Allylation reaction stereochemical outcome

As the results show, the chirality of the a-alkoxy center, as well as the type of allyl metal employed, are the two most important determinants for the stereochemical outcome of the reaction. In other words, the 1,2-asymmetric induction combined with the right choice of the allyl organometallic overrides the influence of the chiral nitrogen substituent. 

Therefore, in order to obtain a 1,4-addition of an allyl residue to an enone, two activation modes can be used reactions take place either under electrophilic conditions with Lewis acid promotion, or in the presence of fluoride ions. This is important as the stereochemical outcome often depends on the activation mode selected. 

The stereochemical outcome of these reactions can be rationalized as arising from attack on a ground-state conformation in which the sulfoxide lone pair and the alkene double bond are syn coplanar2. Such a conformation would minimize 1,3-allylic strain. 

Intramolecular 1,3-cycloadditions of nitrile oxides (INOC) provide a useful tool for the construction of fused cyclic ring systems. The stereochemical outcome of this reaction is presumed to be a consequence of reaction through the transition state that minimizes allylic 1,3 strain (Scheme 8.19).103... 

Although palladium catalysts have played the most prominent role in this area, other metals have also been found to catalyze allylic etherification reactions, often providing complementary stereochemical outcomes. A few ruthenium catalyst systems have been used for the O-allylation of phenols,143,144 including an enantioselective version utilizing [Cp Ru(MeCN)3]PF6 that provides promising ee s, albeit with diminished control of regioselectivity (Equation (25)).145... 

In the reaction of (R,R)-tartrate allyl-boronate with aldehydes, Si attack of the nucleophile on the carbonyl group has been observed, while Re attack occurs in (S, S )-tartrate allyl-boronate reactions. Thus, an (S )-alcohol is produced preferentially when an (R,R)-allyl reagent is used, and the (R)-product can be obtained from an (S.Sj-reagent. assuming that the R substituent in the aldehyde substrate takes priority over the allyl group to be transferred. In fact, no exceptions to this generalization have yet been found in over 40 well-characterized cases where the tartrate auxiliary controls the stereochemical outcome of the allyl or crotyl transfer.72... 

In the carbohydrate chemistry arena, the Tsuji-Trost reaction has been applied to construct N-glycosidic bonds [53]. In the presence of Pd2(dba>3, the reaction of 2,3-unsaturated hexopyranoside 68 and imidazole afforded N-glycopyranoside 69 regiospecifically at the anomeric center with retention of configuration. In terms of the stereochemistry, the oxidative addition of allylic substrate 68 to Pd(0) formed the jc-allyl complex with inversion of configuration, then nucleophilic attack by imidazole proceeded with another inversion of the configuration. Therefore, the overall stereochemical outcome is retention of configuration. 

Obviously, the nature of the organocopper reagent is an important factor with respect to the stereochemical outcome of the cuprate addition. This is nicely illustrated for the cuprate addition reaction of enoate 75 (Scheme 6.15). Here, lithium di-n-butylcuprate reacted as expected by way of the modified Felkin-Anh transition state 77 (compare also 52), which minimizes allylic A strain, to give the anti adduct 76 with excellent diastereoselectivity [30]. Conversely, the bulkier lithium bis-(methylallyl)cuprate preferentially yielded the syn diastereomer 78 [30, 31]. It can be argued that the bulkier cuprate reagent experiences pronounced repulsive interactions when approaching the enoate system past the alkyl side chain, as shown in transition state 77. Instead, preference is given to transition state 79, in which repulsive interactions to the nucleophile trajectory are minimized. 

For the first time, DFT theory has been applied to a study of diastereofacial selectivity in 1,3-dipolar cycloadditions of nitrones to di-3,4-dimethylcyclobutene. ° The stereochemical outcome of the INAC reactions of nitrones (49) and (52) derived from 3-(9-allyl-D-hexoses is dependent only on the relative configuration at C(2,3), and thus 3-(9-allyl-D-glucose (48) and -o-altrose (threo configuration) afford oxepanes (50) selectively whereas 3-O-allyl-D-allose (51) and -D-mannose (erythro configuration) give tetrahydrofurans (53) and (54) selectively (Scheme 18). frani-2-Methylene-... 

Intramolecular Sakurai reaction. Allylic and propargylic silanes can undergo a Lewis acid catalyzed intramolecular Sakurai reaction.1 In cyclization to hydrin-danones, the stereochemical outcome can differ from that obtained by fluoride ion catalysis (presumably kinetically controlled cyclization), equation (I).2... 

An interesting use of the nickel-catalyzed allylic alkylation has prochiral allylic ketals as substrate (Scheme 8E.47) [206]. In contrast to the previous kinetic-resolution process, the enantioselectivity achieved in the ionization step is directly reflected in the stereochemical outcome of the reaction. Thus, the commonly observed variation of the enantioselectivity with respect to the structure of the nucleophile is avoided in this type of reaction. Depending on the method of isolation, the regio- and enantioselective substitution gives an asymmetric Michael adduct or an enol ether in quite good enantioselectivity to provide further synthetic flexibility. 

Comparison of the stereochemical outcomes in entries 2 and 3 of Table 1 implies that the heteroatom functionality directs the oxidation reaction more effectively from pseu-doequatorial orientation (the O—C—C=C dihedral angle is 140°). Selective formation of yyw-epoxides is also known for olefins having carbamate18, acetal19, ether20 and halogen groups21 in allylic positions. There are many cases in the literature where the epoxidation... 

The stereochemical outcome in the elimination reaction of acyclic ( >allylic acetates (7) to the corresponding dienes by the use of [Pd(dppe)2] as a catalyst in the presence of DBU has been elucidated by 2-dimina-tioi1 -30 The unprecedented Z-preference has been rationalized by the syn -effect in the transition state of deprotonation, which arose from a a n interaction. 

Recently, highly functionalized allyl boronates such as 150 were synthesized from allylic acetates 148 and pinacolatodiboron 149. These boronates were employed for the allylation of aldehydes, furnishing the borate esters 151 which undergo lactonization to furnish the a-methylene-7-butyrolactones in high yield and selectivity. The strength of Lewis acids as well as the substituents on the aromatic aldehydes have a profound influence on the stereochemical outcome of the reaction (Scheme 25) <20040L481>. 

---