Hydrogen selective transformations

In recent years, the importance of aliphatic nitro compounds has greatly increased, due to the discovery of new selective transformations. These topics are discussed in the following chapters Stereoselective Henry reaction (chapter 3.3), Asymmetric Micheal additions (chapter 4.4), use of nitroalkenes as heterodienes in tandem [4+2]/[3+2] cycloadditions (chapter 8) and radical denitration (chapter 7.2). These reactions discovered in recent years constitute important tools in organic synthesis. They are discussed in more detail than the conventional reactions such as the Nef reaction, reduction to amines, synthesis of nitro sugars, alkylation and acylation (chapter 5). Concerning aromatic nitro chemistry, the preparation of substituted aromatic compounds via the SNAr reaction and nucleophilic aromatic substitution of hydrogen (VNS) are discussed (chapter 9). Preparation of heterocycles such as indoles, are covered (chapter 10). 

Bose and coworkers [10] have described hydrogenation using ammonium formate as hydrogen donor and Pd/C as catalyst for selective transformations (Tab. 8.1) of /7-lactams 10, as shown in Scheme 8.6. 

Prabhu, A.K. and S.T. Oyama, Highly hydrogen selective ceramic membranes Application to the transformation of greenhouse gases,. Membr. Sci, 176, 233-248, 2000. 

Sasson and Rempel [97] showed that the system [(PPh3)3RuCl2]/secondary alcohol is suitable for the selective transformation of 1,1,1,3-tetrachloro into 1,1,3-trichloro compounds. Similarly, Blum and coworkers [98, 99] employed [(PPh3)3RuCl2] as well as polystyrene-anchored Rh, Ru and Ir complexes for the hydrogen transfer from alcohols to trihalomethyl compounds, leading to dihalo-methyl derivatives. For example, one of the Cl atoms of 2,2,2-trichloro-l-phenyl-ethanol was displaced by H at 140-160 °C in 2-propanol. The polymer-anchored catalysts proved to be resistant to leaching [99]. 

The addition of dopants is found to have beneficial effects. However, they are not restricted only to transition metals. The hydrogenation of acrylic acid can be promoted significantly by the addition of neodymium ions onto the palladium particles [142], The selective transformation of 3,4-dichloronitrobenzene to the corresponding aniline has been selected to test pre-prepared Pt hydrosols as heterogeneous catalyst precursors (see Figure 3.9) [143],... 

Zaccheria et al. have reported the selective transformation of various ketones employing a CU/AI2O3 catalyst with a 8% copper content, without the need for any kind of basic additive [110]. Table 6.8 presents the most interesting results. Of particular interest is the hydrogenation of p-isobutylacetophenone, where the excellent selectivity obtained makes the CU/AI2O3 catalyst competitive with other heterogeneous catalysts reported so far [109]. 

In 2001, Albrecht Berkessel and Nadine Vogl reported on the Baeyer-Villiger oxidation with hydrogen peroxide in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as solvent in the presence of Brpnsted acid catalysts such as para-toluenesulfonic acid (equation 85) . Under these conditions cyclohexanone could be selectively transformed into the corresponding lactone within 40 min at 60 °C with a yield of 92%. Mechanistic investigations of Berkessel and coworkers revealed that this reaction in HFIP proceeds by a new mechanism, via spiro-bisperoxide 234 as intermediate, which then rearranges to form the lactone. The study illustrates the importance of HFIP as solvent for the reaction, which presumably allows the cationic rearrangement of the tetroxane intermediates. 

As was discussed in Sections 11.1.5 and 11.2.3, the stereoselective partial hydrogenation of alkynes to either cis or trans alkenes is of key importance. Chemical reductions can also be applied to achieve both selective transformations. 

A number of oxidation reactions of mono- and difluorosteroid compounds has been reported. In some reactions, the specific influence of a fluoro substituent on the reactivity has been observed the presence of a 9a-fluorine in a 11 /i-hydroxy-A4-3-oxo steroid causes completely stcreospecific alkaline epoxidation with hydrogen peroxide in a much slower reaction (4d vs 4 h) in comparison with the nonfluorinated analog.322 Most oxidations are accomplished by the highest selective biochemical (that is, by bacterial enzymatic) transformations. As the biochemical oxidation systems are not discussed in this section, only a list of selected transformations of steroids is presented in Table 21. For additional information see ref 323. 

