Patai S., Rappoport Z. 2000 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 2 / 0059 196

I. INTRODUCTION

A. Scope and Limitation

From the advent of organic chemistry, dienes (and polyenes) have played a very important role in both the theoretical and synthetic aspects. For example, 1,4-addition of bromine to 1,3-butadiene to form 1,4-dibromo-2-butene rather than 3,4-dibromo-1-butene as the major product was a challenging problem for theoretical chemists, who interpreted the phenomenon in terms of resonance or delocalization of -electrons1.

Later, the structural chemists determined the structure of 1,3-butadiene (the bond lengths C1 C2 and C2 C3 are 1.467 and 1.349 A,˚ respectively, which can be compared with the corresponding values of trans-2-butene (1.508 and 1.347 A),˚ respectively). Another significant aspect of dienes is the Diels – Alder reaction, the reaction between a diene and an olefin with electron-withdrawing substituents to give a six-membered ring2. The reaction is designated as 4 C 2 cycloaddition since the diene has four carbon atoms while the olefin, a dienophile, may represent a two-carbon unit. The mechanism of this useful reaction was not clear until 1964, when Woodward and Hoffmann proposed the so-called Woodward – Hoffmann rule3. This proposal has opened a wide world of electrocyclic reactions in which the symmetry of orbitals plays an important role.

NMR spectroscopy has been extensively used in diene chemistry not only for conventional structural analysis but also in dealing with theoretical problems. Among a variety of examples in which NMR spectroscopy played an important role in the latter application, is the unusually large highand low-field shifts observed for inner protons of cyclic conjugated polyenes (annulenes). Thus, the high-field shifts for [4n C 2]annulenes and corresponding low-field shifts for [4n]annulenes were interpreted as the indication of aromaticity and antiaromaticity, respectively, of these compounds4.

Previously, the NMR spectroscopic data for dienes and polyenes were treated as a part of a chapter on alkenes5, and have not been treated as an independent topic. In view of the important role which dienes and polyenes have generally played in chemistry, the authors believe that the topic can, and should, be treated in an independent chapter.

In this review, chemical shifts and coupling constants of simple dienes will first be summarized, and the theory of chemical shift for dienes and polyenes will then be reviewed. Finally, the recent applications of NMR spectroscopy to a variety of polyenes and dienes and specific systems (allenes, solitons and fullerenes) will be reviewed.

Although we have tried to cover the literature on standard data as much as possible, our emphasis have been focused on new developments of application of NMR spectroscopy to dienes and polyenes. Readers who seek more basic data rather than recent advances are advised to consult books and journal articles dealing with this topic.

B. Chemical Shifts and Coupling Constants

It would be convenient for the readers if a limited amount of selected data are summarized at the beginning of this chapter so that they can have some general idea on the chemical shifts and coupling constants observed for dienes and polyenes.

i. Protons bonded to conjugated carbon atoms. Collections of data on chemical shifts in linear dienes, cyclic dienes and exocyclic multi-methylene systems are given in Table 1 together with references to selected compounds. The characteristic values of the geminal and allylic coupling constants and chemical shifts assembled in Table 1 make these signals very informative. The chemical shifts of some allenic protons are also included in Table 16.

TABLE 1. 1H chemical shifts and coupling constants of linear and cyclic dines, exocyclic methylenes and allenes

C

H H a

1

2

13

126.1 124.6 22.3

8

aAssignment uncertain.

