A-Allyl compounds

A-Allyl compounds can be rearranged to A-propenyl derivatives. Similarly, A-propargyl compounds (969) give A-allenyl derivatives (970). 

Tetraallyl- and tetrabutenyltin (a-allylic compounds) do not act as initiators of butadiene polymerization even under most favorable conditions (presence of donor additives, elevated temperature). In contrast, butenyllithium, which is considered to be a typical compound of dynamic structure, does initiate polymerization even at temperatures below zero. 

However, in the case of the photoreaction of alkylidenepropanedinitriles, a different regioselectivity is observed. The photoreaction of cyclohexylidenepro-panedinitrile with aUyltrimethylsilane in the presence of Phen in acetonitrile gives the a-allylated compounds as major products along with the reduction products (Scheme 39). 

The INS spectra of the a-allyl compound allyl iodide, CH2=CHCH2l, and the r -allyl compound di-p-chloro-bis(r] -allylpalladium), [ Pd(Ti -C3H5) 2(p-Cl)2] [94] and the INS below 800 cm of nickel and palladium allyl complexes [97] have been reported. The structure of the palladium complex is shown in Fig. 7.24 [98]. The INS spectra are shown in Fig. 7.25. Peak positions and assignments are in Table 7.15. 

Other rearrangements (Scheme 38) are exemplified by the conversions of rac-171 into rac-172 (93ACS72) and of the mixed carbonates 173 into A-allylated compounds 155 and 156 (93ACS63). 

Some allyl complexes may react with le ligands to give a-allyl compounds. 

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

TT-Aliylpalladium chloride reacts with a soft carbon nucleophile such as mal-onate and acetoacetate in DMSO as a coordinating solvent, and facile carbon-carbon bond formation takes place[l2,265], This reaction constitutes the basis of both stoichiometric and catalytic 7r-allylpalladium chemistry. Depending on the way in which 7r-allylpalladium complexes are prepared, the reaction becomes stoichiometric or catalytic. Preparation of the 7r-allylpalladium complexes 298 by the oxidative addition of Pd(0) to various allylic compounds (esters, carbonates etc.), and their reactions with nucleophiles, are catalytic, because Pd(0) is regenerated after the reaction with the nucleophile, and reacts again with allylic compounds. These catalytic reactions are treated in Chapter 4, Section 2. On the other hand, the preparation of the 7r-allyl complexes 299 from alkenes requires Pd(II) salts. The subsequent reaction with the nucleophile forms Pd(0). The whole process consumes Pd(ll), and ends as a stoichiometric process, because the in situ reoxidation of Pd(0) is hardly attainable. These stoichiometric reactions are treated in this section. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

Stereochemical features in the oxidative addition and the elimination of /3-hydrogen of cyclic and acyclic alkenes are different. The insertion (palladation) is syn addition. The syn addition (carbopalladation) of R—Pd—X to an acyclic alkene is followed by the syn elimination of 3-hydrogen to give the trans-a ksne 6, because free rotation of 5 is possible with the acyclic alkene. On the other hand, no rotation of the intermediate 7 is possible with a cyclic alkene and the syn elimination of /3-hydrogen gives the allylic compound 8 rather than a substituted alkene. 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

Asymmetric allylation of carbon nucleophiles has been carried out extensively using Pd catalysts coordinated by various chiral phosphine ligands and even with nitrogen ligands, and ee > 90% has been achieved in several cases. However, in most cases, a high ee has been achieved only with the l,3-diaryl-substitiitcd allylic compounds 217, and the synthetic usefulness of the reaction is limited. Therefore, only references are cited[24,133]. 

Wylation under neutral conditions. Reactions which proceed under neutral conditions are highly desirable, Allylation with allylic acetates and phosphates is carried out under basic conditions. Almost no reaction of these allylic Compounds takes place in the absence of bases. The useful allylation under neutral conditions is possible with some allylic compounds. Among them, allylic carbonates 218 are the most reactive and their reactions proceed under neutral conditions[13,14,134], In the mechanism shown, the oxidative addition of the allyl carbonates 218 is followed by decarboxylation as an irreversible process to afford the 7r-allylpalladium alkoxide 219. and the generated alkoxide is sufficiently basic to pick up a proton from active methylene compounds, yielding 220. This in situ formation of the alkoxide. which is a... 

