Solvent effects allylation

They demonstrated that the C2-symmetric bis-benzothiazine (R,R)- 91 was an effective ligand in the asymmetric allylic alkylation reaction. The best result in this case was the reaction of 198 and 199 in the presence of BSA, Pd2(dba)3 and (/ ,/ )-197, which gave the product (S)-200 in 75% yield and 86% ee. More experimental data revealed that solvent effects are very important in this reaction (Scheme 57). Relatively nonpolar solvents resulted in good yields and enantiomeric excesses while reaction in CH3CN and CH2CI2 gave only racemic products in moderate yields (Table 8). 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

It must be stressed, however, that the methyl alkyl sulfoxide rule is not valid for alkyl benzyl and alkyl allyl sulfoxides (222-224), where the electronic, steric, and solvent effects exert influences on the chiroptical phenomena in a way that is difficult to rationalize. This rule was found to be satisfactory and was used for the assignment of absolute configurations of steroidal (200,201,225), penicillin (226), and amino acid (227-230) sulfoxides. 

The Bayhs-Hillman reaction of optically pure azetidine-2,3-diones [235, 236] with methyl vinyl ketone in the presence of l,4-diazabicyclo[2.2.2]octane (DABCO) in acetonitrile at -20°C for 1 h have been reported to give functionalized allylic alcohols, having the p-lactam scaffold, in good yields (80%) and complete diastereoselectivity [237]. In terms of achieving good yields with a reasonable rate of reaction, 50 mol% of DABCO seemed to be the catalyst amount of choice for this reaction. No significant solvent effect was observed in the overall yield (Scheme 108). 

Alberti, M.N. and Orfanopoulos, M. (2006) Stereoelectronic and solvent effects on the allylic oxyfunctionalization of alkenes with singlet oxygen. Tetrahedron, 62 (46), 10660-10675. 

Over the years several different mechanisms for the singlet oxygen ene reaction have been considered as depicted in Sch. 2. Biradical (mechanism B in Sch. 2) [10,11] and zwitterionic (mechanism C in Sch. 2) mechanisms have been considered as very unlikely for a variety of reasons including the facts that Markovnikov directing effects are not observed [4], radical scavengers have no influence on the reaction, only minor solvent effects are observed [12-14], and cis-trans isomerizations of the substrates do not occur [6], A dioxetane intermediate (mechanism D in Sch. 2) was considered very early [6] but this suggestion was dismissed when isolated dioxetanes were shown to cleave to the carbonyl compounds rather than rearrange to the allylic... 

Dirhodium(ll) tetrakis[methyl 2-pyrrolidone-5(R)-oarboxylate], Rh2(5R-MEPV)4, and its enantiomer, Rh2(5S-MEPY)4, which is prepared by the same procedure, are highly enantioselective catalysts for intramolecular cyclopropanation of allylic diazoacetates (65->94% ee) and homoallylic diazoacetates (71-90% ee),7 8 intermolecular carbon-hydrogen insertion reactions of 2-alkoxyethyl diazoacetates (57-91% ee)9 and N-alkyl-N-(tert-butyl)diazoacetamides (58-73% ee),10 Intermolecular cyclopropenation ot alkynes with ethyl diazoacetate (54-69% ee) or menthyl diazoacetates (77-98% diastereomeric excess, de),11 and intermolecular cyclopropanation of alkenes with menthyl diazoacetate (60-91% de for the cis isomer, 47-65% de for the trans isomer).12 Their use in <1.0 mol % in dichloromethane solvent effects complete reaction of the diazo ester and provides the carbenoid product in 43-88% yield. The same general method used for the preparation of Rh2(5R-MEPY)4 was employed for the synthesis of their isopropyl7 and neopentyl9 ester analogs. 

A study of the solvent effects on the rate of thermal racemization of chiral allyl sulfoxides has revealed that polar solvents significantly decelerate the racemization [108], The reaction proceeds by way of a reversible and concerted rearrangement achiral allyl sulfenates are formed as intermediates and an intramolecular a, 7-shift of the allyl group between the sulfoxide oxygen and sulfur termini occurs as shown in Eq. (5-38). 

From the first-order rate constants obtained in different solvents (in sealed ampoules), it is apparent that this isomerization is not very sensitive to the polarity of the medium, in accordance with an isopolar, six-membered activated complex [156]. A similar small solvent effect has been observed for the [3,3]sigmatropic rearrangement of allyl S-methyl xanthate to allyl methyl dithiol carbonate [559]. 

The solvent effect is high in the case of C-allylation to ortho-carborane cluster. In non-polar solvents the C-allylation is accelerated, while in polar solvents the C-allylation is decelerated. 

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