The 13C NMR data for pentafulvene (1) and heptafulvene (2) (Table 3) and for 6,6- dimethylpentafulvene (5) and sesquifulvalene (6), afford evidence of the extent to which polar structures of the types 5a and 6a contribute to the ground state. If the chemical shifts are analyzed on the basis of electron density, these hydrocarbons are to be considered as olefinic systems with only a small contribution (10% at most) from the polar structures

5a and 6a.

iv. Cyclic conjugated polyenes. Table 48 gives the 13C chemical shifts for cycloheptatriene and related compounds. The internal olefinic carbons C3 and C4 of the triene

system are the most deshielded in cycloheptatriene. Methyl substitution at C1 deshields C1 and C7 and shields C2 and C3.

v. Allenes. Allenes form a unique class of compounds because of the extremely low field shift of the central allenic carbon C2 (200 to 220 ppm). Table 58 presents representative data for a number of substituted allenes. For a given alkyl substituent, there is a linear relationship between the number of substituents and the chemical shift of the central carbon. The shielding is regarded as an additive property, a methyl group shields that carbon by 3.3 ppm, an ethyl group by 4.8 ppm and a sec-alkyl group by 7 ppm. Carbons C1 and C3 are shielded by some 30 ppm relative to corresponding ethylene carbons but otherwise display similar substituent effects. Strain in cyclic allenes appears to have little effect.

II. THEORY OF NMR CHEMICAL SHIFTS OF POLYENES

Recently, ab initio shielding calculations based on well-established theories, IGLO (individual gauge for localized orbitals)9, GIAO (gauge including atomic orbital)10 and LORG

a13C – 1H coupling (in hertz) in allene: 1JC – H [167.8], 2JC – H [3.9], 3JC – H [7.7]. bAssignments uncertain.

(localized orbital/local origin) methods11 have been widely used not only to assist signal assignments but also to elucidate the electronic structure and conformation of molecules. The use of high-quality basis sets in these types of shielding calculations leads to reasonable results which are within the experimental accuracy.

Inoue and coworkers12 reported an ab initio calculation of the 13C shieldings for some polyenals and their Schiff bases using the LORG method. They reported the results for some polyenes with basis sets of various quality. It was shown that the introduction of polarization functions substantially improves agreement between experiment and theory. They used the program RPAC9.0, which was developed by Bouman and Hansen13 for ab initio shielding calculations, interfacing to the Gaussian-90 program14. The geometrical parameters of all the molecules studied were optimized by using 6-31G basis sets and planar frameworks were then assumed for the backbone. The basis sets they used for shielding calculations were the Pople type 6-31G, 6-31CG, 6-31CCG, 6-31GŁ , 6-31GŁŁ, 6-311GŁ and 6-311GŁŁ15.

In the LORG theory, occupied orbitals are localized according to the Foster – Boys

criterion16. In the calculations described above they chose the LORG centroid assignment11a.

Table 6 gives the calculated and experimental 13C shieldings for acrolein, crotonaldehyde and hexa-2,4-dienal. The numbering of the carbon atoms is given in Figure 1. The calculated and experimental 13C chemical shift data were converted to the methane reference using the data in Table 7 and in the standard reference17, respectively.

The author examined the correlation between the calculated and experimental isotropic shieldings. The 6-31G shielding data are in qualitative agreement with the experimental data and completely reproduce the relative order of all the carbon shieldings studied. The 6-31G shieldings for the carbonyl carbons shift are about 20 ppm downfield of the experimental values. If the experimental data are converted to the methane reference using the data reported by Jameson and Jameson18, this discrepancy still remains large (about 16 ppm).

The RMS error for the 6-31G data is relatively large (9.8 ppm). The origin of this error has been considered to be attributable to electron correlation effects, which were not included in the calculations. The results for the carbonyl shieldings could be improved by using d-type polarization functions on carbon and oxygen atoms. The results using 6- 31GŁ and 6-31GŁŁ basis sets exhibited a good reproducibility for all the carbons including the carbonyl carbons. The RMS errors for the 6-31GŁ and 6-31GŁŁ results are 3.7 and 3.8 ppm, respectively.

As shown in Table 6, the addition of p-type polarization functions to hydrogen atoms in the 6-31GŁŁ basis has little effect on the calculated data.

FIGURE 1. Numbering system of carbon atoms for the compounds in Table 6