P-Carotene dication

Fig. (24). Structures for p-carotene dication suggested on the basis of theoretical calculations.

Ferric chloride (FeCl3) is the Lewis acid most frequently employed [109,144]. Exclusive formations of dications were reported for carotenoids such as P-carotene (1) and canthaxanthin (16) when the carotenoids were oxidised with FeCl3 [109], The course of formation of canthaxanthin (16) dication and of astaxanthin (17) dication with FeCl3 in hexafluoroisopropanol at low temperature was studied [144]. Recently, we have prepared p-carotene dication (11) by using BF3-etherates in CHC13 at -20°C [11]. 

Whereas free radicals like carotenoid cation radicals are not amenable to NMR analysis, was the first complete NMR analysis of p-carotene dication (11) recently accomplished by our group [11]. 

The detailed structure of the charge delocalised P-carotene dication (11) was established by COSY, HSQC, HMBC and ID and 2D ROESY NMR techniques (600 MHz, CDC13, -20°C), leading to complete... 

An alternative method to gain information on the structure of carotenoid dications is by studying their reactions with suitable nucleophiles [11]. We have recently investigated the products obtained when reacting the p-carotene dication (11) with H2O as a nucleophile in acetone providing isomerised isocryptoxanthin (49), mutatochrome (50) and strongly isomerised isocarotene (51), according to HPLC, Vis, MS and H NMR data. The formation of these products are rationalised in Fig. 

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

A hypsochromic shift with increasing solvent polarity is observed. Absorption curves for the cation radical and dication of [3-carotene (1) are reported [109], see Fig. 23. The molar extinction coefficients for neutral p-carotene (1) and the charged species 11 and 12 are of comparable magnitude, Table 1, Fig. 23. INDO/S calculations for these spectra were performed and discussed. 

It is interesting to note that the dication structure 11 elucidated by NMR incorporates elements predicted by theoretical calculations including a pair of charged solitons (47, 48) and reversal of double and single bonds in the central part of the molecule (47, 48). Rotated C-6,7 and C-6 ,7 single bonds as calculated for the P-carotene cation radical (12), Fig. 21, was established for the dication 11. 

The equilibrium between neutral carotenoid, cation radical and dication was already discussed [142,143], More recently the effect of electrolytes and temperature on carotenoid dications were studied [40]. The stability of the P-carotene (1) dication at -25°C in CHCI3 was remarkable, showing a decrease of less than 20% during 2h, as based on NIR spectra [11]. 

It was observed that electrochemical oxidation of all-trans P-carotene (1) and canthaxanthin (16) in CH2CI2 leads to significant trans-cis isomerisation [105]. It was suggested that the isomerisation mechanism involved cation radicals and/or dications which could easily undergo geometrical isomerisation. This proposal was supported by AMI molecular orbital calculations, which showed that the energy barrier from trans to cis is much lower in the cation radical and dication species than in the neutral carotenoid [105]. 

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