Spiro transition states

Two extreme epoxidation modes, spiro and planar, are shown in Fig. 9 [33, 34, 53, 54, 76-85]. Baumstark and coworkers had observed that the epoxidation of cis-hexene of dimethyldioxirane was seven to nine times faster than the corresponding epoxidation of tran.y-hexene [79, 80]. The relative rates of the epoxidation of cisitrans olefins suggest that spiro transition state is favored over planar. In spiro transition states, the steric interaction for cw-olefm is smaller than the steric interaction for fran -olefm. In planar transition states, similar steric interactions would be expected for both cis- and trans-olefms. Computational studies also showed that the spiro transition state is the optimal transition state for oxygen atom transfer from dimethyldioxirane to ethylene, presumably due to the stabilizing interactions... 

The stereochemistry of the resnlting epoxidation products using chiral ketones, such as ketone 26, could provide new insights about the epoxidation transition states. Studies showed that the epoxidation of trans- and trisubstituted olefins with ketone 26 mainly goes through the spiro transition state (spiro A) (Fig. 10). Planar transition state B competes with spiro A to give the opposite enantiomer [53, 54]. Hence, factors that influence the competition between spiro A and planar B will also affect the enantiomeric excess of the resulting epoxides. Spiro A can be further... 

The oxidation of the simplest symmetrically substituted alkene, ethylene, is noteworthy in that an asymmetric spiro transition state is observed. When constrained to Cs symmetry with eqnal forming carbon-oxygen bond lengths, the energy increases by only 0.1 kcalmol. The spiro TS has the plane of the HO—ONO (or peracid) at right angles to the axis of the C=C bond. In an idealized spiro TS this angle is exactly 90°. While the formation of snlfoxides from snlhdes by peroxynitrons acid is well-established , epoxidations have not yet been observed in solution. 

An interesting intramolecular variant of this epoxidation procedure is represented in the reaction of the unsaturated oxaziridine 167, which undergoes highly stereoselective oxygen transfer through a spiro transition state to provide the epoxyaldehyde 1 <99TL4453>. 

Other advantages include a mechanism that allows one to rationalize and predict the stereochemical outcome for various olefin systems with a reasonable level of confidence utilising a postulated spiro transition state model. The epoxidation conditions are mild and environmentally friendly with an easy workup whereby, in some cases, the epoxide can be obtained by simple extraction of the reaction mixture with hexane, leaving the ketone catalyst in the aqueous phase. 

The stereochemistry of epoxidation by using chiral ketones (3 and 4) as catalysts can be explained by spiro transition state, in which jt-electrons of the olefin attack the lone-paired electrons concurrently attack the it -orbital of double bond to give the epox ide (Figure 6B.4) [12,13]. The observed effect of the size and location of the olefinic substituents on enantioselectivity (Figure 6B.3) is compatible with the proposed transition-state model [10a],... 

AMI and PM3 calculations reveal that epoxidations by DMDO and TFDO involve peroxide-bond cr at a very early stage and that TFDO is the most reactive dioxirane as the CF3 group in it stabilizes this cr level. In accord with previous calculations a spiro transition state is predicted. Furthermore, allene is predicted to be less reactive than alkenes toward epoxidation by DMDO.192 DFT calculations on the oxidation of primary amines by dimethyldioxirane predict a late transition state with a barrier of 17.7 kcal mol-1 which is drastically lowered by hydrogen bonding to the O—O bond to just 1.3 kcal mol-1 in protic solvents.193... 

FIGURE 10.5 Spiro transition states for the epoxidation of cw-P-methylstyrene with ketone 1. 

Calculations [46] and studies of intramolecular oxaziridinium epoxidations [47] suggest that, like their dioxirane counterparts, these epoxidation processes proceed via spiro-transition states. However, the iminium epoxidations are generally more substrate-specific than those using dioxiranes, and models to explain the observed trends in stereocontrol have proved more difficult to construct. One complication is the possibility of formation of diastereomeric oxaziridinium salts from most of the iminium catalysts. Houk has rationalized computationally the observed enantioselectivity with Aggarwal s catalyst 16 [46]. The results of a recent study by Breslow suggest that hydrophobic interactions are important in these processes [48], and aromatic-aromatic interactions between catalyst and substrate may also play a role. 

In the transition state of the epoxidation of alkenes with a percarboxylic acid the C=C axis of the alkene is rotated out of the plane of the percarboxylic acid group by 90° ( spiro transition state ). In this process, four electron pairs are shifted simultaneously shifted. This very special transition state geometry make peracid oxidations of C=C double bonds largely insensitive to steric hindrance. The epoxidation given in Figure 3.20 provides an impressive example. 

Fig. 2. Comparison of planar and spiro transition state geometries for oxidation of cis-and frans-alkenes.

There are two possible transition states spiro and planar. Nearly every example of frans-disubstituted and trisubstituted olefins which were studied with Shi s catalyst is consistent with the spiro transition state. The extent of the involvement of the competing planar transition state depends on the nature of the substituents on the olefins. 

A. L. Baumstark, C. J. McCloskey, Epoxidation of alkenes by dimethyldioxirane Evidence for a spiro transition state. Tetrahedron Lett. 28 (1987) 3311. 

Two transition states have been proposed for the epoxidation of alkenes by dioxiranes and oxaziridines, the spiro and the planar (Fig. 5.8). In the spiro transition state, the alkene approaches the oxaziridinium moiety in such a way that the axis of the carbon-carbon double bond is perpendicular to the carbon-nitrogen bond axis. In the planar transition state, the two components approach one another so that their axes are parallel to one another, and they and the oxygen atom are in the same plane. 

The spiro transition state is now generally accepted as the mechanism in operation during both dioxirane- and oxaziridine-mediated epoxidation. This conclusion is supported by theoretical and computational studies [31-33]. 

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