Ethene diazomethane cycloaddition

A study of the regioselectivity of the 1,3-dipolar cycloaddition of aliphatic nitrile oxides with cinnamic acid esters has been published. AMI MO studies on the gas-phase 1,3-dipolar cycloaddition of 1,2,4-triazepine and formonitrile oxide show that the mechanism leading to the most stable adduct is concerted. An ab initio study of the regiochemistry of 1,3-dipolar cycloadditions of diazomethane and formonitrile oxide with ethene, propene, and methyl vinyl ether has been presented. The 1,3-dipolar cycloaddition of mesitonitrile oxide with 4,7-phenanthroline yields both mono-and bis-adducts. Alkynyl(phenyl)iodonium triflates undergo 2 - - 3-cycloaddition with ethyl diazoacetate, Ai-f-butyl-a-phenyl nitrone and f-butyl nitrile oxide to produce substituted pyrroles, dihydroisoxazoles, and isoxazoles respectively." 2/3-Vinyl-franwoctahydro-l,3-benzoxazine (43) undergoes 1,3-dipolar cycloaddition with nitrile oxides with high diastereoselectivity (90% de) (Scheme IS)." " ... 

Do the transition states of the 1,3-dipolar cycloadditions with diazomethane benefit from a stabilizing frontier orbital interaction Yes Computations show that the HOMOdia zomethm/LUMOethene interaction (orbital energy difference, -229 kcal/mol) stabilizes the transition state of the 1,3-dipolar cycloaddition to ethene (Figure 15.37) by about 11 kcal/mol. Moreover, computations also show that the HOMOethene/LUMOdjazomethane interaction (orbital energy difference, -273 kcal/mol ) contributes a further stabilization of 7 kcal/mol. 

Fig. 15.37. Frontier orbital interactions in the transition state of the 1,3-dipolar cycloaddition of diazomethane to ethene.

Diazomethane is an electron-rich 1,3-dipole, and it therefore engages in Sustmann type I 1,3-dipolar cycloadditions. In other words, diazomethane reacts with acceptor-substituted alkenes or alkynes (e. g., acrylic acid esters and their derivatives) much faster than with ethene or acetylene (Figure 15.36). Diazomethane often reacts with unsymmetrical electron-deficient... 

The combination of the spin-coupled formulation of modem valence bond theory with intrinsic reaction coordinate calculations provides easy-to-interpret models for the electronic rearrangements that occur along reaction pathways. We survey here the information revealed by such studies of the mechanisms of various gas-phase six-electron pericyclic reactions the Diels-Alder reaction between butadiene and ethene, the electrocyclization of cis-l,3,5-hexatriene, the 1,3-dipolar cycloaddition between fulminic acid and ethyne, and the 1,3-dipolar cycloaddition of diazomethane. The fully-variational CASVB strategy proves particularly efficient for such studies. 

The problem of bending diazomethane for the cycloaddition transition state was basically answered by Leroy and Sana s investigations (1975, 1976a). Using ab initio SCF calculations at the STO-3G level, they evaluated some 150 points on the hypersurface of diazomethane, ethene, and the primary cyclization product 1-pyrazoline. The essential results are shown in Scheme 6-12. There is no second transition state. 

For an example, we refer to the cycloaddition of diazomethane to ethene, for which Sustmann and Sicking (1987 a) developed an improved perturbation MO program (see Sect. 6.4) HOMO (CH2N2) - LUMO (C2H4) = - 46.2 kJ mol HOMO (C2H4) - LUMO (CH2N2) =... 

If the dipolarophile is assymmetric (d e in d=e. Sect. 6.2, Scheme 6-5), there are two alternatives for the diazoalkane and all other unsymmetrical 1,3-dipoles in their cycloadditions with that dipolarophile. For example, diazomethane and an ethene derivative with an alkyl substituent R may yield the 3- or the 4-alkyl-l-pyrazolines (6-19), or a mixture of both. These primary products rearrange at higher temperature or in the presence of base to give the corresponding alkyl-2-pyrazolines. 

Buchner accomplished the first synthesis of pyrazole by a cycloaddition in 1889, but not by the direct reaction of diazomethane and ethyne, but with methyl diazoacetate and an electron-deficient dipolarophile, ethynedicarboxylate (see Sect. 6.2). Unsubstituted ethene and ethyne were considered for a very long time to be not sufficiently reactive for synthetic purposes, although von Pechmann (1898 b) had reported a 50 0 yield of pyrazole from diazomethane and ethyne shortly after the discovery of diazomethane. The knowledge of diazomethane being explosive is probably the reason that the obvious advantage of a reaction under pressure in order... 

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