Hydrogenation olefinic substrates

The 0X0 process, also known as hydrofomiylation, is the reaction of carbon monoxide (qv) and hydrogen (qv) with an olefinic substrate to form isomeric aldehydes (qv) as shown in equation 1. The ratio of isomeric aldehydes depends on the olefin, the catalyst, and the reaction conditions. 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

In conclusion, the above summary of oxidation methods shows that there is still room for further improvements in the field of selective olefin epoxidation. The development of active and selective catalysts capable of oxidizing a broad range of olefin substrates with aqueous hydrogen peroxide as terminal oxidant in inexpensive and environmentally benign solvents remains a continuing challenge. 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

BITIANP, were tested as ruthenium ligands for the asymmetric hydrogenation of various olefinic substrates. The results collected in Scheme 8.8 show that these novel ligands were able to induce high enantioselectivities of up to 94% ee. ... 

The use of rhodium catalysts for the synthesis of a-amino acids by asymmetric hydrogenation of V-acyl dehydro amino acids, frequently in combination with the use of a biocatalyst to upgrade the enantioselectivity and cleave the acyl group which acts as a secondary binding site for the catalyst, has been well-documented. While DuPhos and BPE derived catalysts are suitable for a broad array of dehydroamino acid substrates, a particular challenge posed by a hydrogenation approach to 3,3-diphenylalanine is that the olefin substrate is tetra-substituted and therefore would be expected to have a much lower activity compared to substrates which have been previously examined. 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Through continued exploration of the applicability of enantiomeric hydrogenation, phospholane-based catalysts have been reported to be efficient for the reduction of several atypical olefinic substrates (Fig. 24.18). 

Several examples of achiral biphasic hydrogenations are shown in Table 38.1. It can be seen, that the activity of the various phosphine complexes of precious metals rarely exceeds 100 h-1 under mild conditions. In many cases hydrogenation is accompanied by isomerization of the olefinic substrates. 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

When norbornene is treated with trichlorosilane in this manner, quantitative yield of evo-2-trichlorosilylnorbornane is obtained, and (lS,2S,4R)-exo-2-norbornanol can be obtained in 96% ee upon hydrogen peroxide oxidation.28 This reaction can be extended to other olefinic substrates such as 2,5-... 

The BINAP-Rh catalyzed hydrogenation of functionalized olefins has a mechanistic drawback as described in Section 1.2.1. This problem was solved by the exploitation of BINAP-Ru(ll) complexes.Ru(OCOCH3)2(binap) catalyzes highly enantioselective hydrogenation of a variety of olefinic substrates such as enamides, a, (3- and (3,y-unsaturated carboxylic acids, and allylic and homoallylic alcohols (Figure 1.9). " " Chiral citronellol is produced in 300 ton quantity in year by this reaction. ... 

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixtnre in small amonnts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the A-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic substrate to give hydrogen bromide and an allylic radical 3 ... 

Olefin epoxidation by hydrogen peroxide catalysed by MTO on niobia in the presence of urea was successfully applied with better results than under homogeneous conditions, thereby transforming simple olefin substrates to unsaturated fatty acids and esters [56,57]. 

A number of reactions, principally of olefinic substrates, that can be catalyzed by supported complexes have been studied. These include hydrogenation, hydrosilylation, hydroformylation, polymerization, oxidative hydrolysis, acetoxylation, and carbonylation. Each of these will be considered in turn together with the possibility of carrying out several reactions consecutively using a catalyst containing more than one kind of metal complex. 

When NBS is used to brominate non-olefinic substrates such as alkanes, another mechanism, involving abstraction of the hydrogen of the substrate by the succinimidyl radical127 8 can operate.128 This mechanism is facilitated by solvents (such as CH2C12, CHCI3, or... 

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