The Allyl Group

The group CH2=CHCH2— is known as allyl, which is both a common name and a permissible lUPAC name. It is most often encountered in functionally substituted derivatives, and the following compounds containing this group are much better known by their functional class lUPAC names than by their substitutive ones  

The tenn allylic refers to a C=C—C unit. Its -hybridized carbon is called the allylic carbon, and an allylic substituent is one that is attached to an allylic carbon. Conversely, the yp -hybridized carbons of a carbon-carbon double bond are called vinylic carbons, and substituents attached to either one of them are referred to as vinylic substituents. 

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The 5p -hybridized carbon of a C=C—C unit is classified as an allylic carbon and atoms or groups attached to it are allylic substituents. The two 5p -hybridized carbons are vinylic and atoms or groups attached to them are vinylic substituents. 

Allyl is derived from the botanical name for garlic Allium sativum). It was found in 1892 that the major component obtained by distilling garlic oil is H2C=CHCH2SSCH2CH = CH2, and the word a//y/was coined for the H2C=CHCH2— group on the basis of this origin. 

Allylic is also a general term for molecules that have a functional group at an allylic position. Thus, the following compounds represent an allylic alcohol and an allylic chloridcy respectively. 

Conjugare is a Latin verb meaning to link or yoke together, and allylic carbocations, allylic free radicals, allylic anions, and conjugated dienes are all examples of conjugated systems. In this chapter we ll see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone. 

Another way to indicate this electron delocalization is via the dotted-line structures shown. It is important, however, to recognize that the +, -, or above the middle of the dashed line applies to the unit as a whole and is shared only by the end carbons. 

In allylic species that are not symmetrically substituted, the two resonance structures are not equivalent and do not contribute equally to the hybrid. 

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Since the allyl group has rearranged, it may have added to nitrogen first ... 

Alkylation of aldol type educts, e.g., /3-hydroxy esters, using LDA and alkyl halides leads stereoselectively to erythro substitution. The erythro threo ratio of the products is of the order of 95 5. Allylic and benzylic bromides can also be used. The allyl groups can later be ozonolysed to gjve aldehydes, and many interesting oligofunctional products with two adjacent chiral centres become available from chiral aldol type educts (G. Prater, 1984 D. Seebach, 1984 see also M. Nakatsuka, 1990, p. 5586). 

Another method for deallylation of ally esters is the transfer of the allyl group to reactive nucleophiles. Amines such as morpholine are used[415-417], Potassium salts of higher carboxylic acids are used as an accepter of the allyl group[418]. The method is applied to the protection and deprotection of the acid function in rather unstable /f-lactam 664[419,420]. 

The mechanism of the rearrangement catalyzed by Pd(fl), typically by PdCl2(RCN)2, is explained by the oxypalladation of an alkene to form 810 as an intermediate, or cyclization-induced rearrangement. As a limitation, no rearrangement takes place when the allylie ester 812 is substituted at the C-2 position of the allyl group, while a smooth rearrangement of 811 takes place[500]. 

The allyl group was used to protect the nitrogen in a /3-lactam synthesis, but was removed in a four-step sequence. Whether a transition-metal-catalyzed isomerization to the enamide followed by hydrolysis is an effective cleavage procedure remains to be tested and warrants further study. ... 

Further functionalization has been carried out in the mixed benzo-binaphtho crown shown in Eq. (3.55). Using 2-allylcatechol as starting material, the mixed crown was prepared in the usual fashion. The allyl group was isomerized to a propenyl substituent by treatment with potassium f-butoxide in a benzene/f-butanol mixture. Selective ozono-lysis affords the aldehyde. 

Protection for the double bond in the allyl protecting group may be achieved by epoxidation. Regeneration of the allyl group occurs upon... 

The use of Pd(Ph3P)4 and MA -dimethylbarbituric acid removed the allyl group in 98% yield. 

The allyl group was used to protect the nitrogen in a j3-lactam synthesis, but was removed in a four-step sequence. ... 

The possibility that the allyl group CH2=CH-CHi- can act as an ligand was recogni2ed independently by several groups in I960 and since then the held bas flourished, partly because of its importance in homogeneous catalysts and partly because of the novel sleric possibilities and inferconversions that can be studied by proton nmr spectroscopy. Many synthetic routes are available of which the following are representative. 

In (l-allylbenzotriazole)tricarbonyliron, the ligand is coordinated via the site as well as the allyl group attached to the N atom (77JOM(137)207). 

If both ortho positions bear substituents other than hydrogen, the allyl group will further migrate to the para position. This reaction is called the para-Claisen rearrangement. The formation of the para-substituted phenol can be explained by an initial Claisen rearrangement to an ortho-2l y intermediate which cannot tautomerize to an aromatic o-allylphenol, followed by a Cope rearrangement to the p-allyl intermediate which can tautomerize to the p-allylphenol e.g. 6 ... 

If elemicin be heated with alcoholic solution of potash, the allyl group is isomeiised to the propenyl group, and iso-elemicin results. 

Evidence for this mechanism comes from the observation that the rearrangement takes place with an inversion of the allyl group. That is, allyl phenyl ether containing a 14C label on the allyl ether carbon atom yields o-allylphenol in which the label is on the terminal vinylic carbon (green in Figure 18.1). It would be very difficult to explain this result by any mechanism other than a pericyclic one. We ll look at the reaction in more detail in Section 30.8. 

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

Of particular interest are reactions between a-hydroxy ketones and allyl(trifluoro)silanes which proceed with participation of the hydroxy group via a pentacoordinated allylsilane, with intramolecular delivery of the allyl group to the ketone. Excellent stereoselectivity is obtained at up to three contiguous stereogenic centers66. 

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