Combination of enzymes and metal catalysts. A powerful approach in asymmetric catalysis

Enzymes in the synthesis of bioactive compounds: the prodigious decades

The growing demand for enantiomerically pure pharmaceuticals has impelled research on enzymes as catalysts for asymmetric synthetic transformations. However, the use of enzymes for this purpose was rather limited until the discovery that enzymes can work in organic solvents. Since the advent of the PCR the number of available enzymes has been growing rapidly and the tailor-made biocatalysts are becoming a reality. Thus, it has been possible the use of enzymes for the synthesis of new innovative medicines such as carbohydrates and their incorporation to modern methods for drug development, such as combinatorial chemistry. Finally, the genomic research is allowing the manipulation of whole genomes opening the door to the combinatorial biosynthesis of compounds. In this review, our intention is to highlight the main landmarks that have led to transfer the chemical efficiency shown by the enzymes in the cell to the synthesis of bioactive molecules in the lab during the last 20 years.

Enzyme catalysed deracemisation and dynamic kinetic resolution reactions

New catalysts and reaction conditions have been developed for the dynamic kinetic resolution or deracemisation of racemic mixtures of chiral compounds. Specific functional groups that lend themselves particularly well to this approach include chiral secondary alcohols, alpha-amino acids, amines and carboxylic acids. A general theme of these processes is the combination of an enantioselective enzyme with a chemical reagent, the latter being used either to racemise the unreactive enantiomer or alternatively recycle an intermediate in the deracemisation process. In some examples of dynamic kinetic resolution, a second enzyme (racemase) is used to interconvert the enantiomers of the starting material.

One-pot synthesis of enantiopure syn-1,3-diacetates from racemic syn/anti mixtures of 1,3-diols by dynamic kinetic asymmetric transformation

A one-pot synthesis of enantiomerically pure syn-1,3-diacetates starting from readily accessible racemic diastereomeric mixtures of 1,3-diols has been realized by combining (i) enzymatic transesterification, (ii) ruthenium-catalyzed epimerization of a secondary alcohol in a diol or diol monoacetate, and (iii) intramolecular acyl migration in a syn-1,3-diol monoacetate. The in situ coupling of these three processes results in an efficient enantioselective synthesis of acyclic syn-1,3-diacetates via combined deracemization-deepimerization and constitutes a dynamic kinetic asymmetric transformation concept. Several differently substituted unsymmetrical, acyclic syn-1,3-diacetates were obtained in yields up to 73% with excellent enantioselectivities (>99%) and good diastereomeric ratios (>90% syn).

Asymmetric catalysis: an enabling science

Chirality of organic molecules plays an enormous role in areas ranging from medicine to material science, yet the synthesis of such entities in one enantiomeric form is one of the most difficult challenges. The advances being made stem from the convergence of a broader understanding of theory and how structure begets function, the developments in the interface between organic and inorganic chemistry and, most notably, the organic chemistry of the transition metals, and the continuing advancements in the tools to help define structure, especially in solution. General themes for designing catalysts to effect asymmetric induction are helping to make this strategy more useful, in general, with the resultant effect of a marked enhancement of synthetic efficiency.

Chemoenzymatic dynamic kinetic resolution

During the past decade a new concept has appeared in asymmetric catalysis involving the combination of a biocatalyst and a chemocatalyst in one 'pot' leading to efficient deracemization via dynamic kinetic resolution (DKR). Here, we outline the different strategies that have been developed for efficient chemoenzymatic DKR, in particular the powerful combination of an enzyme and a metal catalyst.

Aminocyclopentadienyl Ruthenium Complexes as Racemization Catalysts for Dynamic Kinetic Resolution of Secondary Alcohols at Ambient Temperature

Aminocyclopentadienyl ruthenium complexes, which can be used as room-temperature racemization catalysts with lipases in the dynamic kinetic resolution (DKR) of secondary alcohols, were synthesized from cyclopenta-2,4-dienimines, Ru(3)(CO)(12), and CHCl(3): [2,3,4,5-Ph(4)(eta(5)-C(4)CNHR)]Ru(CO)(2)Cl (4: R = i-Pr; 5: R = n-Pr; 6: R = t-Bu), [2,5-Me(2)-3,4-Ph(2)(eta(5)-C(4)CNHR)]Ru(CO)(2)Cl (7: R = i-Pr; 8: R = Ph), and [2,3,4,5-Ph(4)(eta(5)-C(4)CNHAr)]Ru(CO)(2)Cl (9: Ar = p-NO(2)C(6)H(4); 10: Ar = p-ClC(6)H(4); 11: Ar = Ph; 12: Ar = p-OMeC(6)H(4); 13: Ar = p-NMe(2)C(6)H(4)). The tests in the racemization of (S)-4-phenyl-2-butanol showed that 7 is the most active catalyst, although the difference decreased in the DKR. Complex 4 was used in the DKR of various alcohols; at room temperature, not only simple alcohols but also functionalized ones such as allylic alcohols, alkynyl alcohols, diols, hydroxyl esters, and chlorohydrins were successfully transformed to chiral acetates. In mechanistic studies for the catalytic racemization, ruthenium hydride 14 appeared to be a key species. It was the major organometallic species in the racemization of (S)-1-phenylethanol with 4 and potassium tert-butoxide. In a separate experiment, (S)-1-phenylethanol was racemized catalytically by 14 in the presence of acetophenone.

Mild and efficient palladium(ii)-catalyzed racemization of allenes

Allenes undergo racemization in the presence of catalytic amounts of Pd(OAc)2/LiBr under mild conditions; the reaction proceeds via a bromopalladation-debromopalladation sequence and tolerates various functional groups.

Mechanism of hydrogen transfer to imines from a hydroxycyclopentadienyl ruthenium hydride. Support for a stepwise mechanism

The negligible double kinetic deuterium isotope effect (k(HH)/k(DD)= 1.05) in the reaction where [2,3,4,5-Ph4(eta5-C4COH)Ru(CO)2H (2) transfers a hydride and a proton to N-phenyl-[1-(4-methoxyphenyl)ethylidene]amine (4) indicates that no bond to hydrogen is broken or formed in the rate-determining step.