Carbonyl groups coupling

The aldehydic functional group produces a characteristic band at very low field arising from the resonance of the proton bonded to the carbonyl group. Coupling between this proton and the adjacent aliphatic groups is usually observed in the HNMR spectra of the aliphatic aldehydes. 

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

In general, imines are too reactive to be used to protect carbonyl groups. In a synthesis of juncusol, however, a bromo- and an iodocyclohexylimine of two identical aromatic aldehydes were coupled by an Ullman coupling reaction modi-fied by Ziegler. The imines were cleaved by acidic hydrolysis (aq. oxalic acid, THF, 20°, 1 h, 95% yield). Imines of aromatic aldehydes have also been prepared... 

All carbon signals and resolved couplings can be assigned C-6 is more deshielded (8c = 150.0) than C-7 (8c = 146.5) due to electron withdrawal of the carboxy carbonyl group in para position. 

The initial step of the coupling reaction is the binding of the carbonyl substrate to the titanium surface, and the transfer of an electron to the carbonyl group. The carbonyl group is reduced to a radical species 3, and the titanium is oxidized. Two such ketyl radicals can dimerize to form a pinacolate-like intermediate 4, that is coordinated to titanium. Cleavage of the C—O bonds leads to formation of an alkene 2 and a titanium oxide 5 ... 

Barbituric acid can be considered as a cyclized malonic acid diamide (malonyl-urea). It is therefore a cyclic diketone that may be classified, in the sense of the compounds discussed in Section 12.6, as a coupling component with a methylene group activated by two carbonyl groups in the a- and a -positions. The reaction with arenediazonium salts was studied by Nesynov and Besprozvannaya (1971). These authors obtained coupling products (in good yield) that they considered to be arylhydrazones. Coupling with 4-(phenylazo)benzenediazonium chloride was studied by Chandra and Thosh (1991). The lH NMR spectra of these compounds are consistent with the arylhydrazone structure 12.68. 

Methylene coupling components with carbonyl groups in a,a-positions are very important, particularly acetoacetanilide derivatives (Scheme 12-40, X=NHAr), for the production of yellow monoazo dyes and pigments (Hansa Yellows see Zollinger, 1991, p. 180). 

Another synthetic application of an azo coupling reaction was found by Hecken-dorn (1987, 1990). In aqueous acetone a series of dialkyl (chloroacetamido)malo-nates, which are coupling components with a CH group activated by two carbonyl groups, were found to react with arenediazonium ions as in Scheme 12-44. The primary product cannot be isolated because it cyclizes easily to give a 1,2,4-triazin-5-one derivative (12.90). ... 

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