Phosphoric acid catalysts synthesis

Acid-Gatalyzed Synthesis. The acid-catalysed reaction of alkenes with hydrogen sulfide to prepare thiols can be accompHshed using a strong acid (sulfuric or phosphoric acid) catalyst. Thiols can also be prepared continuously over a variety of soHd acid catalysts, such as seoHtes, sulfonic acid-containing resin catalysts, or aluminas (22). The continuous process is utilised commercially to manufacture the more important thiols (23,24). The acid-catalysed reaction is commonly classed as a Markownikoff addition. Examples of two important industrial processes are 2-methyl-2-propanethiol and 2-propanethiol, given in equations 1 and 2, respectively. 

In 1950 the Fischer-Tropsch synthesis was banned in Germany by the allied forces. Sinarol, a high paraffinic kerosene fraction sold by Shell, was used as a substitute. This ban coincided with the rapid development of the European petrochemical industry, and in due time Fischer-Tropsch synthesis applied to the production of paraffins became uneconomic anyway. After the war there was a steady worldwide increase in the demand for surfactants. In order to continually meet the demand for synthetic detergents, the industry was compelled to find a substitute for /z-paraffin. This was achieved by the oligomerization of the propene part of raffinate gases with phosphoric acid catalyst at 200°C and about 20 bars pressure to produce tetrapropene. Tetrapropene was inexpensive, comprising a defined C cut and an olefinic double bond. Instead of the Lewis acid, aluminum chloride, hydrofluoric acid could now be used as a considerably milder, more economical, and easier-to-handle alkylation catalyst [4],... 

In this paper we report the use of supported heteropoly acid (silicotungstic acid) and supported phosphoric acid catalysts for the acylation of industrially relevant aromatic feedstocks with acid anhydrides in the synthesis of aromatic ketones. In particular, we describe the acylation of thioanisole 1 with iso-butyric anhydride 2 to form 4-methyl thiobutyrophenone 3. The acylation of thioanisole with acetic anhydride has been reported in which a series of zeolites were used as catalysts. Zeolite H-beta was reported to have the highest activity of the zeolites studied (41 mol % conversion, 150°C) (2). 

Temperatures of 300°C to 400°C and pressures of 60 to 70 atm are used with a phosphoric acid catalyst. Both methanol and ethanol are used widely as solvents and as intermediates for further chemical synthesis. 

The continuous increase of isoprene demand has determined the enhancement of researches focused on the elaboraticxi of new and efficient routes for the industrial synthesis of this valuable diene. The acid-catalyzed cc ensation of formaldehyde and isobutene, known as Prins reacticxi, is already a recognized industrial route for the isoprene synthesis, being applied as two-stages process, condensation to form 4,4-dimethyl-l,3-dioxane in the presence of an aqueous sulphuric acid and its decomposition to isoprene using solid phosphoric acid catalysts. 

An enantioselective synthesis of dibenzo[l,4]diazepines 81 has been realized by employing a SPINOL-derived chiral phosphoric acid catalyst 82 in a three-component reaction between 1,2-phenylenediamines 78, aryl aldehydes 80 and cyclohexane-1,3-diones 79 (14ASC2009).The reaction was generally high yielding and enantiomeric excesses up to 88% could be obtained, although in general the enantioselectivity was moderate. 

American Cyanamid operated a plant with a capacity of 14,000 tpa 2-naphthol, for some years prior to 1982 for the oxidation of 2-isopropylnaphthalene. 2-Iso-propylnaphthalene can be obtained from propylene and naphthalene at 150 to 240 °C and 10 bar with a phosphoric acid catalyst using a large excess of naphthalene, followed by isomerization to a mixture of 1- and 2-isopropylnaphthalenes (5 95). The introduction of air (oxygen) at 110 °C produces the a-hydroperoxide of the 2-isomer. The hydroperoxide is cleaved with sulfuric acid, in a manner analogous to the Hock synthesis of phenol the 2-naphthol yield is around 95%. 

Another important set of bi-component reactions involving C-N and C-C bond formation is based on the Pictet-Spengler reaction, consisting in the cycUzation of electron-rich aromatic moieties onto iminium intermediates. This weU-known sequence constitutes an important domino transformation used for the synthesis of bioactive polyheterocycles. Its organocatalytic asymmetric version was pioneered by Jacobsen and revisited by List, who developed two complementary highly enantioselective accesses to tetrahydro- 3-carbolines from tryptamine-derived imines (Scheme 16.33). Thus, Taylor and Jacobsen [64] reported an enantiomerically pure thiourea-catalyzed cyclization of an acyl iminium intermediate, whereas List and co-workers [65] described the cyclization of an iminium diester intermediate in the presence of a chiral phosphoric acid catalyst. Recently, this methodology has been applied to the synthesis of chiral pyrrolopiperazines [66]. 

