Carbon nucleophiles allylic rearrangement

Various S-nucleophiles are allylated. Allylic acetates or carbonates react with thiols or trimethylsilyl sulfide (353) to give the allylic sulfide 354[222], Allyl sulfides are prepared by Pd-catalyzed allylic rearrangement of the dithio-carbonate 355 with elimination of COS under mild conditions. The benzyl alkyl sulfide 357 can be prepared from the dithiocarbonate 356 at 65 C[223,224], The allyl aryl sufide 359 is prepared by the reaction of an allylic carbonate with the aromatic thiol 358 by use of dppb under neutral condi-tions[225]. The O-allyl phosphoro- or phosphonothionate 360 undergoes the thiono thiolo allylic rearrangement (from 0-allyl to S -allyl rearrangement) to afford 361 and 362 at 130 C[226],... 

The carbocations formed as intermediates when allylic halides undergo Stvfl reactions have their positive charge shared by the two end carbons of the allylic system and may be attacked by nucleophiles at either site Products may be formed with the same pattern of bonds as the starting allylic halide or with allylic rearrangement... 

Nucleophilic substitution at an allylic carbon takes place either by an SN2 mechanism or by an Sn2 mechanism. No allylic rearrangement takes place in the former case, but it does occur in the latter case. 

Nucleophilic substitution at an allylic carbon can also take place by an Sn2 mechanism, in which case no allylic rearrangement usually takes place. However, allylic rearrangements can also take place under Sn2 conditions, by the following mechanism, in which the nucleophile attacks at the y carbon rather than the usual... 

This result is consistent with an equilibrium between an initially formed a-substituled allyl tellnroxide (resulting from the well-known regio-controlled electrophilic attack at the y-carbon) and the y-isomer (which is really the main product, as detected by NMR). In the snbseqnent reaction with amines, the more nucleophilic alkylamines attack the central carbon atom, giving rearranged products, whereas the less nucleophilic arylamines attack the terminal carbon, giving the thermodynamic y-product. 

Nucleophilic Substitution at an Allylic Carbon. Allylic Rearrangements... 

Efforts to cause the carbon nucleophile available at C-2 (carbohydrate numbering) of the osulose derivative 66 to displace the methoxy group with allylic rearrangement and with consequent formation of a tricyclic product by use of Pd(0) catalysts [34] were unsuccessful, but the intended reaction proceeds "smoothly when tin(IV) chloride is used together with acetic anhydride in dichloromethane. Clearly, the Lewis acid activates the allylic ether group, and the C-2 nucleophile effects its displacement. Concurrently, acetolysis of the benzylidene ring occurs and the product isolated is the cu-decalin analogue 67 [33],... 

The reaction is a nucleophilic substitution in which the nucleophile (C6H5S ) becomes attached to the carbon that bore the chloride leaving group. Allylic rearrangement is not observed therefore, it is reasonable to conclude that an allylic carbocation is not involved. The mechanism is SN2. 

Interestingly, the allylation of stabilized carbon nucleophiles has been found to be reversible. Complete transfer of dimethyl methylmalonate moiety, involving C—C bond cleavage, from the secondary carbon 115 to the primary carbon 116 was observed by treatment with a Pd catalyst for 24 h, showing that the allylic C—C bond cleavage proceeds slowly [61], Ni(dppb)2 was found a more efficient catalyst for the rearrangement [62],... 

Carbon nucleophiles which do not readily trigger the rearrangement of epoxides include lithiated dithianes [295, 304], lithiated sulfones [238], lithiated diarylphos-phine oxides [240, 305], lithium enolates [306], and allylic organolithium or organo-magnesium compounds [298, 307-310] (Scheme4.67). 

Alternatively, Pd(0) adds oxidatively to the double bond of a glycal derivative resulting in the formation of a ir-allyl complex, which may react with carbon nucleophiles to give C-glycosides with a double bond between C(2) and C(3).26 A rt-allyl complex may also be formed starting from a Ferrier rearrangement product (2,3-unsaturated sugar derivative).22... 

Finally, it is important to recognize that an SN1 reaction that forms an allylic carbo-cation often provides more than one site at which the nucleophile can bond. The nucleophile may bond to either of the carbons that bear the positive charges in the resonance structures. If the allylic cation is not symmetrical, this will result in the formation of two products one normal and one rearranged. An example of such an allylic rearrangement is... 

The reaction of AT-sulfinyl dienophiles with 1,3-dienes yields 3,6-dihydrothia-zine 1-oxides (compare the corresponding reactions with sulfur diimides. Sect. 3.1), which can be ring opened with carbon nucleophiles to yield allylic sulfoxides ready to undergo the ME rearrangement. The seminal paper on this chemistry by Weinreb appeared in 1984 (Scheme 57) [67,140]. 

Allylic systems are interesting substrates for the investigation of new methods of C-C bond formation. Some of these methods have been adapted to the carbohydrate field. Enopyrano-sides are often crystalline compounds readily available, for example, using Fenier rearrangement of tri-O-acetyl glycals with alcohols or its variants with carbon nucleophiles. 

Silane 5 attacks ion 4 or 8 with the double-bond carbon atom that is in y-position to the silicon atom. This generates carbocation 11, which is stabilized by the silicon in the P-position. Nucleophilic attack at the silicon regenerates the double bond with an overall allylic rearrangement of the double bond. 

---