Molybdenum hexacarbonyl [Mo(CO)6]

Abstract

(A) The use of a thermostable catalytic system in combination with Mo(CO)6 allowed the synthesis of benzofuranones starting from bromobenzyl alcohols. Indanones were obtained under similar conditions using ortho-bromo(chloro)styrenes. A variant was performed by using polymer-supported amines for the synthesis of amides from aryl halides. A convenient synthesis of α-methylene-γ-butyrolactones from allenyl carbonyls was performed by a DMSO-promoted carbonylation. These reactions enlighten the role of Mo(CO)6 as a source of CO. (Chemical equation presented). (B) 'Instant' catalysts formed from Mo(CO)6 and phenols have been developed for alkyne metathesis. Pre-heating activation of catalytic systems consisting of Mo(CO)6 and 4-chlorophenol has increased the yields of productive enyne metathesis. This procedure was used to dimerize ortho-alkoxypropynylbenzenes and to afford ring-closing alkyne metathesis (RCAM) products from dipropynyls. The use of 2-fluorophenol in place of 4-chlorophenol led to a more reactive and friendly catalyst, which has been employed not only in RCAM, but also in alkyne homodimerizations (HD) and cross metatheses (CM). (Chemical equation presented). (C) Mo(CO)6 was reported to effect Pauson-Khand reactions of enynes to afford cyclopentenones. Chiral alkynyl allenes afforded enantioenriched α-alkylidene cyclopentenones. Under the same conditions, functionalized difluoroallenes underwent intramolecular [2+2] cycloaddition to afford gem-difluoro bicyclo [4.2.0] systems, instead of the expected Pauson-Khand products. A tandem Pauson-Khand reaction of bisyne-bisallenes to [5.5.5.5] tetracycles has been reported by Cook, who used a saturated solution of Mo(CO)6. (Chemical equation presented). (D) Mo(CO)6 has catalyzed the cycloisomerization of 1-alkyn-4-ols to 2,3-dihydrofurans and the isomerization of epoxyalkynes to furans. Cyclizations of allylphenylethers with skeletal rearrangement to benzopyrans have also been performed. Friedel-Crafts alkylations have been reported, too. (Chemical equation presented). (E) Mo(CO)6 has been used to reduce the N-O bonds of isoxazoles, isoxazolines, isoxazolidines and 1,2-oxazines. Variants employed sub-stoichiometric amounts of Mo(CO)6 in the presence of NaBH4, or a decomplexing work-up on silica gel. The selective reduction of azides, nitro compounds and hydroxylamines to amines and deoxygenation of epoxides have also been accomplished. (Chemical equation presented). (F) Mo(CO)6 behaved as a catalyst for the mild oxidation of 2,5-dialkylfurans to E or Z (depending on the use of a base) enediones using cumyl hydroperoxide (CHP). Diones gave peroxipyranones in moderate yields, irrespective of the C=C bond configuration, when t-butylhydroperoxide (TBHP) was used. This indicates that Mo(CO)6 can be employed not only in reduction processes, but also in oxidations. (Chemical equation presented).