Dual catalysis

SCHEME 2.84 Rh(COD)jOTf and chiral phosphoric acid-cocatalyzed asymmetric reductive coupling reaction. 

SCHEME 2.86 Rhj(OAc) and chiral phosphoric acid-cocatalyzed Mannich-type three- 

SCHEME 2.87 Synthetic applications of a dual catalytic asymmetric Mannich of type three-component reaction. 

SCHEME 2.88 Enantioselective trapping of protic carbamate ammonium ylides with imines under an Rhj(OAc)ychiral Brpnsted acid cocatalysis system. 

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An alternative route to nitriles of high enantiomeric excess, based on the conjugate addition of Me,Si-CN to an activated amide 7, has been reported (J. Am. Chem. Soc. 2004,126,9928) by Eric Jacobsen of Harvard University. The key to success in this case was dual catalysis, with a chiral AI complex and a chiral Er complex. 

Aldimines react with AllylSiMe3 in the presence of iodine in MeCN to afford the products of allylation, but an asymmetric version of this method has not yet been developed [47]. Nevertheless, with a chiral auxiliary attached to the imine nitrogen, dual catalysis was reported. Here, In(III) was employed as a Lewis acid to coordinate the imine nitrogen, while Bu4N+F- was used as a Lewis basic activator of Si, resulting in excellent diastereocontrol [48, 49]. 

Agami C, Puchot C (1986) Kinetic analysis of dual catalysis by proline in an asymmetric intramolecular aldol reaction. J Mol Catal 38 341-343 Agami C, Puchot C, Sevestre H (1986) Is the mechanism of the proline-cata-lyzed enantioselective aldol reaction related to biochemical processes Tetrahedron Lett 27 1501-1504... 

Ni(II)(OAc)2bpy and Co(II)(OAc)2bpy catalyze the Michael addition of nitro-methane, malononitrile, and aniline to a,j8-unsaturated ketones, methyl acrylate, and acrylonitrile in DMF under neutral conditions [116]. FeCls 6H2O is a highly efficient catalyst of Michael reaction of 1,3-dicarbonyl compounds with a,/3-unsaturated ketones under mild and neutral conditions (Sch. 24) [117]. There is literature precedent for this reaction with dual catalysis Ni(II) immobilized on a clay support and FeCl3 to activate the enone [118]. The mechanism proposed for the single-center catalysis involves coordination of the enone to a diketonato complex [119]. The chemo-... 

Strukul has recently shown that platinum catalyst [(triphosPO)Pt]2+ used to promote the Baeyer-Villiger oxidation with H2O2 operated by Lewis acid catalysis and promoted the leaving of the OH group.33 This dual catalysis property was considered unique to Pt catalysts. Chiral platinum systems have been applied to cyclic ketones to give moderate enantioselectivity of lactones.32... 

Consequently, the Rovis group identified a productive dual-catalysis mode that enables the asymmetric a-acylation of tertiary amines with aldehydes facilitated by the powerful combination of chiral NHC catalysis and photoredox catalysis. m-DNB (dinitrobenzene) is likely to induce an oxidative quenching cycle of [Ru(bpy)3] under these conditions, with adventitious oxygen likely being the terminal oxidant (Scheme 7.14). 

A related iridium complex has been used for the decarbo)grlative radical allylation of aminoacids and phenylacetic acids that occurs smoothly at room temperature in the presence of Pd(PPh3)4, irradiating by white LEDs. The proposed scheme (Scheme 6) is based on dual catalysis. Ruthenium tris(phenanthroline) dichloride has been used for visible light catalysis of the mild amidation of ketoacids by ort/zo-substituted anilines using ozygen as terminal oxidant (Scheme 7) ... 

Under the dual catalysis of AgOAc and a chiral phosphoric acid, asymmetric annulation of 3-alkynylacrylaldehydes with 2-hydroxystyrenes affords complex multiring-fused chromans (14JOC7063). A variety of... 

Abstract By combining an ever-increasing number of catalysts or catalytic functions, cooperative catalysis is a research area that grows fast. In the field, early-late heterobimetallic complexes are rather old objects but they still continue to fasdnate chemists because of their latent reactivity. After a brief and ccaicise overview of cooperative catalysis, this review focuses on early-late heterobimetallic complexes that were used in catalysis over the last decades. Examples of dual catalysis using early and late metal partners are also described. This chapter ends with an opening towards therapeutic applications of early-late heterobimetallic complexes. 

