Carbonylative couplings

2 Cross-coupling of Organoboron Compounds with Organic Halides 

The cross-coupling reaction of organoboron compounds with organic halides or related electrophiles represents one of the most straightforward methods of carbon-carbon bond 

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

Low valent titanium McMurry carbonyl coupling is believed to go through the vic-diol. vic-diols are smoothly converted to the corresponding olefins under these conditions. JOC 1976, 41, 896... 

Most metal carbonyls are volatile soflds that sublime easily. The volatility of metal carbonyls coupled with their toxicity is an important safety consideration. The vapor pressure of many metal carbonyls have been tabulated elsewhere (75). 

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.

Xia and co-workers synthesised a number of Pd-NHC complexes (33, 34, 36) for carbonylative Suzuki reactions (Fig. 9.6) [41], Various aryl iodides were carbonylatively coupled (P = 1 atm) with either phenylboronic acid or sodium tetraphenylborate. All the complexes were highly active, but 33 provided the best results with >76% selectivity for ketone in all the reactions. Xia followed this work with the double carbonylation of various aryl iodides with several secondary amines using the catalysts [CuX(Mes)] (37-X) and [Cu(IPr)X] (38-X) (X = I, Br, Cl) (3 MPa, 100°C, 10 h) (Scheme 9.7) [42],... 

Scheme 10 Plausible catalytic mechanism for alkyne-carbonyl coupling as supported by the effect of chiral Bronsted acid catalyst and deuterium-labeling...

A somewhat related process, the cobalt-mediated synthesis of symmetrical benzo-phenones from aryl iodides and dicobalt octacarbonyl, is shown in Scheme 6.49 [100]. Here, dicobalt octacarbonyl is used as a combined Ar-I bond activator and carbon monoxide source. Employing acetonitrile as solvent, a variety of aryl iodides with different steric and electronic properties underwent the carbonylative coupling in excellent yields. Remarkably, in several cases, microwave irradiation for just 6 s was sufficient to achieve full conversion An inert atmosphere, a base or other additives were all unnecessary. No conversion occurred in the absence of heating, regardless of the reaction time. However, equally high yields could be achieved by heating the reaction mixture in an oil bath for 2 min. 

Palladium(0)-catalyzed coupling reactions - i. e. the Heck and Sonogashira reactions, the carbonylative coupling reactions, the Suzuki and Stille cross-coupling reactions, and allylic substitutions (Fig. 11.1) - have enabled the formation of many kinds of carbon-carbon attachments that were previously very difficult to make. These reactions are usually robust and occur in the presence of a wide variety of functional groups. The reactions are, furthermore, autocatalytic (i.e. the substrate regenerates the required oxidation state of the palladium) and a vast number of different ligands can be used to fine-tune the reactivity and selectivity of the reactions. 

Equation 11.15 Carbonylative coupling with molybdenum hexacarbonyl. 

Morimoto, Kakiuchi, and co-workers were the first to show that aldehydes are a useful source of CO in the catalytic PKR [68]. Based on 13C-labeling experiments, it was proposed that after decarbonylation of the aldehyde, an active metal catalyst is formed. This was proven by the absence of free carbon monoxide. As a consequence CO, which is directly generated by previous aldehyde decarbonylation, is incorporated in situ into the carbonylative coupling. The best results were obtained using C5F5CHO and cinnamaldehyde as CO source in combination with [RhCl(cod)]2/dppp as the catalyst system. In the presence of an excess of aldehyde the corresponding products were isolated in the range of 52-97%. 

Carbonylative coupling of iodobenzene with 2-methyl-3-butyn-2-ol under 65 bar carbon monoxide afforded phenylfuranones (double carbonylation) in reasonable yields (Scheme 6.32) [69]. The reaction is thought to proceed through the formation of a benzoylpalladium intermediate which either reacts with the alkynol or liberates benzoic acid hence the formation of considerable amounts of the latter. 

Co complexes, Buchwald reported the Ti-catalyzed carbonylative coupling of enynes-the so-called Pauson-Khand-type reaction [28]-and realized the first such catalytic and enantioselective reaction using a chiral Ti complex [29]. Here, a variety of enynes were transformed into bicyclic cyclopentenones with good to high ee-values however, several steps were required to prepare the chiral Ti catalyst, while the low-valent complex proved to be so unstable that it had to be treated under oxygen-free conditions in a glove box. 

By contrast, in 2000 Shibata reported the Ir-catalyzed enantioselective Pauson-Khand-type reaction of enynes [30aj. The chiral Ir catalyst was readily prepared in situ from [lrCl(cod)]2 and tolBINAP (2,2 -bis(di-p-tolylphosphino)-l,T-binaphthyl), both of which are commercially available and air-stable, and the reaction proceeded under an atmospheric pressure of carbon monoxide. The Ir-catalyzed carbonylative coupling had a wide generality in enynes with various tethers (Z), substituents on the alkyne terminus (R ) and the olefinic moiety (R ). In the case of less-reactive enynes, a lower partial pressure of carbon monoxide achieved a higher yield and ee-value (Table 11.1) [30b]. 

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

Miller PW, Long NJ, de Mello AJ et al (2006) Rapid formation of amides via carbonylative coupling reactions using a microfluidic device. Chem Commun 5 546-548... 

Carbonylative coupling reaction of Ar3Bi and carbon monoxide takes place under rhodium catalysis (Scheme 46) [61, 62]. Ph3Bi reacted with carbon monoxide in the presence of 5 mol% of [RhCl(CO)2]2 in MeCN at room temperature to give benzophenone in 71% yield together with a small amount of biphenyl. The same... 

Scheme 46 Carbonylative coupling reaction of Ar3Bi with carbon monoxide...

A similar carbonylative coupling reaction was applied to the kinetic resolution of secondary alcohols [63]. In the presence of a Pd catalyst ligated by chiral oxazolinylferrocenylphosphine, the pentavalent Ph3Bi(OAc)2 and carbon monoxide effectively benzoylated secondary alcohols, and up to 48% enantiomeric excess (ee) was attained (Scheme 47). Although the enantioselectivity is not satisfactory, this is a unique new procedure for the kinetic resolution. 

Scheme 47 Kinetic resolution of secondary alcohols by carbonylative coupling reaction of Ph3Bi (OAc)2...

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