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Note from the Editor: Tetsuo Nozoe lived his entire life on one of several island nations (Japan and Formosa, subsequently Taiwan, then returning to Japan). Those of us—and that is most of us!—who have never lived on an island nation likely cannot completely appreciate the ‘isolation’ and ‘lack of diversity’ that accompanies—along with many other valuable and happy consequences—such a life experience. As a New Zealander, colloquially as a Kiwi, Brian Halton shares this life experience with Nozoe. Indeed, New Zealand is far more isolated than either Japan or Taiwan in terms of distance to its nearest neighbor, population density, and size of its chemical community. Often during his career, Halton travelled to foreign lands, as did Nozoe, to fill his cup with collegiality and science. And Halton welcomed visitors to his beautiful country, as did Nozoe. Indeed, their mutual friendship was a blessing and honor to each of them. As Brian Halton tells his story of place and of science, I can well imagine Tetsuo Nozoe's response if he could read these words. He surely would have beamed in understanding these shared experiences! —Jeffrey I. Seeman Guest Editor University of Richmond Richmond, Virginia 23173, USA E-mail: jseeman@richmond.edu

The disrotatory ring opening of gem-dibromocyclopropanes provides a versatile and increasingly utilised procedure for the synthesis of a range of natural products. The field is summarised and product formation explained.

The arylmethylidenecyclopropa[b]naphthalene family has been extended to include the 1and 2naphthyl 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-anthraquinodimethanes does not take place.

Reaction of 1,1-dichloro-2,5-diphenylcyclopropabenzene 6 with Meldrum's acid 8 in the presence of pyridine leads to coupling of the cycloproparenyl cation 7 with the stabilized diketo anion 9. Subsequent, spontaneous, base-induced dehydrochlorination gives the alkylidenecyclopropabenzene 11 in a one-pot reaction. Flash vacuum thermolysis of 11 at 650 degrees C ejects acetone and carbon dioxide, giving cyclopropabenzenylidenethenone 12 that is isolated in an Ar matrix at 20 K and characterized by a strong ketene band at 2107 cm(-1) in the IR spectrum. [reaction: see text]

The first simple and stable cross-conjugated π-extended alkylidenecycloproparenes 5, 6 and 15−22 have been prepared as coloured crystalline derivatives from use of (E,E)-1,5-diphenylpenta-2,4-dienone, (E,E)-1,5-diarylpenta-1,4-dien-3-ones and (E,E)-2,5-(diarylmethylidene)cyclopentanones with the relevant 1,1-bis(trimethylsilyl)cyclopropa[b]naphthalene. The permanent dipole moments of the compounds have been measured, and the conformational preference of the rotamers assessed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Dipole moment measurements and calculations show the cyclopropa[b]naphthalene moiety to be an electron donor that is stronger than cycloheptatriene. Crystallographic analyses of cycloheptatrienylidenecyclopropa[b]naphthalenes, with and without electron donating substituents in the cycloproparene, have the seven-membered ring resisting imposition of 8π 7C antiaromaticity by bending significantly out of the plane that carries the cycloproparene. The extent of this distortion depends upon the level of electron donation from the cycloproparene.

Dithiole and cyclopentadiene rings have been introduced into functionalised cyclopropa[b]naphthalenes to provide highly coloured, conjugated and cross-conjugated π-extended derivatives that have varying stability. Cyclopentadienylidene-containing cyclopropanaphthalene 18 and its dithiolylidene analogue 25 are prepared from bromophenylcyclopropanaphthalene 12, and the 1,1-biscyclopentadienylidene 20 from dibromide 13. Use of dithiole-containing ketones with 1,1-bis(trimethylsilyl)cyclopropa[b]naphthalene 10 gives the π-extended cycloproparenes 27 and 29 in Peterson olefinations that are more efficient than Wittig−Horner reactions of cyclopropanthraquinone 31 with dithiolate 24 to give the cross-conjugated cycloproparenes 34 and 35. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Diarylmethylidenecyclopropanaphthalenes 4b-d add diphenylisobenzofuran (DPIBF) and α- pyrone across the exocyclic double bond to give ring expanded products 11b-d and 13b-d that result from subsequent relief of ring strain in the non-isolable spirocyclic intermediates 10 and 12, respectively. The benzene homologues 3b and 3c add DPIBF across the bridge bond to give the norcaradiene adducts 19b and 19c.

