PYRAZOLINES: VI. THE STEREOCHEMISTRY IN THE PYROLYSIS OF PYRAZOLINES FOR THE C₃ AND C₅ POSITIONS OF THE PYRAZOLINE MOLECULE

Abstract

cis-and tra~zs-3,5-Dimethyl-3-carbo11~ethoxy-A~-pyraoline (IV and V) have been separated, and , . product analysis for the pyrolysis and photolysis of these compounds has been completed.I he cyclopropane products are formed with some change in the relative coniig~iration of the cis-trans groups in both reactions. 'The a,@-unsaturated olefir1 prod~rct is formed stereospecifically with IV, yielding methyl 2-methyl-cis-2-pentelloate (XI), and with V, yielding methyl 2-methyl-tra?zs-2-pentenoate 0 ; ) . The results of the pyrolysis and photolysis of cis,tra?ts-3,4,5-trimethyl-3-carbornethoxy-A1-pyrazoline are also described. T h e product s t~~d i e s and some kinetic s t~~d i e s are the basis of a new ~nechanisrn proposed for the pyrolysis of pyrazolines.Recent studies on the pyrolysis of pyrazolines have reopened the question regarding the stereocheinistry of the conversion of pyrazolines into cyclopropanes (1-3). This reactio~l was reported by von Auwers (4) to be essentially stereospecific and this was the accepted view on the reaction for many years (stereospecific here is used t o mean t h a t the substituents on the cyclopropane ring have the same cis-trans relationship as on the initial pyrazoline). \Ian Aulcen and Rinehart (1) showed that the pyrolysis of a pair of isomeric pyrazolines with inethyls cis and trans on Cg and Cq (c0111pounds I and 11) gave cyclopropanes with effective randoinization of this cis-trans relationship. On the other hand, Overberger and Anseline (2) have found that the pyrolysis of trans-3,5-diphenyl-a'-pyrazoline (111) gave stereospecifically the trans-1,2-diphenyl~~rclopropane, and thus the stereocheinistry of this reaction is not clear.\lie have prepared some new pyrazolines, \vhich shoxv a cis-trans relationship a t positions Cs and Cj, in an effort to find out more about the pyrolysis reaction. T h e preparation of a cis-trans mixture of 3,s-dimethyl-3-carboinethoxy-A1-pyrazoline and its pyrolysis was reported earlier ( 5 ) . This mixture has now been separated by distillation through a spinning-band coluilln a t a temperature slightly below the pyrolysis temperature. T h e lower-boiling isoiner has been assigned the structure with the methyl groups cis (I\/) and the higher-boiling isoiner t h a t ~v i t h the methj.1 groups trans (V). T h e preparation and ex m, x = c o , c~, 11, X = COCH, N +

Pyrazolines. VII. Concerning the formation of olefins from the pyrolysis of pyrazolines

“…A similar stereospecificity was noted earlier (1) for the pyrolysis of methyl cis-and trans-3,5- XI and XII), where the hydrogen on C-4,…”

“…Other evidence has supported the possibility that different paths a r e involved in the two reactions. For example, the variation of ~r o d u c t com~osition with the solvent used for the reaction sho~vs a much greater change in the proportion of olefins to cyclopropanes than it does in the proportion of the two cyclopropanes formed (1). In the present study we have further explored the stereochemistry of the olefinforming reaction.…”

“…I n an earlier paper (1) we examined the results of the pyrolysis of a number of pyrazolines, and a t that time separated the discussion on mechanism into that leading to olefin products and that leading to cyclopropanes. This division of the discussion was prompted by the fact that the stereochemical result of the olefin-formingreaction showed a clearly defined stereospecificity, whereas that of the cyclopropane-forming reaction did not.…”

“…thesis of W.-S. Wu. (1)(2)(3)(4)(5)(6). Their work did, however, focus attention on the value of stereochemical studies.…”

Photolysis of some alkyl-1-pyrazolines

“…Carbon-carbon bond cleavage products have been observed in the photolysis of 1-pyrazolines before (14,17). One instance of cleavage has also been observed by Crawford and Mishra (2), in t h~ thermolysis of 4,4-dimethyl-1-pyrazoline.…”