Electrophilic substitution with allylic

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a k bond and a C-Si O bond, to the allylic product with a new K bond and a new o bond to the electrophile. The intermediate cation is mainly stabilized by O donation from the C-Si bond into the vacant p orbital but it has other a-donating groups (C—H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

This reaction, for which the termprototmpic rearrangement is sometimes used, is an example of electrophilic substitution with accompanying allylic rearrangement. The mechanism involves abstraction by the base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more... 

Phenolic derivatives were prepared and then converted into thioether analogs using ethanedithol followed by oxidation of this intermediate to the disulfide. Phenolic resins were prepared by electrophilic substitution of allyl phenol derivatives with formaldehyde and then flee radically copolymerizing with ethanedithol. Epoxidation was performed using epichlarohydrine. 

This section deals, in general, with electrophilic substitution in allylic systems. For more complicated molecular rearrangements which may accompany electrophilic substitution, there is no simple method of description other than by a full statement of the mechanism. The recommended nomenclature is given in square brackets, [ ]. 

In basic aqueous media, a kinetic study of the reaction between stannate(II) ions and alkyl halide shows that mono- and disubstituted organotin compounds are formed (Eq. 6.12a).27 The monosubstituted organotin compound is obtained after a nucleophilic substitution catalyzed by a complexation between the tin(II) and the halide atom. The disubstituted compound results from an electrophilic substitution coupled with a redox reaction on a complex between the monosubstituted organotin compound and the stannate(II) ion. Stannate(IV) ions prevent the synthesis of the disubstituted compound by complexation. Similarly, when allyl bromide and tin were stirred in D2O at 60° C, allyltin(II) bromide was formed first. This was followed by further reaction with another molecule of allyl bromide to give diallyltin(IV) dibromide (Eq. 6.12b).28... 

Feringa and coworkers [258] and O Doherty et al. [259] independently reported palladium-catalyzed glycosylations of 2-substituted 6-acyl-2H-pyran-3(6H)-one derivatives and alcohols (Scheme 5.98). This reaction presumably involves electrophilic Pd 7t-allyl complex intermediate, which was generated by the reaction of 2-substituted 6-acyl-2Ff-pyran-3(6H)-one and Pd(0)/PPh3. It is noteworthy that 2-substituted 6-acyl-2H-pyran-3(6H)-one derivatives were stereoselectively converted into 2-substituted 6 - a I k o x y - 2 H - p y r an - 3 (6 H) -o n e derivatives with complete retention of configuration by this reaction. A two-step reduction/oxidation manipulation after the glycosylation can install new stereocenters in the obtained glycosides. 

Modes of cycloaddition of alkylideneallyl cation are also controlled by the reaction conditions. [4 + 3] Cycloaddition occurs in the reaction with furan. The [4 + 3] cycloaddition with furan was observed for the siloxy-substituted allyl cation 5S, but not for the methoxy-substituted allyl cation 5M. The lower electrophilicity of 5S may prefer the concerted pathway of [4 + 3] cycloaddition in competition with the stepwise pathway to yield a [3 + 2] cycloadduct and an electrophilic substitution product. 

Hence the positional selectivity is different from that of the furan additions to 417 (Scheme 6.90). Assuming diradical intermediates for these reactions [9], the different types of products are not caused by the nature of the allene double bonds of 417 and 450 but by the properties of the allyl radical subunits in the six-membered rings of the intermediates. Also N-tert-butoxycarbonylpyrrole intercepted 450 in a [4 + 2]-cycloaddition and brought about 455 in 29% yield. Pyrrole itself and N-methylpyr-role furnished their substituted derivatives of type 456 in 69 and 79% yield [155, 171b]. Possibly, these processes are electrophilic aromatic substitutions with 450 acting as electrophile, as has been suggested for the conversion of 417 into 442 by pyrrole (Scheme 6.90). 

Extension of this reaction to electrophiles other than aldehydes was unsuccessful [22, 23], However, propargylic boronates were found to react with allylic halides and various carbonyl compounds [23], The boronates were prepared by lithiation of a methyl-substituted alkyne with t-butyllithium followed by treatment with a trialkylborane. The propargylic boronate preferentially reacts with the electrophile at the y-position to yield propargylic products (Eq. 9.20). The methodology has also been applied to alanates with comparable results. 

Electrophilic carbene complexes can also react with organic halides to yield halonium ylides. Reaction of acceptor-substituted carbene complexes with allyl... 

In most allylation reactions, only a catalytic amount of CuCN-2LiCl is required [41]. Use of the chiral ferrocenylamine 104 as a catalyst makes enables asymmetric allylation of diorganozinc reagents to be effected with allylic chlorides (Scheme 2.36) [78]. Related electrophiles such as propargylic bromides [79] and unsaturated epoxides [80] also undergo SN2 -substitution reactions (Scheme 2.37). 

Woodward et al. have used the binaphthol-derived ligand 40 in asymmetric conjugate addition reactions of dialkylzinc to enones [46]. Compound 40 has also been studied as a ligand in allylic substitutions with diorganozinc reagents [47]. To allow better control over selectivity in y substitution of the allylic electrophiles studied, Woodward et al. investigated the influence of an additional ester substituent in the jS-position (Scheme 8.25). 

The scope of allylic electrophiles that react with amines was shown to encompass electron-neutral and electron-rich ciimamyl methyl carbonates, as well as furan-2-yl and alkyl-substituted allylic methyl carbonates. An ort/io-substituted cinnamyl carbonate was found to react with lower enantioselectivity, a trend that has been observed in later studies of reactions with other nucleophiles. The electron-poor p-nitrocinnamyl carbonate also reacted, but with reduced enantioselectivity. Allylic amination of dienyl carbonates also occur in some cases with high selectivity for formation of the product with the amino group at the y-position over the s-position of the pentadienyl unit [66]. Arylamines did not react with allylic carbonates under these conditions. However, they have been shown to react in the presence of the metalacyclic iridium-phosphoramidite catalysts that are discussed in Sect. 4. 

Most iridium-catalyzed allylic substitutions have been performed with allylic esters, which are typically synthesized from allylic alcohols. Reactions of allylic alcohols as electrophiles would alleviate the need to prepare the esters from the alcohol. In a few cases, however, iridium catalyzed allylic substitutions have been conducted with allylic alcohols as the electrophile. As discussed earlier in this... 

Cyclic epoxides such as 124 can react in two ways with strong bases (a) via abstraction of a /3-proton to form allylic alcoholates 125 or (b) by deprotonation at the epoxide carbon atom forming the intermediate 126 and, after electrophilic substitution, the epoxides 128. If there is a suitable C—H bond in the vicinity of the C-Li moiety, intramolecular carbenoid insertion reactions to 127 may take place (equation 27) ° . ... 

Pyrrolo[3,2-f]pyridine can be readily substituted with a variety of electrophiles at C-2 or C-3 after protection of the ring nitrogen atom. Derivatives can be synthesized with substituents such as iodo, methyl, trimethyltin, formyl, and allyl groups. The reactions proceed with excellent yields (47-95%) <200583581 >. 

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