A CIDNP study of the wittig rearrangement of allylic ethers

Stereoconvergent [1,2]- and [1,4]-Wittig Rearrangements of 2-Silyl-6-aryl-5,6-dihydropyrans: A Tale of Steric vs Electronic Regiocontrol of Divergent Pathways

“…Another complication is the inherent “problem” of regioselectivity that arises in (alkoxyallyl)metal species ( A in Scheme 1 ), where the [1,4]-migration competes with the [1,2]-shift, leading to mixtures of products. 5 , 6 Relative to the [2,3]- and [1,2]-shifts, [1,4]-Wittig rearrangements (routes c and d) are unique in their ability to generate stereodefined enolates 7 − 9 (rather than alkyl alkoxides). In addition, [1,4]-Wittig rearrangements have the potential to transfer chirality and stereoselectively form adjacent chiral centers.…”

“…Arguably, the [2,3]-Wittig rearrangement pathway has enjoyed more attention from both mechanistic and synthetic perspectives, resulting in an impressive display of applications such as the stereoselective assembly of adjacent chiral centers, the transfer of chirality, and the formation of olefins with specific geometries. , Although some of these features are also characteristics of the [1,2]-Wittig rearrangement, a narrower range of substrates are capable of efficient [1,2]-migration, perhaps a reflection of the requisite radical-stabilizing groups (i.e., R in Scheme ) for facile C–O bond homolysis. Another complication is the inherent “problem” of regioselectivity that arises in (alkoxyallyl)metal species ( A in Scheme ), where the [1,4]-migration competes with the [1,2]-shift, leading to mixtures of products. , Relative to the [2,3]- and [1,2]-shifts, [1,4]-Wittig rearrangements (routes c and d) are unique in their ability to generate stereodefined enolates − (rather than alkyl alkoxides). In addition, [1,4]-Wittig rearrangements have the potential to transfer chirality and stereoselectively form adjacent chiral centers.…”

Periselectivity between the [1,4] and [3,3] thermal sigmatropic rearrangements of 2-allyloxypyridine N-oxides

“…An experimental distinction between these two pathways was based upon the premise that the concerted reactions would be regiospecific in relation to the allyl group, whereas scrambling of the allyl group might be a consequence of its participation as an allyl radical in a radical pair. In the event, specifically labelled 2-( 1 , 1 -dideuterioallyloxy)pyridine N-oxide (7) was synthesised from 1 , 1-dideuterioallyl Its thermal transformation in dimethylformamide ( 1 0 0 O C ; 25 h) yielded the [1,4] product (8) and the [3,3] product (9) regiospecifically. This result directly supports the view summarised in Scheme 2 that the thermal transformation It is inconceivable that the regiospecificity associated with the [ 1,4] and [3,3] sigmatropic rearrangements of 2-allyloxy-pyridine N-oxide (Scheme 2; R = H) could be satisfactorily interpreted in terms of two independent dissociation-recombination processes involving two topologically different but tight radical pairs.…”