Unsaturated intermediates

The first step consists in the attack of a proton on the W-H bond to yield a labile dihydrogen intermediate (Eq. (3)) that rapidly releases H2 to form a coordi-natively unsaturated complex (Eq. (4)). This complex adds water in the next step to form an aqua complex (Eq. (5)) that completes the reaction by substituting the coordinated water by the X anion (Eq. (6)). Steps (3)-(6) are repeated for each W-H bond and the factor of 3 in the rate constants appears as a consequence of the statistical kinetics at the three metal centers. The rate constants for both the initial attack by the acid (ki) and water attack to the coordinatively unsaturated intermediate (k2) are faster in the sulfur complex, whereas the substitution of coordinated water (k3) is faster for the selenium compound. 

Any proposed mechanism for the unprecedented transformation described by Equation 18 must account for the promotion of this photoisomerization by CO, although CO is not required by the stoichiometry. A possible initial step would be similar to that for the Ru3(CO)i2 fragmentation (Scheme 1). In this a Ru-Ru bond is broken concomitant with the movement of a CO from a terminal to a bridging site to form an unsaturated intermediate analogous to I. A speculative proposal along these lines is presented in Figure 5. The key feature of this proposal would be the formation of III with one unsaturated ruthenium, which could be captured by CO to promote the subsequent steps leading from the /1-methylidyne to the acyl... 

II. Synthesis of Hexitols from Unsaturated Intermediates 1. Methods of Preparation... 

Very little was published on the synthesis of hexitols from unsaturated intermediates between the time of Griner s work and 1933, when it occurred to me that it might be possible to add four hydroxyl groups to divinylglycol by means of a solution of silver chlorate containing a small amount of osmic acid. In carrying out this work I had the assistance of one of my students, Joseph Wiemann. We succeeded far beyond our expectations and obtained allitol, unknown up to that time, and d,l-mannitol. 

Very little work on the synthesis of pentitols had been carried out before 1928. Those pentitols which had been synthesized were made by the reduction of pentoses. Several pentitol intermediates had been synthesized" and it seemed desirable to attempt to complete the synthesis of pentitols from the unsaturated intermediates, using the methods which had been successfully employed in the synthesis of hexitols. 

In terms of hydrogenation, solutions of H2Os3(CO)i0 reduce ethylene stoichiometrically, and the resulting unsaturated intermediate Os3(CO)10 can oxidatively add H2, or further ethylene to give a hydridoalkenyl... 

Figure 8.12 The unsaturated intermediate is thought to be favoured in scC02 due to a car-bonyl-C02 donor-acceptor interaction...

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

At high PPh3 concentrations, where the catalyst resting state is (PPh3)3Rh(CO)H, phosphine dissociation must occur to form the coordinatively unsaturated intermediates 3c and 3t. This dissociation is suppressed by increased PPh3 concentration, which serves to reduce the concentration of active Rh species in the catalytic cycle. 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

Stepwise cyclohexane dehydrogenation revealed the possible importance of unsaturated intermediates in benzene formation 48). 

In addition to the stepwise mechanism, Dautzenberg and Platteeuw proposed another platinum-catalyzed cyclization mechanism (23). This might correspond simply to a disguised stepwise aromatization where the further reaction of unsaturated intermediates is very rapid compared with their desorption. Thus, hydrogen pressure would govern the probability of desorption versus further reaction. Since the cyclization of triene is irreversible, a very low steady-state surface triene concentration must be sufficient to ensure a measurable reaction rate. 

Garin and Gault 82) reported a hydrogen order of - 3.4 for position isomerization of n-pentane over alumina-supported platinum between 240° and 300°C. They concluded therefore that a surface species that lost 3-4 hydrogen atoms would cyclize. It is not clear to what extent this discrepancy (cf. 77 and 82) may be attributed to different experimental conditions, to different catalysts, or actually to different mechanisms that is, C5 cyclization with participation of unsaturated intermediates. 

Stepwise Ce dehydrocyclization was observed over potassia-chromia-alumina as well as potassia-molybdena-alumina catalysts (9, 10). Higher operating temperatures (450°-500°C) of these catalysts facilitate the appearance of unsaturated intermediates in the gas phase. Radiotracer studies indicate a predominant Ce ring closure of C-labeled n-heptane over pure chromia (132,132a). 

In lipid metabolism, ds-trans isomerism is particularly important. For example, double bonds in natural fatty acids (see p.48) usually have a as configuration. By contrast, unsaturated intermediates of p oxidation have a trans configuration. This makes the breakdown of unsaturated fatty acids more complicated (see p. 166). Light-induced cis-trans isomerization of retinal is of central importance in the visual cycle (see p.358). 

Eliminating deamination takes place in the degradation of histidine and serine. H2O is first eliminated here, yielding an unsaturated intermediate. In the case of serine, this intermediate is first rearranged into an imine (not shown), which is hydrolyzed in the second step into NH3 and pyruvate, with H2O being taken up. H2O does not therefore appear in the reaction equation. 

Unsaturation is also important in the metal-catalysed disrotatory ring opening of XXXII ( hexamethyl-Dewar-benzene , HMDB) to hexamethylbenzene. This formally forbidden process is catalysed by monomeric HMDBRhCl (the reaction being of order 1/2 in [HMDBRhCl]2 and order 1 in substrate) Closely related is the conversion of XXXIII(a) to XXXIV, presumably by way of an unsaturated intermediate XXXIII ( ), since free CO or added alkene ligands inhibit the process . ... 

Hydrogenation of benzaldehyde yields benzyl alcohol, condensation with aliphatic aldehydes leads to additional fragrance substances or their unsaturated intermediates. Unsaturated araliphatic acids are obtained through the Perkin reaction, for example, the reaction with acetic anhydride to give cinnamic acid. 

Although 3-chloro- and 3-bromo-hexahydroazocin-2(l//)-one (12) and (13) undergo substitution reactions when treated with thiolate ions to give (14), only elimination products are formed from other anions subsequently the anions sometimes add to the a,(3-unsaturated intermediate (15) to give 4-substituted derivatives such as (16) (81AJC569). 

Several 1,5-dioxocanes, including the parent compound, have been prepared via the unsaturated intermediate (345), which unfortunately is only formed in very low yield from (344) <70LA(736)75). As well as (345), there is also formed the 16-membered cyclic dimer of this compound in 14% yield. Catalytic hydrogenation of (345) gives 3-methyl-1,5-dioxocane (346), whilst ozonolysis provides the ketone (347), v = 1725 cm-1, which is converted to 1,5-dioxocane (348) by a three-step reduction procedure via the alcohol and the tosylate. 

The syntheses of both l-(3-deoxy-/ -D-g/i/cm>-pentofuranosyl-2-ulose)uracil (25c) and l-(3-deoxy-/ -D-g/ /cm>-pentofuranosyl-2-ulose)cytosine (30b) by selective elimination reactions have been reported.4,5 Thus, the reaction of sulfonyl derivatives of the cytosine nucleoside 26, and uracil nucleosides 23a, 23c, and 28, with sodium benzoate in N,N-dimethylformamide (DMF) leads to 3 -deoxy-2 -ke-tonucleosides by way of such (presumed) unsaturated intermediates as 24a, 24c, 29a, and 29b. However, in one instance, the intermediate... 

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