Alkenes cyclic allylic acetates

Although the catalytic reactions described above involve mononuclear Rh and Rh complexes, dinuclear Rh compoimds have also been studied as catalyst precursors in oxygenation reactions. The system [Rh2(p.-OAc)4]/ f-BuOOH is effective in the oxidation of cyclic alkenes such as cyclopentene, cyclohexene and cycloheptene, mainly to o, /i-unsaturated ketones and allylic acetates, but with poor yields (Eq. 4) [30,31]. 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

The enantioselective synthesis of azabicyclic y-lactams starting from 2-azanorbornenones after treatment of a catalytic amount of RuCl2(PCy3)2 (= CHPh) in the presence of ethylene or allyl acetate proceeds also via ring rearrangement—alkene metathesis (ROM-CM-RCM) [41] (Scheme 19). If n = 0 or 3, no RCM occurs and a cyclic dialkenyl compound is formed by cascade ROM-CM reactions. 

Interception of the Tr-allyl palladium complex by soft nucleophiles, particularly malonates, has been described above. Alkenes, alkynes and carbon monoxide can also insert into the Tr-allyl palladium complex, generating a u-alkyl palladium species. When an internal alkene is involved, a useful cycbzation reaction takes place (sometimes called a palladium-ene reaction).Addition of palladium(O) to the allylic acetate 225 gave the cyclic product 226 (1.225). The reaction proceeds via the -ir-allyl palladium complex (formed with inversion of configuration), followed by insertion of the alkene cis- to the palladium and p-hydride elimination. In some cases it is possible to trap the a-alkyl palladium species with, for example, carbon monoxide. 

Allylic C-H Bond Activation. Internal alkenes, in particular cyclic ones, can be transformed into allylic acetates in a palladium-catalyzed oxidation (eq 6). With benzoquinone as stoichiometric oxidant or electron transfer mediator, the allylic acetoxy-lation proceeds with high selectivity for the allylic product and usually in excellent yield. 

The rearrangement is also useful for furan annulations, through enlargement of the starting carbocycle. Thus addition of SnCU to either diastereomer of the allylic acetal (4) produces the cis-fused cycloheptatetrahydrofuran (5) in 48-76% yield (eq 12). Acetals derived from frans-diols rearrange to the same cU-fused bi-cyclics in higher yield. The stereochemistry of a terminal alkene is transmitted to the C-3 carbon of the bicyclic products (eq 13). 

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

The stereochemical outcome of the cycloaddition to 3-butene-1,2-diol derivatives, cyclic acetals, or to related alkenes that possess an allylic nitrogen substituent such as 4-vinyl-oxazolines or -oxazolidines was also rationalized by this model (162) (Table 6.7). In the latter cases, the A-Boc group instead of the a-oxygen prefers the inside position (Scheme 6.24). 

Hwu et al. have examined the dependence of the metal oxidant on the mode of reactivity in silicon-controlled allylation of 1,3-dioxo compounds [95JOC856]. The use of manganese(III) acetate furnished the dihydrofuran product 22 only. On the other hand, use of cerium(IV) nitrate resulted in the formation of both acyclic (23) as well as the cyclized compound, with the product distribution dependent on the nature of the allylsilane. Facile synthesis of dihydrofurans by the cerium(IV) mediated oxidative addition of 1,3-dicarbonyl compounds to cyclic and acyclic alkenes has also been reported [95JCS(P1)187]. 

Two examples using azo compounds in allylic amination reactions will be presented in the following. Leblanc et al. have used the more reactive trichloro derivative 82 of DEAD, 81, and found that the ene reaction proceeds at various temperatures and without any Lewis acid catalyst present, for both cyclic and acyclic alkenes to give allyl amines in good yields. The reaction of the alkene 85 with 82 gave the allylic aminated compound 86 in 85 % yield (trans. cis = 85 15) (Eq. (21)) [53f. The allyl amine 87 was formed in good yield after treatment with a suspension of zinc powder in acetic acid solution. 

Acetyl nitrate (or nitryl acetate, Ac0NO2), prepared from nitric acid and acetic anhydride, reacts with simple acyclic and cyclic alkenes to give complex mixtures of nitro acetates, nitro nitrates and nitroalkenes, which are often difficult to separate12 103, The reaction with unsubstituted cycloalkenes was recently reexamined and a complex mixture of products, including allylic and homoallylic nitroalkenes, 1,2-, 1,3- and 1,4-nitro acetates, and 1,4-nitro nitrates, was identified104. These experimental observations are best accommodated by the proposal, supported by theoretical calculations, that the initial reaction of acetyl nitrate with alkene substrates is a [2 + 2] addition of the nitryl cation to the C-C double bond to form a cyclic cationic intermediate. 

Lead acetate azides, Pb(OAc)4 (N3)n, prepared in situ from lead tetraacetate and azidotrimethylsilane, react with alkenes to yield a variety of products, depending on the structure of the alkene 1,2-diazides. 1,2-acetoxy azides, a-azido ketones, allylic azides, and <5-oxo nitriles (by the oxidative cleavage of cyclohexene rings)97. The diazides and acetoxy azides are formed by preferential syn addition, but the diastereoselectivity (up to 3 1) is far from satisfactory with both acyclic and cyclic alkenes98,99. 

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