Allylic derivatives conjugate substitution

Vinyl substitution occurs with conjugated dienes as well as with alkenes, employing aryl-, vinyl-, methyl-, alkoxycarbonyl- or benzyl-mercury reagents and lithium tetrachloropalladate(II), but the products are usually rr-allylpalladium complexes if the reactions are carried out under mild conditions (equation 8).24,25 The ir-allylic complexes may be decomposed thermally to substituted dienes26 or reacted with nucleophiles to form allylic derivatives of the nucleophile. Secondary amines, for example, react to give tertiary allylic amines in modest yields, along with dienes and reduced dienes (equation 9).25... 

In entry 28 is an example of Fe-H addition to conjugated dienes, while the Si-Fe bond remains intact. With isoprene, the initial product from 1,4-addition is a cr-allyl compound, which then rearranges with loss of CO to a 77--allyl derivative. This opens a useful route to other functionally substituted derivatives. 

Several pyranose derived dienes and pyranose derived conjugated enals were prepared and the stereochemical outcome of their cycloaddition reactions studied. The diene and the enal moiety were incorporated into the carbohydrate ring by different methods depending on the substitution pattern of the pyranose. The pyranosidic ring might act as a diene or dienophile in cycloaddition reactions, and as a 1-oxabutadiene system in hetero Diels-Alder reactions. The factors governing the stereoselectivities for each type of cycloaddition are rationalized. The role of die steieogenic allylic alkoxy substituents in the relative topicity of the reactions is analyzed. 

An interesting and useful property of enamines of 2-alkylcyclohexanones is the fact that there is a substantial preference for the less substituted isomer to be formed. This tendency is especially pronounced for enamines derived from cyclic secondaiy amines such as pyrrolidine. This preference can be traced to a strain effect called A or allylic strain (see Section 3.3). In order to accommodate conjugation between the nitrogen lone pair and the carbon-carbon double bond, the nitrogen substituent must be coplanar with the double bond. This creates a steric repulsion when the enamine bears a p substituent and leads to a... 

For acyclic allylic substrates die situation is mote complex, since a larger number of reactive conformations, and betice corcesponding transition states, compete. Hius, mediyl ciimamyl derivatives 163 tX= O.Acj, upon treatment witli litliiiim dimetliylcuprate, mainly gave tlie S 2 substitution product 166 fentry 1, Tab. 6.6 and Sdieme 6.34) [80]. Hie preference for die S 2 product is expected, since de-conjugation of die alkene system is electronically imfavorable. 

A new synthesis of substituted 1,3-dienes by reductive elimination of allylic nitro derivatives has been reported (Eq. 7.134).180 Tertiary allylic nitro compounds, bearing an acetate group in the (3-position, smoothly undergo reductive elimination to give conjugated 1,3-dienes when treated with chromous acetate and 2,2-dipyridine in DMF at 111-120 °C. 

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). 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

An additional interesting example is the conjugate addition of 1 to activated allylic acetates 18 under the chiral phase-transfer catalysis of 4d, and subsequent elimination reaction, as reported by Ramachandran and coworkers, as this enables the synthesis of various enantiomerically enriched glutamic add derivatives [39]. The utility of this process has been demonstrated by the transformation of (S)-19 (R = Ph) into 4-substituted pyroglutamate (2S,4S)-20, as illustrated in Scheme 2.17. 

Green has also used the method of H+ addition to a -CHO-substituted (allyl)metal complex for the preparation of (s-trans-diene) complex derivatives of a Group 8 metal. The Ru complex 35 was synthesized in that way.33 Stable [(s-tra ,s-r 4-conjugated diene) Ru(acac)2] isomers have been described34 and related [(s-tra s-r 4-diene)Ru(trispyrazolylbo-rate)Cl] systems are stable and isolable.35 Eventually, Mashima et al. have described the reaction of all-tra ,s-l,8-diphenyloctatetraene with [Ru(acac)3] under reducing conditions. A dimetallic complex (36) was isolated and characterized by X-ray diffraction, that contained a linear array of two (s-tra ,s-r 4-diene)Ru subunits36 (see Scheme 10). 

The change to substituted allyl ligands influences the 48 49 equilibrium in the corresponding hafnium systems.53 Silver et al. have used the specific deprotonation at related methylallyl derivatives to generate the conjugated diene ligand in the coordination sphere of the metal. 

---