Methylene cyclopropene

Compute the eigenvalues and draw the energy level diagram for methylene-cyclopropene. 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

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 ... 

Stabilization of the methylene cyclopropene system has to be expected according to the introductory concept of triafulvene stabilization, if (a) the exocyclic C-atom... 

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). 

The inverse functionalization of the two components for a Wittig reaction has been described by Russian authors70 who combined the cyclopropenylide 70 with aldehydes to give the unstable methylene cyclopropenes 71 characterized by protonation as cyclopropenium salts 72 ... 

The Wittig reaction of diphenyl cyclopropenone with the phosphorane 68 failed to give the methylene cyclopropene instead a hydrocarbon of probable structure 67 was obtained71. ... 

Triafulvene 69 was synthesized by an interesting route starting with diphenyl cyclopropenium cation and lithio ethyl acetate72. Although widely used in calicene chemistry (see later) this reaction principle — like the Wittig reaction — did not find general application in methylene cyclopropene synthesis. 

As was shown for the mechanism of quinocyclopropene formation in acetic anhydride75 (see p. 20), acylation of the cyclopropenone is reasonable for the primary reaction step, then the O-acyl-cyclopropenium ion 74 forms methylene cyclopropene 73 through addition of the anion of the C-H acidic component and elimination of acetic acid. 

Open-chain 1,3-dicarbonyl compounds did not lead to methylene cyclopropenes when reacted with the cation 75 by the DIPEA method. However, 4,4-diacyl triaful-venes 83 can be prepared very easily and in high yields (60—80%) from cations 80 and copper (or zinc) chelates 81 of 1,3-dicarbonyl compounds (known to be capable... 

A few examples of vinylogous methylene cyclopropenes are known. Thus, in an interesting reaction mode the spirohexadiene 93, prepared from dimethyl acetylene... 

The first stable derivative of methylene cyclopropene was the quinocyclopropene 108 reported in 1964 by Kende98 it was prepared from the cyclopropenium cation 106 which underwent pyrolysis and bromination with NBS to the p-hydroxy-phenyl substituted cation 107, which gave quinocyclopropene 108 by deprotonation ... 

Similiar results on electron distribution of cyclopropenone, cyclopropenone imine and methylene cyclopropene were obtained in an ah initio calculation (SCF-MO) by Skancke137a. ... 

If the bathochromic effect of geminal dicyano substitution is assumed to be in the same range as found for ethylenic reference compounds79, from the positions of maxima in 182 and 193 the absorption of the parent triafiilvene can be estimated to appear in the region of 205—215 nm. This is in qualitative agreement with MO prediction172 that the rr -> 77 transition for methylene cyclopropene will fall near 200 nm. 

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

The photochemical behavior of methylene cyclopropenes is a subject of current investigation170 Previous results with some 4,4-diacyl and 4,4-dicyano triafulvenes indicate that mainly dimerization, but sometimes additional solvent incorporation and hydrogen abstraction occurs. In the case of the photodimer of 1,2-diphenyl-4,4-diacetyl triafulvene (180) the structure 460 can be assigned from spectral evidence ... 

Thus TONE is reported to give with l,2-diphenyl-4-carbethoxy triafulvene (69) the spirocyclohexene 49569 the (2 + 2) cycloadduct of the semicyclic methylene cyclopropene bond to TCNE ... 

Acetylene dicarboxylate and maleic anhydride failed to react with simple methylene cyclopropenes, but reacted readily with calicene derivatives, as shown by Prinz-bach293. Thus ADD combined with benzocalicene 497 to give the dimethyl tri-phenylene dicarboxylate 499, whose formation can be rationalized via (2 + 2) cycloaddition across the semicyclic double bond as well as (4 + 2) cycloaddition involving the three-membered ring (498/501). The asymmetric substitution of 499 excludes cycloaddition of ADD to the C /C2 triafulvene bond (500), which would demand a symmetrical substituent distribution in the final triphenylene derivative. 

