Axial Ligand Effects: Utilization of Chiral Sulfoxide Additives for the Induction of Asymmetry in (Salen)ruthenium(ii) Olefin Cyclopropanation Catalysts

Abstract

The enantioselective synthesis of compounds containing the cyclopropyl fragment has recently received considerable attention, largely because of the frequent occurrence of cyclopropanes in natural products and their importance as valuable synthetic intermediates. [1][2][3] Although many methods have been developed, transition-metal-catalyzed asymmetric cyclopropanation has emerged as one of the most efficient routes for the formation of optically pure cyclopropanes. [1,4] However, the syntheses of the necessary chiral catalysts for this important transformation are often rather laborious. A general and facile method for the asymmetric synthesis of a cyclopropanation catalyst has yet to be developed.Recently, we have focused on cultivating efficient methods for the development of asymmetric catalysts. We began by improving on our previously reported (salen)ruthenium(ii) asymmetric cyclopropanation system. This catalyst system has been shown to be highly effective for the cyclopropanation of olefins with the carbene precursor ethyl diazoacetate (EDA).[5] However, although the use of chiral catalyst 1 (Scheme 1) has led to exceptional selectivity and high yields for the cyclopropanation of olefins with EDA, these results can not necessarily be extended to all olefin substrates. For some olefins, multiple catalysts containing different chiral diamine backbones must be examined before optimal results can be achieved. The synthesis of a series of chiral salen-based catalysts with differing backbones is both expensive and time consuming so an alternative approach is needed to expedite the screening of new olefins and diazo compounds for the cyclopropanation reaction.Previous work in our group has demonstrated a significant ligand-accelerated catalysis effect for certain substrates through what is presumed to be axial ligand binding and further activation of the ruthenium carbene cyclopropanation intermediate (Scheme 2 a). [6] We hypothesized that this could be used to our advantage and an optically pure Lewis base could be used to bind axially to an achiral catalyst as an alternative to using the preformed chiral (salen)ruthenium(ii) catalyst 1. The Lewis base, if paired correctly with the catalyst (both electronically and sterically), should force an achiral salen ligand into an asymmetric conformation, thus transferring enantioselectivity to the cyclopropanation reaction occurring on the opposite axial face of the catalyst. This approach would allow the use of the inexpensive and readily synthesized achiral catalysts 2-5 and the facile screening of a nearly endless library of chiral Lewis bases L * that can function as ligands to the metal carbene intermediate (Scheme 2 b). Related strategies were previously employed by Katsuki and co-workers with varying degrees of success for the (salen)manganese(iii)-catalyzed asymmetric epoxidation of olefins through the application of chiral amines [7,8] and chiral pyridine N-oxides.[9] Walsh and co-workers also reported considerable enhancement in selectivity when chiral additiv...

Multinuclear Cu(II) Schiff Base Complex as Efficient Catalyst for the Chemical Coupling of CO2 and Epoxides: Synthesis, X-ray Structural Characterization and Catalytic Activity

“…The epoxides (1,2-epoxybutane, propylene oxide, styrene oxide, epichlorohydrin) were tested as substrates and epichlorohydrin was found to be the most reactive epoxide, while propylene epoxide exhibited the lowest activity of the epoxides surveyed in accordance with previous study [21]. The active catalyst contains trimetallic species of the copper in it.…”

“…Lots of studies about metal based complexes as the catalysts for the catalytic conversion of CO 2 into valuable organic products were published in recent years [20][21][22].…”

Nonsymmetrical Salen Ligands and Their Complexes: Synthesis and Applications

“…Axial ligand binding has been employed by Nguyen and co-workers for the induction of asymmetry in the Ru II -salen catalyzed olefin cyclopropanation. [58] Achiral Ru II -salen complexes were axially decorated with chiral sulfoxide ligands and thus provided a nonsymmetrical environment for the salen system. An even more elegant way to afford such a nonsymmetry around a symmetrical salen framework was reported by Carell and others.…”

Syntheses and Crystal Structures of Ruthenium-Salen Complexes Containing Triphenylphosphine Ligands

“…It has been noted that manganese-salen complexes can be widely used as catalysts for alkene epoxidation [8]. Recently, a number of ruthenium-salen complexes have also been found to be active catalysts in various chemical transformations [9,10]. In particular, ruthenium macrocyclic complexes which are stable towards demetalation have been found to show a reversible electrochemistry and provide good model systems for mech-0932-0776 / 11 / 0300-0324 $ 06.00 c 2011 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com anistic investigations of proton-coupled multielectron transfer reactions [11,12].…”