Vicinal hydride shift

Equation 16 illustrates a celebrated example where ring closure competes with vicinal hydride shift (a common form of atom transfer in cations, which does not take place in free radicals or anions). The gas phase reaction was explored by preparing the dimethylfluoronium ion, (CH3)2F" , by y-radiolysis of fluoromethane. Exothermic methylation of a sample of C-yd-phenethyl chloride (where the asterisk in equation 16 symbolizes the labeled position) in the gas mixture gives a vibrationally excited ion that loses chloromethane to form two isomeric ions, a-phenethyl cation and spirooctadienyl cation (sometimes called ethylenebenzenium). Nucleophilic attack by methanol in the reaction mixture yields PhCH(CH3)OCH3, whose isotopic label remains almost entirely at the methyl group. The recovered PhCH2CH20CH3 contains equally distributed between the two methylene positions. The spirooctadienyl ion does not isomerize to a-phenethyl cation, even though DFT calculations predict the latter to be 55 kJ/mol more stable. 

According to the rules of orbital symmetry, vicinal hydride shift in cations is a thermally allowed suprafacial 1,2-sigmatropic shift. Migration of a more distant hydrogen can occur by successive 1,2-shifts, via a bridged structure (such as the cyclooctyl cation in Scheme 7 ), or by corner-to-corner transfer in a protonated cyclopropane (which give net 1,3-shifts). An EBFlow experiment has been able to measure the competition between 1,2- and 1,3-shift in the 1-fluoropropyl system. 

It is also interesting that the calculated energy of a planar 3-methylenecyclobu-tylidene, which could be responsible for formation of methylenecyclobutene by a vicinal hydride shift and for inversion of the propellane, is similar to those of the transition states forming the cyclopropane products. 

Berson, j. a., j. H. Hammons, A. W. McRowe, R. G. Bergman, A. Remanick, and D. Houston The Chemistry of Mcthylnorbomyl Cations. VI. The Stereochemistry of Vicinal Hydride Shift. Evidence for the Nonclassical Structure of 3-Methyl-2-norbornyl Cations. J. Amer. Chem. Soc. 89, 2590 (1967). 

CHAPTER 7 Inversion of Configuration in the Sn2 Reaction 244 Racemization in the Sn1 Reaction 252 Hydride Shift in an Sn1 Reaction 253 Methyl Shift in an Sn1 Reaction 254 Rearrangement in an E1 Reaction 261 Dehydrohalogenation by the E2 Mechanism 304 Stereochemistry of the E2 Reaction 306 E2 Debromination of a Vicinal Dibromide 310... 

With educts containing an axially oriented LG at C-3, in addition to an intramolecular Sn2 reaction with an axially oriented hydroxy or acetamido group in a vicinal position (IS, Table 3, entries 1 [26] and 2 [30]), elimination and/or hydride shift (E or M/E, entries 3 [30],4 [31] and 6 [32]), as well as substitution with retention of configuration (AS, entries 7 and 8 [27b]) have been observed. For the transformation outhned in entry 4, 2,6-di-ferf-butyl-4-methylpyridine was used as acid scavenger, since pyridine per se in a similar reaction had caused Sn2 displacement (entry 5 [33]). Of special interest are the results from the deamination of methyl 3-amino-3-deoxy-/5-D-allopyranoside (entry 6), where the main reaction consists of direct Sn2 displacement with formation of methyl... 

When, under the assumption of conformational homogeneity, looking at the set of reactions possible with educts containing an equatorially oriented LG, hydride shift (MS [H], route f. Scheme 4) and elimination (E[H], route h. Scheme 4) cannot be found. However, besides the direct nucleophilic substitution with inversion of configuration (by the external nucleophile or the solvent, Sn2), all other sorts of participation, from vicinal positions, by antiperiplanarly oriented substituents - including each single ring atom of the pyranose - appear. These are in particular ... 

The above reaction scheme could not explain the marked effect of water on the reaction rate. Another drawback of the above reaction scheme is that if each monomer addition were followed by the shift of a hydride ion and isomerisation, the two charges should always remain in the vicinity and it should allow the elimination of HC1 or the addition of chloride ion to proceed easily otherwise the reaction scheme will not be feasible. 

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