The 1H-cyclopropa[b]naphthalenyl anion reacts with N,N-dimethylamides to provide C-1 acyl derivatives in good yield in what are rare examples of mono-substitution in the cycloproparene series. The availability of these acylcycloproparenes provides for subsequent benzylic proton abstraction and enolate ion interception to give O-silyl and O-phosphinate derivatives. This reaction sequence leads to the first cyclopropa[b]naphthalenylidene enol ethers and provides the only example of exocyclic alkene formation taking place in one step from the benzylic centre. Protonation of the acyl derivatives by mineral acid triggers three-membered ring opening to 2,3-disubstituted naphthalenes rather than 2-monosubstituted analogues that typically arise from protonation at the aromatic ring. Upon thermolysis ring expansion to a naphthofuran occurs.

1H-Cyclopropa[b]naphthalene-3,6-dione 3 adds bromine stoichiometrically across the enedione C=C bond to give dibromodihydrocyclopropanaphthalenedione 10 while with an excess of the reagent (bromomethyl)tribromonaphthoquinone 12 is formed. Typical quinone character is exhibited by 3 in Diels-Alder cycloadditions and it gives the endo-methanocyclopropanthraquinone 14 with cyclopentadiene. With buta-1,3-diene the analogous tetrahydrocyclopropanthraquinone 15 is formed from a temperature dependent reaction. Above 45 degrees C opening of the three-membered ring of 4 also occurs and the cyclopentanthracenedione 16 is obtained from [pi 2+pi 4] and [sigma 2+pi 2] additions; it is the sole product at 100 degrees C. Enolisation of the tetrahydroanthraquinone 15 provides diphenolate 18 and this can be diverted to diether 19 or readily oxidized to the dihydroanthraquinone 20. In turn, 20 is dehydrogenated to the fully aromatic cyclopropa[b]anthracene-3,8-dione 4, the first anthraquinone of the cycloproparene series.

1H-Cyclopropa[b]naphthalene is converted into novel polar olefins 15−19 via reaction of the 1,1-disilyl with anthrone-like ketones. The alkenes so formed are crystalline, polar compounds that range in color from yellow to magenta. Solutions of the 10,10-dimethylanthrylidene 15, the xanthenylidene 16, and N-methylacrydinylidene 18 fluoresce; parent anthrone 19 likely phosphoresces. X-ray crystal structure determinations are reported for 15, 18, and the thioxanthenylidene 17. Theoretical studies using ab initio calculations at the HF/6-31G* level have been performed to provide assessments of the structures, charge distributions, and dipole moments of 15−19.

Desilylation of 1,1-bis(trimethylsilyl)-1H-cyclopropa[b]naphthalene 10 with tert-butoxide or hydroxide ion gives anion 9 as a highly reactive species. Formed with tert-butoxide 9 can be intercepted by iodomethane to give the 1-methyl-1-trimethylsilyl derivative 13. With hydroxide ion and water catalytic didesilylation regenerates cyclopropanaphthalene 6 quantitatively. Formation of 9 with hydroxide ion in the presence of benzophenone, fluoren-9-one or 4-(dimethylamino)benzaldehyde provides a simple and economic route to the methylidenecyclopropanaphthalenes 11, 17 and 18. With benzaldehyde and 4-fluorobenzophenone this revised protocol (to give 19 and 12, respectively) is effective only in the presence of 18-crown-6.

