Remote catalysis

J. Rebek et al. synthesized novel dibenzoheptalene bislactones via a double intramolecular Cannizzaro reaction for condensation polymerization and remote catalysis studies. These bislactones are chiral, atropisomeric molecules. 

B. Delmon, and H. Matralis, The remote control mechanism, general phenomena, possible consequences concerning unsteady state processes, unsteady state processes in catalysis, Y.S. Matros, ed. USF, Utrecht, The Netherlands (1991), p. 25. 

The monetization of remote natural gas has been a key economic driver for catalysis research over the past 20 years. Significant reserves of natural gas exist in remote locations, distant from available gas pipehnes, which cannot be readily brought to market. The conversion of these resources to higher-valued, transportable products, such as methanol or polyolefins can allow the economical utilization of these stranded assets. Other low-valued natural gas streams, such as associated gas from oil production, could also provide feedstocks to such a technology. The conversion of remote gas, typically valued at US 0.50-1.50 per MMBTU, into polyolefins, valued at more than US 1000/t, via methanol has sparked the development of several MTO technologies. 

Addition of a substituent onto the diene terminus remote from the methoxy group ultimately introduces a second chiral centre into the product dihydropyrone. Under zinc chloride catalysis the relative stereochemistry of these two centres is observed to be cis which is consistent with an endo transition state and the pericyclic pathway (Figure Si3.7). Cis products are obtained for a range of aldehydes including simple aliphatic aldehydes and so secondary orbital interactions do not adequately explain all the results obtained. It is postulated therefore that the bulky catalyst/solvent ensemble co-ordinates to the aldehyde trans to its substituent. This places the substituent in the endo position which leads to cis stereochemistry in the product. 

Simple hydrolysis of substrate 36 with base in solution furnishes an essentially equal mixture of 37 and 38, since the t-butyl group is too remote to have any influence. However, catalysis by the artificial enzyme mixture 41 gave 37 only. The geometry of the catalyst-substrate complex directs the attack by water (hydrogen bonded to the imidazole) in a line perpendicular to the ring axis (cf. 42). When we moved the two imidazole groups out further, in catalyst 43, they were then able to deliver the water in a... 

This new style of synthetic catalysis will of course not replace all normal synthetic methods. For many purposes, the standard methods and rules - e.g. aldehydes are more easily reduced than are ketones - will continue to dominate organic synthesis. However, when we require a synthetic transformation that is not accessible to normal procedures, as in the functionalization of unactivated carbons remote from functional groups, artificial enzymes can play a role. They must compete with natural enzymes, and with designed enzyme mutants, but for practical large-scale industrial synthesis there can be advantages with catalysts that are more rugged than proteins. 

Table 2.1), in which 5 with the longer chain was the fastest, supports our previous contention that in transamination the catalytic group also performs the protonation at the amino acid a-carbon. In the imidazole series (7-9) the shortest chain system 7 was also fastest in HC1 elimination. The striking contrast to the data for transamination fully supports the proposition that in our transamination studies the catalysis was sequential, with proton transfer by the catalytic group to the remote position of the intermediate. 

A remote sulfinyl group has also been used to control the stereoselectivity of the hetero-Diels-Alder reaction of the carbonyl group of furfural [176]. Reaction of sulfoxide 236 with Danishefsky s diene in the presence of Ln(OTf)3 (Ln = Yb, Nd, and Sm) yielded cycloadducts 237a and 237b with high de (93-99%). When reactions were conducted under Eu(thd)3 catalysis, the stereoselectivity of the reaction was dramatically inverted (Scheme 107). The influence of the catalysts in the stereoselectivity is not discussed. 

Cyclizations can be achieved if an organomagnesium reagent bears a remote leaving group. A stereoselective substitution without erosion of the optical purity was observed by using copper catalysis (Scheme 35).92... 

There are multiple reasons why this criterion is not met on a routine basis. First, better spectroscopy—reactor cells need to be designed. Section 3 provides details of many of the innovative designs published, it is hoped that this will provide ideas for the much-needed improved designs. Second, XAFS investigations need to be performed at synchrotrons. This requirement necessitates that the catalysis researcher conduct the experiments at a remote location and typically with only a few days to... 

Obviously, the reaction network of the synthesis gas is very versatile and only the rather high cost of syngas itself prevents its wide application. When CO conversion was introduced into industrial practice, coal was the main source of CO/H2. Nowadays, it is mainly CH4 from remote gas fields. Synthesis gas reactions are also a very suitable subject for courses on catalysis. Many interesting aspects of the mechanism, kinetics and technology can be demonstrated using the data available for this reaction. 

Catalysis has little direet impaet on transportation costs for a given produet. However eatalysis can be used to eonvert gaseous and solid materials that are expensive to transport into eheaper to transport liquids. Examples of this inelude the eonversion of methane in remote locations, via synthesis gas, into methanol or liquid hydroearbons [Sections 4.1.2 and 4.7]. 

In the earliest study on the PPI activity of CyP (Fischer et al., 1984), Fischer made the observation that the length of the peptide substrate is important for efficient catalysis. In the series Glt-(AIa) -Pro-Phe-pNA, kJK values are similar for n = 2 and 3, but are about 10-fold larger than n — 1. These results suggest the existence of distinct subsites at the active site of CyP that are able to interact with the amino acid residues of the substrate. Energy released from favorable interactions of the substrate at these remote subsites is then used for catalysis. Thus, CyP utilizes binding energy to stabilize catalytic transition states (Jencks and Page, 1972). 

The popularity of (l/ ,2/ )-1,2-diaminocyclohexane and (l/ ,2R)-diphenylethylen-ediamine as starting materials for these chiral imidazolium salts originates from their ready availability from commercial sources. The question remains to be answered as to whether a different substituent on the backbone (the substituent that makes the backbone chiral) improves the properties of the ligand and thus ultimately increases the chiral resolution in the product of asymmetric catalysis. No study has been proposed yet, probably because such a remote group is not expected to influence the performance of any... 

Grieco, P.A., Nishizawa, M., and Marisovic, N. 1976. Remote double bond migration via rhodium catalysis A novel enone transposition. JAm Chem Soc 98, 7102-7104. 

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