Remote dehydrogenation

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

On the other hand, acyclic ketones, when oxidized in trifluoroacetic acid [82] or in MeCN [83], gave dehydrogenated ketones and products substituted at the remote (y, S, or e) positions. 

Methanol dehydrogenation to ethylene and propylene. In some remote ioca-tions, transportation costs become very important. Moving ethane is almost out of the question. Hauling propane for feed or ethylene itself in pressurized or supercooled vessels is expensive. Moving naphtha or gas oil as feed requires that an expensive olefins plant with unwanted by-products be built. So what s a company to do if they need an olefins-based industry at a remote site One solution that has been commercialized is the dehydrogenation of methanol to ethylene and propylene. While it may seem like paddling upstream, the transportation costs to get the feeds to the remote sites plus the capital costs of the plant make the economics of ethylene and its derivatives okay. 

Another route to ethylbenzene is available for those remote places where olefin plants or refinery crackers are not nearby but a supply of ethane is— catalytic dehydrogenation of ethane to ethylene followed by its reaction with benzene to produce EB. The first of two steps in Figure 8-4 use a gallium zinc zeolyte catalyst that promotes ethane dehydrogenation to ethylerie at 86% selectivity and up to 50% conversion per pass. 

It now remains for us to consider the oxidation of monofunctional alcohols and molecules containing the -OH group remote from other functions. The conversion of methanol to formaldehyde (methanal) can be performed either by dehydrogenation (difficult, see Chapter 9) or by oxidative dehydrogenation according to the equation ... 

Nevertheless, some notable effects have been observed in the metal-mediated activations of alkanols with larger alkyl backbones, where dehydration still persists but activation of internal C-H bonds via remote functionalization also becomes accessible. Thus, regiospecific 3,4-dehydrogenation of l,6-hexandiol/Fe+ [31] is associated with a considerable diastereoselectivity (an SE of about 3.2 can be derived from the experimental data). Similarly, 3,4-dehydrogenation of 1,8-oc-tandiol/Fe+ occurs diastereoselectively [32]. Note, however, that most complexes of the monofunctional alkanols 13 and 16 with Mn+-Co+ show negligible SEs. 

An entirely different approach to specific dehydrogenation has been reported by R. Breslow and by J.E. Baldwin. By means of this approach it was possible, for example, to convert 3a-cholestanol (4) to 5a-cholest-14-en-3a-ol (5), thus introducing a double bond at a specific site remote from any functional group. This was... 

Substitution of Ala 306, a residue in the proteinase-sensitive loop of S. cerevisiae flavocytochrome 62 > by Ser results in small but significant changes in kcat and Km for L-lactate (143) (Table VIII). Thus, a rather minor alteration to a surface residue quite remote from the active site has an effect on lactate dehydrogenation. The result is similar to that obtained by proteolytic cleavage of the region in which the substitution... 

Kinetic studies on the degradation of several cobalt(II) polyamine dioxygen complexes indicate that dehydrogenation is fast when coordinated dioxygen is close to the a-CH- involved in the reaction, and it is slow when dioxygen is remote from that a-CH- group [5,23]. This situation is illustrated by the transition state 9 derived from... 

The design and catalytic activity of dibenzobarrelene-based bifunctional PC(5p )P pincer catalysts for acceptor-less dehydrogenation of primary and secondary alcohols to give carbonylic and carboxylic compounds has been described. The mechanism of the H2 formation involves intra-molecular cooperation between the structurally remote functionality and the metal centre. The feasibility of the complete catalytic cycle was studied using a stoichiometric model. ... 

The a-C(sp )-H functionalization of carbonyl compounds is a well-known tool in organic synthesis, but functionalization of remote 3-C(sp )-H bond remains unusual (Scheme 2.40). For example, Pihko and co-workers developed Pd(II)-catalyzed oxidative 3-C(sp )-H arylation of P-keto esters with electron-rich arenes [233-236]. The Pd(II)-catalyzed dehydrogenative transformation of ketones to a,P-unsaturated ketones allows divergent P-C(sp )-H bond functionalization with a wide range of nucleophiles [237-239]. Two important independent reports on organocatalytic (aminocatalysis and NHC catalysis) P-C(sp )-H bond of aliphatic aldehydes were also achieved by Wang [240] and Chi [241]. 

Structurally related PCP-pincer Ir complexes (Fig. 10) were synthesized by straightforward [4+2] cycloaddition and employed as catalysts in the acceptorless dehydrogenation of alcohols. Such complexes can be easily modified with a functional sidearm that is capable of interacting with the catalytic site, thus making them suitable candidates for catalytic studies involving ligand-metal cooperation. The H2 formation involves an intramolecular cooperation between the structurally remote functionality and the metal center. ... 

Our results rest, on the one hand, on the study of a model reaction (oxygen aided dehydration of N-ethyl formamide (5,6,14,15,16)) which allowed the identification of catalytic sites created through the remote control (namely Brdnsted acidic sites), and, on the other hand, on the oxidation of isobutene to methacrolein and the oxidative dehydrogenation of 1-butene to butadiene. In these studies, the catalysts have been prepared by mectianically mixing oxides prepared separately (about 40 different mixtures were studied). 

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