Experimental and theoretical perspectives of the Noyori-Ikariya asymmetric transfer hydrogenation of imines

Abstract

The asymmetric transfer hydrogenation (ATH) of imines catalyzed by the Noyori-Ikariya [RuCl(η6-arene)(N-arylsulfonyl-DPEN)] (DPEN=1,2-diphenylethylene-1,2-diamine) half-sandwich complexes is a research topic that is still being intensively developed. This article focuses on selected aspects of this catalytic system. First, a great deal of attention is devoted to the N-arylsulfonyl moiety of the catalysts in terms of its interaction with protonated imines (substrates) and amines (components of the hydrogen-donor mixture). The second part is oriented toward the role of the η6-coordinated arene. The final part concerns the imine substrate structural modifications and their importance in connection with ATH. Throughout the text, the summary of known findings is complemented with newly-presented ones, which have been approached both experimentally and computationally.

Scheme 1

Typical routes towards the preparation of 1,2,3,4-tetrahydroisoquinolines (THIQs). Route I proceeds via the Schotten-Baumann acylation of the starting amine, Bischler-Napieralski cyclization of the resulting amide, and reduction of the C=N bond. Route II comprises the formation of an imine which subsequently undergoes the Pictet-Spengler reaction. The steps in which asymmetry originates are marked with asterisks.

Figure 1

Selected structures of N-aryl and N-alkylsulfonyl-DPEN ligands that have been reported.

Figure 2

(a) Interaction of the sulfonyl group with protonated DHIQ via a hydrogen bond (green dashed line). (b) Interaction of the sulfonyl group with protonated amines viahydrogen bonds (green dashed line).

Figure 3

A section of a ¹H-¹⁵N gHMBC NMR spectrum of a mixture of catalyst [RuCl(η⁶-p-cymene)TsDPEN], HCOOH, piperidine-¹⁵N and acetonitrile-d₃[36].

Figure 4

Transition state of hydrogenation of 1-methyl-DHIQ with [RuH(η⁶-p-cymene)(R,R)-TsDPEN] with Et₃NH⁺ hydrogen-bonded to one oxygen atom of the sulfonyl group (optimized geometry).

Figure 5

Transition state of hydrogenation of 1-methyl-3,4-DHIQ with [RuH(η⁶-mesitylene)TsDPEN] featuring double CH/π interaction.

Figure 6

General structures of common imine substrates.

Figure 7

Transition state of hydrogenation of 1-phenyl-DHIQ with [RuH(η⁶-mesitylene)(S,S)-TsDPEN] optimized at the ωB97XD/Def2-SVP level [38].

Scheme 2

Tautomerization of DHIQs.

Scheme 3

Isomerization of acetophenone N-benzylimine.