Transition spiro

The mechanism for the conversion of the A -oxide (94) to the o-methylaminophenylquinoxaline (96) involves an initial protonation of the A -oxide function. This enhances the electrophilic reactivity of the a-carbon atom which then effects an intramolecular electrophilic substitution at an ortho position of the anilide ring to give the spiro-lactam (98). Hydrolytic ring cleavage of (98) gives the acid (99), which undergoes ready dehydration and decarboxylation to (96), the availability of the cyclic transition state facilitating these processes. ... 

Formation of the six-membered ring on cyclization of the ( )- and (Z)-4-nonenylhydroxylac-tams [E)-4 and (Z)-4 in formic acid, occurs completely stereoselectivcly to afford the 6.6-spiro compounds (7R )-5 and (7S )-5, respectively51,52. The reaction supposedly proceeds via a chair-like transition state, as depicted. Depending on the reaction conditions, however, 0.5 % 2 or about 25%51 of the five-membered ring isomers are also formed. 

Fig. 12.7. Alternate orientations of 3-methylbut-3-en-l-ol (3a) in the transition state for Ti-mediated epoxidation. Angle 9 is the inter-ring angle of the spiro rings. Reproduced from J. Org. Chem., 67, 1427 (2002), by permission of the American Chemical Society.

Raman spectroscopy is primarily useful as a diagnostic, inasmuch as the vibrational Raman spectrum is directly related to molecular structure and bonding. The major development since 1965 in spontaneous, c.w. Raman spectroscopy has been the observation and exploitation by chemists of the resonance Raman effect. This advance, pioneered in chemical applications by Long and Loehr (15a) and by Spiro and Strekas (15b), overcomes the inherently feeble nature of normal (nonresonant) Raman scattering and allows observation of Raman spectra of dilute chemical systems. Because the observation of the resonance effect requires selection of a laser wavelength at or near an electronic transition of the sample, developments in resonance Raman spectroscopy have closely paralleled the increasing availability of widely tunable and line-selectable lasers. 

All the reactions were carried out at 0°C, with the substrate (1 equivalent), ketone (3 equivalents), Oxone (5 equivalents), and NaHCC>3 in CH3CN aqueous EDTA for 2 hours. High enantioselectivity can generally be obtained for trans- and trisubstituted olefins. The favored spiro and planar transition states have been proposed for ketone 130-mediated rrans-stilbene epoxidation (Scheme 4-48). 

The similarity of olefin epoxidation by TM peroxo and hydroperoxo complexes with epoxidation by dioxirane derivatives R2CO2 and percar-boxylic acids RCO(OOH) was confirmed by computational studies [73-79]. This similarity holds in particular for the spiro-type transition structure. 

Figure 3.2. Differential calorimetric curves for the molecular glasses (a) Spiro-sexiphenyl (second heating curve) and (b) Spiro-PBD (first and second heating curve). The glass transition is indicated by a characteristic step, the melting point by an endothermic peak. In (a) recrystallization occurs above Tg, which can be seen by an exothermic peak. The material in (b) forms a stable amorphous glass without recrystallization. The melting point from the first heating curve of a crystalline sample (dotted line) disappears in the second heating cycle (solid line). Only the glass transition is visible.

In order to obtain asymmetric spiro compounds, there are two different possibilities. First, one can connect two different chromophores via a common spiro center. The thiophene compounds 39a and 39b are one example [84, 85]. Second, one can connect two equal but asymmetric chromophores. Based on this principle are Spiro-PBD (40), spiro-bridged bis(phenanthrolines) (41) [86], and the branched compounds Octol (42a) and Octo2 (42b) [87]. Because of their symmetry, these molecules are chiral. The glass transition temperatures of 40 and 42b are reported to be 163 and 236°C, respectively [88], Unfortunately, reports on the thermal properties of 39 and 41 are lacking. 

A detailed study of the electronic structure and optical properties was published for the spiro derivative of f-Bu-PBD, Spiro-PBD (40) [108]. The vibronic structure of the lowest energy absorption band is well resolved, in solution as well as in the amorphous him. The 0-0 transition is at 351 nm (3.53 eV), the 0-1 and 0-2 vibronic bands that have a higher oscillator strength, are at 336 nm (3.69 eV) and 318 nm (3.90 eV), respectively. The fluorescence spectrum of this compound is symmetrical to the absorption spectrum with a Stokes shift of 43 nm. 

The very large value of the pre-exponential factor indicates a transition complex which is very loose compared with the highly strained spiro-pentane structure. Inspection of the models of the reactant and product makes it clear that considerable distortion of the reactant must occur on going to the transition complex. A minor reaction path results in the formation of allene and ethylene. These products are primary. 

Peroxynitrous acid, which has an estimated lifetime of 1-3 s at neutral pH, has been studied through ab initio calculations that suggest that peroxynitrous acid, per-oxyformic acid, and dimethyldioxirane have, despite diverse 0—0 bond energies, similar activation energies for oxygen-atom transfer." The transition-state structures for the epoxidation of ethene and propene with peroxynitrous acid are symmetrical with equal or almost equal bond distances between the spiro oxygen and the carbons of the double bond. 

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... 

Fig. 9 The spiro and planar transition states for the dioxirane epoxidation of olefins...

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... 

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