Upon flash vacuum thermolysis at 750 °C fluorenylidenecyclopropa[ b ]naphthalene ( 1 ) undergoes opening of the three-membered ring and rearrangement to give a range of C 24 H 14 polycyclic aromatic hydrocarbons. Dibenz[ e . l ]- and -[ e . k ]acephenanthrylene ( 7 ) and ( 12 ), respectively, have been identified while the plausible naphth[1,2- e ]- and [2,3- e ]acephenanthrylenes ( 9 ) and ( 14 ) were not detected. With diphenylmethylidenecyclopropa[ b ]naphthalene ( 2 ) cyclodehydrogenation and rearrangement also provide C 24 H 14 polycycles; dibenz[ e . k ]acephenanthrylene ( 12 ) is identified and dibenz[ a . e ]aceanthrylene ( 15 ) is a proposed product.

Proton abstraction from the methylene position of a cycloproparene provides the corresponding C1 anion that can be intercepted by a carbonyl-containing compound to provide a wide range of novel methylidenecycloproparenes. The physical and chemical aspects of this comparatively new class of surprisingly stable, strained compounds have been explored. The present account provides a perspective on these developments from the initial experiments in the Utah laboratories.

The general preparation of several new alkylidenecycloproparenes containing functional groups at the exocyclic (C8) position is described.

[reaction: see text] The first derivative of the novel and highly strained rocketene (cyclobuta[a]cyclopropa[d]benzene) 5, namely, the C4 diphenyl-substituted exocyclic alkene 8, has been prepared in 16% overall yield as a yellow oil with properties fully consistent with its highly strained nature.

Diarylmethylenecyclopropabenzenes react in [2+4] cycloaddition at the ‘cyclopropene’ bridge bond rather than at the exocyclic olefin. The results are in accord with computational data.

The reactions of diarylalkylidenecycloproparenes in [2+4] and [2+2] cycloadditions are described.

The behaviour of cyclopropabenzene (1) with 1,3-diphenylisobenzofuran ( dpibf ) is temperature dependent. Addition to the strained bridge bond of (1) gives the Diels -Alder adduct (13) in tetrahydrofuran at 20°C, but in chloroform at the same temperature the ring-expanded 5,10-epoxydibenzocycloheptene (12) is also formed. Compound (12) becomes the sole product from reaction at 30°C or higher. Cyclopropa [b]naphthalene (2) is inert to dpibf under similar conditions but provides the dimer 6,13-dihydropentacene (15). The triazoloindazole (19) results from reaction of (2) with 4-phenyl-3H-1,2,4-triazole-3,5(4H)- dione.

9,9-Dichloro-1,4-dihydro-4a,8a-methanonaphthalene (4) adds the electron-deficient dienophiles 4-phenyl-1,2,4-triazoline-3,5-dione, maleic anhydride and dimethyl acetylenedicarboxylate to the α-face to give adducts (5)-(7) respectively; the addition of the alkyne requires Lewis acid catalysis. Inverse electron-demand addition of 3,6-diphenyl-1,2,4,5-tetrazine to the monoene component of (4) in hydrophilic solvent (ethane-1,2-diol) is thwarted; the CCl2 bridge is ejected and the 1,4-dihydronaphthalene formed is captured to give (ultimately) phthalazine (16). Dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate adds to (4) to give the methanobenzophthalazine (17b).

Treatment of 6,6-dichloro-3-oxabicyclo[3.1.0]-hexane (7) with potassiumt-butoxide generates the ring-strained cyclopropene8 as a reactive intermediate. With 1,3-diphenylisobenzofuran compound8 is trapped as a ∼5∶1 mixture of theexo- andendo-adducts11a and11b, respectively. With furan only the ring expanded vinylcarbene12 is intercepted and spirocycle13a is formed. Carbene12 is also captured by cyclohexene, 2,3-dihydrofuran, 2,5-dihydrofuran, and diphenylethyne to give22–25 respectively. A likely side product in these reactions is the pyran14.

4,4-Dichlorotricyclo[5.4.1.03,5]dodeca-7,9,11-triene (18a) undergoes cycloaddition with 4-phenyl-1,2,4 triazoline-3,5-dione via its norcaradiene valence-bond isomer (18b) to give adduct (21)in 86% yield. By comparison the aromatic tricyclo [5.4.1.03,5]dodeca-2,5,7,9,11-pentaene (6)and its 12-chloro stereoisomer (20) fail to react; this contrasts with the behaviour of other 1,6-methano[10] annulene derivatives.

