Thermolysis of 3,3,5,5,5-Tetramethyl-4-methylene-1-pyrazoline and the Thermal Isomerization of Some Alkylidenecyclopropanes

Abstract

ROBERT J. CRAWFORD and HIROKAZU TOKUNAGA. Can. J. Chem. 52,4033 (1974). The thermolysis of 3,3,5,5-tetramethyl-4-methylene-1-pyrazolne (1) proceeds at 1/63 the rate of 4-methylene-1-pyrazoline. The activation parameters, log (kls-') = (15.53 f 0.3) -(40.7 f 0.4)/0 where 0 = 2.303 R T in kcal mol-', suggest that 1 is undergoing thermolysis by a mechanism different from that for 4-methylene-1-pyrazoline. The 2,2,3,3-tetramethylmethylenecyclopropane (5) produced rapidly isomerizes under the reaction conditions to 2,2-dimethylisopropylidenecyclopropane (4). The four opposed methyl groups of 5 have created sufficient ground state destabilization as to cause its isomerization to be 147 times faster than the conversion of 2,2-dimethylmethylenecyclopropane to isopropylidenecyclopropane.The products are considered in terms of two possible intermediates, one wherein an allylic diazenyl diradical is considered and the second wherein a Chesick type of intermediate is invoked. For the latter it is demonstrated that substituents on the orthogonal radical center can affect the rotational propensities of methylene GS. isopropylidene of the allylic system. ROBERT J. CRAWFORD et HIROKAZU TOKUNAGA. Can. J. Chem. 52,4033 (1974). La thermolyse de la tCtramCthy1-3,3,5,5-m6thylt.ne-4-pyrazoline-l (1) s'effectue avec une vitesse Bgale a 1/63 de la vitesse de dCcomposition de la methylene-4-pyrazoline-1. Les parametres d'activation, log (k/sxl) = (15.53 * 0.3) -(40.7 + 0.4)/0 ou 0 = 2.303 R T en kcal mol-', suggerent que 1 subit la thermolyse selon un mecanisme different de celui de la mithylene-4-pyrazoline-1. Le tetramethyl-2,2,3,3-mCthylenecyclopropane (5) produit, s'isomerise rapidement, dans les conditions de la reaction, pour donner le dimethyl-2,2-isopropylidenecyclopropane (4). Les quatre groupes mithyle opposes de (5) crtent une dCstabilisation de ]'&tat fondamental suffisante pour que son isomerisation soit 147 fois plus rapide que la conversion du dimethyl-2,2-methylenecyclopropane en isopropylid~necyclopropane.On considere les produits en faisant intervenir deux intermediaires possibles, l'un dans lequel un diradical allylique-diazenyl est considere et le second dans lequel un intermtdiaire du type Chesick est invoque. Pour ee dernier, il est demontre que les substituents sur le centre radicalaire orthogonal peuvent affecter la tendance a la rotation du groupe methylene par rapport au groupe isopropylidene du systeme allylique. IntroductionAs a consequence of our work on the thermolysis of 4-alkylidene-1-pyrazolines (1) we have carried out an investigation of the gas phase thermolysis of 3,3,5,5-tetramethyl-4-methylene-1-pyrazoline (1). This structure has the advantage that the tautomerism, which plagued our previous studies, is no longer possible. Our principal objectives in studying 1 were to compare the competition between the isopropylidene group and the methylene group, of the presumed intermediate 2a, with the same competition observed in 26, the implied intermediate in the thermolysis of 4-isopropylidene-1-pyrazoline ...

Synthesis and 1H and 13C nuclear magnetic resonance spectra of some 4-alkylidene-1-pyrazolines and alkylidenecyclopropanes

“…We have used two methods to synthesize the 4-alkylidene-I -pyrazolines : (a) the diazoalkane cycloaddition to an allene has been used to prepare all compounds except 7 and 11 whose syntheses have been reported earlier (2) and (6) the synthesis of 3 was also achieved from the dibromide 35 and the pyrazolidine 36 (Scheme 1). The product was the same as that obtained by the addition of diazomethane to 1,2-butadiene.…”

A relative extrusibility scale and its application to the thermal fragmentation patterns of 5-membered ring diazenes

“…Thermodynamic Cycles Based on the Relative Extrusibility Scale Equation [9] represents a thermodynamic cycle in which model pyrazoline 4a, whose heat of formation has been measured, extrudes isobutene, while the target molecule extrudes CO, in the other direction. Equations [lo]- [12] summarize the algebra (recalling the definition in eq.…”

Reactions of Isomeric Arylchloropyruvates and Glycidates with Hydrazines

“…The azoolefins unsubstituted at the nitrogen are prone to nitrogen elimination giving an olefin, similarly to Kizhner reduction [8]. The various concurrent reactions proceeding in the system chloropyruvate Ia, Ibhydrazine or phenylhydrazine commonly prevent isolation of individual products and their identification at the use of equimolar reagents ratio since the hydrazine and many among its derivatives show abnormal high nucleophilic activity with respect to the sp 2 -hybridized carbon atom linked to the corresponding leaving groups [1,9]. On the other hand, condensation of hydrazine and its derivatives with carbonyl compounds affording hydrazones is well known and is widely used for aldehydes and ketones identification [10].…”