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

Abstract

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.

Zirconocene and Si-Tethered Diynes: A Happy Match Directed toward Organometallic Chemistry and Organic Synthesis

“…Furthermore, the whereabouts of the Cp 2 Zr moiety and the SiMe 2 moiety was determined by successful isolation of the cyclic zirconasiloxane 6. 14 In order to isolate the intermediate I, we found that the substituent tolyl on the Si-tethered diyne was better than phenyl. Thus, as demonstrated in Scheme 9, a green solid 7b was obtained in 70% isolated yield by the reaction of 2b with 1.5 equiv of i-PrCN at 50 °C for 1 h. 14 The further reaction of 7b with i-PrCN generated 5b in a quantitative yield.…”

One-Pot Synthesis of Stable NIR Tetracene Diimides via Double Cross-Coupling

“…Aromatic annulation by double cross-coupling reactions of organometallic reagents and electron-rich acenes with dihalides function groups is an efficient method for region- and stereospecific formation of C–C bonds, which would provide a straightforward and promising method for extension of polycyclic aromatic molecules . However, the efficient synthetic approach based on electro-deficient acenes for big π-delocalized functional molecules are rarely reported.…”

Zirconium‐Mediated Multicomponent Reactions of 1,3‐Butadiynes with Ylidenemalononitriles to Form Functionalized 1,8‐Naphthyridine and Cyclopenta[b]pyridine Derivatives

“…The nitrogen on the hydropyridinyl ring coordinates with Zr to form a four‐membered chelate ring. The bond lengths of Zr1C20 (2.395(2) Å) and Zr1N2 (2.160(2) Å) are in the range of single bonds,2i,j whereas the Zr1N1 distance (2.464(2) Å) suggests a dative interaction 2o. The bond lengths of the C19C20 (1.368(4) Å) and C17C18 (1.376(4) Å) demonstrate strong CC double bond character and C15N1 (1.292(3) Å) corresponds to an imine moiety.…”