Cyclopropenes thermal decomposition

Correlation diagrams include the product orbitals while perturbation approaches require knowledge of the empty orbitals of the reactant. However, the occupied molecular orbitals of diazirine (II), compared with those of cyclopropene (I), do seem to give some indication of a preferred thermal decomposition of (II) compared with the rearrangement of (I). Moreover these molecular orbitals are a typical illustration of the localization obtained in the presence of an electronegativity perturbation. 

A detailed, extensive review of cyclopropene has appeared. Cyclopropene is the last of the small strained ring hydrocarbons to have its thermal decomposition subjected to intensive investigation.158 Cyclopropenes can decompose by a variety of mechanisms involving diradicals, vinylcarbenes, and vinylidenes. Cyclopropene itself has been shown to be an intermediate in the allene-propyne rearrangement. 

Pyrolysis is accomplished by concentrating an aqueous solution of the quaternary hydroxide at temperatures ranging from 30° to 200°. Thermal decomposition on a platinum catalyst at an optimum temperatiue of 325° has been studied in detail in the preparation of cyclopropene (45%). °... 

Scheme 10. Possible thermal decomposition of the cycloadduct between cyclopropene and 2,5-bis(trifluoromethyl)-l,3,4-oxadiazole.

The question as to whether the thermal azirine formation proceeds through a vinyl nitrene intermediate or by a concerted mechanism is not as yet resolved. A nitrene intermediate seems most probable on the basis of its similarity with cyclopropene formation from alkenylcarbenesM>> and because either thermal or photochemical decomposition of vinyl azides yields die azirines (Table XI). 

Functionalized cyclopropenes are viable synthetic intermediates whose applications [99.100] extend to a wide variety of carbocyclic and heterocyclic systems. However, advances in the synthesis of cyclopropenes, particularly through Rh(II) carboxylate—catalyzed decomposition of diazo esters in the presence of alkynes [100-102], has made available an array of stable 3-cyclopropenecarboxylate esters. Previously, copper catalysts provided low to moderate yields of cyclopropenes in reactions of diazo esters with disubstituted acetylenes [103], but the higher temperatures required for these carbenoid reactions often led to thermal or catalytic ring opening and products derived from vinylcarbene intermediates (104-107). 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

The nitrogen extrusion from 1-pyrazolines and 3H-pyrazoles giving cyclopropanes and cyclopropenes, respectively, has been extensively reviewed The cyclopropane synthesis from 1-pyrazolines can be executed thermally as well as photochemically, but the latter method generally gives substantially better results than the former. The major side reaction observed in the thermal process is the production of olefins, which arise in the migration of a substituent from the C(4) to C(3) position. A retro-1,3-dipolar addition producing a diazoalkane and an olefin has been observed in certain cases. The decomposition of 3-acyl- or 3-alkoxycarbonyl-1-pyrazolines is catalyzed by acids, such as perchloric acid and boron trifluoride and by Ce The stereochemical course... 

The photochemical nitrogen extrusion from 3H-pyrazoles provides a major access to cyclopropenes. It has been shown that the 3H-pyrazoles are transformed into diazoalkenes in the first step of the decomposition, and the resultant diazoalkenes give the cyclopropenes by loss of nitrogen". Benzocyclopropenes have been prepared in a similar manner" The thermal or photochemical decomposition of 4-alkylidene-l-pyrazolines produces methylenecyclopropanes". The products obtained especially in the triplet sensitized photolysis are frequently derived by a methylenecyclopropane rearrangement. 

The cyclopropanation of a cyclopropene to produce a bicyclo[1.1.0]butane has often been achieved by dipolar addition to give an isolated 2,3-diazabicyclo[3.1.0]hexane followed by loss of nitrogen in a second step to produce the second cyclopropane. The formation of the diazabicyclo[3.1.0.]hexanes is discussed in Section 1.1.6.1.5.3.1. The decomposition of these to produce bicyclobutanes is described in Section 4.2.1.1.2. This section will only discuss those reactions in which a cyclopropene is converted directly into a bicyclo[1.1.0]butane, by thermal or photochemical methods (Table 20). Metal-catalyzed processes are discussed in Section 1.1.6.3.1.2. 

The question remains will it be possible to isolate the cycloadduct between 1,3,4-oxadiazole and the dienophiles from the reaction mixture Let us examine the case that is likely to be very easy experimentally the addition of cyclopropene to 3,5-bis(trifluoromethyl)-l,3,4-oxadiazole. This cycloadduct actually represents a cyclic azo compound (Scheme 10). Azo compounds are known to decompose very easily producing nitrogen molecules and the corresponding radicals. The energy for the decomposition can be either thermal or photochemical [60]. In... 

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