Stereochemistry sigmatropic shifts

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... 

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. 

According to the Woodward-Hoffmann rales, five concerted transition states are possible for the Claisen as well as the closely related Cope rearrangements chair, boat, twist, cross and plane (Table 6). Only the chair and boat TS have to be considered, as twist, cross and plane are antarafacial-anta-rafacial processes and require highly elevated temperatures. - For the correct prediction of product stereochemistry it is nevertheless crucial to know the preference for chair- or boat-like transition state in the actual 3,3-sigmatropic shift. 

An elegant demonstration of the stereochemistry of a thermally allowed [1,5] sigmatropic shift was reported by Roth and coworkers in 1970. They studied the stereochemistry of the reaction of the optically active starting material,... 

The ester group is transformed into an ethyl group in simple fashion 116 and the position and stereochemistry of all the chiral centres corrected by an Evans-Mislow rearrangement. Oxidation to the sulfoxide 118 allows a [2,3] sigmatropic shift across the top face of the molecule giving an unstable sulfenate ester 119 from which sulfur is removed by a phosphite thiophile to give the alcohol 117 with the correct anti-stereochemistry. The alkene returns to its original position and stereochemistry. The rest of the synthesis involves the construction of the polyene chain.16... 

If these reactions occur in uncatalyzed processes where bond breaking and bond formation are taking place concertedly, and not in two-step pathways via ionic or diradical intermediates, then the stereochemistry of these sigmatropic shifts can be predicted in a qualitative manner 1 -4. According to the Woodward-Hoffmann rules the thermally allowed reaction should take place in an antarafacial fashion across the allylic framework. The shifting hydrogen has to move from one side of the allylic plane to the other as depicted below. 

Suprafacial [1, 5]-sigmatropic shifts of hydrogen and silyl substituents are allowed, generally facile, and proceed without inversion of stereochemistry on the migrating atom. A well-known example is the 1,5-shift of an allylsilyl group in silylcyclopentadienes, which proceeds with retention of configuration281. 

Amination of allosecurinine (32) and securinine (33) with 0-mesitylenesulpho-nylhydroxylamine has been shown to give the N-amino-salts (34) and (37), respectively (Scheme 3). The stereochemistry of (34) and (37) is assigned on the basis of the observed rearrangements of the corresponding ylides (35) and (38) into compounds (36) and (39), respectively. These reactions may be envisaged to proceed by a [2,3] sigmatropic shift mechanism. The stereochemistry of the amination reaction parallels that of the quaternization of the two alkaloids with methyl iodide. 

The results were interpreted in terms of an allowed thermal 1,3-sigmatropic shift with suprafacial-mvQxsiow stereochemistry, and the lower specificity in and higher temperature necessary for the rearrangement of syn material was argued to be due to steric inhibition of the inversion pathway. 

The initial isomerization is formally a 1,3-sigmatropic shift with allowed stereochemistry, but it was proposed that the reaction proceeded by an allowed conrotatory ring opening to a c/5,cw,c/5, ran -l,3,5,9-cyclodecatetraene which electrocyclized in an allowed disrotatory fashion to the major product (Scheme... 

As discussed during the previous frontier molecular orbital theory analysis of [3,3]-sigmatropic shifts, a chairlike transition state is allowed for these reactions. As you will show in Problem 20.48, a boatlike transition state is also allowed by frontier molecular orbital theory. However, chair conformations are more favorable than boat conformations for six-membered cyclic rings (look back at Section 2.5A). This preference influences the stereochemistry of these shifts, as we now show with an example. 

As we have al ready mentioned, two of the important factors in sigmatropic shifts are the stereochemistry of the migrating group and whether migration occurs to the same or opposite faces of the ir system. Here, we look at some examples that verify the predictions given above. 

The orbital symmetry study of sigmatropic reactions will help us to understand the stereochemistry of these reactions. An FMO analysis of this process indicates the interaction between the frontier orbitals of the n system and orbitals of the migrating atom or atoms. Let us consider the simplest case of 1,3-sigmatropic shift of hydrogen. In this case, n system is an allyl radical and the migrating atom is hydrogen. Their frontier orbitals are allyl i2 and hydrogen Is for thermal reaction... 

---