Alcohols carbonylation-dimerization

The carboxytelomerization, a variant to the usual telomerization, is the carbonylation-dimerization of butadiene, carbon monoxide and alcohols. 

Most ozonolysis reaction products are postulated to form by the reaction of the 1,3-zwitterion with the extmded carbonyl compound in a 1,3-dipolar cycloaddition reaction to produce stable 1,2,4-trioxanes (ozonides) (17) as shown with itself (dimerization) to form cycHc diperoxides (4) or with protic solvents, such as alcohols, carboxyUc acids, etc, to form a-substituted alkyl hydroperoxides. The latter can form other peroxidic products, depending on reactants, reaction conditions, and solvent. 

A novel route to azelaic acid is based on butadiene. Butadiene is dimerized to 1,5-cyclooctadiene, which is carbonylated to the monoester in the presence of an alcohol. Hydrolysis of this ester foUowed by a caustic cleavage step produces azelaic acid in both high yield and purity (56). 

Simple carbonylation and dimerization-carbonylation of butadiene take place in alcohol depending on the catalytic species of palladium. When PdCl2 is used as a catalyst with or without PPh3, 3-pentenoate (72) is the sole product (74, 75). On the other hand, when Pd(OAc)2 is used with PPh3, the dimerization-carbonylation takes place to give 3,8-nona-dienoate (73) (76, 77). 

The essential factor which differentiates the monomeric and dimeric carbonylations seems to be the presence or absence of halide ion coordinated to the palladium. The dimerization-carbonylation proceeds satisfactorily with halide-free palladium phosphine complexes. Most conveniently, Pd(OAc)2 is used with PPh3. PdCl2(PPh3)2 can be used as a catalyst with addition of an excess of bases. The reaction is carried out at 1I0°C under 50 atm of carbon monoxide pressure in alcohol. Higher... 

The carbonylation was explained by the following mechanism. Formation of dimeric 7r-allylic complex 20 from two moles of butadiene and the halide-free palladium species is followed by carbon monoxide insertion at the allylic position to give an acyl palladium complex which then collapses to give 3,8-nonadienoate by the attack of alcohol with regeneration of the zero-valent palladium phosphine complex. When halide ion is coordinated to palladium, the formation of the above dimeric 7r-allylic complex 20 is not possible, and only monomeric 7r-allylic complex 74 is formed. Carbon monoxide insertion then gives 3-pentenoate (72). 

No dimerization-carbonylation takes place with isoprene, irrespective of the catalytic species (78). Selective formation of 4-methyl-3-pentenoate (75) was observed in alcohol by the catalysis of either PdCl2 or Pd(OAc)2 and PPh3 ... 

The shape of this wave and the variation with pH are both consistent with fast equ-librium reactions In the pH region lower than the value of pK, for the hydroxyl radical, the reactions of this hydroxyl radical dominate the electrochemical process. Controlled potential reduction at the potential of this first wave indicates a IF process and the principal products are dimers of the hydroxyl radical. The second wave in this acidic region is due to addition of an electron and a proton to the neutral radical. This process competes with dimerization in the mid-pH range where the two polarographic waves merge. Over the pH range 7-9, monohydric alcohol is the principal product. At pH <3 or >12, pinacols are the main products. Unsymmet-rical carbonyl compounds afford mixtures of ( )- and meso-pinacols. Data (Table 10.3) for the ( ) / meso isomer ratio for pinacols from acetophenone and propio-phenone indicate different dimerization mechanisms in acid and in alkaline solutions. 

Esters and acids from simple carbonylation reactions Alcohols, ethers and esters with higher homologous alkyl groups. Hydrocarbons from hydrogenolysis of the alcohol and its homologs. Ethers from dehydration of the substrate. Esters of the reagent alcohol. s)oiefins from dehydration of the alcohols. Isomeric alcohols. Isomer products (linear/branched 50/50 - 60/40). Only 2-methyl butanol Dimers and trimers of i-butene. 

Moreover, no aldol or pinacol-type dimerization of the carbonyl compound was observed, even in the case of easily reducible benzophenone or benzaldehyde derivatives. As observed previously, the nature of the solvent is an important factor for the success of the reaction. Indeed, no homoallylic alcohols were formed in DMF. 

A very short and elegant synthesis of the 16-rtiembered dilactone ( )-pyrenophorin (515) has been accomplished by the dipolar cycloaddition reaction of a trialkylsilyl nitronate (81TL735). Nitromethane was added to 3-buten-2-one and the carbonyl group of the product reduced with sodium borohydride. The nitro alcohol (511) was converted to the acrylate (512) which was then subjected to a dimerization-cyclization reaction by treatment with chlorotrimethylsilane and triethylamine in dry benzene. Hydrogenation of the mixture of isoxazoline products (513) over palladium on charcoal followed by double dehydration of the intermediate bis-/3-hydroxyketone (514) led to ( )- and meso-pyrenophorin (Scheme... 

With rhodium trichloride, the only species observed in solution were Rh2(CO)4Cl2 and HC1 gas. As noted in Table I, no hydroformylation occurred in this reaction mixture. When the carbonyl chloride dimer was used as starting material, the clusters, Rh4(CO)i2 and Rh6(CO)i6, formed along with the anion, Rh(CO)2Cl2. Aldehyde, but no alcohol, was produced. These IR data are in good agreement with earlier reports on amine-free systems (16), and they support the contention that a chloride-free carbonyl (or carbonyl hydride) is the active hydroformylation catalyst. 

---