Cyclopropene, from photolysis

R = R1 = Ph) in very high yield by photolysis, the pyrazole is formed in only 3 % yield from diphenylethyne.50) The formation of cyclopropenes from pyrazoles apparently occurs via the corresponding diazo-compound. Thus photolysis of (53, R = R1 = Ph) at between 330 and 410 nm has been shown to lead to the diazo-compound (56) which on further photolysis or pyrolysis is converted to (54)50). Indeed, photolysis of vinyldiazoalkanes provides a good route to cyclopropenes thus photolysis of E- (57) leads to 3-alkoxycarbonyl-3-cyanocyclopropene51), while (58) leads either to the corresponding cyclopropene or to reversal to the pyrazole5Z). 

Carbenes are considered to be intermediates in the formation of the two indenes (26) and (27) from photolysis of the cyclopropene (28) in benzene. The 4 1 ratio of (26) (27) indicates preferential cleavage of bond (a) over... 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

Photolysis of pyrazoles such as (64) has also been shown to produce indenes as well as cyclopropenes. The indene is apparently derived from a triplet intermediate, while the cyclopropene is singlet derived 54). In a related example, direct photolysis of dimethyl diazomalonate in an alkyne leads to moderate yields of cyclopropene, whereas sensitized photolysis leads to furans by ring closure of an intermediate diradical (65) 55). 

While the photolysis of (164) can lead to reasonable yields of acylcyclopropenes, no cyclopropenes are detected from the corresponding reactions of (165) however, if the photolysis of (165, R = Me, R = H) is carried out in the presence of furan, a 2-oxa-bicyclohexane is isolated. This is apparently derived by addition of the carbene (166) to the 2,3-bond. The oxabicycle in turn rearranges to a single triene (167) 128). 

Photolysis of pyrazolines derived from l,2-bis-trimethylsilyl-cyclopropene-3,3-di-carboxylates has also been shown to provide an efficient route to bicyclobutanes 249). 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

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. 

A variety of cyclopropene derivatives have been prepared by means of thermolysis or photolysis of alkenyldiazomethanes, which are obtainable from a,j5-unsaturated aldehydes or ketones via the corresponding tosylhydrazones (equation 26). Cyclic allylic... 

The most common source of a,j5-unsaturated carbenes for cyclopropene synthesis is from the decomposition of 3H-pyrazoles which are, in turn, available from the addition of a diazo compound to an alkyne. On photolysis these compounds suffer ring cleavage to an a>diazoalkene prior to nitrogen loss and carbene formation. In this way cyclopropene 27 is available from either 25 or 26 by way of common diazo and carbene intermediates. ... 

Singlet excited state reactions of cyclopropenes proceed with low quantum efficiencies due to thermal return of carbene 193 to starting material as elegantly illustrated with optically active esters 234. On photolysis racemization of 234 occurs four times faster than ring cleavage. The photorearranged furan product results from six-electron cyclization of the intermediate carbene and such species have been invoked to account for the interconversion of variously substituted furans By comparison... 

The parent Dewar furan (49) was generated in a similar fashion from the corresponding propellane precursor (48) and trapped with isobenzofuran to give the cycloadducts (51) and (52) (Scheme 7) <85JA7176, 86PACI61). The proportion of isobenzofuran-Dewar furan adducts (51) and (52) was increased at the cost of the cyclopropene aldehyde adducts (53) and (54) as the photolysis temperature was lowered, indicating that (49) is indeed the primary photolysis product. 

The higher members of the [l,n,l]-eliminations are also of preparative importance. Thus, the Doering allene synthesis 14) leads to bicyclobutanes (e.g. 66, 28 %) 45), if bulky substitution as in 65 favours the [l,3,l]-elimination. The related cyclopropene syntheses from 6746) and 6946, respectively (40 and 6% 68 in derivatized form lithiation and carboxylation), are to be classified as [l,3,(2)l]-eliminations of bromine (reductive) as well as of hydrogen chloride. The thermolysis or the photolysis of diazo... 

The sensitized photolysis of tetrasubstituted cyclopropenes that do not have a C-C double bond in the side chain at C3 leads to hydrogen atom abstraction from a 3-alkyl group to form a diradical, which may either cyclize or disproportionate (see Section 1.1.6.2.1.) to give exolendo-l and ( /Z)-2. ... 

The involvement of trimethylenemethane diradicals in deazetization of diazoalkane-allene adducts or trimethylene diradicals in the deazetization of the adducts of acyclic alkenes often leads to mixture of regioisomers and stereoisomers and from the standpoint of cyclopropane syntheses, this is undesirable. Far fewer problems of this type attend deazetization of the adducts of cyclic or polycyclic alkenes and, furthermore, even a modest amount of strain in the system activates the alkene to diazoalkane addition so that there is no need for activating substituents on the double bond. Cyclopropene is highly reactive towards diazoalkanes (see also Section 1.1.5.1.5.3.1.) and cycloaddition reactions of this type provide a ready entry into the bi-cyclo[1.1.0]butane series. The addition of diphenyldiazomethane to cyclopropene gave 4,4-diphenyl-2,3-diazabicyclo[3.1.0]hex-2-ene (1), which on photolysis gave a mixture of 2,2-diphenylbicyclo[1.1.0]butane (2) and 1,1-diphenylbuta-l,3-diene (3). ... 

Thus photolysis of the tosylhydrazone sodium salt of 5//-dibenzo[a,c]cyclohepten-5-one (14) at — 60 °C in the presence of cyclopentadiene or furan with tetrahydrofuran as cosolvent gave the cyclopropene Diels-Alder adducts 17 and 18 in 73 and 47% yields, respectively. If the photolysis was stopped shortly after all the tosylhydrazone salt had decomposed, adduct 17 was the only isomer found in the cyclopentadiene reaction. In the formation of the furan adduct 18, the reaction was not so clean and a number of unidentified products were also formed. Unfortunately, adduct 18 is thermally unstable at the temperature necessary for thermal formation of carbene 15. No trace of adduct 18 was detected when the arylcarbene 19 was generated directly from its tosylhydrazone salt in the presence of furan. ... 

Triplet diphenylcarbene, from the photolysis of diphenyldiazomethane, underwent addition to but-2-yne to give predominantly a cyclopropene 10. ... 

There are few other reaetions in whieh elimination of two ring atoms from a five-membered ring precursor give a cyclopropene. However, a low yield of cyclopropene (1 a) or of 1 -methyl-cyclopropcnc (1 b) was obtained by elimination of carbon dioxide from the corresponding a,/ -unsaturated y-lactone using mercury vapor sensitized photolysis. ... 

Diazopropane is a potential source of ge w-dimethyl groups. It undergoes 1,3-dipolar addition to acetylenes and allenes/ and the adducts can be photolyzed to give cyclopropenes or methylenecyclopropanes/ respectively. In certain cases the adducts from a-substituted acetylenes give good yields of allenes and conjugated dienes on photolysis. ... 

Photolysis of bis(trifluoromethyl)diazomethane in the presence of bis(tri-methylsilyl)acetylene gives a complex mixture from which 3,3-bis(trifluoro-methyl>l,2-bis(trimethylsilyl)cyclopropene has been isolated in 6% yield (seep. 198). ... 

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