Suprafacial allyl inversion

The 1,4- and 3,2-rearrangements are thermal, concerted, orbital symmetry allowed processes possessing suprafacial-suprafacial characteristics. They are facile reactions, occurring readily and under mild conditions. The 3,2-rearrangement will take place with allylic inversion, as demanded by orbital symmetry control. Thus, the sulfonium salt (30) gave the allylic sulfide (31), whereas (32) gave the isomeri-cally pure (33 Scheme 9). ... 

No oxetans were observed as by-products in the thermal rearrangement of (583) to (584), suggesting that the reaction does not proceed via the perpendicular biradical (585). An alternative 1,3-concerted shift could follow two possible modes (i) inversion at the migrating C-4 centre with suprafacial allylic participation or (ii) retention at the migrating C-4 centre and antarafacial allylic inversion. The demonstration that Z-3S-(586) rearranges to -3J -(586) confirms the first of these possibilities. ... 

Maryanoff and McComsey have studied the concerted suprafacial nature of the aza-Claisen rearrangement in the pyrrolo-[2,l-o]isoquinoline system 367 by deuterium labeling studies.Quaternary c -fixsed salt 370 underwent aza-Claisen rearrangement with allyl inversion to yield iminium species 371 which was reduced in situ to amine 372. 

Entries 1 and 2 in Table 8 are examples of an overall antarafacial 1,3-transposition of a hydroxy group by selenium compounds20,21. Treatment of the alcohols with 2-nitrophenyl seleno-cyanate in the presence of tributylphosphine gave the selenide with inversion of configuration. Oxidation with hydrogen peroxide led to the selenoxide, which rearranged suprafacially to the allylic alcohol. 

The earliest studies relative to the mechanism was the stereochemical work of Conforth, Popjak, and their collaborators [49]. They concluded that there was an inversion of configuration at C-1 of the allylic substrate, consistent with a concerted process, the new carbon-to-carbon bond being formed as the carbon-to-oxygen bond is cleaved. They also felt that elimination of a proton from C-2 of the isopentenyl moiety would not be concerted with this, since suprafacial (same side) reactions are generally considered unfavorable. To circumvent this, a 2-stage mechanism involving an electron donor X , with X being covalently linked to the initial condensation product, was proposed. The X residue is then lost simultaneously with elimination of the proton in an anti-mode (Fig. 12). 

Scheme 6.6. (a) Example illustrating the suprafacial nature of the migration across the allyl moiety [52]. (b) Examples illustrating inversion of configuration at the metalated carbon [57,58]. 

Berson first showed that bicyclo[3.2.0]hept-2-enes are converted thermally to bicyclo[2.2.1 ]hept-2-enes, however, it was the determination of the stereochemistry of the migrating carbon in this 1,3-shift that captured the attention of organic chemists in the late 1960s.There was at least a 19-fold preference for inversion in the pyrolysis of x<9-7-deuterio- nJo-6-acetoxybicyclo[3.2.0]hept-2-ene at 307°C. This is consistent with an allowed reaction in which the allylic moiety is used in a suprafacial manner (Scheme 8.44). 

The ground state electronic configuration of allyl radical is Pi -HOMO ( Pi) of this radical has opposite sign on the terminal lobes (Ci symmetry). Suprafacial [1,3] hydrogen shift under thermal condition is forbidden because there is no question of inversion at this atom, which is bonded to the carbon atom through its spherically symmetrical Ir-orbital. Antarafacial [1,3] hydrogen shift is allowed only by the principles of orbital symmetry. The transition state is a highly contorted species and the reaction is forbidden because of the steric inhibition involved in such a process. 

When an allyl group migrates, an additional stereochemical feature arises because of p-orbital of migrating carbon of alkyl group. In this case, the thermal [l,3]-suprafacial alkyl shift is an allowed process with inversion of configuration of migrating alkyl carbon and the thermal suprafacial [l,5]-alkyl shift is also allowed process with retention of configuration of alkyl carbon (Pig. 4.5) [1, 2],... 

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