1.3- Dipoles carbonyl oxides

In contrast to dioxirane oxidation, the transition states for carbonyl oxide oxidation are not affected as much by RHF-UHF wave function instability problems, and there is good agreement between the MP2, MP4 and QCISD(T) barrier heights. Methyl substitution on the carbonyl oxide has very little effect on the barrier heights, but it can be anticipated that methyl substitution of the aUtene would lower the barriers significantly . The calculated changes in the barriers due to solvation are much smaller than for dioxirane oxidation, primarily because the differences between the reactant and transition state dipoles are smaller. 

Cremer, D., J. Gauss, E. Kraka, J. F. Stanton, and R. J. Bartlett, A CCSEHT) Investigation of Carbonyl Oxide and Dioxirane. Equilibrium Geometries, Dipole Moments, Infrared Spectra, Heats of Formation, and Isomerization Energies, Chem. Phys. Lett., 209, 547-556 (1993). 

Fig. 2.3 shows the core structures of the most important 1,3-dipoles, and what they are all called. As with dienes, they can have electron-donating or withdrawing substituents attached at any of the atoms with a hydrogen atom in the core structure, and these modify the reactivity and selectivity that the dipoles show for different dipolarophiles. Some of the dipoles are stable compounds like ozone and diazomethane, or, suitably substituted, like azides, nitrones, and nitrile oxides. Others, like the ylids, imines, and carbonyl oxides, are reactive intermediates that have to be made in situ. Fig. 2.4 shows some examples of some common 1,3-dipolar cycloadditions, and Fig. 2.5 illustrates two of the many ways in which unstable dipoles can be prepared. 

Ah initio calculations suggest that in ozonolysis, as the two fragments formed by dissociation of the primary ozonide start to move apart, a strong electrostatic attraction builds up between newly formed dipoles.157 The torque created causes a flip of one relative to the other, with formation of a dipolar complex which converts to the secondary ozonide. Thus, the authors suggest that the carbonyl oxide and carbonyl are never actually separated to a van der Waals distance. This argument goes some way to explaining some observed experimental stereoselectivities. 

The presence of two O—O bonds renders primary ozonides so unstable that they decompose immediately (Figures 15.47 and 15.48). The decomposition of the permethylated symmetric primary ozonide shown in Figure 15.47 yields acetone and a carbonyl oxide in a one-step reaction. The carbonyl oxide represents a 1,3-dipole of the allyl anion type (Table 15.2). When acetone is viewed as a dipolarophile, then the decomposition of the primary ozonide into acetone and a carbonyl oxide is recognized as the reversion of a 1,3-cycloaddition. Such a reaction is referred to as a 1,3-dipolar cycloreversion. 

Eliminations are mentioned in the preparation of 1,3-dipoles such as diazoalkanes or a-diazoketones (Section 12.5.3) and nitrile oxides (Section 12.5.4), in connection with the decomposition of primary ozonides to carbonyl oxides (Section 12.5.5) and the decomposition of phenylpentazole to phenyl azide (Section 12.5.6). 

Although the data are scattered, a least mean squares value of yc l 8 0.2D can be obtained for the dipole moment of the carbonyl oxidation product. This value Is In reasonable agreement with the accepted value of 2.3D for the carbonyl group, considering the assumptions that have been made In the above analysis. The agreement could, however, be fortuitous. 

Figure D.7 illustrates the application of the rules to delocalized systems A8a-1, A8b-1, A8c-1, A9a-1, A9b-1, and A9c-1. In particular, 1,3-dipoles are prominent members of the tricentric cases. Azomethine ylids, azomethine imines, nitrones, carbonyl ylids (e.g. A8c), carbonyl imines, carbonyl oxides fit prototype A8 bent nitrile ylids, may also be treated as A8. The linear 1,3-dipoles are treated as a triply-bonded systems with two mutually orthogonal sets of paired faces.

