Vicinal hydrogen shift

The configuration and optical purity of the tertiary product (740) from (738) show that it arises entirely by direct exo vicinal hydrogen shift in cation (7J9)523. ... 

Cyclopropene undergoes a thermal isomerization to propyne with log k = 13.25 - 37 400/2.37 r and to allene with log k = 13.25-43 400/23RT The simplest reaction pathway would appear to be Cl-C3 cleavage to a biradical, which is also vinylcarbene, followed by a vicinal hydrogen shift from C2 to either C3 or Cl, respectively (Scheme 4.2). 

However, the energy surface for formation of the acetylene is much more complex. As a result of MRCI-MCSCF calculations with a 4-31G -h DZP basis set, two hydrogens migrate the first is a 1,3-shift to generate a propenylidene and the second is a vicinal hydrogen shift to establish the acetylene as is well known for vinylidenes (Scheme 4.3). Further, the pathway of Scheme 4.2 was found to be nearly 20 kcal/mol higher than the favored one ... 

Tropilidene (1,3,5-cycloheptatriene) is a non-planar, tub-like molecule with only a 6.3 kcal/mol barrier to ring inversion. It is converted to toluene upon heating with log = 13.54 - 51 100/2.3/ T or log = 13.9 - 52200/2.3/ r. Themost reasonable course of this rearrangement is disrotatory electrocyclization to norcaradiene followed by homolytic cleavage of an external cyclopropane bond to a 1,3-biradical and subsequent vicinal hydrogen shift (Scheme 8.8). 

At 400-600°C in a flow system, COT gives 3,8-dihydropentalene (Scheme 9.16). The most reasonable pathway is via the bicyclo[3.3.0]octadiendiyl which undergoes a vicinal hydrogen shift to a dihydropentalene which undergoes 1,5-hydrogen shifts to the observed product. Of interest is the fact that at still higher temperatures, only benzene and acetylene are formed from... 

Further, 3-deuterio-gxo-tricyclo[3.2.1.0 ]oct-6-ene (as a 2 1 syrv.anti mixture) gave exclusively bicyclo[3.2.1]octa-2,6-diene-4-d in a 2 1 ratio of endo to exo isomers. This observation rules out simple vicinal hydrogen shifts in a trimethylene... 

Conformation-different trimethylene biradicals were proposed with the biradical from the syn,anti isomer giving more vicinal hydrogen shifted material than that from the anti, anti isomer, which gives mostly intramolecular 2 + 2 product. Both also gave tetralin and cw-/3-methylstyrene. 

Interestingly, the adduct of cycloheptatrienylidene and styrene gave 9-phenylbicyclo[5.2.0]nona-l,3,5-triene at 75°C, a reaction that appears to be a 1,7-shift but may also involve biradicals. At higher temperatures this product gave 2-phenylindane presumably via a norcaradiene which is also a bicyclo[2.1.0]pentane which could undergo central bond homolysis followed by a vicinal hydrogen shift (Scheme 12.15). 

The naphthocyclopropane derivative of Scheme 14.1 not only was found to give phenalene, but it underwent bridgehead double inversion upon heating. These reactions most reasonably occur via cleavage of the more substituted cyclopropane bond followed by a vicinal hydrogen shift. Subsequently, it was shown that the reactions were not affected by the presence of oxygen which indicates that the biradical is probably in its singlet state. ... 

The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H—Si(lll) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si—H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the... 

The chemical shift differences observed for these 1-fluoro-l-methyl-f-butylcyclohexanes, the cis- and rrans-9-nuomdecalins, and for 1-fluoroadamantane provide insight regarding the significant influence of conformation upon fluorine chemical shifts in fluorocyclohexanes. The relative chemical shifts of these various cyclohexyl fluorides can be rationalized simply on the basis of what is commonly known as an anomeric effect. That is, a vicinal hydrogen that is rigidly anti to a fluorine substituent will exhibit an anomeric double-bond/no-bond... 

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. 

Cyclopentadiene (CPD) undergoes an intramolecular vicinal (1,5-) hydrogen shift. The reaction was first recognized when a mixture of double bond isomers was... 

A further investigation of the photolysis products of 4,5-dihydroxypentyl-(pyridine)cobaloxime and of 5,6-dihydroxyhexyl(pyridine)cobaloxime shows that the initially generated radicals rearrange by intramolecular hydrogen shifts. Furthermore, the primary radicals, which attack the vicinal diols, do so regio-selectively and show considerable primary kinetic isotope effects. In all these features the system thus mimics aspects of the diol-dehydratase enzyme system. 

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