Bose, A.K., Banik, B.K., Barakat, K.J. and Manhas, M.S., Microwave-induced organic-reaction enhancement (more) chemistry. 5. Simplified rapid hydrogenation under microwave irradiation-selective transformations of beta-lactams, Synlett, 1993, 575-576. 

Sometimes, due to special conditions, chain transformation may hardly be induced. An example of this is the reaction of propylene epoxidation. However, intense generation of active sites (H02) in the primary reaction gives the possibility of suppressing acceptor chain transformation to undesired products and simultaneously stimulating the main direction—epoxidation. This is obtained due to chemical induction, which induces and speeds up selective transformation of propylene (acceptor) to a quite high rate. The authors have implemented such a conjugation mechanism in propylene epoxidation by hydrogen peroxide [10]. 

Let us discuss the questions outlined in more detail using the example of conjugated substrate oxidation with hydrogen peroxide. Hydrogen peroxide concentration in the reaction mixture is reduced in the course of the reaction, followed by the change of the key active sites (H02 and OH radical) and, correspondingly, the physicochemical situation is changed not for the benefit of selective transformation of the raw material. 

Such high activity in hydrogen transfer reactions smoothes the way for a series of selective transformations of synthetic interest. Thus, a versatile and efficient catalyst could potentially promote hydrogenation, dehydrogenation, and a combination of the two reactions in order to set up isomerization reactions. 

Hydrogen peroxide is also the oxidant in the halogenations of alkanes under Gif conditions. In these systems, developed by Barton and his coworkers312, alkanes are selectively transformed into alkyl chlorides or bromides by polyhaloalkanes and H202 in the presence of FeCypicolinic acid catalyst in pyridine/acetic acid solvent313-315. It has clearly been established that the reaction mechanism does not involve a free-radical intermediate. 

Acetylbenzofuran or 2-(l-hydroxyethyl)benzofuran was hydrogenated to 2-(l-hy-droxyethyl)-2,3-dihydrobenzofuran in high yields over Raney Ni in ethanol at room temperature and 0.2-0.3 MPa H2 (eq. 12.1 ll).215 The selective transformation of 2-acetylbenzofuran to 2-(l-hydroxyethyl)benzofuran was successful over platinum oxide in ethanol (eq. 12.111), while the hydrogenation over a colloidal platinum on Norit catalyst from chloroplatinic acid and platinum oxide gave a mixture of 2-(l-hy-droxyethyl)benzofuran, 2-ethylbenzofuran, and their 2,3-dihydro derivatives. 

Recent reviews on olefin metathesis [1, 2], nonmetathesis [3], asymmetric hydrogenation [4] and organic synthesis reactions [5] have shown the potential of selected ruthenium catalysts. Among the emerging topics in which ruthenium catalysts play a crucial role are the selective transformations of multiple carbon-carbon bonds. 

Other aminophosphines have also been sought and applied in different enantio-selective transformations, e. g., allylic substitution [56] (up to 95 % ee), and Ir-based imine hydrogenation (88% ee) [57]. Chiral aminophosphines have also been investigated in the asymmetric transfer hydrogenation of ketones (up to 84 % ee for the reduction of aryl ketones) [58],... 

The quantitative and selective transformation of the precursor into the dinuclear complex in catalytic conditions, the different catalytic performance of the mononuclear derivative [(BDPBzP)Ru(CH3CN)j]OTf (lower activity and enantioselectivity), the identical catalytic performance of the precursor and of the T12-H2 complex, and the recognized capability of the structurally related Ru(II) complex [(ri2-H2)(dppb)Ru()a.-Cl)3RuCl(dppb)] to maintain the dimeric structure in olefin hydrogenation reactions [35-37] were taken as proofs for the direct involvement of [(BDPzP)(DMSO)Ru((1-C1)3Ru(ti2-H2)(BDPBzP)] in the enantioselective hydrogenation of acetylacetone [68]. 

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