Alkynes 3+2 dipolar cycloadditions, diazomethane

Trifluoromethyl-substituted pyrazoles are easily obtained using trifluoromethyl-alkynes as dipolarophiles (Table 8.2, entry 9). Thus, treatment of 4,4,4-trifluorobut-2-ynoic acid with excess diazomethane gave methyl 4-(trifluoromethyl)pyrazole-4-carboxylate (45%) accompanied by its N - (32%) and -methylated (6.5%) derivatives (267). Another convenient route to CF3-substituted pyrazoles involves dipolar cycloaddition of appropriately CF3-substituted alkenes followed by eliminative aromatization (76,77,268). For example, the reaction of alkenes such as (CF3)2C=C(H)COAr with ethyl diazoacetate gave 4-aroyl-5-trifluoromethylpyra-zole-3-carboxylates (268). 

The reaction between alkynes and diazomethane, or monosubstituted diazomethanes, has been known for almost a century to yield l//-pyrazoles7 and is an important method for preparing these compounds.2,5,6 It proceeds via a 1,3-dipolar cycloaddition (Scheme 1) giving initially a 3//-pyrazole (4),... 

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... 

Some examples are known of 1,3-dipolar cycloaddition reactions of trifluoromethyl-substituted alkenes and alkynes with other dipoles (see Table 12), such as diazomethane.nitrile imines, " " nitronates," and munchones." Depending on reaction conditions, cycloadditions may occur via a two-step process. 

The rates of 1,3-dipolar cycloadditions of diazoalkanes to alkenes and alkynes have been determined electron-attracting substituents in the latter increase the rate, in accordance with frontier molecular orbital theory, which predicts that these reactions are controlled by the interaction of the highest occupied molecular orbital of the diazo-compound with the lowest unoccupied molecular orbital of the dipolarophile " the kinetics of the reactions of methyl diazoacetate or phenyl diazomethanesulphonate, on the other hand, give rise to U-shaped activity functions, which is also explained by the theory. Diazomethane or... 

Diazomethylene)phosphoranes 33 (Scheme 8.10), which represent another type of diazocumulenes (12) are easily obtained by the oxidative ylidation of the corresponding phosphanyl(trimethylsilyl)diazomethane with CCI4. The increased stability of these compounds as compared with diazocumulenes (R2C=C=N2) is probably due to the ylidic character of the P=C bond. These diazo compounds exhibit the expected dipolar reactivity toward electron-deficient alkenes, alkynes, phosphaalkenes, and heterocumulenes (12). Thus, 33 reacts with TCNE to form A -pyrazoline 35 (60). Furthermore, 33 could be converted into the phosphonio-borate-substituted diazo compound 34, which underwent subsequent cycloaddition with electron-deficient alkenes (e.g., 34 36) (61). 

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