Rearrangements alkyl shifts

In addition to the migration of hydrogen atoms in sigmatropic rearrangements, alkyl shifts also take place. A large number of such reactions occur... 

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 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... 

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 ... 

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. 

We have already seen that in [l, 5] alkyl shifts with symmetry allowed will lead to retention of configuration. Another such example with retention of configuration in a [1, 5] alkyl shift is the rearrangement of spirodienes. 

A search for the structural changes expected in the early stages of a classical 1,2-alkyl shift was also fruitless. A series of mono-substituted / ra 5-cyclohexane-l,2-diols [104], which undergo the pinacol rearrangement... 

In a deep-seated rearrangement like this, it s sometimes easier to work backwards from the product. The n bond at C8=C9 in 12 suggests that the last step is deprotonation of C8 of a carbocation at C9, C. Carbocation C might have been formed from carbocation D by a 1,2-alkyl shift of Cl 1 from C9 to C3. Carbocation D might have been formed from carbocation E by a 1,2-alkyl shift of C13 from C3 to C4. Carbocation E might have been formed from carbocation F by attack of a C3=C4 n bond on a C9... 

In the course of dolastane synthesis (the dolastanes are a group of marine diterpenes) interesting rearrangements catalyzed by Lewis acids were found. Treatment of the trienone 293 with excess (1.5 eq) ethylaluminum dichloride at low temperatures (—5°C, 48 h) gave the tetracyclic enone 295 in 53% yield while the tricyclic dienone 296 (50%) was formed at room temperature (equation 102)156. It was assumed that both products can be derived from the common zwitterion 294 which undergoes intramolecular alkylation at low temperatures (path a) whereas an alkyl shift takes place at elevated temperatures (path b), followed by a 1,2-hydride shift (equation 102). 

When both positions a to the C—Li bond of the lithiooxirane have no hydrogen to shift, a 1,2-alkyl shift (Scheme 58) can occur. This rearrangement has been observed recently in the case of oxiranes 125 derived from cyclopentenols. This 1,2-alkyl shift can occur on both sides of the lithiooxirane (Scheme 58, paths a and b). The resulting lithium enolate then undergoes a -elimination process of Li20, leading to diversely substituted cyclopentenones 126 and 127. 

Recently, interesting examples have been reported of 1,2-alkyl shift (paths a and b in Scheme 58) in the rearrangement of snbstitnted cyclopentenol and -hexenol oxides ... 

This reaction can occur through hydrogen shift, alkyl shift (Cope rearrangement) or Claisen rearrangement. 

Alkyl shift is evident in the Cope rearrangement. A Cope rearrangement is a [3,3] sigmatropic rearrangement of a 1,5-diene. This reaction leads to the formation of a six-membered ring transition state. As [3,3] sigmatropic rearrangements involve three pairs of electrons, they take place by a suprafacial pathway under thermal conditions. 

It must be pointed out that the 1,2-alkyl shift may be subverted by other factors such as steric strain. Thus the cascade rearrangement of a tetraspiroketone on exposure to acid is most aesthetically appealing and synthetically useful for entry into [4.5]coro-nane [201],... 

In the acid-catalyzed rearrangement of bicyclo[3.1.1]heptanones, two different cations, 1 and 2, lead to two different bicycloheptanones via 1234-1243 rearrangement. The ionic process involves sequential 1,2-alkyl shifts. Substituents at C2 and C4 determine which cationic intermediate is more stable. 

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