Keywords Bifunctional catalysis Bimetallic complexes Cooperative catalysis Dual catalysis Early-late heterobimetallic complexes... 

Dual Catalysis from Early and Late Metal Complexes. 175... 

Since the last decades, chemists have described a huge variety of multi-catalytic systems and cooperative effects [6]. First of all, it has been shown that cooperative effects can appear by combining two catalytic functions within the same molecule (bifunctional catalysis) [7] or in two separate molecules (cooperative dual catalysis) [8,9], Both can participate to the same catalytic cycle by activating together the same substrate (double-activation catalysis) or its own substrate. The two catalytic centers can also activate simultaneously different substrates in two directly coupled catalytic reactions for giving a product (synergistic catalysis) [10]. Tandem reactions have been also described [11, 12]. In that case, the two catalytic centers operate consecutively in two independent catalytic cycles, the second catalytic cycle using the product of the first one as an intermediate and converts it as final product. The second catalytic function may also not interact with the substrates but contributes to the stability of the active metal center and acts as redox partner (restorative catalysis) [8]. 

In a fifth contribution, Pierre Le Gendre et al. review the catalytic performances of early-late bimetallic complexes. After an introduction dealing with bifunctional and cooperative dual catalysis in which the cooperative effects arise from two catalytic functions present in the same or two different molecules, this paper focuses on an inventory of early-late heterobimetallic complexes in catalysis. It includes those where the two metal centers belong to the same complex, with bridging ligands as in the following example in which the chiral information is provided by the titanium metal center ... 

Scheme 1.24 Dual catalysis using a secondary amine and a tt-allylpalladium complex...

Metathesis events and substitution reactions can be readily combined in attractive domino processes. Thus, a dual catalysis with palladium and ruthenium complexes... 

SCHEME 8.37. Proposed mechanism of a-alkylation of aldehydes via dual-catalysis. 

More recently, Jacobsen s group [67] developed a similar approach under dual catalysis combining a chiral thiourea and a weak Brpnsted acid for the enantioselective synthesis of A-unsubstituted carboline derivatives. Alternatively, Hiemstra and co-workers [68] considered the use of A-sulfenyliminium ions as intermediates in the asymmetric Pictet-Spengler reaction in the presence of chiral phosphoric acids. Due to the easy acid-mediated removal of the sulfenyl group, this methodology is also powerful for accessing A-unsubstituted products. 

NHCs are also regarded as good candidates to promote organic transformations via cooperative or dual catalysis. In this way, a dual secondary amine/NHC catalytic system has been developed for the formation of highly functionalized cyclopentanones... 

Biologically active tetrahydroquinoline alkaloids 15 were prepared by simple N-methylation of intermediates 14 to lead the desired natural products in good overall yields and high enantioselectivities (for examples of dual catalysis, see [80,81], for more recent examples, see [82-84]). 

Egami H, Ide T, Fujita M, Tojo T, Hamashima Y, Sodeoka M (2014) Dual catalysis with copper and rhenium for trifluoromethylation of propargylic alcohols efficient synthesis of a-trifluoromethylated enones. Chem Eur J 20 12061-12065. doi 10.1002/chem.201403447... 

Du J, Skubi KL, Schultz DM, Yoon TP (2014) A dual-catalysis approach to enantioselective [2 +2] photocycloadditions using visible light. Science 344 392-396... 

Figure 4.29 A cross-section of results for the Rh(lll) dual catalysis involving oxidative coupling of aldehydes by directed C-H functionalization as reported by Seayad and covirorkers [61].

In addition to transition-metal catalysis, biocatalysis and organocatalysis are very important aspects of stereoselective catalysis. Recently, dual catalysis, involving combinations of several types of catalysis, for example, transition-metal catalysis and biocatalysis, has become of interest. Multicomponent and domino reactions are desirable because of the often high atom economy and efficiency. All these aspects have been pursued within the Heidelberg Collaborative Research Network (Sonderforschungsbereich, SFB 623), though most of the effort has been devoted to transition-metal catalysis. 

Fig. 13.9 [(salen)AI]20 Complex and (PyBOXJErCE complex for intermolecular cooperative dual catalysis. 

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