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Abstract

Arene)Cr(CO) 3 ] complexes have found widespread application in organic synthesis as a result of the activating and stereodirecting influence of the p Lewis acid fragment Cr(CO) 3 . The applications include nucleophilic addition to the aromatic ring, to carbonyl and imine functions in side chains, and to configurationally stable benzylic carbocations. Arene lithiation reactions and reactions via benzylic anions also are much more facile in the complex than in the free arene. Diastereoselective and enantioselective variants have been developed for many transformations and they have found application in asymmetric organic synthesis and in the synthesis of chiral ligands for asymmetric catalysis. [1] [(benzene)Mo(CO) 3 ] is readily accessible from Mo(CO) 6 [2] but, surprisingly, analogous reactions to the ones listed above for the Cr(CO) 3 complex have not been reported. Intrigued by this situation, we started a research project on [(arene)Mo-(CO) 3 ] complexes and here report the first results.Thermochemical studies show the arene±Mo bond (68 kcal mol À1 in [(h 6 -C 6 H 6 )Mo(CO) 3 ] (1)) to be stronger than the arene±Cr bond (53 kcal mol À1 in [(h 6 -C 6 H 6 )Cr(CO) 3 ]) (2)). [3] MÀH bonds are also stronger with the second-row transition metal, as shown by the values of 62 kcal mol À1 and 54 kcal mol À1 in [CpMo(CO) 3 (H)] and [CpCr(CO) 3 (H)], respectively. [4] Kinetically, however, the situation is reversed. The metal±arene bond in 1 is far more labile than that in 2. This lability and the resulting difficulty in handling the Mo compounds have retarded their use in synthesis. We anticipated that there would be different selectivities in the reactions of the Cr and Mo complexes and that the higher bond strength of the MoÀC and MoÀH bonds would make possible the isolation of intermediates that have eluded characterization in the Cr-mediated reaction sequence. Both of these hypotheses have now been fulfilled.Addition of 2-lithio-1,3-dithiane [5] (2 a; Scheme 1) to a cold (À78 8C) solution of the benzene±Mo complex 1 in THF*** afforded a yellow precipitate 3 a, which dissolved when the reaction mixture was warmed to À20 8C. The 1 H NMR spectrum of 3 a was consistent with that expected for the anionic cyclohexadienyl complex. Treatment of the THF solution of 3 a with allyl bromide (4 a) produced the [(h 3allyl)(h 5 -cyclohexadienyl)Mo(CO) 2 ] complex (5 a) which was isolated as a yellow solid. In solution, 5 a is present as a mixture of the two rapidly interconverting exo±endo allyl isomers. Analogous reactions with [(h 6benzene)Cr(CO) 3 ] have been shown previously to give directly the decomplexed cyclohexadiene 6. [6] In the Cr-mediated reaction, metal allylation, followed by CO insertion, reductive elimination, and decomplexation occurred readily and intermediates could not be detected. In marked contrast to the above, the Mo±allyl complex 5 a is stable at room temperature. Its intermediacy on the pathway to a trans-disubstituted cyclohexadiene was shown by placing a solution of the complex 5 a under 4 atm of CO ...

Influence of Halide Ion and Lewis Acid in the Demetalation of a Spirolactam Cyclohexadienyl Ruthenium Complex

“…(η 6 -Arene)Cr(CO) 3 complexes have been used extensively in the context of metal-mediated dearomatization. Cyclohexadienyl adducts obtained via nucleophilic addition to arene chromium complexes are anionic and generally quite labile, readily undergoing additional reactions with electrophiles (H + , alkyl and acyl halides). , Recently, analogous arene molybdenum complexes have been shown to exhibit similar reactivity …”

“…1,2 Recently, analogous arene molybdenum com-plexes have been shown to exhibit similar reactivity. 3 Cationic (η 6 -arene)Mn(CO) 3 complexes possess an even more activated arene ligand relative to their Cr(0) and Mo(0) counterparts. Consequently, neutral cyclohexadienyl adducts obtained from aromatic addition reactions still retain electrophilic character and can be further functionalized by treatment with additional nucleophilic reagents to produce metal-free 1,3-dienes.…”

Exploiting Phosphonate Chemistry in Metal-Mediated Dearomatization: Stereoselective Construction of Functionalized Spirolactams from Arene Ruthenium Complexes

“…2 Recent examples of diastereoselective metal-mediated dearomatization involving tricarbonyl Cr(0) and Mo(0) arene complexes have been reported. 3,4 More reactive cationic arene Mn(CO) 3 complexes have also been elaborated to substituted cyclohexadienes. 5 Arene ruthenium complexes, particularly those incorporating a CpRu(II) fragment, are easily prepared in high yield using conditions compatible with functionalized arene ligands.…”

Stereoselective chromium- and molybdenum-mediated transformations of arenes

“…For this reason, we and others have been searching for transitionmetal fragments that form labile complexes with arenes, but that retain electrophilic activation. One approach makes use of Cr(CO) 2 The results of a first study of a sequential nucleophile/electrophile addition sequence to an arene Mo(CO) 3 complex in a nucleophilic/allyl bromide double addition sequence is shown in Scheme 7 [17]. It demonstrates the viability of using π-arene molybdenum complexes in organic synthesis.…”