Benchmark reaction

These ligands were active for allyhc substitutions but the process was not enantioselective in the benchmark reaction (88, in Scheme 49). More structurally constrained chelates led, however, to measurable enantioselectivities 40% ee for 89, 50% ee for 90, and 64% ee for 91 in the test reaction. By further modifications in the structure of these bipyridine-type hgands (see 92 in Scheme 51, a chiral Ci-symmetric 2,2 -bipyridine) [126], enantioselectivities up to 89% were obtained. 

Some selected examples of standard metathesis transformations are shown in Scheme 3.2. These reactions are commonly considered as benchmark reactions when testing the activity of new metathesis catalysts. 

Most recently, a catalyst system based on PEG-modified phosphine ligands was reported to allow for a highly effective C02-induced separation procedure. In this case, the scC02 was used only at the separation stage to precipitate the catalyst and extract the products. The hydrogenation of styrene to ethyl benzene was used as a benchmark reaction, and it was shown that the catalytic active species could be recovered and not only re-used for another hydrogenation but also be subjected as a cartridge to a series of different transformations [43]. 

In most cases, a suitable molecular modification of the catalyst structure is required to obtain the desired thermoresponsive properties. Polyether and in particular PEG substituents are receiving considerable interest in this field. The present study has addressed structure-activity relationships for well-defined low molecular weight model ligands in the rhodiiun-catalyzed hydroformylation of 1-octene as benchmark reaction. Figure 3 summarizes the observed trends. 

This silver salt was used for the halogen exchange with a broad variety of second generation Grubbs catalysts, leading to the catalytic species shown in Scheme 8.9. In the benchmark reaction with DEDAM, TONs up to 830 were achieved [88]. 

Adlerol, i.e. l-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-l-ol, is a well-established dimeric model compound of lignin and, as such, its oxidation to the ketone-derivative Adlerone, i.e. l-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphen-oxy)propan-l-one, has been taken as a benchmark reaction (Scheme 20) to evaluate the efficiency of several chemo-enzymatic procedures. 

The application of such complexes in the cyclopropanation of alkenes was investigated, mostly in benchmark reactions between styrene and diazo esters or aromatic diazomethane derivatives (Scheme 9.19) to form the cis- and trans-cyclo-propene derivatives 23 and 24. 

Scheme 9.19 Benchmark reactions in carbene transfer reactions to alkenes.

Scheme 3.15 Hydroformylation as a benchmark reaction to validate the cartridge catalyst system MeOPEG750-PPh2/[Rh(acac)(CO)2]/scCO2 (Reproduced from Ref. [76], with the permission of John Wiley and Sons)...

In the present section some applications of the previously outlined theoretical framework, recently reported in literature [24,25], will be illustrated. In particular we will focus our attention on two benchmark reactions of computational-theoretical chemistry, namely the carbon monoxide (CO) binding-unbinding reaction in myoglobin (Mb) and the unimolecular tautomeric proton transfer in aqueous malonaldehyde. 

With respect to the application of asymmetric carbene catalysis as a tool for enantioselective synthesis, the last decade s major success is based on substantial improvements in catalyst development. Early reports dealt with implementing chirality in thiazolium scaffolds (Sheehan and Hunneman 1966 Sheehan and Hara 1974 Dvorak and Rawal 1998), but their catalytic performance suffered from either low yields or low ee-values. In this regard, the investigation of triazole heterocycles as an alternative core structure (Enders et al. 1995) has played a crucial role to provide heterazolium precatalysts improving both asymmetric benzoin and Stetter reactions. An intramolecular Stetter reaction yielding chromanones upon cyclization of salicylaldehyde-derived substrates is commonly used as a benchmark reaction to compare catalyst efficiency (Scheme 1 Ciganek 1995 Enders et al. 1996 Kerr et al. 2002 Kerr and Rovis 2004). 

Scheme 1. Intramolecular Stetter reaction as benchmark reaction for catalyst efficiency...

In summary, D-limonene has been exploited with interesting results in polymer chemistry however, it remains to be seen whether its chirality can be used to induce similar effects in small molecule syntheses. Many of the benchmark reactions e.g. Diels-Alder, Michael addition) used in the alternative solvent field are reactions of olefinic substrates and therefore could not be performed successfully in a terpene solvent. 

The Michael addition to nitrostyrenes is, so far, the benchmark reaction where catalyst designers compete in proposing new protocols, including protocols in non-conventional solvents. 

In view of this we have tested several heterogeneous copper catalysts in the benchmark reaction of ethyl diazoacetate (1) with styrene (2). Some of them have been also used in the reactions of ethyl diazoacetate (1) with cyclohexene (4), a-methylstyrene (6), trans-anethole... 

The preparation of 45 was used by Verducci as the benchmark reaction to check polymer 43 containing 4-piperidine pyridine fragments. The activity of this catalyst used in 20mol.% was lower than that of DMAP. Although the catalyst was recycled ten times a 30% decrease in its efficiency was observed. The use of different spacers did not modify significantly the results, but the catalytic activity was clearly decreased by raising the catalyst loading [185]. 

The latter benchmark reaction for the preparation of 84 was also used recently by Reggelin to study a completely different kind of chiral polymeric bases (Scheme 10.15). In their work, the authors used pyridyl N-oxide substituted helically chiral poly(methacrylate)s 82 prepared by helix sense selective anionic polymerization of... 

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