Product stereoconvergence

BINAP-AgF gives good enantioselectivity, especially for the major anti product in the addition of 2-butenylstannanes to benzaldehyde.188 This system appears to be stereoconvergent, suggesting that isomerization of the 2-butenyl system occurs, perhaps by transmetallation. 

Stereoconvergence owing to reactant or product equilibration We also saw several cases where the product composition was the same for stereoisomeric reactants, e.g., for E- and Z-allylic reactants. This can occur if there is an intermediate step in the mechanism that permits E- and Z-equilibration or if the final stereoisomeric product can attain equilibrium. 

These palladium- or nickel-catalyzed reactions are radical reactions leading to an organometallic product. By using a precursor such as 37 as a 1 1 mixture of diastereoisomers, the palladium-catalyzed cyclization provides in a stereoconvergent way the cyclopentylmethylzinc derivative 38 which, after allylation, produces the unsaturated ester 39 in 71% yield". The intermediate radical cyclizes via a transition state A where all the substituents are in an equatorial position. Interestingly, the analogous reaction using Ni(acac)2 as a catalyst allows the preparation of heterocyclic compounds such as 40. The... 

Many such examples are known. In most cases where the stereochemistry has been investigated, retention of configuration is observed,225 but stereoconvergence (the same product mixture from an E or Z substrate) has also been observed,226 especially where the carbanionic carbon bears two electron-withdrawing groups. It is not immediately apparent why the tetrahedral mechanism should lead to retention, but this behavior has been ascribed, on the basis of molecular orbital calculations, to hyperconjugation involving the carbanionic electron pair and the substituents on the adjacent carbon.227... 

Reactions which are apparently stereospecific occur in the nucleophilic displacement of vinylic iodide [31] in the electron-deficient alkenes E- and Z-24 shown in Scheme 9.14. With ethanolic toluenethiolate, the sole detectable product from the reaction of -24 is -25. However, -25 is also the sole detectable product from the reaction of Z-24. This stereoconvergence demands that the stereoisomers react through a common intermediate, and it was reasonably suggested that initial nucleophilic addition of the thiolate anion yields a resonance-stabilised carbanion (26) whose stereoisomerisation, again by rotation about a carbon-carbon single bond, is much faster than the loss of iodide to yield the substitution product ( fy). 

Although the subsequent discussion describes the stereoselection at the steady state through the example of radical reactions, the analysis and principles are general for any reaction profile that fits into the scheme of complex stereoselective reactions. In the process proposed and analyzed by Curran et al., the activation of compounds of type 1 is done, for example, by radical formation. The group selectivity in this first step has again no effect on the stereomeric nature of the product. To obtain a stereoconvergent process it is crucial, however, that the reaction is operating at the steady state. This means that the concentrations of the radial intermediates (compounds in brackets in Scheme 2) is low and stationary, while their absolute concentrations are determined by the different rates of reaction. 

There are two distinct selectivities configurational selectivity relates to syn vs. anti addition regio (or directio) selectivity is concerned with 1-Nu, 2-E+ vs. 1-E+, 2-Nu addition. The resulting orientations will be labelled specific (0 or 100%) when one product or process is exclusive otherwise they may range from highly selective to non-selective (>0 or < 100%) - A reaction will be termed stereoconvergent if the same composition of product isomers is obtained on two or more reaction paths. 

The Mukaiyama aldol reaction of ethyl ketones can lead to the controlled introduction of two adjacent stereocenters. While enolate geometry may not be trans-fened faithfully to the relative stereochemistry of the aldol product syn versus anti), stereoconvergent reactions are possible. In the example shown in Scheme 9-5, it should be noted that 7i-facial control from the chiral aldehyde is strong as both products 7 and 8 arise from Felkin selectivity [5]. 

Stereoconvergent A reaction or reaction sequence is stereoconvergent if stereo-isomerically different starting materials yield the same stereoisomeric product. The sequence may be more specifically labeled either enantioconvergent or diastereoconvergent. 

Yamamoto s CAB catalysts (7.16) (see Section 8.1) have also been used in catalytic aldol reactions. This reaction is stereoconvergent as either geometry of silyl enol ether (7.18) affords syn selectivity in the product (7.19) indicating that the reaction proceeds via an open anticHnal transition state with the minimum of steric interactions between the aldehyde substituent and a-substituent of the enol ether, as depicted in Figure 7.2. ... 

Stereoconvergence can be considered an opposite of stereoselectivity, when the reaction of two different stereoisomers yields a single product stereoisomer. 

Recently, Johnson and coworkers reported that a chiral N-heterocyclic carbene F3-catalysed stereoconvergent crossbenzoin reaction of aldehydes with racemic (3-halo-a-ketoesters. The corresponding fully substituted (3-halo glycolic ester products 17 were obtained with high diastereo- and enantioselectivity (Scheme 20.10). 

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