Aryl-substituted methylidenecyclopropa[ b ]naphthalenes: synthesis and attempted silver(I)-mediated dimerization

The arylmethylidenecyclopropa[ b ]naphthalene family has been extended to include the 1-and 2-naphthyl and 9-anthryl derivatives ( 5-7 ). When subjected to Ag(I) in aprotic media, conditions typically employed for the linear dimerization of the parent cycloproparenes, diarylalkynes and/ or ketones are obtained; in alcoholic media enol ethers are formed. Dimerization to 9,10-anthra-quinodimethanes does not take place.


Introduction
As novel aromatic hydrocarbons, the alkylidenecycloproparenes, e.g. 1 and 2, [1][2][3] have continued to provide a source of fascination 4 since their discovery in 1984, 5 not least because the various derivatives have unexpected polarities, [6][7][8] fluorescence characteristics, 9 and unusual properties. 4,10,11Recently, we described five protocols that allow for the synthesis of an extensive series of 1-aryl-and 1-diaryl-methylidene-1H-cyclopropa[b]naphthalenes, their polarities, and the linear dependence of their cycloproparenyl 13 C NMR chemical shifts upon the Hammett σ p + constant of the remote aryl substituent. 3We also addressed conjugated and cross-conjugated cycloproparene derivatives containing cyclopentadiene and dithiole sub-units, 12 and others with simple π bonds that enhance polarity through extended conjugation. 13Despite these advances there is no recorded attempt to utilize these exocyclic alkenes in what would be a simple and straightforward synthesis of quinodimethanes from ring opening and dimerization as occurs for the parent cycloproparenes.6][17][18] For the simple cycloproparenes, the dimerization reaction entails the dropwise addition of an anhydrous chloroform solution of the cycloproparene to a suspension of AgBF 4 (ca. 1 mol %) in the same solvent at 0 o C. 15 Such reactions are usually complete within a few minutes and, as the anhydrous non-nucleophilic solvent cannot intercept the σ complex, a second equivalent of cycloproparene binds with the ring-opened cation ultimately to yield cycloproparene dimer (Scheme 1).Of the two possible products of dimerization the linear isomer dominates, as dictated by addition of the Ag(I)-complexed cycloproparene to the second molecule of reactant, and it is usually present in excellent yield as illustrated by the 90% conversion of cyclopropabenzene into 9,10-dihydroanthracene (Scheme 1).We report herein the synthesis of the new arylmethylidene-1H-cyclopropa-[b]naphthalenes 5-7 and the outcome of attempted dimerizations.

Results and Discussion
The synthesis of an alkylidenecycloproparene is conveniently performed by subjecting the parent annulated aromatic hydrocarbon to lithiation/silylation sequences that (ultimately) provide the C1 α-silylcycloproparenyl anion for in situ reaction with an aldehyde or ketone.The derived exocyclic alkene is obtained directly from such Peterson olefination in a 'one pot' procedure from cyclopropabenzene, but only from isolation and subsequent desilylation of 1,1-bis-(trimethylsilyl)cyclopropanaphthalene 4 from 3. 19 The precise conditions needed for a given carbonyl compound and 4 1,2 have been the subject of detailed scrutiny, and fall into five distinct procedures that allow for the convenient synthesis of new derivatives. 3While these procedures do not justify further discussion here, use of 'Method 1' has provided easy access to the previously known 1-phenyl-1, 1 1-diphenyl-2, 1 and the hitherto unrecorded 1-(1'-naphthyl)-5, 1-(2'-naphthyl)-6 and 1-(9'-anthrylmethylidene)-1H-cyclopropa[b]naphthalene 7 (Scheme 2). 20ompounds 5-7 are characterized by their C8 vinylic proton resonance (δ H 7.35, 6.75 and 7.59, respectively) and the appearance of H2/H7 as narrowly coupled doublets (J ~1.4 Hz) between 7.3 and 7.6 ppm.The 13 C NMR resonances for C2/C7 fall in the typical range 3,4 and at 108.2-108.6 ppm, and while C8 for 5 and 7 is at δ 102.9 it is at δ 107.3 for 6.The increased shielding of H8 (6.75 ppm) and deshielding of C8 in 6 are fully consistent with the same resonances of 1 (δ H 6.53; δ C 107.1).These reflect the angular (C2') attachment of the naphthalene ring that allows the substituent to lie closer to planarity in 6 than in 5 or 7 in analogy to the phenyl group of 1 that is twisted by about 5 o out of the cycloproparenyl plane. 2,4Me 3 ; 30% ; 43% ; 45%