In some cases the C /C2 double bond in methylene cyclopropenes and calicenes was found to show dienophilic functionality towards diene components. Thus, di-ethylamino butadiene combines with 497 to give the Diels-Alder adduct 507, whose proton-catalyzed elimination of amine interestingly did not lead to the dibenzo heptafulvalene 508, but to the methylene norcaradiene derivative 509293 ... 

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 ... 

It was suggested by Agranat, I., and Bergmann, E. D. Tetrahedron Lett. 1966, 2373, to use the name triafulvene for methylene cyclopropenes as well as for cyclopropenones and their functional derivatives. In this article, the term triafulvene is restricted to methylene cyclopropene. 

The origin of the alkyne in this reaction was not clear 30), but this will be discussed again below. In the same way the methylene cyclopropenes (24) can be obtained from the corresponding 1,1-dibromocyclopropanes, although the products are extremely sensitive to water, undergoing rapid hydrolysis at ambient temperature to produce (25) and (26), presumably by initial hydration of either of the two double bonds31). 

Reaction of the sulphonium salt (52) with cyclopentadienide ion leads to methylene-cyclopropene, though this can only be trapped in low yield by addition to cyclo-pentadiene 47). 

Table 7-5. Excitation energies (eV) at HF/CIS level forselected transitions of methylen-cyclopropene (MCP) and acrolein (ACRO) in dioxane (diox) and acetonitrile (ACN) solutions obtained using linear response (LR), State Specific (SS) and corrected linear response (cLR) approaches...

For this study we have used methylen-cyclopropene (MCP) and acrolein (ACRO) in two solvents, an apolar (dioxane) and a polar one (acetonitrile). The selected transitions can be seen as representative examples of different types of electronic transitions for which different solvent responses can be studied for MCP the first 77 - 77 transition for MCP, and the first n -> 77 and 77 -> 77 transitions for ACRO. We note that in MCP the resulting excited state is characterized by a dipole moment which has an opposite direction with respect to that of the ground state, whereas in ACRO, the n -> 77 and 77 -> 77 transitions are characterized by a decrease and an increase in the dipole moment passing from ground to excited state, respectively. 

Following these examples, methylene cyclopropene and certain 5,6-dihydrocali-cenes are now obtainable. Adding lithio trimethylsilylcyclopropanes to aldehydes also provides alkylidene cyclopropanes (Peterson olehnation route) . An approach to a-methylene y-butyrolactones starting with dibromocyclopropanes has been opened by a suitable manipulation of intermediate cyclopropyl anions (equation 133). ... 

Treatment of the bis(propynyl)zirconocene 759 with B(C6Fs)3 results in a linear G-G coupling of the alkynyl ligands to form the zwitterionic complex 76 0582,583 (Scheme 186). Complex 760 reacts with nitriles R CN to form initially the 1 1 adduct 761 that concurrently equilibrates with 760 and the metallacyclocumulene 762 (and the nitrile-borane adduct) subsequently, irreversible reaction in the presence of excess nitrile yields the methylene-cyclopropene derivative 763.584,585 Calculations have shown that the conversion 760 —> 763 is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate that features coordination of the three-membered carbocycle through one of its G-G cr-bonds. 

Cyclopropenylium ions with heteroatom substituents were transformed into the methylene-cyclopropene derivatives, as exemplified by the following general equations (see also Section... 

The addition of diazoalkanes to methylene cyclopropenes gives pyridazines. A labile 1 1 adduct (41) was isolated in the case of diazoethane, thus indicating the reaction sequence leading to 42 as the product. ... 

Methylene cyclopropene derivatives undergo reaction with aldehydes and imines that are activated by Lewis acid to generate the indene derivative as the major product in most cases, although occasionally dihydropyrans are obtained. Alternatively, if the reaction is conducted at —25 °C, rather than room temperature, the corresponding furan or p3rrolidine derivative is formed (eq 43). A mechanistic rationale for the divergent reaction pathways was provided. ... 

Other carbon nucleophiles, such as the anions derived from 3-diketones, ketoesters or diesters, react with the carbonyl group [e.g. 91,127-129]. Complex mixtures result [128] but methylene-cyclopropene derivatives (triafulvenes) and ring-opened products have been obtained under different conditions, e.g. [91,127]. 

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