Synthesis and Diels–Alder cycloaddition reaction of norbornadiene and benzonorbornadiene dimers

Summary Dimeric forms of norbornadiene and benzonorbornadiene were synthesized starting with known monobromide derivatives. The Diels–Alder cycloaddition reaction of dimers with TCNE and PTAD was investigated and new norbornenoid polycyclics were obtained. All compounds were characterized properly using NMR spectroscopy.


Results and Discussion
One of the starting materials, 2-bromobenzonorbornadiene 4 was synthesized using a procedure described in the literature [15,23] (Scheme 1). Photochemical bromination of benzonorbornadiene 2 with 1,2-dibromotetrachloroethane gave isomeric dibromides 3 in high yield. Dehydrobromination reaction of dibromides 3 with potassium tert-butoxide resulted in the formation of monobromide 4. The other starting material 5 was obtained using the reported procedures based on the use of potassium tert-butoxide/n-butyllithium super-base by starting with commercially available norbornadiene [24][25][26][27].
When 2-bromobenzonorbornadiene 4 was treated with n-BuLi at −78 °C and the resulting anion was quenched with trimethyltin chloride, a single trimethyltin derivative 6 was observed in the crude reaction mixture and was finally isolated in 91% yield. Copper salts have been successfully employed for Stille-type hetero-coupling between unsaturated halides and stannanes [28,29]. Treatment of 6 with Cu(NO 3 ) 2 ·3H 2 O in dry THF at r.t. afforded the first synthesis of the expected dimers 7 and 8 in 25% yield in a 3:4 ratio, respectively, besides benzonorbornadiene 2 after column chromatography. The Diels-Alder cycloaddition of dimers 7 and 8 with PTAD (9) and TCNE (10) resulted in the formation of the corresponding products 11-14 in high yields (Scheme 2).
Similarly, tin compound 15 was synthesized by the reaction of monobromide 5 with n-BuLi followed by reaction with trimethyltin chloride. Reaction of 15 with Cu(NO 3 ) 2 ·3H 2 O resulted in the formation of dimers 16 and 17 [30]. This reaction offered an alternative synthetic route to norbornadiene dimers 16 and 17. The isomers 16 and 17 could not be separated, but after cycloaddition reaction of the mixture, the corresponding addition products 18-21 were isolated by chromatographic methods (Scheme 3).

Structural Analyses
The determination of the structures of dimers 7, 8 and dimers 16, 17 by spectroscopic methods was not simple because the C s symmetry of the syn dimers and the C 2 symmetry of the anti dimers and the free rotation around the central σ bond make For the symmetric addition products 11, 12, 18 and 19, there are two possibilities: exo adduct or endo adduct ( Figure 1). The coupling constants between the relevant protons in the norbornene unit are very informative to assign the correct configuration of the substituents [9,10]. The high value of J 34 and J 3′4′ (2.5-3.5 Hz) in the Diels-Alder addition products is uniquely accommodated by the exo orientation of the protons (endo orientation of -A-A-ring) at C 3 and C 3′ carbon atoms. For example, though there is coupling between the protons H 3 and H 4 , there is no measurable coupling between the protons H 3′ and H 4′ in anti structures ( Figure 1). On the other hand, the absence of any coupling between the related protons confirms the endo orientation of protons at C 3 and C 3′ , which in turn proves the exo-orientation of the rings in adduct 11, 12, 18 and 19. The coupling between the protons H 3 (H 3′ ) and H 7syn (H 7′syn ) (M or W orientation) also confirms the exo structures for 11, 12, 18 and 19 ( Figure 1).
In summary, the synthesis and cycloaddition reaction of norbornadiene and benzonorbornadiene dimers was investigated and new norbornanoid polycyclic compounds, which open up several synthetic and mechanical investigations, were obtained.

Experimental
General: Melting points are uncorrected. Infrared spectra were obtained from solution in 0.1 mm cells or KBr pellets on a regular instrument. The 1 H and 13 C NMR spectra were recorded on 400 (100) and 200 (50) MHz spectrometers. Apparent splitting is given in all cases. Column chromatography was performed on silica gel (60-mesh, Merck) TLC was carried out on Merck 0.2 mm silica gel 60 F 254 analytical aluminum plates. All substances reported in this paper are meso-compounds or racemates.