Synthesis and structures of the dinuclear tin(II) complexes [R = Ph, X(Z) = CPh; R = SiMe3, X(Z) = PPh2] and of related compounds

Tin(II) compounds containing the ligands [CH(C₆H₃Me₂-2,5)C(Bu^t)NSiMe₃]⁻ (≡ L¹), [CH(Ph)C(Ph)NSiMe₃]⁻ (≡L²), [CH(SiMe₃)P(Ph)₂NSiMe₃]⁻ (≡ L³),

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(≡ L⁴), [C(Ph)C(Ph)NSiMe₃]²⁻ (≡ L⁵), and [C(SiMe₃)P(Ph)₂NSiMe₃]²⁻ (≡ L⁶) are reported: the transient SnBr(L¹) (1) and SnBr(L²) (2), Sn(L¹)₂ (3) [P.B. Hitchcock, J. Hu, M.F. Lappert, M. Layh, J.R. Severn, J. Chem. Soc., Chem. Commun. (1997) 1189], the labile Sn(L²)₂ (4), [Sn(L⁵)]₂ (5), SnCl(L³) (6), Sn(L³)₂ (7), [Sn(L⁶)]₂ (8), Sn(L⁴)₂ (9) and Pb(L⁴)₂ (10). They were prepared from (i) SnBr₂ and K(L¹) (1, 3) or K(L²) (2, 4, 5); (ii) SnCl₂ and Li(L³) (6–9); or (iii) PbCl₂ and Li(L⁴) (10). Each of 1, 3 and 5–10 has been characterised by multinuclear NMR spectra; 3, 5, 6, 8, 9 and 10 by EI-mass spectra, but only 3, 5, 8, 9 and 10 were isolated pure and furnished X-ray quality crystals. Of greatest novelty are the title binuclear fused tricyclic ladder-like compounds 5 and 8. Quantum chemical calculations, on alternative pathways to 5 from 2 and to 8 from 7, are reported.     

Synthesis of bis N-heterocyclic carbenes,derivatives and metal complexes

A method for the synthesis and isolation of 1,1′-methylene-bis-(3-aryl-imidazol-2-ylidene) ligands, aryl = 2,6-diisopropyl-phenyl (DiPP), L^DiPP, mesityl (mes), L^mes, is reported, which provides synthetically useful quantities of high purity. Derivatisation of L^DiPP with chalcogenides gave the adducts L^DiPPE₂, E = S, Se, Te. Reaction of L^DiPP with [Pd(tmeda)Me₂], [Pt(μ-SMe₂)Me₂]₂, [Ir(1,5-COD)(μ-Cl)]₂/KPF₆ and [NiBr₂(dme)] gave [Pd(L^DiPP)Me₂] (1), [Pt(L^DiPP)Me₂] (2), [Ir(L^DiPP)(1,5-COD)](PF₆) (3) and [Ni(L^DiPP)Br₂] (4), respectively. The latter was reduced in the presence of CO to [Ni(L^DiPP)(CO)₂] (5). The structures of L^mes, L^DiPPTe₂, and 1–5 are also reported.     

Synthesis of an octamethyl-18-crown-6 derivative and the X-ray crystal structure of its 2:1 complex with borane-ammonia

The synthesis of 2,2,3,3,11,11,12,12-octamethyl-1,4,7,10,13,16-hexaoxacyclooctadecane (2) from pinacol (3) by a sequence of reactions (3→4→5→6+5→2) involving alkylation (3→ 4), ozonolysis and reduction (4→5), tosylation (5→6), and cyclisation (5+6→2) is reported. With borane-ammonia, the octamethyl-18-crown-6 derivative 2 forms a crystalline 2:1 complex, (BH₃NH₃)₂ · 2. X-Ray crystallography reveals the two guest BH₃NH₃ molecules are hydrogen bonded in a centrosymmetric manner to the opposite faces of the host 2, which adopts an all-gauche (ag⁺a ag^-a)₃ conformation.     

