Allylic carbon product mixtures

This reaction showed a remarkable selectivity with respect to the solvent and base used. In THF and EtOAc using TEA as the base, a 1 1 mixture of the allylic carbonate and bisacylated products is obtained, but when CH2CI2 is used as solvent, the reaction favors the allylic alcohol by a factor of 97 3 (mono/bis). In THF or MTBE, the use of TMEDA as the base also results in a 97 3 mono/bis ratio. ... 

When the allylic cation reacts with Br to complete the electrophilic addition, reaction can occur either at Cl or at C3 because both carbons share the positive charge (Figure 14.4). Thus, a mixture of 1,2- and 1,4-addition products results. (Recall that a similar product mixture was seen for NBS bromination of alkenes in Section 10.4, a reaction that proceeds through an allylic radical.)... 

A combination of a Tsuji-Trost and a Michael addition was used for the synthesis of (+)-dihydroerythramine 6/1-269, as reported by Desmaele and coworkers [128]. The Pd-catalyzed reaction of the allylic acetate 6/1-267 with the nitromethylarene 6/1-266 in the presence of Cs2C03 as base led to the domino product 6/1-268 as a 4 1 mixture of two diastereomers in 79% yield. Further manipulation of 6/l-268a yielded the desired dihydroerythramine 6/1-269 (Scheme 6/1.70). Interestingly, using the corresponding allylic carbonate without additional base gave the mono-alkylated product only. 

A first example of a combination of a Rh-catalyzed allylic substitution and a Pau-son-Khand annulation reaction has also been developed by the same group [222]. Thus, [RhCl(CO)dppp]2 is able to catalyze both transformations at different reaction temperatures. Treatment of the allylic carbonate 6-106 with the alkyne derivative 6-107 led to a diastereomeric mixture of 6-108 and 6-109 in 63-84% yield, with 6-108 as the main product (Scheme 6/2.23). 

Similarly, 73-allylpalladium complexes were also employed as promoters for the cyclization.39,39a,39b The palla-dium(0)-catalyzed reaction of alkyne 168 with allylic carbonate in THF at 60 °G gave the A-allyl product 170 instead of the desired product 169. However, without isolation of 170, the reaction mixture was heated at 80 °G with 5 equiv. of K2CO3, providing 169 in 77% yield (Equation (7)). The nitrogen atom intervenes in the process as a nucleophile in the A-allylation step and as a leaving group in the cyclization step. 

Mechanistic studies showed that metalacycle la is competent to be a catalyst in asymmetric allylic substitution reactions. The reaction of benzylamine with methyl ciimamyl carbonate catalyzed by a mixture of LI and [Ir(COD)Cl]2 occurs with an induction period and forms product in 84% yield and 95% ee, whereas the same reaction catalyzed by a mixture of metalacycle la and [Ir(COD)Cl]2 occurs without an induction period in just 2 hours to form the substitution product in 81% yield and 97% ee. The latter reaction was conducted with added [Ir(COD)Cl]2 to trap the -bound LI after dissociation. This ligand must dissociate to provide a site for oxidative addition of the allylic carbonate. 

Oxidation of the allylic carbon of alkenes may lead to allylic alcohols and derivatives or a, 3-unsaturated carbonyl compounds. Selenium dioxide is the reagent of choice to carry out the former transformation. In the latter process, which is more difficult to accomplish, Cr(VI) compounds are usually applied. In certain cases, mixture of products of both types of oxidation, as well as isomeric compounds resulting from allylic rearrangement, may be formed. Oxidation of 2-alkenes to the corresponding cc,p-unsaturated carboxylic acids, particularly the oxidation of propylene to acrolein and acrylic acid, as well as ammoxidation to acrylonitrile, has commercial importance (see Sections 9.5.2 and 9.5.3). 

Extension of the above oxidation studies to alkenes such as limonene gave a complex mixture of products that resulted from all possible ene reactions to the trisubstituted double bond (Fig. 30) [165], However, use of NaY zeolite as the microreactor and in the presence of a small amount of pyridine, the photosensitized oxidation of the alkenes is regioselective, yielding only the cis and trans products that result from hydrogen abstraction from the least hindered allylic carbon center. These studies illustrated that a microreactor can provide unprecedented opportunities to conduct selective oxidation of olefins. 