The reactions of halogenated [n.m.1]propellenes with potassium t-butoxide have been investigated. Propelladiene 9,9-dichloro-1,4,5,8-tetrahydro-4a,8a-methanonaphthalene affords 1,6-methano[10]-annulene, 4-methylazulene, and 5H-benzocycloheptene, while the related tetracyclic compound-1,1,8,8-tritrachloro-1a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa[b]naphthalene reacts to give 12-chlorotricyclo[5.4.1.03,5]dodeca-1,3(5),6,8,10-pentaene. Treatment of 8,8-dichloro-2,3,4,7-tetrahydro-3a,7a-methano-1H-indene with the same base produced a mixture of the chlorodiene 8-chloro-2,3-dihydro-3a,7a-methano-1H-indene, azulene, and 4-methylazulene. The structure of the chlorodiene was established by an X-ray crystallographic study of its Diels–Alder adduct with 4-phenyl-4H-1,2,4-triazole-3,5-dione. While reaction of tetracycle 1,1,syn-8-trichloro-1 a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa[b]naphthalene with base failed to produce any characterisable products, under the same conditions epimer 1,1,anti-8-trichloro-1a,2,3,6,7,7a-hexahydro-2a,6a-methano-1H-cyclopropa [b] naphthalene was converted into the cycloproparene tricyclo-[5.4.1.03,5]dodecs-1,3(5),6,8,10-pentaene. Mechanistic proposals which account for the observed conversions have been advanced and are supported by 13C-labelling studies. The initial steps in most of the reaction pathways of the substrates are probably 1,4-elimination processes involving abstraction of an allylic hydrogen, fragmentation of the strained propellene σ-bond, and ejection of the halogen in an endo-relationship to the abstracted proton. The primary product of these processes, bridgehead dienes such as bicyclo[4.4.1]undeca-1,3,6(11),8-tetraene, 4,4,12-trichloro-tricyclo[5.4.1.03.5]dodeca-1 (11), 7(12),9-triene. and 10-chlorobicyclo[4.3.1]deca-1,3,6(10)-triene, then undergo further reaction involving, amongst other things, 1,3-prototropic shifts. Attempts to probe the mode of formation of the C11-4-methylazulene from the C10-precursor 8,8-dichloro-2,3,4,7-tetrahydro-3a,7a-methano-1H-indene have uncovered a novel methylation reaction of azulene by the dimsyl anion.

The reactive bicyclo[n.1.0]alk-1(ω)-enes (n=3-6) 9a-d are formed by dehydrochlorination of 8a-d and trapped by diphenylisobenzofuran as exo Diels-Alder adducts 10a-d. With 8c the major adduct, 18 results not from interception of 9c but from diene 16 that is formed by dehydrochlorination/rearrangement sequences. Attempted interception of cyclopropenes 9a-d by furan provides a [4+2] product 19 from 9b (n=4) only

The dichloromethylene -bridged annulene (5) is available in four steps (47%) from 1,4,5,8-tetrahydronaphthalene (11) via the dibromide (15); the minor (5%) product of dehydrobromination of (15) is 2-bromo-9,9-dichloro-1,4-dihydro-4a,8a-methanonaphthalene (16). Dichlorocarbene adds to the norcaradiene form of (5) to give adduct (6) with 80% efficiency. Diene (6) resists both [π4+π2] cycloadditions and reductive dechlorination. Neither (5) nor parent 1,6-methano[10] annulene (18) adds methylene from Simmons-Smith cyclopropanation procedures.

The reactions of diarylalkylidenecycloproparenes in [2+4] and [2+2] cycloadditions are described.

Diarylmethylenecyclopropabenzenes react in [2+4] cycloaddition at the ‘cyclopropene’ bridge bond rather than at the exocyclic olefin. The results are in accord with computational data.

