Equilibrating rotamers

To rationalize the stereochemical outcome of the reactions illustrated as well as a host of other examples, the following arguments have been put forth. Firstly, since the cycloaddition reactions are exothermic and have an early, reactant-like transition state, those factors which influence the relative energy of the equilibrating rotamers (23)-(25) also influence the transition-state energies. Examination of... 

Table 4 shows the results of such an attempt, where the molecular mechanics method was used to estimate the relative population of equilibrating rotamers and to compute population-weighted vicinal H/H coupling constants by means of a Karplus-type equation. The calculated coupling constants are weight-averaged over three C—C rotamers for 4 27 for 5 81 for 6a 27 for 6b and 6c, and 243 for 7. It can be seen that... 

As has been discussed, ordinary formamides have a barrier of about 21 kcal/ mol, which is a little less than that required for the isolation of atropisomers at room temperature. This means that, at a temperature slightly lower than ambient, it may be possible to obtain stable rotamers. This possibility was first realized by Gutowsky, Jonas, and Siddall (40). They used a uranyl nitrate complex of N-benzyl-N-methylformamide (4) crystallized from dichloromethane. When the crystals were washed with ice water to strip off the uranyl nitrate, a mixture of E and Z forms (Z/E = 1.6) was obtained. Since the equilibrium mixture gives a Z/E value of 0.8, it was possible to perform a kinetic study of equilibration... 

Fig. 10) has been carried out (52) and the result was compared with those of the monomethyl acetals 50 and 1, and the gem-dimethyl acetals 52 and 53. Each pair of isomers gave essentially an identical result, i,e. 33% of the equatorial and 77% of the axial isomer ( 2%) after equilibration in benzene at 70°C. This corresponds to a aG of =0.8 kcal/mol, a value close to that predicted for the relative stability of rotamer A over E (0.6 kcal/mol). 

It should be pointed out (a) that the half-life of each rotamer in forma-mides 139 and 140 is several minutes at room temperature, (b) that the basic hydrolysis of the imidate salts 137 and 138 is completed in less than one minute at that temperature. Consequently, the production of the equilibrium mixture of the amides rotamers comes from the direct fragmentation of tetrahedral intermediates. It cannot come from a subsequent equilibration of the amide rotamers. 

The presence of isoretulinal (111) in the root bark of Strychnos variabilis de Wild, has been confirmed,79 and it has also been shown that retulinal (112), with which it is very easily equilibrated, occurs in smaller amounts in the same extracts both alkaloids exist in two forms, which are amide rotamers, according to their n.m.r. spectra. The 12-hydroxy-derivatives, (113) and (114), form a similar equilibrating pair of alkaloids, and were also isolated from this source. The occurrence of these easily interconvertible C-16 epimers explains the simultaneous occurrence of retuline and isoretuline derivatives in Nature, in both the monomeric and dimeric alkaloid series. 

Iqbal N, Akula MR, Vo D et al (1998) Synthesis, rotamer orientation, and calcium channel modulation activities of alkyl and 2-phenethyl 1, 4-dihydro-2, 6-dlmethyl-3-nltro-4-(3- or 6-substltuted-2-pyrldyl)-5-pyridinecarboxylates. J Med Chem 41 1827-1837 LI RWS, Tse CM, Man RYK et al (2007) Inhibition of human equilibrative nucleoside transporters by dihydropyridine-type calcium channel antagonists. Eur J Pharmacol 568 75-82 Cosconatl S, Marlnelli L, Lavecchia A et al (2007) Characterizing the 1,4-dlhydropyrldlnes... 

The photochemically induced ring closure of (1) to (2) has also found relatively little synthetic use due to tile seemingly complex nature of the photopnxlucts obtained upon irradiation of a hexatriene. " Some of the following principles or factors which can affect tile course of a photochemical process have been reviewed by Laartioven and these include the following, (a) The NEER (non-equilibration of excited state rotamers) principle indicates tiiat each conformer of a polyene yields its own characteristic... 

In order to explain the stereochemical results in terms of the diradical mechanism, one must assume that the diradical intermediates collapse to dienes before rotamer equilibration is achieved. This demand led to the conclusion that the concerted mechanism accommodates the results with less stringent demands. Thermochemical calculations provide additional support for the conclusion that there is only a small probability for a diradical pathway. 

The photochemistry of vitamin D (see also Special Topic 6.4 above and Scheme 6.8) has also played a central role in the development of modern organic photochem-istry.564,598,617 618 The concept of non-equilibration of excited rotamers (NEER Section 6.1.1) has been used to explain the excitation-wavelength dependence of E Z isomerization (Section 6.1.1) of previtamin D3 (41).619 Whereas the quantum yield for E Z isomerization decreases with increasing wavelength, the formation efficiencies of the 6jt-electron conrotatory ring-closure products, diastereomeric 7-dehydrocholester-ol (provitamin D) (64) and lumisterol (65) (Scheme 6.22), increase dramatically. This was found to occur on the basis of a participation of both the Si and S2 excited states in the photoreaction.620 For example, the quantum yields of 64 and 65 formation were [Pg.244]

All the examples to be discussed are of square-planar platinum(II) complexes, recorded in Scheme 2. Thus Boucher and Bosnich (8) have prepared a series of olefin complexes derived from ( -tolylmethylsulfoxide)dichloroplatinum(II) in which the ligand is S-bonded. The S-methyl protons provide a convenient and sensitive NMR probe for the determination of diastereomer ratios. In the cases of but-l-ene, 3-methylbut-l-ene, styrene and 3,3-dimethylbut-l-ene crystals of a single diastereomer can be isolated in all but the last of these, however, there is rapid equilibration between diastereomers in solution, with relatively little discrimination between them (typically 55-75% of the major species). Since several rotamers are possible in the sulfoxide ligand it is not easy to specify the origin of chiral discrimination. 

A comparison of the line-shape method with an equilibration method has shown good agreement in results (the Arrhenius plots for both methods are colinear). The example chosen was the interconversion of rotamers in A7 -methyl-A(-benzylformamide, which between 90° and 170°C shows averaged spectra. One of the rotamers can be obtained substantially pure at low temperatures, and the return to equilibrium was studied in the range —2 5° to 20°C. A similar study using AT-benzyl-A -2,4,6-tetramethylbenzamide and N,AT-2,4,6-penta-methylbenzamide has also been made. °... 

It is not a priori excluded that one of the pathways of relaxation of a certain hl ly excited rotamer could lead to another rotamer. Essentially the principle says no more and no less than that equilibration between the excited rotamers does not occur. 

---