Scheme 2
6][17][18] To date this gap has not been bridged.Thus complexation of, e.g. 2, with Ag(I) opens the strained three-membered ring σ bond in direct analogy with the parent hydrocarbon of Scheme 1, and the methoxystyrene 8 is obtained in 78% yield from capture of the complex by the nucleophilic solvent (Scheme 3). 23When the analogous reaction was attempted in chloroform it was far from spontaneous.Only after a 2 h reflux period did the yellow fluorescence characteristic of unchanged 2 fade.Conventional work-up gave colorless crystals of product that is identified as ethoxystyrene 9 from its analytical and spectroscopic data (Experimental section) and it arises from capture of the silver complex by the ca.2% ethanol used to stabilize chloroform!Colorless crystals of product were again obtained from 2 and Ag(I) in freshly distilled chloroform from which ethanol had been carefully removed.However, infrared stretching at 1658 cm -1 indicates the presence of a conjugated (aryl) carbonyl function and the product, formed in 85% yield, is characterized as 1-(2-naphthyl)-2,2-diphenylethanone (11). 23The formation of 11 is again rationalized by Ag(I)-mediated opening of the lateral three-membered ring bond to give the σ complex but, with no nucleophile present, intercep-tion can only be by water during work-up and this leads to 11 via enol 10 as shown in Scheme 3. It is clear that the σ complex is not captured by an unopened molecule of 2 as no evidence was gained for the presence of dimer 13, even in trace quantities.In all probability the steric requirements of the exocyclic substituents disfavor formation of the silver-bridged dimeric ion 12.However, the involvement of Ag(I) with the slightly polar hydrocarbon 2 is assumed as the reaction does not appear to take place on standing at room temperature.

Scheme 3
In similar vein, use of the less sterically demanding phenylmethylidene homologue 1 did not afford dimer 14.In this case the reaction provided a separable 3:1 mixture of 1-(2-naphthyl)-2-phenylethyne (15) 23 and 1-(2-naphthyl)-2-phenylethanone (17) 23 in 83% combined yield, along with unchanged substrate 1 (15%) (Scheme 4).Alkyne 15 has been isolated previously from 1, but in only 31% yield, by reaction with Ag(I) in tert-butanol where relief of ring strain by proton transfer is facilitated by the metal ion; 23 the size and nucleophilicity of the tert-butyl group does not allow for capture to give enol but does provide for a more complex product mixture. 23The formation of benzyl naphthyl ketone 17 from 16 during aqueous work-up matches that of ethanone 11 from 10 as described above.In the absence of Ag(I), but in chloroform, 1 and 2 are stable for periods longer than the reaction times involved.Because of the failure of 1 (and 2) to provide dimer, analogous reactions with the sterically more demanding new arylmethylidene compounds 5-7 have not been performed.

Scheme 4
An alternative route to linear alkylidenecycloproparene dimers could commence with disilylcycloproparene 4. Thus Ag(I)-mediated dimerization would lead to 6,6,13,13-tetrakis(trimethylsilyl)pentacene that could be subjected to Peterson olefination in direct analogy with the procedure depicted by Scheme 2. In the event, disilane 4 failed to dimerize and it was recovered almost quantitatively, even after reflux for two days with AgBF 4 in anhydrous chloroform.That the reaction conditions employed herein are appropriate for dimerization has been confirmed from successful dimerization of 3 to 6,13-dihydropentacene in 73% yield. 15,17The steric constraints present at C1 of the exocyclic alkenes 1-5 (and disilane 4) are too large to allow dimerization as only products of ring opening (or unchanged starting material) are recorded .

Experimental Section
General Procedures.The general procedures followed and the spectrometers used have been described previously. 3