Intramolecular catalyst recyclability

The same type of porphyrin-Ru complex was immobilized by coordina-tive adsorption on aminopropylsilicas (Fig. 26) as either amorphous or crystalline supports [79]. Mesoporous crystalline MCM-48 was the best support, as shown by the improved results obtained in the epoxidation of styrene with 2,6-dichloropyridine N-oxide (TON > 13 000 and 74% ee). The versatility of this catalyst was demonstrated in the intramolecular cyclopropanation of frans-cinnamyl diazoacetate. TON was ten times higher than that obtained in solution and 85% ee was observed. The solid was recycled and reused, although partial loss of selectivity occurred. 

Compounds in this class are usually made from specific precursors with the ultimate functionalities appearing in the starting materials. Syntheses in which one new bond is formed (cyclizations) usually involve substitution or condensation reactions. Two new closures with potentially some generality have been noted in this chapter. One of the intramolecular Diels-Alder reactions (see Section 13.12.8.1) and the other RCM (see Section 13.12.8.1). The Baeyer-Villiger reaction is still very much used for the preparation of lactones. A supported catalyst and a recyclable catalyst carried in an ionic liquid have been introduced for use in Baeyer-Villiger oxidations from other areas. 

The dibenzo- 1,5-oxazocine 540 was obtained, in nearly quantitative yield, via intramolecular carbonylation of ortho-substituted aniline 539 with 100 psi CO in the presence of recyclable palladium-complexed dendrimer on silica (Gl-Pd) as catalyst and diisopropylethylamine (DIPEA) (Equation 23) <2005JA14776>. 

Reproduced with permission from Alma D, Alonso DA, N jera C. Prolinamides versus proiine-thioamides as recyclable catalysts in the enantbselective solvent-free inter- and intramolecular aldol reactions. Adv Synth Catal 2008 350 2467-72. Copyright (2008), Wiley. 

Punniyamurthy and coworkers reported a ligand-free nano-CuO-catalyzed synthesis of substituted benzo-azole derivatives via intramolecular cross-coupling (Saha et al, 2009). Various benzimidazoles, 2-aminobenzimidazoles, 2-aminobenzothiazoles, and benzoxazoles were efficiently assembled from the corresponding o-bromoaryl substrates using heterogeneous CuO nanoparticles as the catalyst in dimethyl sulfoxide (DMSO) under air (Scheme 4.37). The procedure is quite simple, versatile, and efficient. The nano-CuO could be recycled without loss of its activity, and no external chelating ligands were required. 

In the same year, a series of benzoxazoles were synthesized through the intramolecular C-O coupling of o-halobenzanilides using CuFAP as the catalyst (Scheme 4.38) (Kantam et al., 2009b). A variety of 2-substituted benzoxazoles were generated in good to excellent yields at 110°C. The catalyst can be readily recycled three times without loss of its activity. 

Recently, polystyrene-supported pyrrole-2-carbohydrazide (PSP) was combined with Cu(l) to make up a recyclable catalytic system (Cu(l)/PSP) for the N-(hetero)ary-lation of amines and imidazole (Huang et al., 2013). This heterogeneous catalyst could also be successfully applied to the assembly of imidazo[l,2-a]quinoxaline through the intramolecular cyclization of A -(2-iodophenyl)-l//-imidazole-2-carboxamide (Scheme 4.39). 

Najera el al. have demonstrated that L-prolinethioamides were more effective catalysts for the asymmetric aldolisation of ketones with aldehydes than the corresponding L-prolinamides under solvent-free conditions. " Thus, the use of a novel recyclable L-prolinethioamide, derived from L-proline and (7 )-l-aminoindane, allowed a wide range of chiral aldol adducts to be obtained with a combination of excellent yields and stereoselectivities in general, and at a low catalyst loading of 5 mol % (Scheme 2.24). Furthermore, an intramolecular version of this process could be developed, providing chiral bicyclic diketones with enantioselectivities of up to 88% ee. 

The authors also designed and performed extensive experiments, such as three-phase and catalyst filtration tests, to prove the heterogeneous nature of the SBA-15 supported Pt DENs in the intramolecular hydroalkoxylation reaction. No leaching of the active catalytic species was detected in these tests. PVP-capped Pt nanoparticles were also tested for use in this reaction, but they could not be recycled. 

---