Zhu and co-workers [77] have successfully developed the first organocatalytic enantioselective three-component Povarov reaction for the efficient synthesis of enantiomerically enriched (2,4-cis)-4-amino-2-aryl(alkyl)-tetrahydroquinolines. To illustrate the power of this novel catalytic enantioselective three-component Povarov reaction, they applied this methodology to the short and efficient synthesis of torcetrapib (188), a potent cholesteryl ester transfer protein (CETP) inhibitor (Scheme 17.31). Reaction of 4-trifluoromethylaniline 184, propionaldehyde 18, and enecarbamate 185 using phosphoric acid catalyst 186 afforded tetrahydroquino-line 187 in 57% yield with 93% ee. Ethoxycarbonylation, deprotection/acylation, and benzylation provided torcetrapib (188) in four steps with 32% overall yield. 

A hydroamination/N-sulfonyliminium cyclization cascade was carried out enantioselectively with gold and BINOL-phosphoric acid catalysts to generate fused indole 188. The method can also be extended to furnish lactams with up to 93% ee (13OL4330). Polycyclic indoline 189 was prepared via an indium/TFA-catalyzed tandem cycloannulation of P-oxodithioesters with tryptamine in yields up to 92% (130L1974). Dixon and colleagues utilized an enantioselective Michael addition/iminium ion cyclization of trypt-amine-derived ureas in the synthesis of tetracyclic indole 190 (22 examples, up to 78% yield and 96% ee) (130L2946). 

In 2010, another example of the use of phosphoric acid catalyst was reported [214]. In this case, 10mol% of 3,3 -bis(3,5-bis(trifluoromethyl)phenyl)-l,l -8-binaphtyl-2,2 -diyl hydrogenphosphate 81, in toluene at 40 °C, allowed the synthesis of the corresponding a-amino phosphates with modest to good enantioselectivities (31-87% ee) and good yields (61-91%). 

Phenol is an important raw material for the synthesis of petrochemicals, agrochemicals, and plastics. Examples of the uses of phenol as an intermediate include the production of bisphenol A, phenolic resins, caprolactam, alkyl phenols, aniline, and other useful chemicals. Today, almost 95% of worldwide phenol production is based on the so-called cumene process which is a three-step process (the conversion of benzene and propylene to cumene using supported phosphoric acid catalysts, the conversion of cumene to cumene hydroperoxide with air, and the decomposition of hydroperoxide to phenol and acetone with sulfuric acid). The great interest in the oxidation reaction of benzene to phenol is Unked to some disadvantages of the cumene process (environmental impact, production of an explosive hydroperoxide. 

The hydration of oleflns is important for the direct synthesis of alcohols from olefins in the pietroleum industry and has been extensively studied over various solid acid catalysts. In the case of ethanol synthesis from ethylene and water, silicotungstic acids, silicophosphoric acids, solid phosphoric acids, metal sulfates, " and metal oxides have been studied as solid acid catalysts. In its industrial process, a solid phosphoric acid catalyst (Shell patent) is widely used throughout the world. The nature of the active (acidic) sites which exhibit high catalytic activity and selectivity is discussed below together with the hydration mechanism involving the catalytic behavior. 

SCHEME 3.31 Asymmetric synthesis of piperidines nsing phosphoric acid catalysts [29]. 

The synthesis of chiral piperidines has been accomplished through an intramolecular cyclization reaction using chiral phosphoric acid catalysts (Scheme 3.31) [29], Treatment of protected amines bearing tethered unsaturated acetals with catalytic amounts of a chiral phosphoric acid catalyzed the formation of chiral piperidines with high selectivity. 

The synthesis of resolved allylic amines has been achieved using the combination of a chiral palladium catalyst and a resolved phosphoric acid catalyst (Scheme 3.41 and Example 3.6) [39]. The overall reaction generated highly resolved allylic amines after simply stirring at 25 °C. While the precise interaction between the catalyst components... 

The first enantioselective synthesis of 4-aza-podophyllotoxin derivatives (211) has been achieved using a chiral biphenyl phosphoric acid catalyst (212) through a partial transfer hydrogenation of lactone-fused quinolines (210) (Scheme 56)7 ... 

BINOL-derived phosphoric acid catalyst affording the P-amino ketone (231) in 96% yieldand92% ee (Scheme 11.51) [156]. The total synthesis of (-)-anabasine (232) was completed in three more steps in 55% overall yield (i) reduction of the conjugated double bond, (ii) lednction of the ester moiety with an excess DIBAL-H resulting in the formation of the piperidine ring, and (iii) removal of the PMP-protective group. 

Br0nsted Acids Chiral Phosphoric Acid Catalysts in Asymmetric Synthesis... 

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