Alkyl and hydride shifts

By analogy to carbenium ions, it can be expected that alkylnitrenium ions will experience 1,2-alkyl and hydride shifts. These would presumably be orbital symmetry allowed reactions that convert the nitrenium ion into a more stable iminium ion. This idea is supported by several theoretical studies suggesting that simple... 

The observation of apparent 1,2-alkyl and hydride shift products was the basis of much discussion of alkylnitrenium ion chemistry several decades ago. In fact, several reactions (e.g.. Fig. 13.12) reported by Gassman and others, were ascribed to nitrenium ions. Unfortunately, it is in practice very difficult to determine whether such products resulted from reaction of a free nitrenium ion or whether they occurred conceitedly with the dissociation of the leaving group. 

In understanding these reactions, it is helpful to view the metal-alkene tt complex as an incipient carbocation (just as tt complexes of halogens are incipient carbocations). Alkyl and hydride shifts then bear analogy to carbocation rearrangements. This may be an oversimplification but it makes the chemistry easier to follow. 

For some reactions, it will be necessary to consider both alkyl and hydride shifts in order to account for the products formed. 

Lewis acid complexes of p-substituted a,p-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one can not be induced to undergo ene reactions as acrolein and MVK do. 34 The presence of a substituent on the p-carbon stabilizes the enal- or enone-Lewis acid complex and sterically retards the approach of an alkene to the p-carbon. However, we have found that a complex of these ketones and aldehydes with 2 equivalents of EtAlQ2 reacts reversibly with alkenes to give a zwitterion. 34 This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane or undergoes two 1,2-hydride or alkyl shifts to irreversibly generate a p,p-disubstituted-o,p-unsaturated carbonyl compound (see Figure 19). The intermolecular addition of an enone, as an electrophile, to an alkene has been observed only rarely. The specific termination of the reaction by a series of alkyl and hydride shifts is also very unusual. 35 The absence of polymer is remarkable. 

Carbocation Rearrangements hy Alkyl and Hydride Shifts (Section 9-3)... 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

The mechanistic subtypes presented throughout this book include those related to the acid-base properties of organic molecules. These are protonations, deprotonations, and proton transfers. Mechanistic types based on solvation effects include solvolysis reactions, SN1, and El processes. Additional mechanisms utilizing ionic interactions include SN2, SN2, E2, 1,2-additions, 1,4-additions, and addition-elimination processes. Finally, those mechanistic types dependent upon the presence of cationic species include alkyl shifts and hydride shifts. 

An important difference between Fnedel-Crafts alkylations and acylations is that acyl cations do not rearrange The acyl group of the acyl chloride or acid anhydride is transferred to the benzene ring unchanged The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could con ceivably arise from it by a hydride or alkyl group shift... 

Strategy A Friedel-Crafts reaction involves initial formation of a carbocation, which can rearrange by either a hydride shift or an alkyl shift to give a more stable carbocation. Draw the initial carbocation, assess its stability, and see if the shift of a hydride ion or an alkyl group from a neighboring carbon will result in increased stability. In the present instance, the initial carbocation is a secondary one that can rearrange to a more stable tertiary one by a hydride shift. 

One of the most characteristic properties of carbonium ions is their great tendency to undergo rearrangements. These rearrangements include 1,2-alkyl shifts, hydride shifts, cyclopropylcarbinyl rearrangements, Wagner-Meerwein rearrangements, and others. 

Hydride and 1,2-alkyl shifts represent the most common rearrangement reactions of carbenes and carbenoids. They may be of minor importance compared to inter-molecular or other intramolecular processes, but may also become the preferred reaction modes. Some recent examples for the latter situation are collected in Table 23 (Entries 1-10, 15 1,2-hydride shifts Entries 11-15 1,2-alkyl shifts). Particularly noteworthy is the synthesis of thiepins and oxepins (Entry 11) utilizing such rearrangements, as well as the transformations a-diazo-p-hydroxyester - P-ketoester (Entries 6, 7) and a-diazo-p-hydroxyketone -> P-diketone (Entry 8) which all occur under very mild conditions and generally in high yield. 

Monosubstituted Alkenes. Simple unbranched terminal alkenes that have only alkyl substituents, such as 1-hexene,2031-octene,209 or ally Icy clohexane230 do not undergo reduction in the presence of organosilicon hydrides and strong acids, even under extreme conditions.1,2 For example, when 1-hexene is heated in a sealed ampoule at 140° for 10 hours with triethylsilane and excess trifluoroacetic acid, only a trace of hexane is detected.203 A somewhat surprising exception to this pattern is the formation of ethylcyclohexane in 20% yield upon treatment of vinylcyclohexane with trifluoroacetic acid and triethylsilane.230 Protonation of the terminal carbon is thought to initiate a 1,2-hydride shift that leads to the formation of the tertiary 1-ethyl-1-cyclohexyl cation.230... 

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

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