Alkyl-shift

The preferred alignment of orbitals for a 1,2-hydride or 1,2-alkyl shift involves coplanarity of the p orbital at the carbocation ion center and the a orbital of the migrating group. 

The conversion of 14 to 15 provides a test of the prediction that 1,3-alkyl shifts should occur with inversion of configuration ... 

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

For the reaction in the gas phase, Emmons proposes a liomolytic O—N fission and (possibly simultaneous) alkyl shift [Eq. (28)]. The... 

In the case of an appropriate substrate structure, the carbenium ion species can undergo a 1,2-alkyl shift, thus generating a different carbenium ion—e.g. 4. The driving force for such an alkyl migration is the formation of a more stable carbenium ion, which in turn may undergo further rearrangement or react to a final product by one of the pathways mentioned above—e.g. by loss of a proton to yield an alkene 3 ... 

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. 

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

The product of the rearrangement may be stable or may react further, depending on its nature (see also pp. 1396). An ab initio study predicts that a [l,2]-alkyl shift in alkyne anions should be facile. ... 

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. 

As these results show, 1,2-shift only occurs if there is a substituent on the double bond. In such a case, rearrangement can lead from a bent secondary vinyl cation, 199, to a more stable linear secondary vinyl cation, 200, whereas in the absence of a double-bond substituent, a similar 1,2-alkyl shift would lead to an unstable primary vinyl cation. Interestingly, triflate 205 reacts only by methyl shift and shows no ring-contraction. On the other hand, triflate 206 reacts via... 

Epoxides are ol course as easy to make from un-symmetrical, e.g. (29) as from symmetrical, e.g. (26), alkenes. The more stable carbonlum ion is again formed. Hydride shift is preferred to alkyl shift except in the favourable 6 5 ring contraction. 

Now is where it gets interesting. One of the alkyl groups migrates over (an alkyl shift) to kick off hydroxide ... 

Innumerable reactions occur in acid catalyzed hydrocarbon conversion processes. These reactions can be classified into a limited number of reaction families such as (de)-protonation, alkyl shift, P-scission,... Within such a reaction family, the rate coefficient is assumed to depend on the type, n or m cfr. Eq. (1), of the carbenium ions involved as reactant and/or product, secondary or tertiary. The only other structural feature of the reactive moiety which needs to be accounted for is the symmetry number. The ratio of the symmetry number of the... 

If a potential carbocation intermediate can undergo a hydride or alkyl shift, this shift occurs in preference to closure of the five-membered ring. 

Photolysis of the first known cyclic a-diazo-p-oxophosphine oxide 49 is unsuccessful with regard to phosphene formation. There is no evidence for a P/C-phenyl shift, which should lead to 51, nor for a P/C-alkyl shift, which would afford 52 via ring contraction, since none of the expected phosphinic esters could be isolated in methanolM). 

Rearrangements that involve carbanions are found to be very much less common than formally similar rearrangements that involve car bo cations (p. 109). This becomes more understandable if we compare the T.S. for a 1,2-alkyl shift in a carbocation with that for the same shift in a carbanion ... 

Rearrangements that involve radicals are found to be much less common than otherwise similar rearrangements that involve carbo-cations. In this they resemble carbanions (cf. p. 292), and the reason for the resemblance becomes apparent when we compare the T.S.s for a 1,2-alkyl shift in the three series ... 

While no 1,2-alkyl shifts have been observed in solution, the cool flame oxidation of Me3CH (in the gas phase at 480°) is found to... 

The formation of this ketone is believed to proceed via internal abstraction of H in the initial peroxy radical (128 cf. p. 328), followed by migration of Me. It may be that the vigorous conditions employed now make a 1,2-alkyl shift feasible, or that the shift of Me may involve fragmentation followed by re-addition, rather than direct migration. 

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. 

The similar insertion of 93 into 1,2-diphenylcyclopropenone results in cy-clobutenones or o- and p-alkoxyphenols [49]. In the reaction of 93 with 1-alkynylcyclobutenols, 2-alkenyl-4-cyclopentene-l,3-diones are obtained via alkyl shift-ring expansion [50]. (Scheme 33)... 

Polymeric hydrocarbon by-products accompany the products of the latter two reactions. The structures of the products are clear evidence of the occurrence of 1,2-alkyl shifts leading to more stable carbocationic intermediates. ... 

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