Labelling carbonyl reduction

Scheme 20 Fully aqueous preparation of a [Cp99mTc(CO)3] labelled biomolecule, enabled by the carbonyl group directly attached to the cyclopentadienyl-ring a two-step approach, b direct one-step procedure and c observed by-product from carbonyl reduction by bora-nocarbonate...

The asymmetric reduction of raeso-epoxides with hydrogen or hydrides has been scarcely explored, despite the synthetic utility of the chiral secondary alcohol products. The lone example has been provided by Chan, who treated the disodium salt of epoxysuccinic acid with H2 or MeOH as the reducing agent in the presence of a chiral rhodium catalyst (Scheme 13) [27]. Deuterium labeling experiments established that the reduction proceeded through direct cleavage of the epoxide C-O bond, rather than isomerization to the ketone followed by carbonyl reduction. 

At the time of writing, most of the metabolites examined by the C satellite method also contained labelled carbons not directly bonded to protons, and these labelled nuclei cannot be detected by the satellite method. In some cases, this problem may be circumvented by chemical transformation. For example, in fusaric acid, protons were chemically introduced on to a labelled carbon by reduction to the alcohol i.e COjH CH OH). This procedure could, perhaps, be applied to other labelled carbonyl groups and olefinic carbons ... 

The mechanism of the reduction remains uncertain. The work of E. D. Williams, K. A. Krieger and A. R. Day (1953) using deuterium-labelled aluminium isopropoxide, shows that hydrogen atoms are transferred predominantly from the central carbon atom of an isopropoxide group to the carbon atom of the carbonyl group undergoing reduction, the process probably involving a cyclic complex ... 

The introduction of tritium into molecules is most commonly achieved by reductive methods, including catalytic reduction by tritium gas, PH2], of olefins, catalytic reductive replacement of halogen (Cl, Br, or I) by H2, and metal pH] hydride reduction of carbonyl compounds, eg, ketones (qv) and some esters, to tritium-labeled alcohols (5). The use of tritium-labeled building blocks, eg, pH] methyl iodide and pH]-acetic anhydride, is an alternative route to the preparation of high specific activity, tritium-labeled compounds. The use of these techniques for the synthesis of radiolabeled receptor ligands, ie, dmgs and dmg analogues, has been described ia detail ia the Hterature (6,7). 

Three different methods have been discussed previously (sections III-C,III-D and IV-A) for the replacement of a carbonyl oxygen by two deuteriums. However, in the conversion of a 3-keto steroid into the corresponding 3,3-d2 labeled analog, two of the three methods, electrochemical reduction (section ni-C) and Raney nickel desulfurization of mercaptal derivatives (section IV-A), lead to extensive deuterium scrambling and the third method, Clemmensen reduction (section III-D), yields a 2,2,3,3,4,4-dg derivative. 

The N-labeled amines produced by partial and total reduction of the nitro groups in 2,4,6-trinitrotoluene reacted with carbonyl groups (quinones and ketones) in humic acid to produce a range of products (Thom and Kennedy 2002). 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

X-ray structure analyses of Rh(COCH3)(I)2(dppp) (14) and [Rh(I I)(I)(//-I)(dppp)]2 (15), where dppp l,3-bis(diphenylphosphino) propane, were reported. Unsaturated complex (14) possesses a distorted five-coordinate geometry that is intermediate between sbp and tbp structures.69 Under CO pressure, the rhodium/ionic-iodide system catalyzes either the reductive carbonylation of methyl formate into acetaldehyde or its homologation into methyl acetate. By using labeled methyl formate (H13C02CH3) it was shown that the carbonyl group of acetaldehyde or methyl acetate does not result from that of methyl formate.70... 

Although a cobalt-catalyzed intermolecular reductive aldol reaction (generation of cobalt enolates by hydrometal-lation of acrylic acid derivatives and subsequent reactions with carbonyl compounds) was first described in 1989, low diastereoselectivity has been problematic.3 6 However, the intramolecular version of this process was found to show high diastereoselectivity (Equation (37)).377,377a 378 A Co(i)-Co(m) catalytic cycle is suggested on the basis of deuterium-labeling studies and the chemistry of Co(ll) complexes (Scheme 81). Cobalt(m) hydride 182, which is... 

That specific hydride transfer from carbon to carbon does occur, was established by showing that use of labelled (Me2CDO)3Al led to the formation of RjCDOH. The reaction probably proceeds via a cyclic T.S. such as (47), though some cases have been observed in which two moles of alkoxide are involved—one to transfer hydride ion, while the other complexes with the carbonyl oxygen atom. The reaction has now been essentially superseded by MH reductions, but can sometimes be made to operate in the reverse direction (oxidation) by use of Al(OCMc3)3 catalyst, and with a large excess of propanone to drive the equilibrium over to the left. This reverse (oxidation) process is generally referred to as the Oppenauer reaction. 

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