Cyclopropene derivatives

Photochemical Transformation of the Obtained 3H-Pyrazoles into Cyclopropene Derivatives... 

The analogous photochemical reaction (300 nm) of 3H-pyrazole 64 in dichloromethane at room temperature led to cyclopropene derivative 66, possessing a a-hydroxy group, isolated in 70% yield (Scheme 18). 

The above two consecutive transformations provide straightforward access from propargyl alcohols to cyclopropene derivatives with an a- or /1-hydroxy group. This simple method is complementary to the access to 3-hydroxymethylcyclopropenes, via Rh2(OAc)4 catalyzed addition of diazoacetate to alkynes followed by reduction of the ester group, a route that is restricted to the access of primary cyclopropenyl alcohols [57], and is an alternative to the use of 2,2-dibromo-l-chlorocyclopropane via cyclopropenyl Uthium. 

Ethyl diazopyruvate, under copper catalysis, reacts with alkynes to give furane-2-carboxylates rather than cyclopropenes u3) (Scheme 30). What looks like a [3 + 2] cycloaddition product of a ketocarbenoid, may actually have arisen from a primarily formed cyclopropene by subsequent copper-catalyzed ring enlargement. Such a sequence has been established for the reaction of diazoacetic esters with acetylenes in the presence of certain copper catalysts, but metallic copper, in these cases, was not able to bring about the ring enlargement14). Conversely, no cyclopropene derivative was detected in the diazopyruvate reaction. 

It should be pointed out that there are some methylene cyclopropene derivatives, whose stability is ascribed mainly to inductive effects brought about by strongly electron-withdrawing substituents. Thus, l,2-bis(p-tolyl)-4,4-(bis-trifluoromethyl)-triafulvene (63) synthesized recently by Agranat66 is a perfectly stable molecule with a dipole moment (7.42 D) comparable to that of l,2-diphenyl-4,4-dicyano-triafulvene (64) of the resonance-stabilized type (l)67 (7.9 D). 

As seen from Table 10, both the methyl resonance in dimethyl cyclopropenone (7.75 r) and the separation of CH2 units a and 0 to the three-ring in di-n-propyl cyclopropenone (0.85 ppm) compare well to corresponding values for the covalent cyclopropene derivatives, but differ strongly from those of the positively charged cyclopropenium species. 

Tobey58 has reported further 19F-NMR studies comparing bis(p-fluorophenyl)-cyclopropenone (212) to its dichloride 214 and the cations 215-217. Although the cyclopropene 218 should be a less ambigous reference than the dichloride 214, it can be concluded that the deshielding effect of the cyclopropenone system is related closer to the covalent cyclopropene derivative than to the cationic species. This is in qualitative accordance with the findings in Chapter 5 (a). 

In contrast to cyclopropenones, most methylene cyclopropene derivatives do not undergo clearly defined thermal transformations neither cycloreversion analogous... 

The carbene obtained by heating compound 68 with DMAD at first gave a cyclopropene derivative, which underwent further transformations (z/rro-substitution and cyclization) to afford tricyclic product 69 in 40% yield <1999TL1483>. The thermolysis carried out in the presence of ArCH=C(CN)2 and DMAD used in excess led to the formation of highly functionalized cyclopentene derivatives 70 <2003TL5029, 2005TL201>. 

The formation of the cyclopropene derivative Z-36 is very similar to the isomerization of p-phenylenebisnitrene (13) to 14 (Scheme 2) and both processes are likely to be photochemical, rather than thermal. Further irradiation of Z-36 causes its isomerization to another compound, which according to DFT simulations, is compatible with the structure of its E-isomer (E-36, Scheme 7). 

Whereas the repeated lithiation-trimethylsilylation sequence of trimethyl-silylbicyclopropylidene 43 a yielded predominantly the cyclopropene derivative 51 [54], bicyclopropylidenecarboxylates 42-Me, 42-fBu after repeated deprotonation and carboxylation retain the bicyclopropylidene moiety and give 2,2-disubstituted products 52-R only (Scheme 9) [55]. So far, alkylbicyclopropyl-idenes 43 e, f, g have not been induced to undergo deprotonation and a second substitution [56a]. The urethane 53 with a nitrogen directly attached to the skeleton, more easily than any other bicyclopropylidene derivative, rearranges to... 

