Rates of Cycloadditions to Electron-Rich Alkenes

In order to probe this phenomenon quantitatively in an uncharged system where solvation effects are known to be relatively unimportant, the rates of cycloadditions of two relatively electron-deficient 1,3-dipoles, p-methoxy and p-nitro benzonitrile oxides (MBNO and NMBO, respectively) to a series of simple alkenes — propene, 1-butene, 1-pentene, 1-hexene, 3-methyl-l-butene, 3,3-dimethyl-l-butene and isobutene — were measured70. In each of these reactions, only the 5-alkyl-3-aryliso-xazolines were formed. 

The rates of these reactions were determined by following the disappearance of the nitrile oxide band at 2290 cm-1 in the infrared spectrum, under conditions where the rates of dimerization of the nitrile oxides are negligible compared to the rates of 

The propene rate was measured at —12.5 °C and was found to be identical (0.090 x 10 3) to that of 1-butene at the same temperature. 

However, this interaction should also be increased by alkyl substituents, which lower the alkene IP, or, equivalently, raise the alkene HOMO energy. Experimentally, there is either no change in rate, or a small decrease, as the IP of the alkene decreases. Thus, an apparent contradiction is revealed in these examples dipole LUMO-alkene HOMO control nicely accounts for regioselectivity and the nitrile oxide substituent effect, but does not explain the decrease in rate for increasing alkyl substitution. More potent electron-donors do, indeed, accelerate the reaction, but only feebly. For example, butyl vinyl ether reacts 2.1 times faster than ethylene with BNO at 0 °C, while styrene reacts only 1.2 times faster than ethylene with BNO, in spite of the low IP of styrene (8.48 eV)72.  

Much larger effects of this type are observed in cycloadditions of enol ethers to tetrazines (Fig. 26), a reaction shown by Sauer and co-workers to be an example of a Diels-Alder reaction with inverse electron demand 75. The rates of 3,6-di-(2 -pyridyl)-s-tetrazine to various enol ethers and styrenes are summarized in Fig. 27. These were obtained by measuring the disappearance of the 540 nm band in the absorption spectra of the tetrazine76. These results are of particular interest, since there is little or no correspondence between the electron-donor ability of the enol ether, as measured by the ir ionization potentials (Table 5), and the rate of reaction of the enol ether. For example, although the conversion of methyl vinyl ether to 1,1-dimethoxyethylene results in a 4.3 times increase in rate, in line with the 0.2 — 0.3 eV decrease in IP, the 1,2-dimethoxyethylenes are 13 to 25 times less reactive than methyl vinyl ether, even though the IPs of these molecules are much lower 

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