An Aluminum Fluoride Complex with an Appended Ammonium Salt as an Exceptionally Active Cooperative Catalyst for the Asymmetric Carboxycyanation of Aldehydes

Abstract

Al-F bonds are among the most stable σ bonds known, exhibiting an even higher bond energy than Si-F bonds. Despite a stability advantage and a potentially high Lewis acidity of Al-F complexes, they have not been described as structurally defined catalysts for enantioselective reactions. We show that Al-F salen complexes with appended ammonium moieties give exceptional catalytic activity in asymmetric carboxycyanations. In addition to aromatic aldehydes, enal and aliphatic substrates are well accepted. Turnover numbers up to around 10 were achieved, whereas with previous catalysts 10 -10 turnovers were typically attained. In contrast to Al-Me and Al-Cl salen complexes, the analogous Al-F species are remarkably stable towards air, water, and heat, and can be recovered unchanged after catalysis. They possess a considerably increased Lewis acidity as shown by DFT calculations.

Asymmetric Carboxycyanation of Aldehydes by Cooperative AlF/Onium Salt Catalysts: from Cyanoformate to KCN as Cyanide Source

“…The intermediates are labeled consecutively with the respective letter.T he catalystw ithout substrates or reactants is labeled K and the product is labeled P.P athwayBis equivalent to the initially assumed simplified mechanism depicted in Scheme 1. [17] PathwayA is an alternative possible mechanism that was found during the computational studies. Relative free energies at 223 K( À50 8C) of all intermediates are given in Scheme2.A dditional data (e.g.,t ables of bond lengths) are provided in the SupportingI nformation.…”

“…The catalyst without substrates or reactants is labeled K and the product is labeled P . Pathway B is equivalent to the initially assumed simplified mechanism depicted in Scheme . Pathway A is an alternative possible mechanism that was found during the computational studies.…”

“…Recently, we reported a new strategy for the carboxycyanation of aldehydes to permit unprecedented TONs . In this approach a Lewis acidic Al center cooperates with an internal ammonium salt moiety .…”

“…[14][15][16] Recently,w er eported an ew strategy for the carboxycyanation of aldehydes to permitu nprecedented TONs. [17] In this approach aL ewis acidic Al centerc ooperates with an internal ammonium salt moiety. [18] The idea behind was that al oosely boundc yanides hould be highly reactive in the 1,2-addition step to aL ewis acid activated aldehyde.…”

Chiral Salen Complexes for Asymmetric Heterogeneous Catalysis: Recent Examples for Recycling and Cooperativity

“…Enantioselective carboxycyanation of aldehydes was similarly realized with an Al-Salen complex linked to an ammonium moiety (35,Scheme 22). [79] Of major importance is the presence of an AlÀ F complex delivering an excellent activity for this reaction (TON values of up to 10 14 ) due to its high Lewis acidity associated to significant catalyst stability. Various substrates (21 examples) were transformed with up to complete conversion and 97 % ee, at a very low loading of 35.…”