S) pyrolyzed polymers synthesized using 4-methyl-1-pentene and 4-d-4-methyl-1 -pentene. The mass, infrared, and HNMR spectra of the pyrolyzates, as well as those of a polymer produced by Ziegler-Natta initiator indicated that the cationically produced polymer contained about 70% 13 and 1,4 units, with 1,4 units predominating, and that the 1,4 structure most probably resulted from two consecutive hydride shifts rather than a single hydride jump". 

The same technique was used950 to generate the l -(ferf-butyldimethylsilyl)bicy-clobutonium ion 529, which undergoes fast 1,3-hydride shift upon increasing temperature to yield 3-e ferf-butyldimethylsilyl)bicyclobutonium ion 530 [Eq. (3.135)]. Ion 530 has a static structure, which is due to the efficient stabilization by the y-ewiio-trialkylsilyl substituent. 

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