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. 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

With respect to the olefinic substrate, various functional groups are tolerated, e.g. ester, ether, carboxy or cyano groups. Primary and secondary allylic alcohols, e.g. 14, react with concomitant migration of the double bond, to give an enol derivative, which then tautomerizes to the corresponding aldehyde (e.g. 15) or ketone ... 

The initial step is the protonation of the aldehyde—e.g. formaldehyde—at the carbonyl oxygen. The hydroxycarbenium ion 6 is thus formed as reactive species, which reacts as electrophile with the carbon-carbon double bond of the olefinic substrate by formation of a carbenium ion species 7. A subsequent loss of a proton from 7 leads to formation of an allylic alcohol 4, while reaction with water, followed by loss of a proton, leads to formation of a 1,3-diol 3 " ... 

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. 

The oxidation of alkenes and allylic alcohols with the urea-EL202 adduct (UELP) as oxidant and methyltrioxorhenium (MTO) dissolved in [EMIM][BF4] as catalyst was described by Abu-Omar et al. [61]. Both MTO and UHP dissolved completely in the ionic liquid. Conversions were found to depend on the reactivity of the olefin and the solubility of the olefinic substrate in the reactive layer. In general, the reaction rates of the epoxidation reaction were found to be comparable to those obtained in classical solvents. 

Che et al. have reported the use of iodobenzene diacetate as an alternative to lead tetraacetate in the original Rees-Atkinson reactions of a relatively narrow range of olefin substrates (primarily styrenes) [11]. 

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. 

Nothing is known about the identity of the iron species responsible for dehydrogenation of the substrate. Iron-oxo species such as FeIV=0 or Fem-OOH are postulated as the oxidants in most heme or non-heme iron oxygenases. It has to be considered that any mechanistic model proposed must account not only for the observed stereochemistry but also for the lack of hydroxylation activity and its inability to convert the olefinic substrate. Furthermore, no HppE sequence homo-logue is to be found in protein databases. Further studies should shed more light on the mechanism with which this unique enzyme operates. 

In what follows we will frequently refer to the properties of IOS 1517, IOS 1720, and IOS 2024. These were synthesized according to the procedure described under Sec. II.A for a-olefinsulfonates. For IOS 2024, sulfonates composed 77% of the nonvolatile materials compared to 91% for AOS 2024. The olefin substrates were of similar molecular weight. The darker color of the internal olefinsulfonates was not an issue due to its intended application enhanced oil recovery. 

Evidence supporting the presence of the two isomers was obtained by McBride, Jungermann, and Clutter using gas chromatography [177]. Further support for the above mechanism was provided by Cremer and Corvat [180] who varied the olefinic substrate. 

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 zirconocene catalysts described above are very oxophilic, which provides several synthetically useful transformations. Oxygen substitution at the al-lylic or homoallylic position of an olefin substrate allows for excellent regio-and diastereocontrol in the ethyl magnesiation reactions of a-olefins and dienes [21]. When 29 is substituted with a hydroxyl group (29a), syn 30a is favored over anti in a 95 5 ratio, while substitution with OCH3 (29b) reversed the diastereoselectivity to 11 89 (Eq. 6). Use of THF in place of diethyl ether as the reaction solvent for the reaction of 29a lowered the overall diastereo-... 

The osmium-catalyzed dihydroxylation reaction, that is, the addition of osmium tetr-oxide to alkenes producing a vicinal diol, is one of the most selective and reliable of organic transformations. Work by Sharpless, Fokin, and coworkers has revealed that electron-deficient alkenes can be converted to the corresponding diols much more efficiently when the pH of the reaction medium is maintained on the acidic side [199]. One of the most useful additives in this context has proved to be citric acid (2 equivalents), which, in combination with 4-methylmorpholine N-oxide (NMO) as a reoxidant for osmium(VI) and potassium osmate [K20s02(0H)4] (0.2 mol%) as a stable, non-volatile substitute for osmium tetroxide, allows the conversion of many olefinic substrates to their corresponding diols at ambient temperatures. In specific cases, such as with extremely electron-deficient alkenes (Scheme 6.96), the reaction has to be carried out under microwave irradiation at 120 °C, to produce in the illustrated case an 81% isolated yield of the pure diol [199]. 

The primary product of hydroformylation, as it is usually practiced, consists of aldehydes with one more carbon atom than the olefin substrate. 

This scheme is shown with ethylene as the olefin substrate. If the olefin is substituted, i.e., RCH=CH2, the possibility exists for the formation of the isomers RCH2CH2Co(CO)3 or RCH(CH3)Co(CO)3 in Eq. (8). These isomers, which result from the insertion of olefin into the Co—H bond, then produce the isomeric aldehydes RCH2CH2CHO and RCH(CH3)CHO. The understanding of the factors which determine these pathways and control the desired product, has been the motivation for much study. 

Further progress in providing linear aldehydes from olefinic substrates has been provided by modified rhodium catalysts. Without modifiers, the product from the hydroformylation has very low normal iso isomer ratios 1-octene gave only 31% of the linear isomers in one example (28). 

High ligand concentrations and/or low partial pressures of carbon monoxide cause a predominance of species substituted by more than one phosphorus ligand. These species containing multiple ligands present a greater sterically hindered environment for the olefin substrate and favor the linear product (24). Trialkylphosphines, the more basic ligands of the... 

Information published from several sources about 1970 presented details on both the halide-containing RhCl(CO)(PPh3)2- and the hydride-containing HRh(CO)(PPh3)3-catalyzed reactions. Brown and Wilkinson (25) reported the relative rates of gas uptake for a number of different olefinic substrates, including both a- and internal olefins. These relative rates are listed in Table XV. 1-Alkenes and nonconjugated dienes such as 1,5-hexadiene reacted rapidly, whereas internal olefins such as 2-pentene or 2-heptene reacted more slowly by a factor of about 25. It should also be noted that substitution on the 2 carbon of 1-alkene (2-methyl-l-pentene) drastically lowered the rate of reaction. Steric considerations are very important in phosphine-modified rhodium catalysis. 

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... 

A. Rhodium Catalysts with Chiral Phosphines 1. Olefinic Substrates... 

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