Allylic regiochemistry

The difficulty in controlling the regiochemistry during radical-denitradon of allylic nitre compounds is well known The migradon of the double bond is a serious problem, as shown in Eq 7 96 This problem is overcome by ahydnde transfer reacdon in the presence of a palladiuir catalyst fEq 7 97 ... 

For unsymmetrical allylic systems both the regiochemistry and stereochemistry of the substitution are critical issues. The palladium normally bonds anti to the acetate leaving group. The same products are obtained from 2-acetoxy-4-phenyl-3-butene and 1 -acetoxy-l-phenyl-2-butene, indicating a common intermediate. The same product mixture is also obtained from the Z-reactants, indicating rapid ,Z-equilibration in the allylpalladium intermediate.118... 

Allylic silanes show a pronounced tendency to react at the a-carbon in the presence of bidentate ligands.148 This regiochemistry is attributed to the preferential stabilization of cationic character by the silyl substituent. The bidentate ligands enhance the electrophilic character of the TS, and the cation stabilization of the silyl group becomes the controlling factor. 

The acyclic version of Larock s heteroannulation was successfully applied to the synthesis of highly substituted pyridines [166]. The annulation of rert-butylimine 210 with phenyl propargyl alcohol produced pyridine 211 regioselectively in excellent yield. The regiochemistry obtained was governed by steric effects. Furthermore, the choice of imines was crucial to the success of the heteroannulations. terr-Butylimine was the substrate of choice, since all other imines including methyl, isopropyl, allyl and benzyl imines failed completely to produce the desired heterocyclic products. 

Syntheses of primary ally lie amines have been reviewed183. The regiochemistry of the reaction of iron carbonyl complexes with nucleophiles such as morpholine has been investigated. The (r 3-crolyl) Fe+(CO)4 BF4- complexes 172 (R1 = H R2 = Me or R1 = Me R2 = H) undergo preferential attack at the less substituted allyl terminus to yield allylic amines 173. The (/j2-crotyl acetate) Fe(CO)4 complex 174, on the other hand, does not react with morpholine184. 

In 1999, Clennan and Sram reported a study of the photo-oxidations of a series of tetrasubstituted alkenes (Fig. 5) in methylene blue-doped zeolite Y [11], The ene regiochemistries are very sensitive to the size of the allylic substituent, R, in solution. The A/B ratio increases from 0.49 to 2.4 as the substituent, R, is changed from methyl to ferr-butyl. This phenomenon has been attributed [12] to a sterically induced lengthening of the carbon-2 oxygen bond in the perepoxide intermediate I and subsequent preferred opening of this long bond (Fig. 5). 

The regiochemistry of the reaction of unsymmetrical allylic esters was addressed by the introduction of a silyl group, equivalent to H, on the carbon of the allylic terminus. Eor example, 25 bearing a BuMe2Si group afforded 26 as the sole product in the three-component coupling. Since the subsequent protodesilylation of 26 selectively formed 27 (Scheme 6.6), this sequential procedure provides a practical method to control the regiochemistry in the substitution of unsymmetrical allylic substrates [13 c]. 

Cycloaddition of CO2 with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiochemistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /J-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

A silocyclopropane reacts with an allylitr chloride in the presence of AgF in aqueous ethanol to give an allylated product Eq. (55) [55]. In this case, the structure of the product does not reflect the regiochemistry of the nucleophilic attack on the starting allyl halide. 

As expected, allylic alcohols and ethers afford v/c-diol derivatives upon hydroboration. However, in the presence of Wilkinson s catalyst, a reverse regiochemistry for the hydroboration is observed [108]. There is the possibility that boron, rather than hydrogen, is transferred from the metal to the olefin. In other words, the boron acts as a donor. 

The regiochemistry of the addition of phenylselenenyl chloride to allylic alcohols and their esters [109] can simply be rationalized by using the polarity alternation rule except in those cases where steric factors become the controlling parameters. 

The strong directive effect of an allylic silicon in a diene on the regiochemistry of the Diels-Alder reaction proved to be a blessing in synthesis of 11-deoxycarminomycinone [383],... 

This was a detailed study of the regiochemistry of Se02 allylic oxidation. 

The chiral ir-allyl-Pd(II) intermediates shown in Scheme 84 undergo epimerization. The efficiency of this step and the regiochemistry of the nucleophilic attack to the exo face are very important for obtaining enantioface selection (Scheme 89). Bosnich analyzed the general characteristics of the asymmetric alkylation in terms of the properties of the allylic acetate substrates and of the 7T-allyl-Pd(II) intermediates, which undergo facile o-tc-o rearrangement, readily switching the face of Pd coordination (208). Examination of the dynamic equilibria of a series of cationic ir-allyl-Pd-chiral phosphine complexes has indicated that the 7r-allyl intermediates epimerize 10-100 times faster than the nucleo-... 

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