Two types of rare earth–organic frameworks constructed by racemic tartaric acid

Hydrothermal reactions of rare earth oxides with racemic tartaric acid (H₂tar) yielded 7 rare earth(III) MOFs with general formulas [R₂(tar)₃(H₂O)₂]_n (R=Y (1), Sm (4), Eu (5), Tb (6), Dy (7)) and [R₂(tar)₂(C₂O₄)(H₂O)₂]_n·4nH₂O (R=La (2), Nd (3)). X-ray powder diffraction analysis and single-crystal X-ray diffraction analysis reveal that they present two different structural types. MOFs 1, 4, 5, 6 and 7 are isostructural and crystallize in the orthorhombic non-centrosymmetric space group Iba2, and feature unusual fsc-3,4-Iba2 topology. MOFs 2 and 3 are isostructural and crystallize in monoclinic P2₁/c space group and display rare fsx-4,5-P2₁/c topology containing hydrophilic channels bounded by triple helical chains along a axis. MOFs 3, 4, 5, 6 and 7 exhibit intense lanthanide characteristic photoluminescence at room temperature.     

Preparation and reaction of cyclopropyltriphenylphosphonium salt

Cyclopropyltriphenylphosphonium bromide (2a) was conveniently prepared from 3-bromopropyltriphenylphosphonium bromide (1a). The Wittig reaction of cyclopropyltriphenylphosphonium bromide (2a) with carbonyl compounds gave alkylidenecyclopropanes (4, 6 and 7). Successive treatment of 1a with two equivalents of base and carbonyl compounds gave alkylidenecyclopropanes (4 and 5) without isolation of intermediary 2a. 2-Methylcyclopropyltriphenylphosphonium bromide (2b) was prepared and allowed to react with carbonyl compounds.     

Tetrathiafulvalene based electroactive ligands and complexes: Synthesis,crystal structures and antifungal activity

The synthesis of two tetrathiafulvalene-appended pyridinehydrazone pyrimidine ligands, namely (Z)-4-(2-((5-([2,2′-bi(1,3-dithiolylidene)]-4-yl)pyridin-2-yl)methylene) hydrazinyl)-6-chloropyrimidine L1 and (Z)-4-(2-((6-([2,2′-bi(1,3-dithiolylidene)]-4-yl)pyridin-2-yl)methylene) hydrazinyl)-6-chloropyrimidine L2 is described. Ligand L1 was reacted with cobalt(II) to yield a cationic metal complex [Co(L1)₂] while ligand L2 was reacted with zinc(II) to afford a neutral metal complex [ZnL2Cl₂]. The crystal structure analysis of [Co(L1)₂] indicate that Co(II) ion is coordinated by six nitrogen atoms from two perpendicular ligands while in [ZnL2Cl₂], Zn(II) is coordinated by two chlorine atoms and three nitrogen atoms. The electrochemical behavior indicate that ligands L1 and L2 and the zinc(II) complex are suitable fort the preparation of crystalline radical cation salts. Finally the determination of MIC₈₀ values against C. albicans, C. glabrata, C. parapsilosis, C. krusei and E. dermatitidis revealed that the cobalt(II) metal complex [Co(L1)₂] is active against all the studied fungi.     

Structure and properties of BETS salts: κ-(BETS)8(Cu2Cl6)(CuCl4), θ-(BETS)2(CuCl2) and (BETS)2(CuCl4)

κ-(BETS)₈(Cu₂Cl₆)(CuCl₄) (1), θ-(BETS)₂(CuCl₂) (2), (BETS)₂(CuCl₄) (3) (BETS = bis(ethylenedithio)tetraselenafulvalene) have been prepared by diffusion-electrocrystallisation of BETS and (AsPh₄)₂(Cu₂Cl₆) solutions in chlorobenzene–ethanol. 2 has also been obtained by simple diffusion of BETS and (AsPh₄)₂(Cu₂Cl₆) solutions. 1 and 2 exhibit metal-like behaviour, down to 40 K for 1 and 4 K for 2. 3 behaves as an insulator. The crystal structures of 1, 2, and 3 are determined by X-ray diffraction methods. The structures of 1 (at 140 and 25 K) and 2 are characterised by a strong disorder of their respective anions. The crystal structure of 3 shows an unusual packing of the BETS molecules, consisting of slipped stacked (BETS)₂ dimers, leading to insulating properties. Based on the structures of 1 (at 140 and 25 K), 2 and 3, molecular and band structure calculations are carried out for the interpretation of the physical behaviours of these phases.     

