Hydride shifts, seven

A quite different-looking rearrangement is the hydride shift 1.16 —> 1.17, also long known from the chemistry of vitamin D. In this case, the end of the H-C bond attached to the hydrogen atom (H-l1) remains attached to the hydrogen, but the other end moves seven atoms (C-l to C-7) along the conjugated carbon chain. This reactions is therefore called a [1,7] shift. 

A rather complex mixture of seven alcohols can be detected (besides alkenes and nitrites). The product ratio of the alcohols (normalized to 100 in Table 7-3) is dependent on the initial concentration of the amine 7.74, on the nitrosating reagent and on the mineral acid used. The product ratio in HCIO4, to a lesser extent also in HCl, is dependent on the concentration of the amine, and is obviously also influenced by the cmc. The influence of micelles can be seen in the yields for 7.75, i. e., the product of straightforward substitution, and also in the configuration of the compounds formed by a 1,2-hydride shift, 3-methylhexan-3-ol (7.76) and... 

The stereochemistry of a sigmatropic hydride shift depends on the highest occupied molecular orbital (HOMO) of the T framework. In the transition state, a three-centered bond is required, which involves overlap between the s orbital of the hydrogen and lobes of p orbitals of the two terminal carbons. From the HOMO configuration of a seven-carbon allylic radical in the ground state, we see that the symmetry-allowed shift is in an ahtarafacial manner (see Figure 1). 

Sketch (a) the transition state for a concerted metal atom-assisted 3,9 hydride shift (b) two PNP ligands (c) the ligand used for selective dimerization of butadiene (d) a general structure for molybdenum- and tungsten-based metathesis precatalyst (e) a six-coordinate rathenium precatalyst for metathesis (f) a solid isolated from the reaction between Pd(OAc)j plus PRj (R = o-tolyl) (g) a T-shaped palladium complex and a two-coordinate palladium complex with a monodentate phosphine (h) an iron complex with a seven-membered metallacycle (i) the transition state for metal-catalyzed cyclopropanation (j) a rhodium and a copper precatalyst used in cyclopropanation reactions. 

Sometimes several of these rearrangements occur in one molecule, either simultaneously or in rapid succession. A spectacular example is found in the triterpene series. Friedelin is a triterpenoid ketone found in cork. Reduction gives 3p-friedelanol (47). When this compound is treated with acid, 13(18)-oleanene (48) is formed. In this case seven 1,2 shifts take place. On removal of H2O from position 3 to leave a positive charge, the following shifts occur hydride from 4 to 3 methyl... 

Successive Wagner-Meerwein rearrangements have been utilized in triterpenoid syntheses. A unique and fascinating example of this can be foimd in Corey s synthesis of the triterpenoid oleanene. When 3P-friedelanol (35) is treated with acid, a total of seven 1,2-alkyl and 1,2-hydride migrations occur leading to the formation of 37. The stereospecific shifts are driven by the apparent decrease in steric strain due to the original location of the axial substituents. Intermediate products have been isolated, supporting the assertion that at least some of the steps are not concerted. 

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