Abstract-The PMR spectra of cyclopentadiene endo-dimer, its 1 y-acetoxy-derivative and the dimer of spiro-heptadiene have been thoroughly studied. The assignment is proved by the use of spindecoupling experiments and the INDOR-technique. The relative signs of several coupling constants were found and used for the assignment. General trends in the proton chemical shifts and coupling constants are discussed for the derivatives of the cyclopentadiene dimer and for the substituted cyclopentadiene dirners.THE CYCLOPENTADIENYL compounds (cyclopentadiene itself1 and its alkylated2 or silylated3 derivatives) undergo a Diels-Alder dimerization. If they are allowed to react under mild conditions, the endo-product is usually formed. *
(1)The dimerization of cyclopentadiene (C,H,) is a classic and well-documented example of the Diels-Alder reaction. Where substituted cyclopentadienes are concerned, however, the literature data are rather scarce. This is due to several reasons: (i) the compounds mentioned above are prone to prototropic rearrangement and there generally exists a mixture of isomer~,5*~,~ (ii) the position of a substituent in the cyclopentadienyl ring is not easy to locate and this problem has only recently been solved,"* (iii) the structure of a substituted cyclopentadiene dimer or of a substituted derivative of (1) is also difficult to deduce.The difficulties mentioned above are inherent in the study of the dynamic processes in cyclopentadienyl compounds of ~ilicon.~ In particular, both the prototropic rearrangement and dimerization in the compound C,H,SiCH,CI, [and other compounds of the series C,H,Si(CH,),CI,-,Ihave rate constants of comparable value, which makes the procedure for estimating these rather tedious. In the course of studying the dynamics of slow rearrangements in these compounds we have reached the conclusion that the main difficulties are inherent in the structural assignment of the dimers of cyclopentadienyl compounds. We thought it natural to apply PMR spectroscopy 7-an effective approach to various stereochemical problems.ll * The results of an X-ray study of a substituted derivative of (1) are described in Ref. 4. t Some attempts to use PMR spectroscopic data in studying a substituted derivative of (1) are reported in Refs. 9 and 10. In this paper we report the results of a thorough study of the PMR spectra of the cyclopentadiene endo-dimer (l), spiro-heptadiene endo-dimer (2), ly-acetoxy-derivative of the cyclopentadiene endo-dimer (3) and the dimers of a silicon derivative of cyclopentadiene (C,H,SiCH,CI,) (4). The results of the analysis of these spectra are discussed, together with literature data on the related compounds (norbornene, cyclopentene etc.), in order to formulate suitable rules for the analysis of similar compounds. E X P E R I M E N T A L The PMR spectra were measured on a Varian HA-1OOD spectrometer. Frequency sweep was used in all double resonance experiments. The line positions were measured from TMS as internal reference. Spectra of all compounds were taken in CCla...
