Hydrogen shift suprafacial

Enamides 163 undergo photochemical conrotatory six-electron electrocyclic reactions to yield the dihydro intermediate 164, which in turn yields the fraws-fused cyclic product 165 (equation 105) by a (l,5)-suprafacial hydrogen shift. Several natural product syntheses like that of benzylisoquinoline and indole type alkaloids can be achieved by this type of photocyclization (equations 106163, 107164, 108165 and 109166). 

Thermal sigmatropic 1,5 hydrogen shifts are quite common in certain allene and diene systems, cis- 1,3-Dienes have the proper geometric arrangement to undergo a thermally allowed suprafacial hydrogen shift, cis-1,3-Hexadiene, for instance, gives... 

The stereochemistry has been proved to follow the allowed pathways. Heating the diene 5.5 induces a suprafacial hydrogen shift to give the diene 5.6, a suprafacial deuterium shift then converts this diene into the diene 5.7, and another suprafacial deuterium shift converts it into a fourth isomer 5.8. The major components at equilibrium are the isomers 5.6, with an E trisubstituted double bond and S at the stereogenic centre, and 5.8, with a Z trisubstituted double bond and R at the stereogenic centre. Neither of the other possible isomers is evident, showing that no [1,5] antarafacial shifts had occurred. 

Under carefully specified conditions the vacuum pyrolysis of 2-( 1,3-butadienyl)furan smoothly provides 4,5-dihydrobenzofuran by way of a nonaromatic intermediate and a 1,5-sigmatropic suprafacial hydrogen shift. As a check, deuterium at the terminal methylene group of the butadiene chain was shown to reside at position 4 in the product.231... 

When treating dimethyl-1,5-nonadicne with A -bromosuccinimide the bromonium ion attacks the nonhalogenated. electron-rich double bond. After Markovnikov ring closure the resulting cation is stabilized by a [1,2] suprafacial hydrogen shift, and dibromo-m-hydrindanone 1 is formed stereoselectively32. 

Fast suprafacial hydrogen shifts, considered to be of [1,5] sigmatropic nature have been observed with the C60-fullcrenc-morpholine adduct C60H6[N(ClI2CH2)2O]6 (14) in chloroform solution38. Above — 25 3C, a reversible, temperature-dependent change in the H-NMR chemical shifts of the C60-bound hydrogens in 14 is noticed (AH < 10 keal/mol)38. 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

The isomerization of l,4-bis(7-cycloheptatrienyl)benzene (35, Figure 11.37) to 36, 37, and 38 can be rationalized as a series of thermal [1,5] and photochemical [1,7] suprafacial hydrogen shifts. A [1,7] hydrogen shift has also been seen in the conversion of previtamin D3 (39) to vitamin D3 (40, Figure 11.38). The stereochemical nature of these reactions is not apparent in the absence of labeling, but the [1,7] hydrogen shift was shown to be antarafacial in previtamin D3 model compounds, as in the conversion of deuterium-labeled ds-isotachysterol 41 to 42 and 43 in Figure 11.39. ... 

The greater flexibility of the linear dienes should - and does - show up in a negative entropy of activation, but should not raise the energy of activation unless the transition state is quite strained that for a suprafacial hydrogen shift in pentadiene is not. 

Analysis of a 1,7-hydrogen shift process indicates that the suprafacial hydrogen shift is symmetry forbidden in a thermal reaction. Photochemically, [1,7] suprafacial shift of hydrogen is symmetry allowed (Fig. 4.4) [1,2]. 

Thermal [1,3]-suprafacial hydrogen shift is orbital symmetry forbidden process, but [1,3]-suprafacial alkyl shift is symmetry allowed process with inversion of configuration of migrating alkyl carbon. For example, the thermal rearrangement of bicyclo-[3.2.0]-heptene 1 to bicyclo-[2.2.1]-heptene 2 [4]. 

Methyl-cycloheptatriene 34 on heating undergoes slow [l,5]-suprafacial hydrogen shift rather that [l,7]-antarafacial H-shift to yield a mixture of methyl-substituted isomers [9]. 

A bonding interaction can be maintained only in the antarafacial mode therefore, the 1,3-sigmatropic suprafacial hydrogen shift is considered forbidden. Since the geometry required for the orbital symmetry-allowed antarafacial shift is very contorted, this shift, too, is of high energy, and the concerted process in unlikely under conditions of thermal activation. 

In recent years, it has been suggested that not all the excitation wavelength dependence of quantum yields in the vitamin D field can be accounted for by the NEER principle. We consider below the competing mechanisms that have been proposed to account for photochemical observations and describe some of the strategies that have been employed to improve the photochemical production of the previtamins from the provitamins. Optimization of the previtamin yields improves vitamin yields as the latter are formed thermally from the previtamins via 1,7-suprafacial hydrogen shifts. Readers interested in the rich photochemistry leading to overirradiation products should consult previous reviews. ... 

---