further coupled to the adjacent bridgehead by 2.8-3.0 Hz. The corpling between the bridgehead and the bridge protons (normally in the order of 1-2 Hz) as well as the geminal coupling between the bridge protons (normally -8 Hz) was not determined in these compounds. The ABX patterns of the vinyl substituent of compounds Ig and IIg were analyzed, and it was found that the sum of the coupling constants (JAB JAx J B X )is larger for the endo vinyl group than the exo vinyl group (cf. footnotes 6 and c in Table 111). The typical splitting pattern of the 3n proton is shown in Figure lb. The signal normally appears as a double doublet in compounds with structure I. However, in compound IC (cf. Figure IC)there is a double triplet with a separation of 1.6 Hz between the lines within the triplet, caused by virtual coupling. The 3n proton is coupled to one of the bridge protons (7s), and because 7s and 7 a are strongly coupled to each other and have the same chemical shift, the splitting pattern of 3n is affected. The triplets can be collapsed to singlets by irradiating the bridge protons with a second radio frequency. With compound Ia an eight line pattern is expected for the 3n proton. However, the 60 MHz spectrum is second order (Figure la) and coupling constants cannot be determined from the spectrum. The 100 MHz spectrum is simpler and allows the determination of the coupling constants. Analytical Aspects. Norbornene derivatives can be synthesized by Diels-Alder condensation between cyclopentadiene and ethylene derivatives. However, this reaction results in a mixture of isomers. Two epimers (ex0 and endo) are formed when monosubstituted ethylenes are used as dienophiles. In most cases, the endo compound is the major product according to Alder’s rule (2). With 1,l-disubstituted ethylenes, two norbornene derivatives can be formed, and the isomer distribution depends on the directing influence of the substituents. We have been interested in using NMR to distinguish between epimeric 2-substituted-5-norbornenes,and we found earlier (12) that the following four absorptions could be used for this purpose: C-2 proton, C-2 substituent, olefinic pattern, and C-3n proton. The absorption of the C-2 proton could not be used in the present case because the compounds studied are 2,2-disubstituted and therefore do not contain any C-2 proton. The ends C-2 substituent always appeared at a higher field than the corresponding exo C-2 substituent. The methyl substituent could be used for the calculation of epimer ratio in all cases. It gave rise to an intense singlet for each isomer. The difference in chemical shift was 0.3 ppm. This signal could easily be recognized because of its intensity. We also had the possibility of using the absorption of the R group. The separation of the signals was easy with the formyl and acetyl derivatives because these groups gave rise to singlets. A detailed study showed that the vinyl and propenyl groups also could be used for the calculation. The olefinic absorption from the norbornene system was very useful in our previous analysis of mono-substituted norbornenes (12). This pattern was also used by Kuivila and Warner (17) in the analysis of trimethylsilyl-norbornenes. In our study the nitrile could be analyzed as usual (12) by using the four upfield olefinic peaks of compound Ib us. the total olefinic absorptions (cf. Figure 2a and 26). However, with the formyl derivative (cf. Figure 3) the downfield half of the
+
+
(17) H. G. Kuivila and C . R. Warner, J. Org. Chem., 29, 2845 (1964).
olefinic absorption of compound IChad to be used because the upfield half was overlapped by the absorption of the compound IIc. An additional long range coupling of 0.8 Hz appeared in the olefinic pattern of IC. The olefinic absorption of the norbornene system could not be used to determine the epimer ratio in the remaining compounds. We have observed earlier that the 3n proton in endo-2(substituted methyl>5-norbornenes absorbs at a surprisingly high field (0.45-0.60 ppm downfield from TMS). This suggested that the 3n proton of the 2,2-disubstituted derivatives with an endo methyl group might also absorb at such a high field that it could be used for calculation of epimer ratio. In fact, this was observed, and the results are shown in Table I. The absorption of the 3n proton of compounds with an endo methyl group appears as a double doublet. This is a result of the large geminal coupling to the 3x proton (in the order of 11-12 Hz) and the stereospecific long range coupling to the 7s proton of 2.4 Hz. As expected, we do not observe any coupling to the adjacent bridgehead proton because of the unfavorable dihedral angle. In addition to the previous methods for distinguishing between epimers, we also found that the absorption of the C-3x proton is useful in the analysis of 2-substituted-2-methyl-5norbornenes. The 3x proton of exo-2-substituted-2-endomethyl-5-norbornenes is shifted so far downfield (to -2.3 ppm) that it can be used for the calculation of epimer ratio in most of the compounds in this study. We also conclude that one further absorption, i.e., of the C-1 proton (bridgehead), is useful in distinguishing between certain epimers of 2-monosubstituted-5-norbornenes. The exo-epimer can be identified by the upfield shift of the adjacent bridgehead proton in the case of the methyl-, vinyl-, and propenyl derivatives (cf. Tables I and 11). ACKNOWLEDGMENT
A discussion with Prof. L. M. Jackman on virtual coupled systems was appreciated. The authors express their thanks t o P. Paterson for recording the NMR spectra and to F. J: Cassidy for the G C separations.
RECEIVED for review January 25, 1971. Accepted June 18. 1971.
Corrections Example of Flame Photometric Analysis for Methyl Parathion in Rat Whole Blood and Brain Tissue In this article by Joe Gabica, Joe Wyllie, Michael Watson, 43, 1102 (1971)], on page and W. W. Benson [(ANAL.CHEM., 1103, column 2, line 1 should read “SE-30 3%, on Chromoport XXX,. . ..”
Combined High Speed Liquid Chromatog raphy and Bioassay for the Evaluation and Analysis of an Organophosphorus Larvacide In this article by R. A. Henry et al. [(ANAL.CHEM., 43,1053 (1971)l on page 1057, Figures 7 and 8 should be transposed. The captions are correct but the chromatograms as published do not correspond to the captions.
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