SYNTHESIS OF HETEROCYCLES VIA PALLADIUM-CATALYZED CARBONYLATION OF ORTHO-SUBSTITUTED ORGANIC NITRO-COMPOUNDS IN RELATIVELY MILD CONDITIONS

Carbonylation of o-nitroaniline in the presence of aldehydes catalyzed by the Pd(TMB)2/TMPhen/TMBH catalaytic system TMBH=2,4,6-trimethyl benzoic acid; TMPhen = 3,4,7,8-tetramethyl-1,10-phenanthroline) at 140-180-degrees-C and 20-40 atm of carbon monoxide, gives the corresponding benzimidazoles, with a selectivity ranging from 10 to 80%, strongly depending on the aldehyde used. The main by-product was shown to be the 2(1H)benzimidazolone, derived by direct carbonylation of o-nitroaniline. The same catalytic system has been used in the carbonylation of o-nitrostyrenes at 120-180-degrees-C and 20-60 atm of carbon monoxide, obtaining the corresponding indoles with a selectivity generally ranging from 70 to 100%. Interestingly, in the latter case the presence of TMBH was not necessary in order to obtain good conversions and selectivities. On the contrary, the presence of excess TMBH is indispensable when nitrobenzene is catalytically converted to phenyl isocyanate. Moreover, in the catalytic synthesis of indoles no carbonylated products are formed. These observations suggest a catalytic cycle different from what has been proposed in the synthesis of phenyl isocyanate from nitrobenzene, catalyzed by palladium.