No 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 +

Methyl cis-4-methyl-2-pentenoate (1) has been found to rearrange at 252 "C by a simple first-order and presumably unimolecular mechanism to methyl 4-methyl-3-pentenoate (3) with a rate constant k of 4.28 x s-'. The heat of activation AH?, was determined to be 37.5 + 2 kcal/mole and the entropy of activation, AS?, was determined to be -8.2 + 4 e.u. These activation parameters and the steric requirements of the reaction support a mechanism involving a cyclic 1,5-transfer of hydrogen. Studies on the isomerization of methyl cis-2-pentenoate, methyl cis-2,4-dimethyl-2-pentenoate, and methyl cis-2-methyl-2-pentenoate to the corresponding 3-pentenoates suggest that the 1,Shydrogen transfer mechanism could well be general for the equilibration of a#-and p,y-unsaturated esters.An isopropyl group cis to a methyl or a carbomethoxy group on a double bond is shown by nuclear magnetic resonance to prefer a conformation with the methyls of the isopropyl group held away from the methyl or carbomethoxy group. A cis hydrogen does not restrict the conformation of the isopropyl group.

In aqueous sulfuric acid, cis-l,3,4,5-tetramethylcyclopentenyl cation ( l a ) has been found to rearrange not only to the trrrrzs isomer ( l b ) but also to 1,2,3,4-tetramethylcyclopentenyl cation ( l c ) . The lo -, Ib and lallh -t lc rearrangements follow H , with slopes of 0.94 and 0.45 respectively. The la -, 112 rearrangement involves a neutral intermediate, I,2,4,5-tetramethylcyclopentadiene, whereas the lullb -, l c rearrangement involves a terrnolecular or intramolecular push-pull process. Le passage la + lb se fait par un intermediaire neutre tetramethyl-1,2,4,5 cyclopentadiene, tandis que les rearrangements lallb +lc se font selon un processus push-pull terrnoltculaire et intramoleculaire.[Traduit par le journal]Can.

Two isomeric 1,2,4,5-tetramethylpentadienyl cations ( 4 1 and 4b) have been observed in FSO3H-SOzCIF at -125". They undergo stereospecific thermal cyclizations to cyclopentenyl cations in accord with the Woodward-Hoffmann rules.Les deux cations isomtres ( 4 1 et 5 ) du tetramethyl-1.2,4,5 cyclopentadienyle ont CtC observes dans un melange de FSOsH-SOzCIF a -125". Ils subissent des cyclisations thermiques sterCospCcifiques qui les transforment en cations cyclopenttnyles suivant les rkgles de Woodward-Hoffmann.[Traduit par le journal]