Allylic trans effect

Chiral Lewis-basic catalysts (Figs. 7.1 and 7.2), in particular phosphoramides 8-12 [9, 14c, 15c, 22-24], formamide 13 [17], pyridine N,N -bisoxides 17 and 18 [25-27], N-monoxides (19-26) [27-32], and N,N N"-trisoxides (27) [33] exhibit good to high enantioselectivities for the allylation of aromatic, heteroaromatic, and cinnamyl-type aldehydes (1) with allyl, trans- and ds-crotyl, and prenyl trichlorosilanes (2a-d). Chiral formamides (with the exception of 13, as discussed below) [17], pyridine-oxazolines [34], urea derivatives [19] and sulfoxides [35] are effective only in stoichiometric quantities (or in excess) and, as a rule, exhibit lower enantioselectivities. 

Pt) to its coordination environment established that the acetamide was O bonded rather than N bonded. These results, which are supported by nmr measurements, could have implications to the mode of action of Pt(II) anticancer drugs. Also relevant to this field, the reaction (first order in each reagent) of allyl alcohol with fra s-[PtCl2(NH3)2] proceeds much more quickly than with the cis isomer, predicted from relative trans effects. This has been developed into a simple spectrophotometric method for estimating the amount of each isomer in solution. 

Simpler chiral pyrrolidine thioethers, reported in 2004 by Skarzewski et al., proved to be effective ligands in the test reaction. The sense of the stereoinduction was in agreement with the nucleophilic attack directed at the allylic carbon located trans to the sulfur atom in the intermediate complex (Scheme 1.40). 

A few examples will illustrate the case. The parent trans-diene derivatives 31a and 3235 have nearly planar chromophores, but the Cotton effects are quite strong and opposite in sign (+15 and —27.9, respectively). This can be attributed mainly to the allylic axial C—CH3 bonds, which provide a positive contribution for compounds 31 and a negative for 32. Furthermore, the As values of P-chiral s-trans-31 are strongly dependent on the polarizability of the allylic C—X bond. 

Although the steric environment around each terminus of the allyl unit is virtually identical, Faller showed that addition of a soft nucleophile occurs exclusively trans to the carbonyl ligand. It was clear that electronic rather than steric factors were responsible for this remarkable selectivity, and, even though the precise origin of the effect was not immediately understood, the principle of electronic differentiation had been experimentally demonstrated.1221... 

Wittig reactions have been studied and extended by sonochemists. Starting from allylic phosphoranes, it was shown that the initial deprotonation step could be effected with an increased efficiency using butyl lithium in TH F or even in benzene (Scheme 3.23) [122]. The insoluble phosphorane disappears after 5-15 min sonication. This superposition of the two essential roles of sonication is reflected by the change in the stereoselectivity, larger proportions of the trans-diene being formed under irradiation. 

The oxygen rebound mechanism was supported by experimental evidence including (1) high kinetic isotope effects, (2) partial positional or stereochemical scrambling, and (3) allylic rearrangements. For instance, in the presence of [Fe(TPP)Cl] and PhIO, dx-stilbene was stereospecihcally epoxidized. In addition, it was found that cis-stilbene was 15 times more reactive than trans-stilbene in competitive epoxidations. (see Figure 7.20). " ... 

The effect of the BF3 activation on the regioselectivity of the ring opening of vinylic and acetylenic oxiranes is dramatic, as in these conditions the reaction occurs exclusively on the allylic " or the propargylic " position, and still with inversion (Scheme 40). This excellent regioselectivity allows the direct stereospecific preparation of homoallylic and homopropargylic alcohols in excellent yields. In the case of acetylenic oxiranes, a remarkable difference in the reactivity of cis and trans oxiranes has been evidenced, the former being more reactive. 

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