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic 7r-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

When allylic compounds are treated with Pd(0) catalyst in the absence of any nucleophile, 1,4-elimination is a sole reaction path, as shown by 492, and conjugated dienes are formed as a mixture of E and Z isomers[329]. From terminal allylic compounds, terminal conjugated dienes are formed. The reaction has been applied to the syntheses of a pheromone, 12-acetoxy-1,3-dode-cadiene (493)[330], ambergris fragrance[331], and aklavinone[332]. Selective elimination of the acetate of the cyanohydrin 494 derived from 2-nonenal is a key reaction for the formation of the 1,3-diene unit in pellitorine (495)[333], Facile aromatization occurs by bis-elimination of the l,4-diacetoxy-2-cyclohex-ene 496[334],... 

BU3P. A rapid redox reaction takes place to yield the active Pd(0) species and tributylphosphine oxide. The Pd(0) thus generated is a phosphine-free cata-lyst[341]. Severe reaction conditions are necessary, or no reaction takes place, when Pd2(dba)3 is used in the elimination reaction of cyclic allylic compounds with an excess of -Bu3P[342]. 

Furthermore, the regioselective hydrogenolysis can be extended to internal allylic systems. In this case, clean differentiation of a tertiary carbon from a secondary carbon in an allylic system is a problem. The regioselectivity in the hydrogenolysis of unsymmetrically substituted internal allylic compounds depends on the nature and size of the substituents. The less substituted alkene 596 was obtained from 595 as the main product, but the selectivity was only... 

A number of useful resins have been prepared from allyl compounds, i.e. derivatives of allyl alcohol CH2 = CH CH20H. One of these, diethylene glycol... 

A -Halogenated compounds such as iV-chlorotnfluoroacetamide, A -chloro-imidosulfuryl fluonde and N N dichlorotnfluoromethylamine add across C=C bonds to form saturated amides [14] tmidosulfury I fluorides [15] and amines [16], respectively Allylic halogenation also occurs with the use of A-bromo- or A-chIo roperfluoroamides The primary amine A,A-dichlorotrifluororaethylamine selectively affords 11 or 2 1 adducts with either tetrafluoroethylene or chlorotrifluoroethylene [16] (equation 7) The reaction mechanism is believed to involve thermal free radicals, with control achieved principally by reaction temperature The 1 1 adduct is formed even in the presence of a large excess of olefin... 

Myristicin, CjjHjgOg, is 4-allyl-6-methoxy-l. 2-methylenedioxybenzene. It is found in oil of nutmeg and in parsley oil. It is a fragrant compound having the following characters —... 

By boiling with alcoholic potash for six to eight hours, the allyl group is changed into the propenyl group, and iso-dillapiol results. This body is a crystalline compound, melting at 44° and boiling at 296°. 

Reduction of vinylic and allylic compounds without hydrogenolysis may present a problem. The ratio of olefin saturation to hydrogenolysis depends importantly on catalyst, temperature, solvent, and pH. In both classes of compounds, hydrogenolysis is favored by polar solvents, acid, and elevated temperatures hydrogenation, by nonpolar solvents and low temperatures. 

A frequent problem is selective reduction of an acetylene to the olefin in the presence of other easily reducible functions. Usually the reaction can be done without difficulty because of the relatively strong and preferential adsorption of the acetylenic function on the catalyst. Functions adjacent to the triple bond may cause special problems if the resulting allylic compound is itself susceptible to facile hydrogenolysis (18,23). The product composition is, as expected, sensitive to steric effects (82). 

Benzyl-type carbanions and their metallo compounds, derived from aromatic or hetero-aromatic precursors, bearing carbon- or hetero-substituents, are readily available with variable substitution patterns due to their mesomeric stabilization (see Section 1.3.2.2)2. Even dicarbanions are accessible without difficulty3,4. The equilibrium acidities of many aromatic hydrocarbons have been determined5-7. The acidities of a-hetero-substituted toluenes8 are similar to those of the corresponding allylic compounds and can usually be generated by the same methods. 

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