The synthesis of shihunine and related compounds from ortho-carboxyphenyl cyclopropyl ketone

Reaction of dicyclopropyl cadmium and phthalic acid monochloride monomethyl ester gives o-carboxyphenyl cyclopropyl ketone (2). Reaction of the methyl ester of 2 with methylamine gives 2 - methyl - 3 - hydroxy-3-cyclopropyl-1-isoindolinone (4b), which is converted by hydrogen halides in chloroform to the rearranged homoallylic halides 5a–c. Thionyl chloride in chloroform converts 2 to 3-(3-chloropropylidene) phthalide (7) which upon reaction with methylamine gives isoshihunine (8). Heating of keto acid 2 with aniline leads to N-phenyl-N-norshihunine (9), while upon heating of 2 with methylamine spiro [(1 - methylpyrrolidine) - 2 - 3′ - (2′ - methyl - 1′ - isoindolinone)] (10) is obtained. 10 is converted to shihunine (1) by 48% HBr solution. The mechanisms of the reactions are discussed.     

(Pyrazol-1-ylmethyl)pyridine palladium complexes: Synthesis,molecular structures,and activation of small molecules

Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1) and 2-(3,5-di-tert-butylpyrazol-1-ylmethyl)pyridine (L2) with either [PdClMe(COD)] or [PdCl₂(COD)] gave the mononuclear palladium complexes [PdCl₂(L1)] (1), [PdClMe(L1)] (2) [PdCl₂(L2)] (3) and [PdClMe(L2)] (4) in good yields. All compounds were characterized by NMR spectrometry, mass spectrometry, elemental analyses and also by single crystal X-ray crystallography for complexes 1, 3, and 4. The reaction of 2 with NaBAr₄ in NCMe gave the salt, [[PdMeNCMe(L3)]BAr₄ (5), in good yield. This salt was used as a catalyst to oligomerize ethylene at high pressures to branched polyethylene, but catalytic activity was low. The reaction of 2 with SO₂ and CO formed the respective insertion products [PdClS(O)₂Me(L1)] (6) and [PdClC(O)Me(L1)] (7).     

Three new two-dimensional metal-organic coordination polymers derived from bis(pyridinecarboxamide)-1,4-benzene ligands and 1,3-benzenedicarboxylate: Syntheses and electrochemical property

Three new metal-organic coordination polymers, [Co(3-bpcb)(1,3-BDC)]·H₂O (1), [Co(4-bpcb)(1,3-BDC)]·2H₂O (2) and [Cu(4-bpcb)(1,3-BDC)]₂·0.5(4-bpcb) (3), have been hydrothermally synthesized using N,N′-bis(3-pyridinecarboxamide)-1,4-benzene (3-bpcb) or N,N′-bis(4-pyridinecarboxamide)-1,4-benzene (4-bpcb) and 1,3-benzenedicarboxylate (1,3-H₂BDC) mixed ligands and characterized by elemental analyses, IR, TG, XRPD and single-crystal X-ray diffraction. Complexes 1–2 exhibit the similar two-dimensional (2D) network with different undulation degrees and dimensions, owing to different N positions from the 3-bpcb and 4-bpcb ligands. 1,3-BDC ligand in complexes 1 and 2 shows two coordination modes. The adjacent 2D layers for 1–2 are further linked by hydrogen bonding interactions to form a three-dimensional (3D) supramolecular network. Complex 3 possesses infinite 3-fold interpenetrating 2D network composed of three kinds of Cu-4-bpcb one-dimensional (1D) chains and 1,3-BDC ligands, in which 1,3-BDC only shows one coordination mode. The 2D network is further extended into 3D supramolecular framework by hydrogen bonding interactions. The non-coordinated 4-bpcb ligands existing in the 2D network connect with adjacent 2D layers through the hydrogen bonding interactions. In addition, the electrochemical behaviors and the fluorescence property of complexes 1–3 have been reported.     

