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I i Crosslinkage

Scheme 7. Preparation of polymer-supported borane amino alcohol complexes as catalyst for asymmetric Diels-Alder reactions.

The cycloaddition reaction occurs smoothly, even at -78° C, by the addition of a mixture of methacroleine 43 and cyclopentadiene 44 in dichloromethane to a slurry of the freshly prepared catalyst (Scheme 8). The catalyst's performance was strongly dependent on its crosslinking structure, due to the specific microenvironment created by the ethylene glycol chains. High exo/endo selectivities (up to 96:4) were achieved with enantiomeric excesses up to 95 % ee (Scheme 8) [31]. This is comparable with the selectivity obtained for the unsup ported catalyst in solution (up to 86 % ee) [32],The catalyst was recovered and could be used several times without loss of activity - an observation \yhich triggered investigations into continuous flow processes. Rinsing a catalyst-filled column with a mixture of methacrolein 43 and cyclopentadiene 44 in dichloromethane at -30° C allowed the isolation of the (R)-prod-uct 45 with 71 % ee at a scale of up to 138 mmol [31].

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