Proton abstraction from the methylene position of a cycloproparene provides the corresponding C1 anion that can be intercepted by a carbonyl-containing compound to provide a wide range of novel methylidenecycloproparenes. The physical and chemical aspects of this comparatively new class of surprisingly stable, strained compounds have been explored. The present account provides a perspective on these developments from the initial experiments in the Utah laboratories.

The X-ray crystallographic structures of crystalline fluorenylidenecycloproparenes 7 and 8 and of dibenzocycloheptatrienylidene 9 are reported. Theoretical studies, using ab initio methods at the HF/6-31G(d,p) and the correlated MP2/6-31G(d,p)) levels, have been used to provide assessments of the structure, charge distribution, dipole moment, and thermodynamic stability of the unknown methylidenecyclopropabenzene 3, the derived parent tria-, penta-, and heptafulvalene derivatives 4−6, and the crystalline derivatives 7, 8, and 9. The hydrocarbons are found to be polar, and the cycloproparenylidene moiety acts as electron donor in all but the cyclopropenylidene 4; this compound is the only fulvalene hydrocarbon in the series calculated to have a negatively polarized cyclopropareneyl unit.

1-(Trimethylsilyl)-1H-cyclopropa[b]naphthalene (10) and its 1-methyl derivative (11) have been isolated as pure compounds from use of a lipophilic size exclusion gel. Acylation of the 1H-cyclopropa[b]naphthalenyl anion (2) is effected with N,N -dimethyl-benzamide and -acetamide to give (5) and (6), respectively. Analogous reactions with the 1-(trimethylsilyl)-1H-cyclopropa[b]naphthalenyl anion (9) do not yield the 1-acyl-1-(trimethylsilyl)-1H-cyclopropa[b]naphthalenes (12) and (13); instead the novel 6-methyl- 7H-dibenzo[b,g]fluorene (15) results from attempted acetylation. Compound (15), a formal dimer of 1H-cyclopropa[b]naphthalene (1), is formed also from anion (9) both in toluene and from addition of hydrocarbon (1); its structure is assigned from spectroscopic data and confirmed by single-crystal X-ray analysis.

The 1H-cyclopropa[b]naphthalenyl anion reacts with N,N-dimethylamides to provide C-1 acyl derivatives in good yield in what are rare examples of mono-substitution in the cycloproparene series. The availability of these acylcycloproparenes provides for subsequent benzylic proton abstraction and enolate ion interception to give O-silyl and O-phosphinate derivatives. This reaction sequence leads to the first cyclopropa[b]naphthalenylidene enol ethers and provides the only example of exocyclic alkene formation taking place in one step from the benzylic centre. Protonation of the acyl derivatives by mineral acid triggers three-membered ring opening to 2,3-disubstituted naphthalenes rather than 2-monosubstituted analogues that typically arise from protonation at the aromatic ring. Upon thermolysis ring expansion to a naphthofuran occurs.

The reactive bicyclo[n.1.0]alk-1(ω)-enes (n=3-6) 9a-d are formed by dehydrochlorination of 8a-d and trapped by diphenylisobenzofuran as exo Diels-Alder adducts 10a-d. With 8c the major adduct, 18 results not from interception of 9c but from diene 16 that is formed by dehydrochlorination/rearrangement sequences. Attempted interception of cyclopropenes 9a-d by furan provides a [4+2] product 19 from 9b (n=4) only

Treatment of 6,6-dichloro-3-oxabicyclo[3.1.0]-hexane (7) with potassiumt-butoxide generates the ring-strained cyclopropene8 as a reactive intermediate. With 1,3-diphenylisobenzofuran compound8 is trapped as a ∼5∶1 mixture of theexo- andendo-adducts11a and11b, respectively. With furan only the ring expanded vinylcarbene12 is intercepted and spirocycle13a is formed. Carbene12 is also captured by cyclohexene, 2,3-dihydrofuran, 2,5-dihydrofuran, and diphenylethyne to give22–25 respectively. A likely side product in these reactions is the pyran14.

The first cyclopropa-fused anthraquinone, 1 H -cyclopropa[ b ]anthracene-3,8-dione, is available in four steps from homologation of 1 H -cyclopropa[ b ]naphthalene-3,6-dione.