Morken and Lavastre used the formation of a colored side product to identify catalysts for the allylation of /i-dicarbonyl compounds [8]. The researchers employed 1-naphthyl allyl carbonate 5 as an allyl source and the diazonium salt of fast red as an indicator. Formation of the active 7z>allyl complex furnishes C02 and 1-naphthoxide which deprotonates the 1,3-dicarbonyl compounds which can, in turn, react with the 71-allyl metal complex. 1-Naphthol is the only species in the reaction mixture that can react with the diazonium salt 6 to generate the bright red azo dye fast red. Thus the red color is indicative of successful formation of the active re-allyl complex (Figure 5.4.3). 

The reaction of benzaldehyde with unsymmetrical allylic bromides in the ionic liquid bmim also proceeds regioselectively to realize a carbon-carbon bond formation at the more substituted allylic carbon (Scheme 5). In the coupling of crotyl bromide with benzaldehyde, the product is a nearly 50 50 mixture of antilsyn diastereomers. Cinnamyl bromide predominantly gives the //-diastereomer. The regio- and diastereoselectivity is similar to that observed for these in aqueous media.104... 

Halogenation at an allylic carbon often results in a mixture of products. For example, bromination of 1-butene under radical conditions forms a mixture of 3-bromo-1 -butene and 1-bromo-2-butene. 

The table, which collects representative examples, shows that monosubstituted epoxides afford homoallylic alcohols resulting from the attack to the less substituted carbon atom (runs 1, 5 and 7). Homoallylic alcohols are useful intermediates in several important total synthesis." Disubstituted epoxides fail to react (run 4). Styrene oxide leads to a mixture of homoallylic alcohols (run 2) and allylic epoxides give mixture of 1,2- and 1,4-opening product, with predominance of the 1,4 product (run 3, 6 and 8). 

In comparison, the iodolactonization of 49 proceeds with very high diastereos-electivity and a 95 5 mixture of the products 50 and 51 is obtained [27, 28]. The diastereoselectivity is tightly controlled by A(1,3) strain between the methyl group on the terminal olefinic carbon and the other methyl group on the allylic carbon so much so that it allows the reaction to proceed primarily through the transition state 50a. 

An admixture of the unsaturated ester 59 with the K-enolate 60 generates yet another K-enolate, which could exist as a mixture of the conformers 61a, 61b and 61c. On account of A(1,3) interaction between either of the substituents, except hydrogen, on the allylic carbon and the solvated OK (or even OMe if one considers the other geometry of the enolate), the conformer 61c will be considered to be the most stable. In 61c, the ctc h at the allylic carbon is nearly cisoid to the double bond of the enolate to avoid A(1,3) strain. The electrophile, Mel in the present instance, approaches the double bond from anti to the large CH2C02Bul substituent and the predominant product 62 is formed [32]. 

Allyl-2-ethylhexanal 475 A solution of 1-(2-ethyl-l-hexenyl)pyrrolidine (13.95 g, 0.08 mole) in acetonitrile (30 ml) is treated, slowly and with stirring, with an equimolar amount (9.3 g, 0.08 mole) of allyl bromide. The mixture is then boiled under reflux for 1 h and set aside at room temperature for 12 h. Most of the solvent is removed and the residue is hydrolysed with cold dilute hydrochloric acid (25 ml). The product is then removed in ether, washed with sodium hydrogen carbonate solution, dried, and distilled, giving 2-allyl-2-ethylhexanal (9.75 g, 75 %), b.p. 83-85°/10 mm. 

Furthermore, allyl carbonate [10] was reacted with l,2,3,4-tetra-C)-acetyl-a-D-glucopyranose and 2,3,4,6-tetra-C>-benzyl-a-D-glucopyranose. By palladium(O)-catalysis, the products [31] and [32] are obtained as mixtures of regioisomers. In both reactions, predominantly the isomer bound to C-11 of the earbohydrate is formed (Eq. 16). The ratio of diastereomers could not be determined exactly because... 

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