The palladium-catalyzed vinylation of aryl halides, also known as the "Heck reaction" (eq 1) is a very useful synthetic

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The first five-membered titanacyclic cumulene C p , T i -c (tBu)=C=C=k(tBu) (5) was prepared by treatment of the titanocene generator Cp2Ti(Me3SiC2SiMe3) with the di-tertbutylbutadiyne in a molecular one-to-one ratio. The reaction of 5 with "CpzZr" or of the analogous complex Cp2-krC(tBu)=C=C=k(tBu) with "Cp,Ti" leads to cleavage of the central C-C bond in the metallacyclocumulene complexes to afford the same product, the heterodimetallic u,n-alkynyl-bridged titanocene-zirconocene complex CpzZr(p-q' :q2-In cyclizations of unsaturated compounds using the metallocene fragments "Cp2M" (M = Ti or Zr) ['], which are frequently employed preparatively, little is known about reactions with 1 ,Cdisubstituted 1 ,3-butadiyne~[~I.Recently, we have found that two equivalents of the titanocene complex Cp2Ti(Me3SiC2SiMe3)[31, an excellent titanocene source, react with one equivalent of disubstituted butadiynes to give dinuclear complexes of type 114] or Z5]. The obtained types of complexes are well-known[6], but we have found a new way of preparation in which, depending on different substituents, the central C-C single bond of the starting butadiyne is cleaved. R" R"With the complex Cp2Zr(THF)(Me3SiC2SiMe3)[7a] as zirconocene synthon the cleavage reaction to complexes of type l[7bl is more favoured, but when Cp2Zr(Py)(Me3SiC2-SiMe3) (Py = pyridine)[sal was used five-(3) and sevenmembered (4) zirconacyclic cumulene complexes were formed, depending as well on the substituentsLsb1. However, C=CtBu)Cp,Ti(p-q' :q2-C=CtBu) (6), in which each alkynyl group is a-bonded to one and rr-bonded to the other metal as shown by X-ray diffraction studies, In solution the two alkynyl groups become equivalent. The coupling reaction of "Cp2Ti" with bis(trimethylsily1)butadiyne in a one-to-two molecular ratio leads to the unsymmetrically substituted tita-analogous cumulene complexes of titanocene have been unknown so far.In the present paper we report on the synthesis, spectral characteristics, and some chemical properties of the first five-membered metallacyclic cumulene complex of titanocene which can act as an intermediate in the cleavage reaction of diynes. Results and DiscussionThe reaction of Cp2Ti(Me3SiC2SiMe3) with equimolar amounts of tBuC=C-C=CtBu in hexane at room temperature affords green solutions from which the metallacyclic cumulene 5 can be isolated by cooling to -40 "C. Also small amounts of the dinuclear complex 2a are formed after a reaction time of I to 2 days.

Die Substitution des THF‐Liganden in Cp2Zr(THF)(Me3SiCCSiMe3) (1) durch Pyridin ergibt den Komplex Cp2Zr(η1‐NC5H5)(Me3SiCCSiMe3) (3). Im Gegensatz zu 1 ist Komplex 3 stabil in Kohlenwasserstoffen. In temperaturabhängigen 1H‐ und 13C‐NMR‐Spektren zeigt 3 in Lösung dynamisches Verhalten durch eine Rotation des Alkin‐Liganden. Zusätzlich tritt beim Lösen von 3 in Pyridin ein Austausch zwischen koordinierten und freien Pyridin‐Molekülen auf. Die Kristallstrukturanalyse von 3 belegt eine tetraedrische Koordinationsgeometrie am Zirkonium‐Zentrum bestehend aus den Cp‐, Alkin‐ und Pyridin‐Liganden. Der Komplex 3 reagiert wie Verbindung 1 mit Wasser und Kohlendioxid zu gleichen Produkten.

COMMUNICATIONSan intense band at 515 nm ( E ~1 0 0 0 0 ~-' c m -' ) , while 2e absorbs only weakly in this region (490 nm, E z l 0 0 0 M-lcm-'). The absence of ring closure in the case of the 3,5-disubstituted azobenzene derivatives 1 c and 1 d could be due to the greater spatial requirements of the substituents (R3 = Me, C1) in comparison to those of 1 a and 1 b (R3 = H), which should make the formation of a planar transition state more difficult. Accordingly, only the 2.3-dihydrocinnoline derivative is formed from 3,3'dimethylazobenzene. Esper inien t al ProcedureAll operations were carried out under an atmosphere of nitrogen, and with anhydrous. N,-saturated solvents. General procedure. To a melt of two parts tolane and one part of the azo compound dded. in portions, the appropriate amount of catalyst (in the case of I a, 1 b. and I d 0.25 equivalents. for 1 c 0.066 equivalents and for I e 0.33 equivalents). The mixture was then stirred for two hours at 85 C. For 1 a.

The new cyclopalladated, phosphine-free imine complexes 1-3 are exceptional catalysts for the Heck arylation, leading to more than a million turnovers in some cases; the catalysts are very thermally and air stable and are recovered unchanged after the catalysis.

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Unerwartet war das Ergebnis der Reaktion von “Cp2Zr” mit Di‐tert‐butylbutadiin: Es entstand 1, das kleinste mögliche cyclische Cummulen. Andere disubstituierte Butadiine reagierten mit “Cp2M” unter Bindungsspaltung, Dimerisierung oder Bildung von Zweikernkomplexen.