Sigmatropic 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... 

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

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 is a [1,5]-sigmatropic rearrangement. The figure T in the square brackets shows that the same atom is at one end of the new o bond as was at one end of the old c bond. One atom has moved in a 1,5 manner and these are often called [1,5]-sigmatropic shifts. This is often abbreviated to [1,5]H shift to show which atom is moving. This particular example is important because sadly it prohibits a most attractive idea. The cyclopentadiene anion is.very stable (Chapter 8) and can easily be alkylated. The sequence of alkylation and Diels-Alder reaction looks very good. 

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

In a sigmatropic reaction, movement of a cr bond takes place, producing rearrangement (tropic is from the Greek word tropos, to turn). Common types of sigmatropic reactions are the familiar 1,2-hydride or alkyl shifts in carbocations and the Cope rearrangement. 

Four-electron, [1,3] sigmatropic rearrangements are very rare. Occasionally an alkyl group can undergo a concerted 1,3-shift, but H atoms never undergo concerted [1,3] sigmatropic rearrangements. 

The non-classical version of the intermediate carbenium ion (also referred to as a carbonium ion) resulting upon dissociation of the pyrophosphate anion from famesylpyrophosphate explains the cyclization to several cyclic carbenium ions as demonstrated for some sesquiterpenes (Fig. 3). Additional diversity arises from 1,2-hydride and 1,2-alkyl shifts (Wagner-Meerweest rearrangements) sbA sigmatropic reactions (COPE rearrangements) on the one hand, and on the other hand from the formation of diastereomers and enantiomers provided that the cyclizations generate new asymmetric carbon atoms (Fig. 3)... 

Another class of reactions that can occur by sigmatropic rearrangements involve alkyl group shifts ... 

---