CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction

Abstract

The transient intermediate η 2 -cyclopropene/bicyclobutane niobium complex [Tp Me2 Nb(η 2 -c-C 3 H 4 )(MeCCMe)] A, generated by an intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [Tp Me2 NbMe(c-C 3 H 5 )(MeCCMe)] (1), is able to cleave the CH bond of a variety of unsaturated hydrocarbons RH in a selective manner to give the corresponding hydrocarbyl complexes [Tp Me2 NbR(c-C 3 H 5 )(MeCCMe)] (R = 2-furyl, 2-thienyl, 1-alkynyl, 1cyclopentenyl, 1-ferrocenyl (Fc), pentafluorophenyl). The activation of the C−H bond occurs stereospecifically via a 1,3-CH addition across the Nb(η 2 -cyclopropene) bond of A. Full characterization of several of these complexes includes multinuclear NMR spectroscopy, X-ray diffraction, UV/vis spectroscopy, and electrochemical data. A charge transfer between the ferrocenyl moiety and the niobium center is responsible for the characteristic purple color of the bimetallic complex [Tp Me2 NbFc(c-C 3 H 5 )(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively the strength and the pK a of the CH bond that is cleaved. The pentafluorophenyl complex [Tp Me2 Nb(C 6 F 5 )(c-C 3 H 5 )(MeCCMe)] undergoes cyclopropyl ring opening and alkyne coupling to give two isomeric η 4 -butadienyl complexes, with [Tp Me2 Nb(C 6 F 5 )(η 4 -CMeCMeCHCHMe)] as the major isomer.

Allyl Complexes of Tungsten from the Rearrangement of Transient Cyclopropyl Precursors

“…In the 1 H NMR spectra, the accompanying resonances for the Hα and inequivalent diastereotopic Hβ, all appearing as complex multiplets, are identified by 1 H‐ 13 C HMQC experiments. These NMR data match those seen for similar cyclopropyl Nb or Zr complexes , , . The other alkyl group of 2‐R is similarly identified by NMR spectroscopy.…”

“…Zirconium and niobium η 2 ‐cyclopropene/metallabicyclobutane intermediates thermally generated from well‐defined cyclopropyl derivatives such as [Cp 2 Zr( c ‐C 3 H 5 ) 2 ] and [Tp Me2 NbR( c ‐C 3 H 5 )(MeCCMe)] [Tp Me2 = hydrotris(3,5‐dimethylpyrazolyl)borate] can cleave a variety of CH bonds of unsaturated hydrocarbons R'H. The products are [Cp 2 ZrR′( c ‐C 3 H 5 )] and [Tp Me2 NbR′( c ‐C 3 H 5 )(MeCCMe)] . Structural and mechanistic studies established that the products are formed by the mechanistic reverse of the β‐H abstraction reaction namely the 1,3‐addition and cleavage of the C–H bond of R′H across the [M(η 2 ‐ c ‐C 3 H 4 )] unit.…”

“…The ring‐opening by distal CC bond cleavage in model cyclopropyl samarocene is found computationally to occur with a prohibitively high energy barrier . Products arising from coupling of the alkyne ligand with a ring‐opened cyclopropyl ligand have been observed in selected cases in [Tp Me2 NbR( c ‐C 3 H 5 )(MeCCMe)] but no mechanistic details are available …”

Coupling and Dearomatization of Pyridines at a Transient η2‐Cyclopropene/Bicyclobutane Zirconocene Complex

“…Capitalizing on known niobium chemistry, we showed recently that dicyclopropylzirconocene [Cp 2 Zr( c ‐C 3 H 5 ) 2 ] ( 1 ) readily activates the C−H bonds of heteroaromatics such as furan and thiophene to give the furyl and thienyl derivatives [Cp 2 Zr( c ‐C 3 H 5 )(2‐XC 4 H 3 )] (X=O, S) through 1,3‐C−H addition across a Zr−C bond of the η 2 ‐cyclopropene/bicyclobutane intermediate [Cp 2 Zr(η 2 ‐ c ‐C 3 H 4 )] ( A ) (Scheme ) . Transient A is generated from 1 by an intramolecular β‐H abstraction of cyclopropane.…”

“…[19][20][21] With am ore strongly bound aryne as in the emblematic benzyne complex [Cp 2 Zr(h 2 -C 6 H 4 )],C ÀHb ond activation of arenes, [11] and ar ich insertion chemistry [12] are realized, but to the best of our knowledgen or eactions with pyridine derivatives have been described. [ Capitalizing on known niobiumc hemistry, [22][23][24][25] we showed recentlyt hat dicyclopropylzirconocene [Cp 2 Zr(c-C 3 H 5 ) 2 ](1)readily activates the CÀHb onds of heteroaromatics such as furan and thiophene to give the furyl and thienyl derivatives [Cp 2 Zr(c-C 3 H 5 )(2-XC 4 H 3 )] (X = O, S) through 1,3-CÀHa ddition across aZ r ÀCb ond of the h 2 -cyclopropene/bicyclobutane intermediate [Cp 2 Zr(h 2 -c-C 3 H 4 )] (A)( Scheme 1). [26] Transient A is generated from 1 by an intramolecular b-H abstraction of cyclopropane.…”