Sulfoxides, allyl metallation

Addition of Metalated Allylic Phosphine Oxides, Phosphonates, Sulfones, and Sulfoxides and Sulfoximines to a,/i-l nsaturated Carbonyl Compounds... 

That sulfides coordinate to palladium(II) more strongly than the corresponding sulfoxides has received an interesting confirmation in a study of the complexes (19)-(21) (R = 4-MeQjHt).149 The complex (19) is the best catalyst both for cyclotrimerization of diphenylethyne and for isomerization of allyl ethanoates. In both processes a vacant metal coordination site is essential and the thioether sulfur is too strongly bound (in 20 and 21) for facile dissociation. 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

S. G. Pyne, Addition of Metalated Allylic Phosphine Oxides, Phosphonates, Sulfones, Sulfoxides and Sulfoximines to a,/3-Unsaturated Carbonyl Compounds , in Stereoselective Synthesis (Houben-Weyl) 4th ed. 1996, (G. Helmchen, R. W. Hoffmann, J. Mulzer, E. Schaumann, Eds.), 1996, Vol. E21 (Workbench Edition), 4, 2068-2086, Georg Thieme Verlag, Stuttgart. 

Another condition which is essential in enhancing the reactivity of 77-allyl compounds towards substitution is the easiness of the 7r,<7-rearrange-ment for the allyl ligand (135, 140-146). When the TT-allyl ligand is converted entirely to its u-bonded state there occurs a consequential increase of the metal s coordinative unsaturation. Hence dimethyl sulfoxide readily substitutes all the ligands in the electronically and coordinatively saturated complex (7r-C3H5)Fe(CO)jI (147). 

Metallations of allyl sulfoxides have been achieved at -78 C with n-butyllithium in THP or more efficiently with LDA under similar conditions. - - Alkylations with primary alkyl iodides - - - and allylations with allyl - or phenyl bromides proceed in good yield, usually below -20 C and in less than 6 h. The reaction takes place predominantly at the a-site but there is often competition from y-alkylation leading to a-alkenyl sulfoxides (Scheme 98 Scheme 99). Changing the solvent, or introducing metal ion complexing agents, such as TMEDA were unsuccessful in changing the a y ratios observed in THF. 

Alkylations of 1-alkenyl aiyl sulfoxides have attracted little attention compared to other cases. - However it has been found that a-metallation occurs on reaction with LDA and that alkylation, allylation and benzylation occur there as a result (Scheme 101). Interestingly, -y-alkylation resulting from a competitive y-metallation reaction has not been observed. The last mentioned reaction, coupled with treatment... 

Ion-pair effects, as in the case of the unsubstituted allyl alkali metal compounds 23a-d, do not markedly influence the rotational barriers of the 1,3-diphenylallyl-lithium species 27a-f, although the reason is different 27a-f are solvent-separated ion pairs 36). Addition of HMPT to the THF solution of 27a raises the AG3 c value by 0.9 kcal/mol which corresponds to a rate retardation of 5-6 times. In the case of the methyl-substituted allyl anion 27b HMPT slows down the rate by a factor of only 2-3. With the 2-cyano anion 27c the AG values of the Li+ compound in THF and the Li +, Na+ andK+ species in dimethyl sulfoxide (DMSO) are the same18). In the case of 27e and 27f the rate of the rearrangement is not affected if HMPT or TMEDA are added to the THF solutions. The observations of some solvent dependence in the case of the sterically less hindered 27a and b, but of no effect with the more crowded 27e and f are in line with the general observation that solvent-separated ion pairs are favored with respect to contact ions pairs by increasing steric hindrance 25). Hence, these experimental results could be interpreted to mean that in the case of 27a and b contact ion pairs participate in the allyl anion rearrangement reaction. 

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