Reactivity of a Seven‐Membered Zirconacyclocumulene towards CN Multiple Bonds – Formation of Metallaheterocycles by Insertion of C≡N and C=N Groups

Abstract

The reactions of the seven-membered zirconacyclocumulene [Cp 2 Zr{η 4 -Me 3 SiC 4 (SiMe 3 )-C(C 2 SiMe 3 )=CSiMe 3 }] (1) with nitriles (RCϵN, R = Me, Ph) started with an insertion of a cyanide group into the metal-butadiyne bond followed by an alkyne-nitrile coupling and formation of 1-zircona-2-azacyclopenta-2,4-dienes. The reaction product for R = Me is stabilized by dimerization to give a compound containing a four-membered -N-Zr-N-Zr-ring and two identical fivemembered 1-metalla-2-aza-cyclopentadiene rings. In case of R = Ph, a metallacycle could not be isolated, and a pyrimidine[a] Leibniz-5304 was formed. The catalytic potential of this reaction was investigated. For PhN=C(H)Ph, again after insertion and coupling, a corresponding 1-zircona-2-azacyclopenta-4-ene was established. The reaction of this compound with carbon dioxide led to ring expansion and formation of a seven-membered product, which released CO 2 at higher temperature and to give back the reactant. The above-mentioned products were characterized by NMR spectroscopy and singlecrystal X-ray structure analysis.ing from these alkyne complexes, five-membered, all-carbon metallacycles, such as 1-metallacyclopenta-2,4-dienes (metallacyclopentadienes A, Scheme 1), 1-metallacyclopent-3-ynes (metallacyclopentynes B, Scheme 1), and 1metallacyclopenta-2,3,4-trienes (metallacyclocumulenes C, Scheme 1), were synthesized to study their reaction behavior. [2,3] In many cases, coupling reactions of two alkynes were observed to yield 1-metallacyclopenta-2,4-dienes A. [5] We were now interested in the formation of the analogous heterocycles by the coupling of nitriles. The corresponding 1-metalla-2,5-diazacyclopenta-2,4-dienes D (Scheme 1), which result from a nitrile-nitrile coupling, are very rare. [6] Lately, these metallacycles were synthesized by reaction of arylnitriles with [Cp* 2 M(η 2 -Me 3 SiC 2 SiMe 3 )] (M = Ti, Zr; Cp* = η 5 -pentamethylcyclopentadienyl). [7] Additionally, there were hints for the formation of the corresponding five-membered 1-metalla-2-aza-cyclopenta-2,4-dienes E (Scheme 1), established by an alkyne-nitrile C-C coupling, which were assumed as intermediates in the synthesis of heteroaromatic compounds. [8] To the best of our knowledge, only in the case of 3-ferrocenyl-2-propyne-nitrile and zirconocene [7b] and for an aluminum complex [6a] has the corresponding metallacyclopentadiene E been isolated by alkyne-nitrile coupling.

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4–8 transition metal catalysts

“…30 ). 126 Under elevated temperature the precatalyst can be activated to IM54 by releasing one equivalent of diyne. The resulting zirconacyclopentene can undergo twofold nitrile insertion to IM56 , followed by reductive elimination of 66 and regeneration of the low-valent zirconocene.…”

Take a “Snapshot” of New Syntheses, Reactions, and Characterizations from Unusual Unsaturated Ring Strained Group 4 Metallacycles

“…In a recent review Tonks and coworkers summarized several examples for multicomponent syntheses of 5‐ and 6‐membered aromatic heterocycles by Group 4–8 transition metal catalysts [3c] . In this context they mentioned that a 7‐membered zirconacyclocumulene can serve with benzonitrile as a catalyst for the formation of the tetrasubstituted 4‐silylethynyl‐3‐silylpyrimidine [3d] . Again the dissociation of one diyne (Scheme 3) via a zirconacyclocumulene and a zirconacyclopropene with a coordinated benzonitrile was assumed.…”

Protolysis of Seven-Membered Zirconacyclocumulene Complexes of Zirconocene

“…1 H NMR (C 6 D 6 , δ, ppm): 0.14 (s, 9H, SiMe 3 ), 0.37 (s, 9H, SiMe 3 ), 0.40 (br s, 9H, SiMe 3 ), 0.45 (br s, 9H, SiMe 3 ), 5.88 (s, 1H, CH), 6.01 (s, 10H, Cp); 13 C NMR (C 6 D 6 , δ, ppm): −0.75, −0.54, −0.41, 0.48 (SiMe 3 ), 101.0, 106. 8,114.1,115.7 (CC),111.8 (Cp),138.0 (CH),143.2 (Cq),164.7,178.7 (br,Cq). Raman spectrum (ν, cm −1 ): 2134, 1979 (CC, CCCC).…”

“…The aforementioned resonance description of the electronic structure of seven-membered zirconacyclocumulene complexes explains satisfactorily not only the abnormally high thermal stability of these compounds but also the peculiarities of their reactivity. Thus, it turned out that in the interaction with acetylenes, nitriles, , diarylketones, and N -benzylideneaniline, , the Buchwald zirconacycle 1 reacts in its zirconacyclocumulene resonance form, whereas in the reactions with benzil, acenaphthenequinone, and S 2 Cl 2 , , the same zirconacycle behaves as if it was in the zirconacyclopentadiene resonance form.…”