Assembly of an R3N52− Chain by Cycloaddition of a Hydrazinediide and an Azide at Zirconium and its Thermal Fragmentation

Gehrmann, Thorsten; Lloret‐Fillol, Julio; Wadepohl, Hubert; Gade, Lutz H.

doi:10.1002/ange.200805631

“…This intermediate 9 is thermodynamically unstable and would undergo skeletal rearrangement by a 1,2-shift of the {Cp 2 Zr} moiety in the azazirconacyclic ring to afford the key intermediate 6, which is stable enough at room temperature and could be characterized by 1 H and 13 C NMR. Although the proposed intermediates 8 and 9 could not be characterized owing to the fast insertion of the C N bonds to both Zr À C and Si À C bonds and the skeletal rearrangement, the insertion chemistry of the CN triple bond of organonitriles into ZrÀC bonds to afford azazirconacycles [11][12][13][14][15][16] and into SiÀC bonds [17] has been documented. Insertion of the CN triple bond of the third organonitrile R 3 CN to the ZrÀC bond in 6 would lead to the formation of 4.…”

mentioning

confidence: 99%

Zirconium‐ and Silicon‐Containing Intermediates with Three Fused Rings in a Zirconocene‐Mediated Intermolecular Coupling Reaction

Zhang

¹

,

Zhang

²

,

Sun

³

et al. 2009

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The isolation and reactivity of important intermediates in transition-metal-mediated reactions are of interest in both organometallic and synthetic organic chemistry. Studies of these intermediates not only play an important role in the indepth understanding of seemingly complicated reaction mechanisms, but can also lead to the discovery of new synthetically useful reactions. We have reported a zirconocene-mediated intermolecular coupling reaction of one molecule of a silyl-tethered diyne with three molecules of organonitriles, which afforded pyrrolo[3,2-c]pyridine derivatives (5-azaindoles) upon hydrolysis of the reaction mixture [Eq. (1)]. [1,2] The most interesting feature of this above one-pot reaction is that five components are involved and integrated in a perfect selective manner via an unknown pathway, which tempted us to investigate what is really going on.[3] We anticipated that novel and important reaction patterns might be involved. Indeed, we have now finally discovered that the process that combines all the five components together is very unusual, unpredictable, and of general importance to metalmediated reactions involving the cleavage of CN triple bonds and Si À C bonds.[4] Complicated reactive intermediates were found to be formed by surprising mechanisms. Synthetically useful applications of these reactive intermediates were also achieved based on deep understanding of the reaction mechanism.The first reaction step between [Cp 2 ZrBu 2 ] [5] and 1 a, as previously reported by Takahashi and co-workers, [6] generates a zirconacyclobutene-silacyclobutene fused complex 3 a (Scheme 1). [6,7] To investigate the reaction mechanism in a pure and controllable system, we prepared and successfully isolated the intermediate 3 a in 93 % yield by a modified procedure.[8] Then, 3.5 molar equivalents of iPrCN was added to a toluene solution of 3 a. After the reaction mixture was stirred at 50 8C for 1 h, a red powder was isolated in 90 % yield, which was confirmed to be the unexpected complex 4 a (Scheme 1).Single crystals of 4 a suitable for X-ray analysis were grown in benzene at room temperature. X-ray analysis of 4 a (Figure 1) reveals that it consists of three fused rings: one sixmembered ring containing silicon and nitrogen, one fivemembered pyrrolo ring, and one six-membered zirconacycle.[9] The zirconium center is bonded to two cylcopentadienyl (Cp) rings, one imine nitrogen atom, and one nitrogen of the pyrrolo ring. The silicon atom is bonded to one quaternary carbon atom, one imine nitrogen atom, and two methyl groups. Two imine carbon atoms neighboring the silicon and zirconium atoms in 4 a result in a singlet at d = Scheme 1. Formation of zirconocene-containing intermediate 4 a and its reaction with water.

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“…This intermediate 9 is thermodynamically unstable and would undergo skeletal rearrangement by a 1,2-shift of the {Cp 2 Zr} moiety in the azazirconacyclic ring to afford the key intermediate 6, which is stable enough at room temperature and could be characterized by 1 H and 13 C NMR. Although the proposed intermediates 8 and 9 could not be characterized owing to the fast insertion of the C N bonds to both Zr À C and Si À C bonds and the skeletal rearrangement, the insertion chemistry of the CN triple bond of organonitriles into ZrÀC bonds to afford azazirconacycles [11][12][13][14][15][16] and into SiÀC bonds [17] has been documented. Insertion of the CN triple bond of the third organonitrile R 3 CN to the ZrÀC bond in 6 would lead to the formation of 4.…”

mentioning

confidence: 99%