To summarize, gwi-dihalocyclopropanes may serve as starting materials for the preparation of cyclopropane and cyclopropene derivatives, they can lead to compounds with bicyclobutane and spiropentane structures, provide allenes and... 

Intermolecular free-radical addition of iodoalkyl sulfones to vinylsilanes yields regios-electively the adduct 146 which can be further transformed to cyclopropene derivatives (equation 122)214. 

Obata, N., and J. Moritani Photochemical transformation of acyl-cyclopropene derivatives. Tetrahedron Letters 1966, 1503. 

The mechanistic aspects of the silver(I)-promoted rearrangement of cyclopropene derivatives have been investigated, confirming preferential attack of Ag + on the a bond to give an argentocarbenium ion. This intermediate is trapped by methanol, leading to a vinylsilver intermediate that is deuterated (Scheme 3.20).33,34... 

The ruthenium-catalyzed addition of diazo compounds to alkynes has led to the selective synthesis of functional 1,3-dienes by the combination of two molecules of diazoalkane and one of alkyne [106] (Eqs. 82,83). The stereoselective formation of these conjugated dienes results from the selective creation of two C=C double bonds rather than leading to the cyclopropene derivative. This is expected to be due to the possibility for the C5Me5RuCl moiety to accomodate two cis carbene ligands. 

From three-membered rings In pro tic media, l-acyl-2-cyclopropene derivatives undergo a ring expansion reaction to cyclobutenols [7]. - Ring expansion of cyclopropylmethanols to fluorinated cyclobutans [8]. 

Addition of a rhodium carbenoid to an alkyne leads to a cyclopropene derivative. In an intramolecular context, the fused cyclopropene moiety is unstable and undergoes ring opening to generate a rhodium vinyl carbenoid entity, which can then undergo cyclopropanation or cyclopropena-tion, carbon hydrogen insertion, and ylide generation. This is illustrated... 

Quenching experiments (anthracene) as well as sensitisation (xanthone) show that in that reaction a triplet is involved 78b>. This is not clear in the photolysis of A, which leads also to a cyclopropene derivative B 78c>. 

A number of Ni(0) complexes containing a cyclopropene ligand have been prepared and shown to catalyze the oxidative coupling of cyclopropene derivatives to compounds with a cyclobutane core (Scheme 21). A recent study of the kinetics of these reactions has led to the isolation... 

When electron deficient alkenes are added to cyclopropene derivatives (74 equation 33) and (77 equation 34) in the presence of [Ni(COD)2], vinylcyclopropanes are formed in good yields. For example, dialkyl fumarate or maleate reacts with 3,3-dimethylcyclopropene in the presence of [Ni(COD)2] to give 2,3-bis(alkoxycarbonyl)-l-(2-methyl-l-propenyl)cyclopropanes (75), (76), (78) and (79), in which alkene stereochemistry is chiefly retained, in 50-73% yields. Reaction of methyl acrylate with 3,3-dimethylcyclopropene results in the formation of several products, while reaction of methyl acrylate with 3,3-diphenylcyclopropene gives vinylcyclopropane derivatives (80 equation 35) in 85% yield. Under similar conditions, methyl crotonate reacts with (74a) to give (82) in low yield (equation 36). Catalysis with nickel(0)/PR3, 2 [Ni(CO)4], 3 [Pd(DBA)2] or [Pd(DBA)2]/PlV33 gives mainly... 

The acetophenone-sensitized photorearrangement of the benzonorbor-nadiene (49) yielded the tricycloheptene (50a) as the exclusive product." Analogous products (50b—d) were also formed by sensitized irradiation of the cyclopropene derivatives (51a—c), respectively. 

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