Macrocyclic Cyclooctene‐Supported AlCl–Salen Catalysts for Conjugated Addition Reactions: Effect of Linker and Support Structure on Catalysis

Abstract

AlCl-salen (salen=N,N'-bis(salicylidene)ethylenediamine dianion) catalysts supported onto macrocyclic oligomeric cyclooctene through linkers of varying length and flexibility have been developed to demonstrate the importance of support architecture on catalyst activity. The role played by the support and the linkers in dictating catalyst activity was found to vary for reactions with contrasting mechanisms, such as the bimetallic cyanide and the monometallic indole addition reactions. While the flexible support significantly enhanced the cyanide addition reaction, most likely by improving salen-salen interactions in the transition state, it lowered the reaction rate for the monometallic indole reaction. For both reactions, significant increase in catalytic activity was observed for catalysts with the longest linkers. The effect of the flexible macrocyclic support on catalysis was further exemplified by the enhanced activity of the supported catalyst in comparison with its unsupported analogue for the conjugate addition of tetrazoles, which is known to be catalyzed by dimeric mu-oxo-salen catalysts. Our studies with the cyclooctene supported AlCl-salen catalysts provides significant insights for rationally designing highly efficient AlCl-salen catalysts for a diverse set of reactions.

Multinuclear cobalt-salen complexes with phenylene linker for epoxide polymerizations

“…Epoxides for the polymerizations were dried over CaH 2 and distilled under argon. The compounds rac ‐ 4 , 5 , rac ‐ and ( R,R )‐ 6 , 4‐(dimethylamino) pyridinium p ‐toluenesulfonate (DPTS), and ( R,R )‐ and ( S , S )‐ 11 were synthesized according to the literatures.…”

Enantioselective ring opening of epoxides with TMSN3 by macrocyclic oligomer-supported Cr(III)–salen catalysts

“…Some other factors may also be involved, such as the influence of linker length on the solubility of the catalysts. 24 As mentioned in the introduction, Weck et al developed macrocyclic oligomer supported Al(III)-salen catalysts with different linker lengths and tested them in the bimetallic mechanism based cyanide addition to unsaturated imide. 24 The catalyst with the longest linker (10-bond) afforded both the highest yields and the highest enantioselectivity, which was attributed to the flexibility of the long linker.…”

“…The catalyst with the longest linker afforded both the highest yield and the highest enantioselectivity. 24 Here, by borrowing this macrocyclic cyclooctene supporting system, we synthesized three supported Cr(III)-salen catalysts with 3-, 8- length, and studied their catalytic properties in the asymmetric ring opening and the kinetic resolution of epoxides with TMSN 3 as the nucleophile. Herein, we address the following questions:…”

“…17 Weck et al developed a series of macrocyclic oligomeric cyclooctene supported salen complexes showing high activity in various asymmetric catalytic reactions. [23][24][25][26] Macrocyclic oligomeric cyclooctene supported Al(III)-salen with linkers of three different lengths (3-, 7-or 10-bond) were tested in the bimetallic mechanism based cyanide addition to unsaturated imides. The catalyst with the longest linker afforded both the highest yield and the highest enantioselectivity.…”

Evaluation of enantiopure and non-enantiopure Co(III)-salen catalysts and their counter-ion effects in the hydrolytic kinetic resolution (HKR) of racemic epichlorohydrin

“…Metal-salen catalysts are now ubiquitous in catalysis and organic synthesis [1][2][3], being used in a variety of enantioselective reactions, including olefin epoxidation [4][5][6], epoxide ring-opening [7][8][9], nitroaldol reactions [10], and conjugate additions [11], as recent examples. The hydrolytic kinetic resolution (HKR) 2 of racemic epoxides is a well-established technique to resolve epoxides, specifically terminal epoxides, into a single enantiomer via a ring-opening reaction using water as the nucleophile (Scheme 1) [7].…”