Oxidation induced carbonyl substitution

Analogous to its reaction with carbonyl compounds (see 6.3.4), benzyltrimethyl-silane undergoes a fluoride-induced nucleophilic substitution reaction on pyridine-1-oxides and quinoline-l-oxide to form 2-benzylpyridines (>70%) and 2-benzyl-quinoline (65%), respectively [57], Allyltrimethylsilane reacts with pyridine-l-oxide to produce 2-propenylpyridine (56%). 

Substitution of CO ligands in clusters is most commonly realized in the same way as in the case of mononuclear metal carbonyls. Substitution may be induced by one of the following, most frequently utilized methods thermal, electrochemical, chemical (reactions with N-oxide of trimethylamine or Bu"PO), photochemical, catalysis by radicals, catalysis by transition metal compounds, etc. ... 

As shown in the synthetic sequence, to induce isomerisation of the c -B/C junction to a trans configuration, a double bond between C(6) and C(7), that can transmit the electronic effects of carbonyl group at C(5) must be created ex profeso. Yhis relatively simple structural modification requires no less than five different synthetic operations i) bromination at C(6) ii) substitution of the bromine atom by an OH group iii) oxidation iv) 0-methylation with dimethyl sulphate, and... 

Chromium hexacarbonyl is extremely photolabile (equation 6) therefore photochemical substitution is an efficient means of preparing derivatives. Oxidation of the Cr center requires nitric or sulfuric acid, or chlorine. Alternatively, some hgands induce complete carbonyl dissociation with concomitant oxidation, for example, acetylacetonate. Chemical reduction with alkali or alkaline-earth metals or electrochemical reduction proceeds in two-electron steps with loss of two CO molecules to first give [Cr2(CO)io]" and then [Cr(CO)s]. Nucleophilic attack at CO generates a number of stable (Nu = R) and unstable (Nu = N3, OH, H, NEt2) products. The stable [(OC)5CrCOR] ion is a carbene precursor. 

Manganese(III) can oxidize carbonyl compounds and nitroalkanes to carboxy-methyl and nitromethyl radicals [186]. With Mn(III) as mediator, a tandem reaction consisting of an intermolecular radical addition followed by an intramolecular electrophilic aromatic substitution can be accomplished [186, 187). Further Mn(III)-mediated anodic additions of 1,3-dicarbonyl and l-keto-3-nitroalkyl compounds to alkenes and alkynes are reported in [110, 111, 188). Sorbic acid precursors have been obtained in larger scale and high current efficiency by a Mn(III)-mediated oxidation of acetic acid acetic anhydride in the presence of butadiene [189]. Also the nitromethylation of benzene can be performed in 78% yield with Mn(III) as electrocatalyst [190]. A N03 radical, generated by oxidation of a nitrate anion, can induce the 1,4-addition of aldehydes to activated olefins. NOj abstracts a hydrogen from the aldehyde to form an acyl radical, which undergoes addition to the olefin to afford a 1,4-diketone in 34-58% yield [191]. 

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