Hydroalkylation

Keywords Bismuth Friedel-Crafts alkylation Green chemistry Hydroalkylation Hydroarylation... 

This first example of a Bi(OTf)3-catalyzed Friedel-Crafts alkylation originated in the following procedures, including benzylations of 2,4-pentanediones or hydroarylation and hydroalkylation reactions. A related procedure was simultaneously developed by Bonrath et al. [39]. The authors utilized Bi(OTf)3 in the synthesis of (all-rac)-a-tocopherol (Vitamin E) [39], Besides rare earth metal triflates, such as Ga(OTf)3, Hf(OTf)3, Sc(OTf)3 and Gd(OTf)3, Bi(OTf)3 was shown to be the most efficient catalyst for the Friedel-Crafts-type reaction between trimethylhydroquinone acetate 10b and isophytols 11a, b. With only 0.02 mol% Bi(OTf)3 (substrate to catalyst ratio 5,000 1) the desired a-tocopherols 12a and 12b were isolated in excellent yields (Scheme 10). 

On the basis of previous results, the Rueping group started to study alkylations with activated alkenes and nucleophiles other than arenes. Recently, Lewis- and Brpnsted-acid-catalyzed hydroalkylation procedures with 1,3-dicarbonyl compounds as nucleophiles have been developed [69, 70]. These reactions primarily utilized Pd... 

Scheme 31 Bi(OTf)3-catalyzed hydroalkylation of 2,4-pentanediones with activated double bonds...

Scheme 33 Hydroalkylation of 4-hydroxycomarin yields highly desirable warfarin derivatives...

Product predictions for nucleophilic additions to C q, based on AMI calculations, show that among the many possible isomers a few are energetically favored [29]. Two areas within the molecule can be distinguished, which are the inert belt at the equator and the more reactive CgQ-like double bonds at the poles (Figure 3.4). Experimentally, hydroalkylation and hydroarylation reactions of C q under quantitative HPLC control yield predominantly one isomer of each C7oHR[4]. 

An example of an intermolecular consecutive reaction is the hydroalkylation of olefins [139, 140]. Ti02 photocatalyzes the reaction of benzyltrimethylsilane... 

Figure 13. Hydroalkylation of maleic acid by benzyltrimethylsilane C-Si bond cleavage of the intermediate radical cation followed by intermolecular addition of the generated radical to maleic acid.

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Depezay, J-C., Dureault, A, Double induction asymetrique hydroalkylation diastereoselective de la L-alanine par le L-glyceraldehyde, Tetrahedron Lett., 25, 1459-1462, 1984 see also Ref. [84]. 

To the best of our knowledge, there are few reports focused on the hydroalkylation of P-dicarbonyl compounds in the ionic liquids [32, 33] some of the obtained results are tabulated in Table 2.2. It is obvious that the use of the ionic liquids [BMIM][PFg] and [BMIMjOTf as the green solvents promotes the hydroalkylation... 

Qureshi ZS, Deshmttkh KM, Tambade PI et al (2010) Amberlyst-15 in ionic liquid an efficient and recyclable reagent for the benzylation and hydroalkylation of P-dicarbonyl com-potmds. Tetrahedron Lett 51 724-729... 

Lanman HE, Nguyen RV, Yao X et al (2008) Evaluating Lewis acid catalyzed hydroalkylation of alkenes in neat and in ionic Hqttids. 1 Mol Catal A Chem 279 218-222... 

Transition metal catalyzed asymmetric hydrocarboration reactions are addition reactions forming one C—C and one C—H bond. Prominent examples are hydrovinylation, hydroformylation, hydroacylation, hydrocarboxylation, and hydrocyanation. Various related conversions, such as hydroalkylation, hydroarylation, conjugate addition, reductive dimerization, and metal induced ene reactions are collected in Section 1.5.8.2.6. dealing with miscellaneous methods of this type. Some of these methods are not exclusively mediated by metal catalysts and therefore are also covered in other sections of this volume. 

Oligomerizations, polymerizations and telomerizations are covered in other parts of this section, since C-C/C-C bond forming additions (dicarborations) are involved. Metal-mediated hydroalkylations of olefins with CH acidic compounds and hydroarylations with formal addition of aromatic C—H bonds to olefins are also known2, however, only a few examples of stereoselective applications have been reported (Section 1.5.8.2.6.). 

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