The thermal chemistry of vinyldiazo compounds as a method for the generation of vinylmethylenes

Abstract

. Can. J. Chem. 57,1403Chem. 57, (1979. The thermal chemistry of a series of substituted 1-aryl-3-methyl-2-phenyl-1-diazobut-2-enes has been examined. These vinyldiazo compounds react in two parallel pathways: cyclization to a 3-aryl-5,5-dimethyl-4-phenyl-5H-pyrazole or loss of nitrogen and cyclization to a l-aryl-2-phenyl-3,3-dimethylcyclopropene presumably by a vinylmethylene intermediate. The kinetics of these processes reveal quite different substituent effects. The relation of these results to the stability of the vinylmethylene produced and to the singlet photochemistry of cyclopropene derivatives is discussed. JAMES A. PINCOCK et KIM P. MURRAY. Can. J. Chem. 57,1403Chem. 57, (1979. On a ktudie la thermochimie d'une skrie d'aryl-1 mkthyl-3 phenyl-2 diazo-1 butbnes-2 substituks. Ces composks vinyldiazo rkagissent par deux voies paralltles: une cyclisation conduisant a l'aryl-3 dimkthyl-5,5 phknyl-4 5H-pyrazole ou une perte d'azote et une cyclisation en aryl-1 phknyl-2 dimkthyl-3,3 cycloproptne probablement par un intermkdiaire vinylmkthylbne. Des ktudes cinetiques de ces processus rkvtlent des effets de substituants importants. On discute de la relation qui existe entre ces rksultats et la stabilite du vinylmkthylene qui est produit ainsi que la photochimie singulet des dkrivks du cycloproptne.[Traduit par le journal] IntroductionHowever, recent results for cases with unsymmetrical The photochemical reactivity of the singlet excited substitution on the double bond (1, 2, 4, 5) have state of cyclopropene derivatives has generally been led to considerable speculation as to the effect of explained in terms of o bond cleavage and the these substituents on ring cleavage. Some examples chemistry of the vinyl methylenes formed (1-3). are summarized in reactions [I] to [4].

Flash Vacuum Pyrolysis of Pyrazoles as an Alternative Way to Study Vinylcarbenes

“…It is interesting to note that the reaction products isolated in these reactions (5)(6)(7)(8)(9)(10) are isomers arising from nitrogen extrusion reactions, like in the case of 1. As it will be discussed later, they are formed from the two different tautomers present in pyrazole 2.…”

“…Vinylcarbenes are normally produced by ring opening of cyclopropenes (thermally and photochemically) and from vinyl-diazo compounds (photochemically). 11 The problem with these last compounds is that in thermal reactions, two competitive reactions are present, that is, cyclization to a 3H-pyrazole and nitrogen extrusion, the first one being the lower in energy, as has been demonstrated by Pincock et al 10 From these evidences, and our experience in kinetic measurements in FVP reactions, we thought that gasphase thermal reactions of pyrazoles (N-unsubstituted to avoid competitive reactions) may provide information on two obscure topics in this field: (i) the influence of different substituents on the activation energy of vinylcarbenes, and (ii) the participation of different substituents in vinylcarbene rearrangement.…”

Photochemical and thermal rearrangements of some 3H-pyrazoles

“…The isolation of the N-methylpyrazoles 5b-d from reaction [7] effectively rules out the occurrence of any unsuspected rearrangement during the course of reactions [5], [6], or [7].…”

“…This reaction can be viewed as a 6e electrocyclic ring opening. Photochemical decomposition of the vinyl diazo compound (4) then leads to the cyclopropene, whereas thermal activation can lead to decomposition to give the cyclopropene (4) and/or to a concerted recyclization back to the 3H-pyrazole (5,6). In those cases studied in detail, there was little or no direct loss of nitrogen from the 3H-pyrazole; that is, without the intermediacy of the vinyl diazo compound (4).…”

“…Reactions [4] and [5] can be thermally allowed suprafacial [1,5]-sigmatropic r'earrangements, although other mechanisms may be involved under some conditions (6). There are now numerous reports of these reactions in the literature (7)(8)(9).…”