The Establishment of Configuration in Diels–alder Adducts by N.M.R. Spectroscopy

Abstract

It has been shown that the ~nag~letically anisotropic double bond in bicyclic Diels-Alder adducts exerts a paramagnetic effect on protons in an ezo config~~ratio~l and a diamagnetic effect on protons in an endo configuration. Thus the configuration of a proton or protonbearing substituent can be ascertained by observing the change in its chemical shift when the double bond is removed by hydrogenation. However, caution nus st be used in applying the method to compounds in which the double bond bears a magnetically anisotropic substituent.The Diels-Alder reaction between cyclic dienes and derivatives of ethylene inay produce either of two diastereomeric adducts, the endo or the exo isomer. In nlost instances the endo isomer predominates in accordance with Alder's rule (I). However, exceptions to this rule have been observed (2, 3) so that each new adduct synthesized requires proof of its stereochemistry. Several chemical methods have been employed to establish the configuration of Diels-Alder adducts (4). Lactonization (5), iodolactonization ( G ) , and titrimetric analysis (4) have been used for this purpose with carboxylic acid derivatives of bicyclo[2.2.l]hept-2-ene. The first two suffer the disadvantage that rearrangements do occur under the reaction conditions. Classical degradative procedures (7) are often unambiguous but exceedingly arduous.The use of physical methods obviates both of these difficulties. Alder and co-workers (3) have employed infrared spectroscopy to assign configurations to the two 5-cyanobicyclo[2.2.2]oct-2-enes forined from cyclohexadiene and acrylonitrile. Nuclear nlagnetic resonance has already proved itself capable of differentiating between diastereoisomers in cyclol~exanes (8) and cyclopentanes (9) and between geonletrical isomers ill inany olefin derivatives (10, 11). I t therefore seeined worth while to investigate the ability of n.nl.r. to be used in assigning structures t o diastereomeric Diels-Alder adducts.I t has already been found that in the cai11phane-2,3-diols (12), in a and a' chlorocainphor (13), and in 3,8-c~~clocampl1or (14) the inagnitude of the coupling of a n exo proton with the adjacent bridgehead proton is larger (4-5 cycles/sec) than when the proton is endo (0-1 cycles/sec). Thus when such a proton is well separated fro111 the rest of the absorption pattern, the size of its spin-spin interaction with the bridgehead proton will establish the configuration of the compound. A inore general method would be possible if the chemical shift of a proton could be correlated with its endo or exo position. T o explore this possibility we have exanlined the spectra of five derivatives of bic~~clo[2.2.l]hept-2-ene whose configurations had been previously established (22).These are the two diastereoineric adducts of a-methacrylic acid and c>-clopentadiene, the two adducts of trans-crotonic acid and cyclope~itadiene, and the eso aclduct of methyl methacrylate and cyclopentadiene. After reilloval of the double bond by hydrogenation, the spectra of the five dihydrocoinpou...

Retardation of AlH2Cl Hydrogenolysis of the Ethylene Ketals of 5-Norbornen-2-one and 2-Norbornen-7-one Due to the Presence of the Double Bond

“…spectrum of the exo-and endo-2-(5-norbornenyloxy)ethanols. In agreement with the well-known fact that the 2-exo proton is deshielded relative to the 2-endo proton in norbornene due to the anisotropy of the double bond (8,9), the signal of the C-5 proton of endo-2-(5-norbornenyloxy)ethanol occurs as a quintet centered at T 5.90 while the signal of the C-5 proton of the exo isomer lies between T 6.3 and 6.7 in the region where the four methylene protons of the substituent absorb.…”

N.M.R. Studies of Bridged Ring Systems: Ii. Unusual Magnetic Deshielding Effect on the Bridge Methylenes of Norbornadiene and Benzonorbornadiene

“…By this effect, the signal of a protoil which lies above the plane of the sp2-bonds is shifted to upfield, whereas a downfield shift is experienced by the proton which lies in the sp2-bond plane. Fraser (14) demonstrated these facts in comparison of the signals of the protons H5 and H6 on some Cg-or Ce-substituted norbornenes with those of the corresponding norbornanes.…”

The norbornene mystery revealed

“…1) derivatives results in exceptional characteristics, which have both puzzled and intrigued theoreticians and experimentalists for decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In addition to the non-planarity of the double bond resulting in an unusually high exo-reactivity (the Huisgen's X-factor), [4][5][6]9 other anomalies of the electronic structures are revealed by the nuclear magnetic resonance spectra [18][19][20] as exemplified by the downfield chemical shift of the bridging methylene 6,20 and extra large 1 J(C-C) couplings. 15 Several explanations, e.g.…”