Using modern analytical methods, a number of transient intermediates and byproducts could be verified [19, 20]. The first step in the mechanism of ozonolysis is the 1,3-dipolar cycloaddition of the dipole ozone to the double bond of OA. A 1,2,3-trioxolane is formed, the unstable primary ozonide or molozonide. The primary ozonide collapses in a 1,3 dipolar cycloreversion to a carbonyl compound and a carbonyl oxide, the so-called Criegee zwitterion. Since OA is substituted with two diverse groups at the double bond, two different opportunities exist for the formation of carbonyl compound and carbonyl oxide. Again, a 1,3-dipolar cycloaddition of these intermediates leads to three different pairs of 1,2,4-trioxolane derivatives (cisltram), the secondary ozonides, which are more stable than the primary ones. Their oxidative cleavage results in AA and PA. 

Examples of 1,3-dipoles include ozone, nitrones, and carbonyl oxides. Table 11.2 lists some of the 1,3-dipoles that can undergo 1,3-dipolar cycloaddition. ... 

Type II Because of the similar energy gap in either direction, HOMO of the dipole can interact with LUMO of the dipolarophiles or HOMO of the dipolarophile can interact with LUMO of the dipole. The situation is referred to as a HOMO—LUMO-controlled dipole or an ambiphilic dipole and includes nitrile imine, nitrone, carbonyl oxide, nitrile oxide, and azide. 

The mechanism, which was worked out by Criegee in 1953, involves somewhat fancy steps. Thus, the first step is a 1,3-dipolar addition of ozone (the 1,3-dipole) to the carbon-carbon double bond (the 1,3-dipolarophile). The product so formed, called a ntolozonide, then undergoes a retro-1,3-cycloaddition to yield a carbonyl oxide and a carbonyl compound ... 

The carbonyl compound then flips its position and undergoes a second 1,3-dipolar addition, with the carbonyl oxide as the 1,3-dipole, to form an ozonide ... 

FIGURE 10.54 Like diazo compounds and ozone, a carbonyl oxide is a 1,3-dipole and adds to Jt systems to give five-membered ring compounds. 

The overall reaction is a series of three 1,3-dipolar additions (Fig. 10.56). First, ozone adds to the alkene to give the primary ozonide. Then the primary ozonide undergoes a reverse 1,3-dipolar addition to give a carbonyl compoimd and a new 1,3-dipole, the carbonyl oxide. Finally, the carbonyl oxide turns over and re-adds to the carbonyl compound in the opposite sense to give the new ozonide. The reaction is a sequence of three reversible 1,3-dipolar additions driven by thermodynamics toward the relatively stable ozonide. 

Cox, R.A., Tyndall, G.S. Rate constants for the reactions of CH3O2 with HO2, NO and NO2 using molecular modulation spectrometry. J. Chem. Soc., Faraday Trans. 2(76), 153-163 (1980) Cremer, D., Gauss, I, Kraka, E., Stanton, J.F., Bartlett, R.J. A CCSD (T) investigation of carbonyl oxide and dioxirane. Equilibrium geometries, dipole moments, infrared spectra, heats of formation and isomerization energies. Chem. Phys. Lett. 209, 547-556 (1993)... 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

For the reactions of other 1,3-dipoles, the catalyst-induced control of the enantio-selectivity is achieved by other principles. Both for the metal-catalyzed reactions of azomethine ylides, carbonyl ylides and nitrile oxides the catalyst is crucial for the in situ formation of the 1,3-dipole from a precursor. After formation the 1,3-di-pole is coordinated to the catalyst because of a favored chelation and/or stabiliza-... 

Since the dipole moments of cyclopropenones are enlarged with respect to simple ketones and compare to other polar systems, e.g. trimethylamine oxide in Table 4, there seems to be evidence for considerable charge separation in the carbonyl group, which was expressed in terms of a cyclopropenium oxide contribution to the ground state. 

Tetracyano ethylene oxide, however, which represents a potential 1,3-dipole of the carbonyl ylide type, reacts with diphenyl cyclopropenone to give a cycloadduct of probable structure 415/417263, which may arise from insertion into the cyclopropenone C1(2)/C3 bond. 

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