Latent biphasic catalysis

Keywords Soluble polymers Thermomorphic Biphasic catalysis Latent biphasic catalysis Separation... 

A second example of latent biphasic catalysis used the polymer-bound trifunctional base catalyst 129 as a dimethylaminopyridine analog in acylation of 2,6-dialkylphenols by (Boc)20 in a 1 1 heptane-ethanol solvent mixture. After acylation of the phenol was complete, the addition of 10 vol% H2O perturbed the system. The yields of product carbonate from Eq. 66 were 35,66,89,99%, 99, and 99% through the first six cycles. 

Poly(4-tert-butylstyrene) copolymers containing nucleophilic catalysts or ligands can be prepared by modification of 120 and are also useful in latent biphasic catalysis [166]. For example, a triarylphosphine can be attached to a PtBS (132) support and can be used to catalyze the Michael addition of 2-ni-tropropane to methyl acrylate (Eq. 69). 

A second nucleophilic catalyst supported by PtBS is the polymer-bound di-methylaminopyridine analog that was also used in latent biphasic catalysis with the poly(JV-alkylacrylamide) support 129 [131]. This example of a nucleophilic catalyst (133) was used to catalyze formation of a t-Boc derivative of 2,6-di-methylphenol (Eq. 70). In this case, the extent of recovery of the catalyst and the yields of product were directly comparable to those seen with thermomorphic systems. The isolated yield for the first five cycles of this reaction were 34.3, 60.9,82.2,94.6, and 99%. In this case we reused catalyst 133 through 20 cycles. Yields after the first few cycles were essentially quantitative (ca. 93% average for each of 20 cycles). Separation of the polymer from the aqueous ethanol phase was quantitative as judged by either visual observation or UV-visible spectroscopic analysis. 

A second example of latent biphasic catalysis used the polymer-bound trifunctional base catalyst 129 as a dimethylaminopyridine analog in acylation of... 

A latent biphasic system is a miscible solvent mixture that will become biphasic by the addition of a small amount of an additive. For example, a mixture of 10 mL of heptane, 9.2 mL of ethanol, and 0.8 mL of water would be miscible near room temperature. However, addition of a small amount (200 ]iL) of water or the addition of some salt would make this mixture biphasic. Such solvent mixtures that are at the cusp of immiscibility are useful as homogeneous media for catalysis and, after perturbation, as biphasic systems for separation. If a soluble polymer-immobilized catalyst is present that is by design phase-selectively soluble in one or the other phases of the biphasic mixture, it is possible to design recoverable reusable homogeneous catalysts with such latent biphasic systems. 

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