Hydrogen sequences

With the excephon of methanol and ethanol, most alcohols are produced from olefins, either through hydration [10] or via a hydroformylation-hydrogenation sequence (Scheme 4-1) [11]. 

Hence the 30 % that are lost could be on the support and slowly react with 1-hexanol to form HA. However when aniline is reacted there is no significant loss of material, which suggests that aniline cannot interact directly with the surface hydroxyls. This suggests that the interaction between aniline and the support hydroxyls is not as simple as shown above, rather it is more likely that the reaction operates via a spillover mechanism involving an intermediate in the nitrobenzene hydrogenation sequence rather than aniline. The alkylation reaction between aniline and 1-hexanol takes place on the metal function, therefore the reaction with the missing aniline associated with the support will be slow as it requires a reverse spillover and a diffusion across the support surface. 

Scheme 10.32. General scheme for multicomponent domino Knoevenagel/hetero-Diels-Alder hydrogenation sequence.

TABLE 3.4. Results from switching between hydrogenation, hydroformylation and hydroformylation-hydrogenation sequence reactions using sol-gel immobilised [Rh(2)CO]+ a... 

This means that the less stable intermediate alkene complex reacts faster in the subsequent reactions. The next step in the hydrogenation sequence involves the oxidative addition of dihydrogen to the alkene complex (Figure 4.10). 

The rhodium complex with bis(diphenylphosphino)phenoxazine was immobilized on silica using the sol-gel technique or by a direct grafting to commercially available silica [127]. Under standard hydroformylation conditions (CO/H2 atmosphere), a neutral hydridic complex (57) and cationic species (58) (Scheme 4.35) coexist on the support and act as a hydroformylation/hydrogenation sequence catalyst, giving selectively 1-nonanol from 1-octene 98% of 1-octene were converted to mainly linear nonanal which was subsequently hydrogenated to 1-nonanol. The... 

The reaction of alkenes (and alkynes) with synthesis gas (CO + H2) to produce aldehydes, catalyzed by a number of transition metal complexes, is most often referred to as a hydroformylation reaction or the oxo process. The discovery was made using a cobalt catalyst, and although rhodium-based catalysts have received increased attention because of their increased selectivity under mild reaction conditions, cobalt is still the most used catalyst on an industrial basis. The most industrially important hydrocarbonylation reaction is the synthesis of n-butanal from propene (equation 3). Some of the butanal is hydrogenated to butanol, but most is converted to 2-ethylhexanol via aldol and hydrogenation sequences. 

Fig. 5.5. Products of the multicomponent domino-Knoevenagel-hetero-Diels-Alder-hydrogenation sequence with a, fi- and y-amino aldehydes with different 1,3-dicarbonyl compounds.

Scheme 2.26 Tandem metathesis/hydrogen transfer/hydrogenation sequence.

Scheme 9 Proposed mechanism and substrate scope for tandem alcohol oxidation/Wittig reaction/alkene hydrogenation sequence...

This C - H activation event is reversible, and is required to achieve catalytic turnover [62], A series of alcohols, mostly secondary benzylic examples, have been oxidized using this catalyst. The catalytic activity does not match that of the Ir examples described above, but it has been used in several tandem reactions that feature both dehydrogenation and hydrogenation steps to achieve interesting transformations. One example is a tandem alcohol oxidation/Wittig reaction/alkene hydrogenation sequence (Scheme 9) [61,62],... 

A one-pot PKR-hydrogenation sequence was the key step in a non-racemic synthesis of (-)-dendrobine (222) published recently. Starting from (+)-trans-... 

Thus, when at a certain stage of the hydrogenation sequence the substrate leaves the metal catalyst [M], with [M] being a homo- or a heterogeneous catalyst, it may coordinate with either side upon return to [M] (e.g., 4 4a). Thereby, a mixture of diastereoisomers (e.g., 5 and 5a) is obtained, even if each individual hydrogenation step were entirely syn-speci c. 

Mechanistic studies have mostly focussed on the 1-alkene formation by polymerization of surface CFI2 (methylenes). The formation of the CH2 species by a deoxygenation/hydrogenation sequence of adsorbed CO is not well understood as there are few convincing organometallic models, but it is usually depicted as shown in Figure 16. The path involves stepwise de-oxygenation to a surface carbide followed by sequential H transfer to make various intermediate CiHx(ad) (x = 0 - 4) species and finally methane which is liberated. 

Cyclopropane ring scission occurs readily either under reducing conditions or upon the action of electrophilic or nucleophilic agents. These possibilities offer multiple options for the synthetic utilization of the cyclopropane moiety in organic synthesis. One of the most important applications is based upon the use of the cyclopropanation-catalytic hydrogenation sequence as a method for the creation of the gem-dimethyl moiety, a fragment frequently encountered in many naturally occurring compounds. A typical example is shown in Scheme 2.161. 

Guerbet reaction. Condensation of alcohols at high temperature and pressure in the presence of sodium alkoxide or copper by a dehydrogenation, aldol condensation, and hydrogenation sequence. 

Dorta [25]. Their mechanistic studies on hydrogenation sequence of C=C and C=N bonds of quinolines revealed that the hydrogenation mechanism of quinolines involves 1,4 hydride addition to quinoline, isomerization, and 1,2 hydride addition, and the catalytic active species may be Ir(III) complex with chloride and iodide [3]. 

Seconday and tertiary alcohols readily react with Martin s sulfurane to yield trisubstituted alkenes. These eliminations preferentially provide more highly substituted and conjugated olefins. In his recent synthesis of monopyrrolinone-based HIV-1 protease inhibitors, Smith used sulfurane 1 as part of a three-step carbonyl addition-elimination-hydrogenation sequence (43—>44—>45). The sulfurane-mediated elimination step proceeded... 

Use of a chromium-fiee catalyst during the hydrogenation sequence. 

The total plasma-chemical process of methane synthesis (9-63) can be considered a continuation of the carbon monoxide hydrogenization sequence, which starts with the production of formaldehyde from syngas (9-56), then proceeds to production of methanol from formaldehyde (9-57), and finally leads to the production of methane (9-62) in the following reaction ... 

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