Reactions 1 Carbonylative Coupling

Keywords. Organotin compounds. Palladium catalyst. Cross-coupling reaction, Carbonylative coupling... 

Carbonyl Coupling Reactions (McMurry Reaction) (Smith Ch. 13.7.F)... 

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). 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

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... 

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 ... 

Although analogous to the direct coupling reaction, the catalytic cycle for the carbonylative coupling reaction is distinguished by an insertion of carbon monoxide into the C-Pd bond of complex A (see A—>B, Scheme 31). The transmetalation step-then gives trans complex C which isomerizes to the cis complex D. The ketone product E is revealed after reductive elimination. 

Scheme 31. Catalytic cycle for the Stille reaction carbonylative coupling.

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

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). ... 

Kishimoto et al. (1974, 1981) found a general acid catalysis by protonated pyridines in coupling reactions of the 1-naphthoxide ion if weakly electrophilic diazonium ions were used. In this case it is likely that the general acid protonates the carbonyl oxygen of the o-complex, with a concerted or stepwise deprotonation at the 4-position (transition stage 12.150). 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

Radical-based carbonylation procedures can be advantageously mediated by (TMSlsSiH. Examples of three-component coupling reactions are given in Reactions (74) and (75). The cascade proceeds by the addition of an alkyl or vinyl radical onto carbon monoxide with formation of an acyl radical intermediate, which can further react with electron-deficient olefins to lead to the polyfunctionalized compounds. ... 

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