Homogeneous catalysis transfer hydrogenation conditions

In a very recent computational study, Diggle et al. have calculated the activation barriers for C(aryl)-X activation (X = H, F, OH, NH, CH3) as 0 (H), 9 (F), 12 (OH), 20 (NH ) and 21.3 kcal mol (CH3), respectively [155]. In comparison, the activation barrier for C(sp3)-H is 6.6 kcal moF [156]. C-X activation occurs under reaction conditions relevant for homogenous catalysis [157], but does not always result in decomposition as C-H activation is often reversible and can be exploited in catalytic transfer hydrogenations involving alcohols [156]. 

Hydrogenation with homogeneous catalysis has received increasing attention over the last few decades. Hexarhodium hexadecacarbonyl (181 Scheme 35) under water gas shift conditions forms 1,2,3,4-tetrahydroquinoline (128) or N-formyl-1,2,3,4-tetrahydroisoquinoline (182) with the parent hete-roaromatic. When hydrogen is substituted for carbon monoxide, 4-methylquinoline is reduced to 4-methyl-5,6,7,8-tetrahydroquinoline (183) exclusively.Isoquinoline behaves similarly generating A/-formyl-l,2,3,4-tetrahydroisoquinoline (184). Similar reductions under water gas shift or synthesis gas conditions using transition metal carbonyls derived from Mn, Co and Fe, have been recorded.Promotion by phase transfer agents is observed in some cases. 

Because of the success experienced in organic synthesis by using phase transfer reaction conditions IS), it was expected that this would be an excellent method for hydrogenating organic soluble dienes by using the water soluble catalyst K3[Co(CN)5H]. Others recently have carried out homogeneous catalysis processes under phase-transfer reaction conditions 14,15,16). 

The publication (70) in 1976 of the preparation of optically active epoxyketones via asymmetric catalysis marked the start of an increasingly popular field of study. When chalcones were treated with 30% hydrogen peroxide under (basic) phase-transfer conditions and the benzylammonium salt of quinine was used as the phase-transfer catalyst, the epoxyketones were produced with e.e. s up to 55%. Up to that time no optically active chalcone epoxides were known, while the importance of epoxides (arene oxides) in metabolic processes had just been discovered (71). The nonasymmetric reaction itself, known as the Weitz-Scheffer reaction under homogeneous conditions, has been reviewed by Berti (70). 

Weak acid-base chiral complex formation represents hydrogen bond catalysis (see Chapter 9) and deprotonation followed by cation/anion association under homogeneous, and also under phase-transfer conditions (see Chapter 4) [14, 65],... 

Electrophilic acetylenes 227 (R = Et02C, CHO or CN R = EtC02, Ph or (EtO)2CH) add the elements of hydrogen fluoride on treatment with caesium fluoride under phase-transfer catalysis to yield mainly (Z)- fluoroalkenes 228 dimethyl acetylenedicarboxylate affords a 85 15 mixture of (Z)- and ( )-adducts under these conditions. No addition occurs in homogeneous solutions ... 

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