Hydrogen with carbocations

Hydride shift (Section 5 13) Migration of a hydrogen with a pair of electrons (H ) from one atom to another Hydnde shifts are most commonly seen in carbocation rearrange ments... 

As with carbocation-initiated polyene cyclizations, radical cyclizations can proceed through several successive steps if the steric and electronic properties of the reactant provide potential reaction sites. Cyclization may be followed by a second intramolecular step or by an intermolecular addition or alkylation. Intermediate radicals can be constructed so that hydrogen atom transfer can occur as part of the overall process. For example, 2-bromohexenes having radical stabilizing substituents at C(6) can undergo cyclization after a hydrogen atom transfer step.348... 

A mixture of exo- and endo-isomers of 5-methylbicylo[2.2.1]hept-2-ene is hydrogenated with the aid of five equivalents of triethylsilane and 13.1 equivalents of trifluoroacetic acid to produce a 45% yield of < <7o-2-methylbicylo[2.2.1] heptane (Eq. 71). The same product is formed in 37% yield after only five minutes. The remainder of the reaction products is a mixture of three isomeric secondary exo-methylbicylo[2.2.1]heptyl trifluoroacetates that remains inert to the reaction conditions. Use of triethylsilane-l-d gives the endo-2-methylbicylo-[2.2.1]heptane product with an exo-deuterium at the tertiary carbon position shared with the methyl group. This result reflects the nature of the internal carbocation rearrangements that precede capture by the silane.230... 

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... 

On this hypothesis, benzylic alcohols should exhibit the fastest reaction rates, owing to the higher stability of their corresponding carbocations. It is therefore apparent that the incomplete conversion observed under our experimental conditions is due not to an intrinsically poor reactivity of the substrates but to the inadequacy of styrene as hydrogen acceptor, unable to prevail over the aromatic aldehyde formed, highly activated toward hydrogenation with Cu/... 

As a result of carbocation rearrangement, two alkyl halides are formed—one from the addition of the nucleophile to the unrearranged carbocation and one from the addition to the rearranged carbocation. The major product is the rearranged one. Because a shift of a hydrogen with its pair of electrons is involved in the rearrangement, it is called a hydride shift. (Recall that H is a hydride ion.) More specifically it is called a 1,2-hydride shift because the hydride ion moves from one carbon to an adjacent carbon. (Notice that this does not mean that it moves from C-1 to C-2.)... 

Another reaction of radicals is rearrangement. Radicals are generally less susceptible to rearrangement than are carbocations, and the 1,2 hydrogen or carbon shifts seen with carbocations are not observed with radicals. However, apparent phenyl migration has been observed (Scheme 4.37). Treatment of neophyl chloride with phenyhnagnesium bromide and cobaltous chloride produced isobutylbenzene... 

Rearrangements also occur in the acid-catalyzed hydration of alkenes, especially when a carbocation formed in the first step can rearrange to a more stable carbocation. For example, the acid-catalyzed hydration of 3-methyl-l-butene gives 2-methyl-2-butanol. In this example, the group that migrates is a hydrogen with its bonding pair of electrons, in effect, a hydride ion H . 

Step 2 1,2 Shift. The less stable 2 carbocation rearranges to a more stable 3 carbocation by migration of a hydrogen with its pair of bonding electrons (in effect... 

As shown in the following mechanism, reaction is initiated by heterolytic cleavage of the carbon-chlorine bond to form a 2° carbocation, which rearranges to a considerably more stable 3° carbocation by shift of a hydrogen with its pair of electrons (a hydride ion) from the adjacent benzylic carbon. Note that the rearranged carbocation is not only tertiary (hyperconjugation stabilization) but also benzylic (stabilization by resonance delocalization). 

Methane can be substituted in many ways through replacement of one or more hydrogens with another atom or groups of atoms. In principle, removal of a hydrogen from methane can lead to the methyl anion ( CH3), the methyl radical ( CH3), or the methyl cation ( CH3) depending on the nature of the hydrogen removed ( "H, H or H). In this chapter, we have discussed only the shapes of these intermediates—reactions are coming later. It will be important to remember that carbocations are flat and s/> hybridized and that simple carbanions are pyramidal and approximately s/> hybridized. 

Of course, we should analyze the carbon-hydrogen bonds as well. If we do this, we find that the differences in the strengths of the C—H bonds should favor the 1-propyl radical. As with carbocations, it is the C—C bonds that dominate and determine the issue. As a practical matter, it appears that we can get away with an analysis of the C—C bonds alone. 

Hydroboration-oxidation (Sections 6.8-6.9) This two-step sequence converts aikenes to alcohols with a regioselectivity opposite to Markovnikovs s rule. Addition of H and OH is stereospecific and syn. The reaction involves electrophilic addition of a boron hydride to the double bond, followed by oxidation of the intermediate organoborane with hydrogen peroxides. Carbocations are not intermediates and rearrangements do not occur. 

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