Carbonyl compounds, parent hydrides

The B-alkyl-9-BBN undergoes an interesting reverse reaction to afford the parent alkene when treated with benzaldehyde. Consequently, the reaction is uniquely employed for the synthesis of exocyclic olefins (Chart 24.3). The hy-droboration of cyclic olefins with an internal double bond, followed by homologation with carbon monoxide in the presence of lithium trimethoxyaluminum hydride afford B-(cycloalkylmethyl)-9-BBN. This intermediate on treatment with benzaldehyde leads to an exocyclic methylene compound (Chart 24.3) [16]. Since the synthesis proceeds from the cycloalkene, thus it provides a valuable alternative to the customary methylenation of carbonyl compounds by Wittig and related procedures. The method also provides a clean synthesis of deuterium-labeled compounds (Eq. 24.10) [16], without positional scrambling or loss of label. Consequently, methylmethylene-d -cyclopentane in 52% isolated yield is obtained. 

The transformation of propargylsilyl ethers into dienylsilyl ethers is catalysed by the ruthenium hydride complex [RuH(Cl)(Ph3P)2] [equation (23)]. Dienylmethyl ethers are readily cleaved to the parent carbonyl compound by... 

Primary halides (and some secondary cases) may be oxidized directly to carbonyl compounds by reaction with chromate ion, either in the presence of crown compounds, or with the chromate associated with a polymer matrix. Reduction of low molecular weight halides to hydrocarbons has been reported in superacid media [e.g. HF-TaFs), and a hydride transfer pathway is suggested. Allyl iodides may be reduced with triphenylphosphonium hydriodide (equation 10), but acid sensitive groups may not survive these conditions. Vinylic and aryl halides are converted into the parent hydrocarbons by reaction with Grignard reagents and a catalytic quantity of manganese salts a reaction mechanism has been proposed. 

Derivatives of 1-methyl-3//-l,4-benzodiazepine-2,5(l/I,4/I)dione (193) were synthesized by ring closure of substituted 2-(A-chloro-acetyl-A-methylaminoJbenzamides with sodium methoxide in methanol.209 Treatment of 193 with lithium aluminum hydride led to reduction of both carbonyl groups.209 The parent tetrahydro system (194) has been prepared by reaction of the tosylate (195) with 1,2-dibromoethane followed by hydrolysis.210 The preparation of 194 by another route had previously been noted.204 Reaction of 194 with formaldehyde or benzaldehyde gave a compound formulated as 196 (R = H or C6H5).210 Hydrolysis of 196 (R = C6H5) with 0.1 N hydrochloric acid gave 194 while 196 (R = H) was not hydrolyzed at this acidity. 

The parent compound in this family, the rhenocene hydride ReCp2H (83) is a milestone in organometallic chemistry, since it was the first isolated carbonyl-free hydride and the first hydride to be characterized by H NMR in 1958. However, it has a more limited chemistry compared to the other groups 4, 5, and 6 bent metallocenes (see Metallocene Complexes) ... 

Vincatine (mp 111-112°) gave a molecular ion at m/e 370, in agreement with a molecular formula of C22H30N2O3. The IR spectrum showed carbonyl absorptions at 1705 and 1735 cm-1 and the UV spectrum indicated the presence of a 2-oxindole. Four aromatic protons were observed in the PMR spectrum together with JV-methyl (3.20 ppm) and carbomethoxyl (3.62 ppm) groups. Lithium aluminum hydride reduction gave a diol 164 but acetylation of this compound gave only a monoacetyl derivative. The structure of vincatine (163) was deduced on the basis of a rationalization of the mass spectra of the parent compound and its various derivatives, particularly the ions at m/e 297, 211, 182, and 124 in the parent compound (Scheme 3) and the base peak at m/e 184 (165) in the mass spectrum of the diol. 

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