Cyclopropenes arylation

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... 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Cycloaddition reactions of phenyl and tosyl azides to the strained double bond in cyclopropenes have been investigated.170il The reaction products from 3,3-dimethylcyclopropene indicate that the initially formed intermediate is a normal 1,3-dipolar adduct. Tetrachlorocyclopropene yields the primary adducts with several aryl azides. 70b However, cyclopropenedicarboxyl ester gives only unstable triazolines with phenyl and methyl azides.170 ... 

The first examples of direct palladium-catalysed arylation and heteroarylation of cyclopropenes with Arl, catalysed by (AcO)2Pd, have been reported. Mechanistic studies strongly suggest an electrophilic mechanism for this Heck-type transformation.78... 

Saito reported the extension of Billups tandem silver-catalyzed ring-opening cycloaddition reaction methodology to addition to imines (Scheme 2.45).77 Naphtho- /) cyclopropene 167 added to substituted aryl imines 168 at room temperature in the... 

Problems with subsequent reaction of a 1-halocyclopropene may be avoided by reduction of the dihalocyclopropane to a monohalocyclopropane prior to dehydrohalogenation. Thus 3,3-dimethylcyclopropene may be obtained in multi-gram quantities by treatment of l-bromo-3,3-dimethylcyclopropane with potassium t-butoxide in DMSO at —78 °C19). Dehydrohalogenation of a series of related 3,3-dialkyl substituted monochloro- or monobromo-cyclopropanes leads to moderate yields of cyclopropenes (11, R = alkyl, alkenyl, aryl, CN, Cl, Rl = H, Ph, t-Bu, R2 = H)20) Indeed, dehydrobromination of (12) leads to either mono- or di-cyclopropenes 2l>. Reaction of a dihalocyclopropane with an alkyl lithium at low temperature followed by carboxylation of the derived 1-lithio-l-halocyclopropane provides a convenient source of 1-halocyclopropane carboxylates dehydrohalogenation leads to cyclo-... 

When the cyclopropene has a single carbomethoxy or aryl substitunt at the 3-position, the predominant stereoisomer of the enone obtained has the substituent cis-to the ketone 260 261). One possible explanation is the formation of a bicyclobutane by peracid attack from the side away from this substituent, followed by a stereocontrolled rearrangement. If the 3-substituent is hydroxymethyl, the reverse stereochemistry is observed in the enone, in agreement with a peracid attack directed by this group to the same face of the cyclopropene, followed again by rearrangement 262). 

The dehydrohalogenation of a monobromo- or monochlorocyclopropane provides one of the simplest routes to a range of cyclopropeneslb. For 3,3-disubstituted cyclopropanes carrying alkyl or aryl groups, the reaction is normally achieved using potassium /-butox-ide or potassium hydroxide in DMSO and in some cases the cyclopropene distils directly from the reaction mixture (Table 1). 

Pyrazoles are photochemically converted into cyclopropenes, and 3//-indazolcs react similarly, e.g., (448 449). If a 3-aryl group is present, an indene can be formed, e.g., 450 451. 

Indenyl complexes of Pd are prepared by the reaction of indenyl sodium with anhydrous PdCl2. Alternatively, they are available by ring-opening reactions of aryl-substituted cyclopropenes with PdCl2(PhCN)2 (equation 49). 

Cyclopropenes, even unactivated ones, exhibit extraordinarily high reactivity in both electrophilic and nucleophilic addition reactions. They are also good dienophiles and react with a variety of conjugated dienes including acyclic 1,3-dienes, alicyclic 1,3-dienes, anthracenes and furans. An endo selectivity is usually observed. An alkyl or aryl substituent at the 3-position of cyclopropene sterically hinders the Diels-Alder addition and thus 3,3-dialkyl- and 3,3-diarylcyclopropenes exhibit a reduced dienophilicity (equation 131) . On the other hand, numerous Diels-Alder reactions have been reported for 3,3-dicyano- and 3,3-dihalocyclopropenes. The reactions of 3-monosubsti-tuted cyclopropenes with the diene take place stereoselectively from the less crowded side of the substrates (equation 132) °. 

