Hydrogenation proposed reaction sequence

The first examples of a homogeneous reduction of this type were reported in 1971. Cobalt carbonyl was found to reduce anhydrides such as acetic anhydride, succinic anhydride and propionic anhydride to mixtures of aldehydes and acids. However, scant experimental details were recorded [94]. In 1975, Lyons reported that [Ru(PPh3)3Cl2] catalyzes the reduction of succinic and phthalic anhydrides to the lactones y-bulyrolaclone and phthalide, respectively [95], The proposed reaction sequence for phthalic anhydride is shown in Scheme 15.15. Conversion of phthalic anhydride was complete in 21 h at 90 °C, but yielded an equal mixture of the lactone, phthalide (TON = 100 TOF 5) and o-phthalic acid, which is presumably formed by hydrolysis of the anhydride by water during lactoniza-tion. Neither acid or lactone were further hydrogenated to any extent using this catalyst system, under these conditions. 

An unusual reaction leading to the formation of 4-thioxopyrazolo[3,4-d]-pyrimidines has been reported. 1-Substituted 4-cyano-5-aminopyrazoles (103, R = H) react with phenyl isothiocyanate in dimethylformamide (DMF) saturated with hydrogen chloride to yield 1-substituted 4(5//)-pyrazolo[3,4-d]-pyrimidinethione (110). A proposed reaction sequence involved an initial nucleophilic addition of phenyl isothiocyanate to the protonated o-amino-nitrile to give an o-aminothioamide (108), followed by formylation by the dimethylformamide-hydrogen chloride mixture affording 109, which then cyclizes to the final product 110 (70MI1). 

Gas chromatography-mass spectrometry analysis of the dimer and trimer fractions of the polymerisation products of propene and but-l-ene also suggested intramolecular hydride and methanide shifts as the source of the isomers formed. A mechanistic scheme for the oligomerisation of propene has been proposed, Reaction sequence (2.4) [31]. In most of the studies described above, the products have been analysed after hydrogenation. However, a study analysed the Cy product of a propene/but-l-ene copolymerisation before hydrogenation and confirmed the role of hydride and methanide shifts in determining the structures of the products [32],... 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

Au "=0 species are postulated, inter alia, as active intermediates in the oxidation of alkanes with hydrogen peroxide catalyzed by gold(III) and gold(I) complexes [115]. The reaction sequence is proposed in Scheme 2.8. 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

As mentioned earlier, practically all reactions are initiated by bimolecular collisions however, certain bimolecular reactions exhibit first-order kinetics. Whether a reaction is first- or second-order is particularly important in combustion because of the presence of large radicals that decompose into a stable species and a smaller radical (primarily the hydrogen atom). A prominent combustion example is the decay of a paraffinic radical to an olefin and an H atom. The order of such reactions, and hence the appropriate rate constant expression, can change with the pressure. Thus, the rate expression developed from one pressure and temperature range may not be applicable to another range. This question of order was first addressed by Lindemann [4], who proposed that first-order processes occur as a result of a two-step reaction sequence in which the reacting molecule is activated by collisional processes, after which the activated species decomposes to products. Similarly, the activated molecule could be deactivated by another collision before it decomposes. If A is considered the reactant molecule and M its nonreacting collision partner, the Lindemann scheme can be represented as follows ... 

A large variety of reducing agents have been proposed for this reduction. However, zinc and sodium hydroxide offer the most common system, and lithium aluminum hydride merits consideration. The reduction of azoxy compounds with lithium aluminum hydride has value mainly in structural determinations. Its importance as a preparative procedure is limited normally such a reaction sequence would be a matter of putting the cart before the horse. The reduction of azines has potential value because of the accessibility of azines unfortunately, only under specialized circumstances has it been possible simply to add the required gram-molecule of hydrogen to the structure. Usua-ally, chlorine is added to an azine structure to produce dichloro azo compounds. An extension of the reaction permits the preparation of a,a -diacyl-oxyazoalkanes from azines. 

Proposed Mechanism for Butadiene Reduction. The above results are compatible with the reaction sequence illustrated below. In the absence of a hydrogen atmosphere, CoH, formed via the aging reaction of cyanocobaltate(II), reacts reversibly with butadiene to yield Co(C4H7) which reacts further with CoH and/ or undergoes hydrolysis to yield butenes. The over-all result is oxidation of cyano-cobaltate(II) to cyanocobaltate(III) with concomitant reduction of butadiene to butenes. 

The polymeric pyrrolic autoxidation products probably result from the oxidized monomeric systems, which are analogous in structure to those isolated from photooxidation and peroxide oxidation reactions. Thus, for example, analysis of the products of the autoxidation of 1-methylpyrrole (Scheme 47) would suggest that 1 -methyl-A3-pyrrolin-2-one (153) is initially formed from a radical reaction of the pyrrole with triplet oxygen. This reaction sequence should be compared with that proposed for the oxidation of pyrroles with hydrogen peroxide (Scheme 50), which yields (181), (182) and (183) as the major isolable products. The acid-catalyzed reaction of a pyrrole with its oxidation product e.g. 153) also results in the formation of polymeric material and the formation of pyrrole black is probably a combination of oxidation and acid-catalyzed polymerization processes. 

In the above diagram, H-D refers to hydrogenation-dehydrogenation centers and A to acidic centers on the catalyst. The reaction sequence involves successive ring contraction and expansion steps, similar to the mechanism proposed by Pines and Shaw (P4) to account for transfer of tagged carbon from the side chain to the ring when ethylcyclohexane was contacted with a nickel-silica-alumina catalyst. 

The question remains open as to whether the surface complexes as proposed in (36)- (39) can be formed under FT conditions, especially at the higlt temperatures and (he low- CO partial pressures used [4], The search for surface chemisorbed formyl species has been unsuccessful 1114], Tlius, the interaction of formaldehyde, glyoxa) and CO/Hj with Al Oi supported rhodium gave no IR-detectahle traces of formyl species [ 169]. The insertion mechanism proposed by Hcnrici-Oliv and Olive is closely related to the Pichler Schul/. mechanism [40]. A reaction sequence based on the oxidative addition of hydrogen and reductive elimination of water is assumed Only one metal center is required, however, the mechanism of water elimination is not explained in detail,... 

Therefore, a new integrated paradigm for lipid oxidation is proposed in which the major alternative pathways are added to the classic free radical chain (Figure 15). The traditional reaction sequence involving hydrogen abstractions is presented vertically down the center of the scheme because most radicals formed in alternative reactions ultimately abstract hydrogens to propagate the chain. This is the core of the oxidation process. Pathways that compete with H abstraction are... 

Since it has been shown that hydrogen migration across the catalyst surface is unlikely, 7 it follows that in this STO procedure each site reacts only once and, thus, there is a 1 1 relationship between the number of specific sites present and the number of molecules of each product formed over these sites. The number of butane molecules produced by the initial reaction of butene with the hydrogen covered catalyst corresponds to the number of direct saturation sites . It has been proposed that these sites give butane by the 3mH2 reaction cycle shown in Scheme 3.4 and, thus, they have been labeled sites. l The formation of butane by reaction of the second pulse of hydrogen with the metalalkyl (Step 3) occurs, presumably, by way of the MH reaction sequence shown in Scheme 3.2. These two-step saturation sites are labeled mH. 

The formation of 18 as the sole product of this reaction has been attributed to a reaction pathway which again involves a ring cleavage/ ff-hydrogen elimination step. Interestingly, in this case the cyclization has been proposed as the final step of the reaction sequence. 

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