Grignard reagents regioselectivity

In a simplified picture, the mechanism of the Zr-catalyzed ethylmagnesation can be rationalized as shown in Scheme 1 [8]. At first, the zirconocene-ethene complex 12 is generated from the catalyst precursor Cp2ZrCl2. Complex 12 can also be regarded as a metallacyclopropane 16. After coordination and insertion of the alkene 10, a metalla-cyclopentane 13 is formed, which subsequently reacts with the Grignard reagent regioselectively to the open-chain intermediate... 

The reaction of Grignard reagents with epoxides is regioselective m the same sense Attack occurs at the less substituted carbon of the ring... 

When 4-substituted 2,2-dimethyl- 1,3-dioxolanes react with Grignard reagents, the bond that is broken is the one at the oxygen attached to the less-substituted a-carbon. What factor(s) are likely the cause for this regioselectivity ... 

Stereo- and regioselective synthesis of trienes and tetraenes has been reported by palladium-catalysed coupling of (E)- or (Z)-l-alkenyl boronates with (E)- or (Z)-2-bromo-1-phenylthio-l-alkenes followed by treatment with a Grignard reagent in the presence of a nickel catalyst (equation 146)259. 

The electrochemical oxidation of l-MeO-7-alkyl-CHT in MeOH yields 2-alkyltropones, while the thermal rearrangement of 3-MeO-7-alkyl-CHT to l-MeO-4-alkyl-CHT followed by its anodic oxidation in MeOH affords 4-alkyltropones (equation 21). l-MeO-7-alkyl-CHT is prepared by the regioselective alkylation of 7,7-diMeO-CHT with a Grignard reagent and Cul, while 3-MeO-7-alkyl-CHT is also regioselectively prepared by alkylation of 7,7-diMeO-CHT with an alkyl lithium. 

In the presence of 5 mol% CuBr SMe2 and 6mol% 17b, acyclic aliphatic enones can also react with Grignard reagents to provide / -substituted linear ketones in high yields (62-91%), good regioselectivities (>94 6), and excellent enantioselectivities (90-98%) (Scheme 10).24... 

The 1,5-substitution of l-chloro-2-en-4-ynes with Grignard reagents has been described by Dulcere and co-workers [41] but lacks generality with regard to the nucleophile (see Section 2.3). In contrast, the regioselective reaction of enyne acetates 47 with various lithium cuprates proceeds smoothly in diethyl ether, furnishing exclusively vinylallenes 48 with variable substituent patterns (Scheme 2.17) [42],... 

Ethynyl-/3-propiolactone (75) reacts regioselectively with Grignard reagents in the presence of the Cul catalyst to afford 3,4-alkadienoic acids 76 in high yields (Scheme 3.38) [84],... 

In the early 1980s, one of the first preparations of substituted allenes was reported, which employed a palladium-catalyzed cross-coupling reaction of allenyl halides [9]. In this study, allenyl bromides 13 and various Grignard reagents 14 were coupled in the presence of catalytic amounts of a Pd(0) species, generated in situ by reduction of a Pd(II) salt. Trisubstituted allenes 15 were obtained with high regioselectivity (allene 15 alkyne 16 = 90 10 to 99 1) (Scheme 14.5). 

Initial studies on the application of these catalysts to allylic substitution reactions showed that the arenethiolate moiety functions as an excellent nontransferable group, and that the regioselectivity can be completely reversed by suitable changes in the reaction parameters [33]. If the reaction between geranyl acetate and n-BuM gl was carried out inTHFat—30°C with fast addition of the Grignard reagent to the reaction mixture, complete a selectivity was obtained. Raising the tempera-... 

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