Hydride shifts preparation

Scheme 10.3 gives some examples of pinacol and related rearrangements. Entry 1 is a rearrangement done under strongly acidic conditions. The selectivity leading to ring expansion results from the preferential ionization of the diphenylcarbinol group. Entry 2, a preparation of 2-indanone, involves selective ionization at the benzylic alcohol, followed by a hydride shift. 

Recently, we have shown that iminium ions can induce a hydride shift to form a new carbocation which then reads with a nucleophile. By this way the novel unusual bridged steroid alkaloids 25 were prepared from the secoestron derivative 19 (scheme 5) M Treatment of 20 obtained from 19 by hydrogenation with aniline or an aniline derivative 21 containing an dedron-withdrawing group in the presence of the Lewis add BF3-OEt2 leads to the iminium... 

Another convenient method for the preparation of functionalized cyclobutanol derivatives is by treatment of 1,2-diphenylethylene acetals containing a 1,3-dithiane moiety in the y-position, e.g. 14c. with butyllithium. The isolation of 2,2-(propane-l,3-diyldisulfanyl)cyclobutanol (15c) together with benzyl phenyl ketone in 90 and 92 % yield, respectively, indicates that the reaction mechanism should involve the intramolecular attack of the metalated dithiane on the acetal carbon atom with concomitant hydride shift at the acetal group.15... 

Bridgehead bicyclo[4.4.0]decyl, bicyclo[4.3.0]nonyl, and bicyclo[3.3.0]octyl cations are found to be rapid equilibrating ions (see Section 3.5.2.1).188 The isomeric bridgehead congressane (diamantane) cations 52 and 53 have been prepared and observed.182 The diamant-4-yl cation 52 rapidly rearranges to the diamant-l-yl cation 53 at —60°C, possibly through intermolecular hydride shifts. Bridgehead bicyclo[3.3.3]undec-l-yl cation 54 has also been observed by 1H and 13C NMR spectroscopy.189... 

Oxidation of veatchine (48) with Sarett reagent gave veatchinone (53), which, when reduced with NaBD4 in CH3OH, afforded an epimeric mixture of (54) and (55). This mixture was acetylated with acetic anhydride in pyridine and the acetates, (56) and (57), were separated by preparative-scale t.l.c. Treatment of (56) with 10% HC1 and re-acetylation gave (58). The 13C n.m.r. data indicated that there was no deuterium present at C-16 in (58) therefore, this rearrangement did not take place by a 15 —> 16 hydride shift. 

The 2-methyl-2-butyl cation [68] also shows degenerate properties. Prepared by hydride abstraction from n-pentane or isopentane in FSOjH-SbFj, its H-nmr spectrum was observed by Olah and Lukas (1967). The temperature dependence of the spectrum indicated scrambling of the methyl groups. A mechanism was proposed with an initial 1,2-hydride shift to the secondary... 

Kirchen and Sorensen (1978) prepared a carbocation from 1,6-dimethyl-cyclodecan-l-ol in FSOaH-SbFj-SOgClF at —120°C. The six peak C-nmr spectrum could be attributed to a tertiary carbocation [76] undergoing rapid 1,6-hydride shifts. The lowest field carbons at 8 142 ( ./isch = 36 Hz) were decoupled by irradiation at the unusual position (8 — 3.9, one proton) of a small broad peak in the H-nmr spectrum. Expected chemical shift for such... 

Epoxides can isomerize under the influence of transition metal catalysts. This formal 1,2-hydride shift is a method to prepare unsaturated carbonyl compounds from epoxides (Equation 54) <1998T1361>. This method has been extended as a double epoxide isomerization-intramolecular aldol condensation (Equation 55) <1996JOC7656, 1998TL3107>. m-Epoxides are isomerized to /ra r-epoxides under ruthenium catalysis <2003TL3143>. 

The synthesis and relative stability of 3,5-diacyl-4,5-dihydro-l//-pyrazoles prepared by dipolar cycloaddition of enones and a-diazoketones has been published <2004JOC9085>. 3-Acyl-4-aryl-2-pyrazolines have been synthesized by the reaction of a,/3-unsaturated ketones with diazomethane <1996IJB1091>. Ethyl diazoacetate added to 1,3-diarylpropenones in a regioselective fashion to give the intermediate 4,5-dihydto-3//-pyrazole derivative 1,3-hydride shift in the latter led to the formation of the isomeric ethyl 4-aryl-5-aroyl-4,5-dihydro-l//-pyrazole-3-carboxylate and ethyl 4-aryl-3-aroyl-4,5-dihydro-l/7-pyrazole-5-carboxylate in a ratio of 5 1 <2001RJ01517>. 1,3-Dipolar cycloaddition of 2-diazopropane with diarylideneacetones afforded diastereomeric bis-A -pyrazolines <1999T449>. 

Besides the work done on solvolysis of 2-norbomyl compounds, the 2-norbornyl cation has also been extensively smdied at low temperatures there is much evidence that under these conditions the ion is definitely nonclassical. Olah and co-workers have prepared the 2-norbomyl cation in stable solutions at temperamres below 150°C in SbFs—SO2 and FSO3H SbF5 S02, where the stmcmre is static and hydride shifts are absent Studies by proton and NMR, as well as by laser Raman spectra and X-ray electron spectroscopy, led to the conclusion that under these conditions the ion is nonclassical. A similar result has been reported for the 2-norbomyl cation in the sohd state where at 77 and even 5 K, NMR spectra gave no evidence of the freezing out of a single classical ion. ... 

The formation of the sultone (160) probably involves addition of the complex across the alkene double bond, a 1,2-hydride shift and an intramolecular nucleophilic substitution reaction. The sultone (161) is formed by addition of sulfur trioxide to give the unstable p-sultone which rearranges to the more stable y-isomer (161). Another useful route to sultones is by metallation of alkanesulfonate esters for example, butane-1,3-dimethylsulfonate (162), prepared from butanel,3-diol, yields the 8-sultone, namely 6-methyl-l,2-oxathiin-2,2-dioxide (163) (Scheme 67). 

Eliminations of (3,4-epoxybutyl)stannanes under the action of EtAlCl2 give rise to cyclo-propylmethyl alcohols <87SC78l, 91JOC2066,91JOC2076). This method can be used reliably to prepare bicyclo[3.1.0]hexanes from the corresponding spirocyclic epoxy stannanes (Equation (9)). These eliminations appear to be concerted when inversion can take place at both centers. In other cases, the 1,3-eliminations are stepwise and must compete with 1,2-hydride shifts. 

Fast Wagner-Meerwein rearrangement and degenerate 1,2-hydride shifts have been extensively investigated under superacid conditions to probe the nature of intermediate carbocations. The 2-butyl cation (3) has been prepared from 2-chlorobutane in SbF5-SO2ClF at -100°C in a vacuum line by Saunders et al. 2 with very little contamination from the rprFbutyl cation (4) (Scheme 6.13). 

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