The Woodward-Fieser Rules for Dienes

In butadiene, two possible transitions can occur 2 and 2 We have already discussed the easily observable Y2 transition (see Fig. 10.12). The transi- 

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A transition at 175 nm lies below the cutoff points of the common solvents used to determine UV spectra (Table 10.1) and therefore is not easily detectable. Furthermore, the s-trans conformation is more favorable for butadiene than is the s-cis conformation. Therefore, the 175-nm band is not usually detected. 

In general, conjugated dienes exhibit an intense band ( = 20,000 to 26,000) in the region from 217 to 245 mn, owing to a — r transition. The position of this band appears to be quite insensitive to the nature of the solvent. 

In cyclic dienes, where the central bond is a part of the ring system, the diene chromophore is usually held rigidly in either the s-trans (transoid) or the s-cis (cisoid) orientation. Typical absorption spectra follow the expected pattern  

Homoannular diene (cisoid or s-cis) Less intense, e = 5,000-15,000 X longer (273 nm) 

Heteroannular diene (transoid or s-trans) More intense, e = 12,000-28,000 X shorter (234 nm) 

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TABLE A3-2 The Woodward-Fieser Rules for Conjugated Dienes Values for Auxochromic Groups... 

The Woodward-Fieser rules for conjugated ketones and aldehydes appear in Table A3-3. Note that bathochromic effects of alkyl groups depend on their location 10 nm for groups a to the carbonyl and 12 nm for groups in )3 positions. Contributions from additional conjugated double bonds (30 nm) and exocyclic positions of double bonds (5 nm) are similar to those in dienes and polyenes. 

Sets of empirical rules, often referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 7.11) and enones and dienones (Table 7.12) to be predicted. To the respective base values (absorption wavelength of parent compound) are added the increments for the structural features or substituent groups present. When necessary, a solvent correction is also applied (Table 7.13). 

Using the Woodward-Fieser rules, the of dienes and trienes, a,p-unsaturated aldehydes and ketones (enones), and for aromatic carbonyl compounds can be reliably calculated and predicted. 

Empirical rules based on thousands of laboratory observations have been developed over the years relating the wavelengths of the UV absorption maxima to the structures of molecules. R.B. Woodward in 1941 and then L. Fieser and M. Fieser developed rules for predicting the absorption maxima of dienes, polyenes, and conjugated ketones by studying terpenes, steroids, and other natural products. The rules are known as Woodward s Rules or the Woodward-Fieser Rules. 

The following examples show how the Woodward-Fieser rules predict values of Amax variety of conjugated ketones and aldehydes. Notice that the molar absorp-tivities (s) for these transitions are quite large (>5000), as we also observed for tt — tt transitions in conjugated dienes and polyenes. 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

Structural analysis from electronic spectra yields little information because of their relative simplicity. In the 1940s, however, before the advent of more powerful identification techniques, UV/VIS visible spectroscopy was used for structural identification. The study of a great number of spectra of various molecules has revealed correlations between structures and the positions of absorption maxima. The most widely known empirical rules, due to Woodward, Fieser and Scott, involve unsaturated carbonyls, dienes and steroids. Using incremental tables based on various factors and structural features, it is possible to predict the position of the n —> n absorption bands in these conjugated systems (Table 11.3). Agreement between the calculated values and the experimentally determined position of absorption bands is usually good, as can been seen by the following four examples ... 

The chiroptical behaviour of cisoid conjugated dienes, already known to be controlled by a combination of diene helicity and axial chirality effects, is further influenced by methyl substituents on the unsaturated carbon atoms. The observed methyl group contributions are not susceptible to a simple interpretation, for in some cases [e.g. at C-2 or C-3 in a 2,4-diene (24), or at C-3 in a 19-nor-l,3-diene (25)] the signs of methyl group contributions to Ae and to the rotatory strength (R) are opposite. Computer resolution showed that the c.d. and u.v. absorption curves are of composite form, with up to five vibronic components. Moreover the wavelength shifts accompanying methyl substitution do not accurately follow the Fieser-Woodward rules. It is concluded that... 

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