No abstract

An unusual linkage: The zirconocene‐catalyzed hydrooligomerization of norbornene proceeds by σ‐bond metathesis to form a 2‐exo,7′‐syn linkage (as evident in the pentamer shown). The oligomers were characterized by gas chromatography, NMR spectroscopy , X‐ray crystallography, and DFT calculations.

Some time ago [1] we reported a metallocene-catalyzed hydrooligomerization of norbornene (NB) in which a C7 linkage is formed as a result of s-bond metathesis. Based on the crystal structure of the pentamer, we were able to show that two of the norbornene molecules are joined by 2-exo-7'-syn linkages and we proposed a mechanism in which a change of conformation of the chain brings the syn hydrogen atom on C7 of the second last inserted monomer into the vicinity of the Zr atom of the metallocene unit so that a s-bond metathesis takes place (Scheme 1: 1!2!2'!3).The crucial question remained unanswered-whether this s-bond step is repeated regularly in the growing chain and in this way determines the as yet unknown structure of the polynorbornene and its properties. Herein we show through norbornene oligomerizations in a deuterium atmosphere and structural analysis of the higher oligomers that this is indeed the case.The oligomerization reactions were carried out in a 250-mL Büchi autoclave under a 0.5-1.25 bar hydrogen or deuterium atmosphere.[2] The oligomers were preparatively fractionated according to their hydrodynamic volumes (! 100 mg; gel permeation chromatography (GPC)) and the samples obtained were analyzed by NMR spectroscopy.First let us consider the hydrotrimer (Scheme 2): If 2 (Scheme 1 with R = D) were present at the moment of chain termination in a deuterium atmosphere, then C3a and C3c would both be deuterated, the rings A and C would be enantiotopic, and the product would have a mirror plane. Figure 1 shows the 13 C{ 1 H} NMR spectra of the meso,mesocoupled NB hydrotrimers prepared in a hydrogen atmosphere and a deuterium atmosphere.As can be clearly seen, the resonance at d = 33.68 ppm of the bridging carbon atom C7b of the second monomer to be inserted (Figure 1; upper spectrum) has almost disappeared in the deuterated compound (Figure 1; lower spectrum), while a new signal with the characteristic 1:1:1 triplet of a carbon