Alkyl, vinyl, and aryl Grignard reagents add to the double bond of cyclopropene in a cis fashion under mild conditions (equation 133) In addition, 1-alkylcyclopropenes... 

Dipolar cycloadducts 176 are available from the reactions of cyclopropenes with azomethine ylids, pyridinium ylids aryl nitrile oxides " ", alkyl nitrile... 

Additional evidence for the presence of cyclopropene intermediates in the interconversion of aryl and aromatic carbenes comes from the products of dehydrohalogenation of gem-dichlorocyclopropanes For example, treatment of cyclopropane 229 with base gives a ca. 1 2 mixture of the ethers 231 and 233, products which are easily rationalized by the pathways shown in equation 76. Of particular significance here is the observation that products derived from both 230 and 232 are obtained. This represents the first definite evidence that bicycloheptatrienes open to both cycloheptatrienylidenes and arylcarbenes under mild conditions, i.e. at ambient temperatures. 

Further support for the intervention of vinylcarbenes in such reactions stems from the cyclopropenes 239 which provide diene products, the proportions of which reflect the facility of the aryl ring to stabilize the developing carbenic centre. The formation of... 

Vinylcyclopropenes, e.g. 245a-c, undergo rearrangements analogous to those of their 3-aryl counterparts Direct photolyses of the cyclopropenes 127 afford preparatively... 

Tetrachlorocyclopropene in the presence of Lewis acids or the trichlorocyclopropenyl cation are important starting materials in cyclopropenone syntheses In this method the cyclopropene or salt is reacted with benzene or derivatives of benzene bearing functional groups such as alkyl, alkoxy, hydroxy, or halogen to yield diaryl-substituted cations. Upon hydrolysis these cations yield the cyclopropenone. In some cases the monoaryl cation (32) can be obtained and converted to the aryl chlorocyclopropenone 33. Alternatively, the monoaryl cation can be reacted with a second aromatic species to give cyclopropenones with different aryl groups (equation 30). 

ARYL BROMIDES Thallium triacetate. ARYLCYCLOPROPANES AND ARYL-CYCLOPROPENES Lithium 2,2,6,6-tetra-methylpiperidide. 

Cyclopropenes bearing a vinyl or an aryl group in the 3-position easily undergo... 

Cycloaddition of methyl diazoacetate or of aryl azides to 3,3-dimethylcyclopropene initially resulted in the formation of 1 1 adducts which then ring opened to diazoalkenes these then underwent addition to a second molecule of cyclopropene, e.g. formation of 1 and Similar products are formed from 1,3,3-trimethylcyclopropene. ... 

The stereoselectivity of the cycloaddition of arylcarbonitrile (V-oxides to 3,3-dimethyl-, 3-aryl-3-methyl-, 3-methyl-3-vinyl- and 3-cyano-3-methylcyclopropenes has been studied. In the case of the 3-vinylcyclopropene, addition to the external double bond can occur in competition with addition to the cyclopropene. 

Nitrile imines, generated in situ from the corresponding hydrazonic acid chloride and triethylamine, undergo 1,3-dipolar cycloaddition to cyclopropene and to 3,3-dimethylcyclo-propene to give 2-aryl-2,3-diazabicyclo[3.1.0]hex-3-enes 10. 

Using this route, a variety of tricyclo[2.1.0.0 ]pentanes are available from cyclopropenes, with the latter being most easily prepared from the metal-catalyzed reaction of a diazo compound and an alkyne (see Section 1.2.1.2.1. for related reactions). Alkyl- and aryl-substituted cyclopropenes 1 react with copper catalysts, such as copper powder, to yield the tricyclic products 2 in low to moderate yield. ... 

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