Thio-acids—V : Some reactions of 3-oxo-1,2-dithioles

Diphenyldiazomethane with compound (1) gave dibenzoyl, while 2 and 3 gave the corresponding 3-oxo(2H)thiophenes 5. With copper-bronze 1 gave 2,2′-di-(thiobenzoate) (4), while 2 gave 2,7-diphenylthiepin (6a) and 2,5-diphenylthiophene (7a), but 3 gave only 2,5-di-(p-methoxyphenyl)thiophene (7b). With Grignard reagents 1 gave the corresponding methanol derivative 14, while 2 gave the thiobenzoylethylenes 13a and b, but 3 gave 2,7-di-(p - methoxyphenyl) - 4,4,5,5- tetraphenyl(4H)thiepin (15). The reaction mechanisms are discussed.     

A convenient ultrasound-promoted synthesis of some new thiazole derivatives bearing a coumarin nucleus and their cytotoxic activity

Successful implementation of ultrasound irradiation for the rapid synthesis of a novel series of 3-[1-(4-substituted-5-(aryldiazenyl)thiazol-2-yl)hydrazono)ethyl]-2H-chromen-2-ones 5a-h, via reactions of 2-(1-(2-oxo-2H-chromen-3-yl)ethylidene) thiosemicarbazide (2) and the hydrazonoyl halides 3(4), was demonstrated. Also, a new series of 5-arylidene-2-(2-(1-(2-oxo-2H-chromen-3-yl)ethylidene)hydrazinyl)thiazol-4(5H)-ones 10a-d were synthesized from reaction of 2 with chloroacetic acid and different aldehydes. Moreover, reaction of 2-cyano-N'-(1-(2-oxo-2H-chromen-3-yl)ethylidene)-acetohydrazide (12) with substituted benzaldehydes gave the respective arylidene derivatives 13a-c under the conditions employed. The structures of the synthesized compounds were assigned based on elemental analyses and spectral data. Also, the cytototoxic activities of the thiazole derivative 5a was evaluated against HaCaT cells (human keratinocytes). It was found that compound 5a possess potent cytotoxic activity.     

Fine structures of 8-G-1-(arylethynylselanyl)naphthalenes (G = H, Cl, Br): Factors to control the linear alignment of five GSe–CC–CAr atoms in crystals and the behavior in solution

8-G-1-(p-YC₆H₄CCSe)C₁₀H₆ [2 (G = Cl) and 3 (G = Br): Y = H (a), OMe (b), Me (c), F (d), Cl (e), CN (f) and NO₂ (g)] have been prepared and the NMR spectra measured, in addition to 1 (G = H). Structures have been determined by X-ray crystallographic analysis for 2b, 2e and 2g, which are all type B (B), where the Se–C_sp bond is placed in the naphthyl plane in B. The type is classified as A if the Se–C_sp bond is perpendicular to the naphthyl plane. Structures around the p-YC₆H₄ (Ar) group are pd (perpendicular) for Y = OMe (2b) and Cl (2e) and pl (planar) for Y = NO₂ (2g), where the Se–C_Nap bond is placed in the aryl plane in pl and perpendicular to the plane in pd. The 1b (A: pd) structure changes dramatically on going to 2b (B: pd) with G = Cl at the 8-position. The effect is called the G-dependence in 2. The G-dependence arises from the energy lowering effect of the n_p(Cl)σ^*(Se–C_sp) 3c–4e interaction. Structures are both (B: pd) for 1e and 2e and both (B: pl) for 1g and 2g. One may realize that the structures are unchanged by G = Cl in place of G = H for Y = Cl and NO₂ at a first glance. However, the B structures in 2e and 2g must be much more stabilized by the G-dependence of the n_p(Cl)σ^*(Se–C_sp) 3c–4e interaction or the GSe–C_sp–C_sp–C_sp2 5c–6e type interaction. The structures of 2 and 3 are examined in solution based on the NMR parameters. The results show that 2 and 3 behave very similarly to each other and the structures are predominantly B, with some equilibrium between pd and pl around the aryl groups in solution. Quantum chemical calculations support the observations.     

