Hydrogen transfer benzyl derivatives

Catalytic hydrogenation of l,2,3,4-tetrahydro-ll//-pyrido[2,l-6]quina-zolin-ll-one and its 6-, 7-, 8-, and 9-methyl derivatives over Pd/C catalyst in ethanol gave 1,2,3,4,6,7,8,9-octahydro-ll //- derivatives (87JMC1543). A solution of a 1 1 mixture of 9-methyl-l,2,3,4-tetrahydro-ll/f-pyrido-[2,l-6]quinazolin-11 -one (73) and 2-benzyl-9-methyl-l,2,3,4-tetrahydro-ll//-dipyrido[l,2-a 4,3-catalyst yielded a complex reaction mixture containing 15% of 9-methyl-l,2,3,4,6,7,8,9-octahydro-ll//-pyrido[2,l-/)]quinanolizin-Tl-one and other products (87T1157). The double bonds of the pyridine of compound 73 was also saturated by intermolecular catalytic hydrogen transfer from 2-(4-cyclohexenyl)-l,2,3,4,5,6,7,8-octahydroquinazolin-4-one in the presence of Pd/C catalyst [85H(23)3095]. 

In an intermolecular hydrogen transfer reaction catalyzed by palladium on charcoal, 9-methyl-1,2,3,4-tetrahydro-l l//-dipyrido[l,2-n 4,3-rf]pyrimidin-ll-one in refluxing xylene is converted into the 6,7,8,9-tetrahydro isomer as the sole product, whereas the /V-benzyl derivative yields a 4 6 mixture of the 6,7,8,9-tetrahydro and the 1,2,3,4,6,7,8,9-octahydro compounds. With nitrobenzene as the hydrogen acceptor, the main product is the fully aromatic derivative.529,530... 

These oxidations probably occur zna mechanisms initiated by electron transfer to the quinone. Rahman and Kobayashi have proposed a plausible mechanism of oxidation of benzyl derivatives, which can be extended to other species (02MOL840). They propose formation of an ion pair 128, probably resulting from initial SET followed by hydrogen atom transfer (Scheme 34). Combination of the ion pair then forms an ether 129 and further oxidation and cyclisation gives the observed product via the cation 130. 

Nitrogen on heterocycles has frequently been protected as the Af-benzyl derivative. Thus, 2-benzylimidazole 399 was successfully debenzylated to 400 in excellent yield (Scheme 118).[i i It should be noted that the benzylic ketone in 399 was not hydrogenated. Transfer hydrogenolysis with ammonium formate was also effective for the removal of the benzyl group in A -benzyl-2-methylimidazole. 

Finally, allene derivatives were also convenient unsaturated substrates allowing carbon-carbon bond formation from benzylic alcohol via hydrogen transfer processes. With these substrates, the best catalytic systems were based on RuHCl (CO)(PPh3)3 in the presence of an equimolar amount of phosphine ligand such as bis(diisopropylphosphino)ferrocene (dippf) [63], bis(dicyclohexyphosphino) ferrocene or PCyPh2 [64], Some examples of selective formation of homoallylic alcohols using this reaction are reported in Scheme 59. 

H-displaccment at the benzylic carbon (Ceraulo et aL 1991). Usually the EE precursor is bifunctional, so that the site of the charge moves (Equation 8.112 Seiler and Hesse 1968). The characteristic periodicity of (CH2)4 in the C H2 i02 ion series in the spectra of fatty-acid derivatives ( i-alkyl-COOR) may in part be due to displacement reactions combined with favored p- and y-hydrogen transfers to the radical sites. Thus the mass spectrum of octadecanoic acid (Figure 8.9 see also Figure 3.17) shows maxima in this series at mjz 73,129,185 and 241. The methyl alkanoates (Figure 3.18) show maxima at mjz 87,143, 199, 255,... 

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