Molybdenum Hexacarbonyl [Mo(CO)6]

(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 a-methylene-gbutyrolactones from allenyl carbonyls was performed by a DMSO-promoted carbonylation. These reactions enlighten the role of Mo(CO)6 as a source of CO. (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.4c,d 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). (C) Mo(CO)6 was reported to effect Pauson-Khand reactions of enynes to afford cyclopentenones. Chiral alkynyl allenes afforded enantioenriched a-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. (D) Mo(CO)6 has catalyzed the cycloisomerization of 1-alkyn-4-ols to 2,3dihydrofurans 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. (E) Mo(CO)6 has been used to reduce the N-O bonds of isoxazoles, isoxazolines, isoxazolidines7f 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. (F) Mo(CO)6 behaved as a catalyst for the mild oxidation of 2,5dialkylfurans 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 tbutylhydroperoxide (TBHP) was used. This indicates that Mo(CO)6 can be employed not only in reduction processes, but also in oxidations.