Cyclopropene from pyrazole

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). 

The photochemical study of 3H-pyrazoles was carried out in the search for a route to cyclopropenyl tertiary alcohols. Irradiation of 63a in dry dichloromethane at 300 nm and at room temperature for 0.5 h led to the exclusive formation of the gem-dimethylcyclopropene 65 (Scheme 17). The formation of cyclopropene 65 arises from the loss of N2 and cycUzation of the vinylcarbene intermediate (III). 

H-Pyr azoles readily undergo photochemically induced elimination of nitrogen to yield the corresponding cyclopropenes, often by way of detectable vinyl diazo intermediates. l-Aryl-3-methyl-2-phenyl-l-diazobut-2-enes have, in fact, been prepared in this way from the appropriate pyrazoles.345 The vinyl diazo compounds 416, obtained by irradiation of the 3//-pyrazoles 417, were further converted to cyclopropenes 418 via vinylmethylene intermediates 419 by irradiation at 10°C.346 1-Acylcyclopropenes have been... 

Analogous reactivity was observed by Jones303 with methylene cyclopropene 554, which on treatment with diazomethane yielded the pyrazole 556, thought to arise from thermal isomerization of pyrazolenine 555 formed initially ... 

As shown in Scheme 8.67, the cyclization of diazoalkenes 273 requires thermal activation and not only affords 3/7-pyrazoles 274, but also cyclopropenes 275 that are formed from carbene intermediates (319). The activation parameters for cyclopropene formation (i.e., N2 elimination from 273) have been determined (320). A novel example involves the cyclization of the 3-nitro-l-diazoprop-2-ene derivative 276 into pyrazolopyridine derivative 277 (45). 

Calculations have indicated that direct thermal extrusion of nitrogen from 3//-pyrazole to give cyclopropene is an unfavorable process, though more favorable in terms of heat of reaction than cycloreversion to diazomethane... 

Section I1,D,1 The kinetics have been studied for competitive formation of 3//-pyrazoles and cyclopropenes thermally from the isomeric vinyldiazo compounds 2,3-dimethyl-l-phenyl-l-diazo-2-butene and 4-methyI-3-phenyl-2-diazo-3-pentene. The higher (12 kJ/mol) ground state energy of the latter accounts almost entirely for its larger (x68) rate of cyclization to a 3H-pyrazole, relative to its isomer.172... 

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). 

Starting from alkynes, cyclopropenes were obtained under mild reaction conditions at room temperature (30-51 %). Aziridines were isolated in 40-90 % yield with catalyst 15 using [(p-tolylsulfonyl)imino]phenyliodinane. Phi = NTs. The first chiral tris(pyrazolyl) ligand was prepared as early as 1992 by Tolman and co-workers starting from a camphor-pyrazole derivative and... 

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 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. ... 

The cycloaddition of a diazo compound to an alkyne followed by extrusion of nitrogen under various conditions from an isolated pyrazole represents one of the standard routes to cyclopropenes (Section 3.2.1.1.). In certain cases the reaction is carried out under conditions which might involve direct addition of a carbene derived from the diazo species, e.g. formation of 7, 8, and9. For8(R = cyclopropyl), the product rearranged under the reaction conditions to 1-cyclopropyl-1-trimethylsilylallene. ... 

Not all vinylcarbenes will cyclize to the cyclopropene. For example, those with an acyl substituent on the carbene-bearing center preferentially undergo a Wolff rearrangement, but the most significant limitations to the scope of this synthetic route arise from the problems of making the required 3//-pyrazoles. Many routes exist, but as far as cyclopropene synthesis is concerned, the 3/f-pyrazoles have been made (1) by addition of diazoalkanes to alkynes (2) from dihydropyrazoles (3) from cyclization of a,/l-unsaturated tosylhydrazones and finally (4) by modification of the structure of other 37/-pyrazoles. 

Table 1. Cyclopropene Derivatives from the Addition of Diazoalkanes to Acetylenes Followed by Photochemical Deazetization of the Intermediate 3//-Pyrazoles...

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