Strategies to Design Chemocatalytic Racemization of Tertiary Alcohols: State of the Art & Utilization for Dynamic Kinetic Resolution

The synthesis of enantiomerically pure tertiary alcohols is an important issue in organic synthesis of a range of pharmaceuticals including molecules such as the anti-HIV drug Efavirenz. A conceptually elegant approach to such enantiomers is the dynamic kinetic resolution of racemic tertiary alcohols, which, however, requires efficient racemization strategies. The racemization of tertiary alcohols is particularly challenging due to various side reactions that can occur because of their high tendency for elimination reactions. In the last few years, several complementary catalytic concepts for racemization of tertiary alcohols have been developed, characterized by efficient racemization and suppression of unwanted side-reactions. Besides resins bearing sulfonic acid moieties and a combination of boronic acid and oxalic acid as heterogeneous and homogeneous Brønsted-acids, respectively, immobilized oxovanadium and piperidine turned out to be useful catalysts. The latter two catalysts, which have already been applied to different types of substrates, also have proven good compatibility with lipase, thus leading to the first two examples of chemoenzymatic dynamic kinetic resolution of tertiary alcohols. In this review, the difficulties in racemizing tertiary alcohols are specifically described, and the recently developed complementary concepts to overcome these hurdles are summarized.

Iridium-Catalyzed 1,3-Rearrangement of Allylic Alcohols

The allylic alcohol structural motif is prevalent in many important molecules and valuable building blocks. The rearrangement reaction is one of the most important transformations, however there are only a few reports for the 1,3-rearrangement of allylic alcohols. Herein, a 1,3-rearrangement of allylic alcohols catalyzed by an Ir(III) dihydride complex is described. This reaction could provide the corresponding less accessible allylic alcohols regio- and stereoselectively from readily available E/Z mixtures of the substrates. Furthermore, a tandem alkene isomerization followed by 1,3-rearrangement of homoallylic alcohols was also realized. In addition, this rearrangement reaction could be used to synthesize the natural product Navenone B. Mechanistic investigation indicated that the reaction pathway involved a π-allyl-Ir(V) intermediate and that the dihydride in the iridium catalyst acts as a hydrogen switch to modulate the valence of the iridium center.

Asymmetric Chemoenzymatic Reductive Acylation of Ketones by a Combined Iron-Catalyzed Hydrogenation-Racemization and Enzymatic Resolution Cascade

A general and practical process for the conversion of prochiral ketones into the corresponding chiral acetates has been realized. An iron carbonyl complex is reported to catalyze the hydrogenation-dehydrogenation-hydrogenation of prochiral ketones. By merging the iron-catalyzed redox reactions with enantioselective enzymatic acylations a wide range of benzylic, aliphatic and (hetero)aromatic ketones, as well as diketones, were reductively acylated. The corresponding products were isolated with high yields and enantioselectivities. The use of an iron catalyst together with molecular hydrogen as the hydrogen donor and readily available ethyl acetate as acyl donor make this cascade process highly interesting in terms of both economic value and environmental credentials.

Evaluation of a new protocol for enzymatic dynamic kinetic resolution of 3-hydroxy-3-(aryl)propanoic acids

The application of tandem metal–enzyme dynamic kinetic resolution (DKR) is a powerful tool for the manufacture of high-value chemical commodities.

The regio- and stereospecific intermolecular dehydrative alkoxylation of allylic alcohols catalyzed by a gold(I) N-heterocyclic carbene complex

A 1:1 mixture of [AuCl(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine) and AgClO(4) catalyzes the intermolecular dehydrative alkoxylation of primary and secondary allylic alcohols with aliphatic primary and secondary alcohols to form allylic ethers. These transformations are regio- and stereospecific with preferential addition of the alcohol nucleophile at the γ-position of the allylic alcohol syn to the departing hydroxyl group and with predominant formation of the E stereoisomer. The minor α regioisomer is formed predominantly through a secondary reaction manifold involving regioselective γ-alkoxylation of the initially formed allylic ether rather than by the direct α-alkoxylation of the allylic alcohol.