Abstract
On photoisomerization of previtamin D - a steroid Z-triene - produced in situ by ring opening of 7-dehydrocholesterol in a cold matrix, it was found in A. M. Müller et al. [Angew. Chem., Int. Ed., 1998, 37, 505-507] that the product (tachysterol) had rotated not only its central double bond but also an adjacent single bond. This is called a Hula twist (HT) due to the alternative description, in which it is just one central CH group that rotates. It was pointed out that the results directly support the calculated molecular structure at a conical intersection, which mediates the Z-E isomerization of polyenes. With a more sophisticated technique, Saltiel et al. (J. Phys. Chem. Lett., 2013, 4, 716-721) confirmed this tachysterol rotamer as the main product but found two additional conformers. They believed to have seen also three previtamin D conformers, suggested to be a result of hot-ground-state reactions from the primary rotamer, and interpreted all tachysterol products to be a result of a double-bond twist (DBT), not a HT. On the basis of published circular dichroism data and consideration of other reactions, it is here shown that under these conditions hot-ground-state reactions are unimportant or even negligible and that there is practically only a single conformer of previtamin D after ring opening. All products can be easily understood on the basis of an HT-type conical intersection, which is thus further supported. Invoking a published pretwist model even rationalizes product ratios. The two twists in HT are concerted. Furthermore HT is fully consistent with the NEER principle (nonequilibration of excited rotamers) and even offers additional possibilities for conformer control.
Highlights
The Z–E photoisomerization of previtamin D (Pre, a steroid Z-triene; structures are given below) to tachysterol (Tachy, the corresponding E-triene) was studied by our group in 1998 in Garching in a matrix (EPA : diethyl ether, isopentane, ethanol 5 : 5 : 1) at 90 K.1 Because Pre was prepared in situ in the cold glass matrix by photochemical ring opening of 7-dehydrocholesterol (Dehy, a cyclohexadiene derivative; called provitamin D3), it was reasonable to assume that it is a single conformer, still ring-like, with the two single bonds adjacent to the central double bond in the cis form
The same difference must be assumed for the – nonplanar – triene chromophores of the two resulting cZc-Pre-s.) They found from Lumi only one Pre conformer but two Tachy conformers, whose identification seems to be less clear-cut than on starting from Dehy
A clear-cut example of the photochemical type is rotamerization of butadiene in a cryogenic matrix:[64] Irradiation at 214 nm (s-trans) produced s-cis, whereas excitation near 230 nm (s-cis) resulted in s-trans; by contrast, the product composition should be very similar with the two wavelengths, if it were a matter of a ground-state reaction (as supposed by Saltiel (p. 2336 of ref. 9)). (Liu interprets this and other examples as an HT process.20–27) More recent examples of the photochemical type are the substituted hemithioindigos of ref. 40, where the conformer pairs A/B and C/D could individually be rotamerized as in eqn (1) and (2). This criterion can be used to answer whether Hula twist is concerted: Saltiel repeatedly[8] suggested that observed cases of HT are photochemical double-bond twist (DBT) followed by rotamerization in the hot ground state
Summary
The Z–E photoisomerization of previtamin D (Pre, a steroid Z-triene; structures are given below) to tachysterol (Tachy, the corresponding E-triene) was studied by our group in 1998 in Garching in a matrix (EPA : diethyl ether, isopentane, ethanol 5 : 5 : 1) at 90 K.1 Because Pre was prepared in situ in the cold glass matrix by photochemical ring opening of 7-dehydrocholesterol (Dehy, a cyclohexadiene derivative; called provitamin D3), it was reasonable to assume that it is a single conformer, still ring-like, with the two single bonds adjacent to the central double bond in the cis form (cZc-Pre). In 2013 the Saltiel group in Tallahassee/Florida reinvestigated this system (at 77 K in EPA) with a more elaborate technique and found two additional Tachy conformers: cEc and cEt besides the main product tEc.[8] They believed to have seen three Pre conformers (cZc, tZc and cZt) as products of the Dehy ring opening They interpreted each Tachy conformer as a result of a simple double-bond twist (DBT) of the corresponding Pre conformer; they concluded ‘‘DBT, not HT’’.8. The quantum-chemical calculations for the CI that controls Z–E isomerization of polyenes[2,3,4,5,6,7] are widely accepted At this CI the molecule has twisted two neighboring CC bonds by (much) less than 1801; i.e., it has performed an incomplete HT, and 6776 | Phys. Further related questions will be considered such as the NEER principle and conformer control, volume constraints and reactions in the hot ground state
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