Preparation and reactivity of CpRu(OMe)PCy3 and CpRuHL2 (L = PCyPh2, PCy2H) from [CpRu(OMe)]2 and bulky phosphines

The reaction of [CpRu(OMe)]₂ (1) with PCy₃ yields the 16-electron alkoxo derivative, CpRu(OMe)(PCy₃) (2). 2 reacts with H₂ and HBF₄ to give the known CpRuH₃PCy₃ (3) and [CpRu(C₆H₉PCy₂)]BF₄ (4). The reaction of 1 with one or two equivalents of L yields CpRuHL₂ (L = PCyPh₂ (5), PCy₂H (6)) through a β-elimination process. Upon protonation, 5 and 6 are converted into [CpRuH₂L₂]BF₄ (L = PCyPh₂ (7), PCy₂H (8)).     

Synthesis, photophysical properties and sugar-dependent in vitro photocytotoxicity of pyrrolidine-fused chlorins bearing S-glycosides

Eight S-glycosylated 5,10,15,20-tetrakis(tetrafluorophenyl)porphyrins (1a′, 1b′, 1a and 1b (a: S-glucosylated, b: S-galactosylated)) and their 1,3-dipolar cycloadducts, i.e. chlorins 2a′, 2b′, 2a and 2b were prepared by nucleophilic substitution of the pentafluorophenyl groups with S-glycoside. These photosensitizers were characterized by ¹H, ¹³C and ¹⁹F NMR spectroscopies and elemental analysis. The photocytotoxicity of the S-glycosylated photosensitizers and the parent porphyrin (1) and chlorin (2) was examined in HeLa cells. Photosensitizers 1, 2, 1a′, 1b′, 2a′ and 2b′ showed no significant photocytotoxicity at the concentration of 0.5 μM, while the deprotected photosensitizers 1a, 1b, 2a and 2b were photocytotoxic. The strong inhibition by sodium azide of the photocytotoxicity of these photosensitizers suggested that ¹O₂ is the main mediator. The S-glucosylated photosensitizers 1a and 2a showed higher photocytotoxicity than S-galactosylated 1b and 2b, respectively. The cellular uptake of 1a and 2a increased up to 24 h, while that of 1b and 2b was saturated by 12 h.     

Copper(II) and zinc(II) complexes with 2-formylpyridine-derived hydrazones

In the present work 2-formylpyridine-para-chloro-phenyl hydrazone (H2FopClPh) and 2-formylpyridine-para-nitro-phenyl hydrazone (H2FopNO₂Ph) were obtained, as well as their copper(II) and zinc(II) complexes [Cu(H2FopClPh)Cl₂] (1), [Cu(2FopNO₂Ph)Cl] (2), [Zn(H2FopClPh)Cl₂] (3) and [Zn(H2FopNO₂Ph)Cl₂] (4). Upon re-crystallization in DMSO:acetone conversion of 2 into [Cu(2FopNO₂Ph)Cl(DMSO)] (2a) and of 4 into [Zn(2FopNO₂Ph)Cl(DMSO)] (4a) occurred. The crystal structures of 1, 2a, 3 and 4a were determined.     

The chemistry of N-benzylidene-1,4-phenylenediamine palladacycles: The crystal and molecular structure of the first tetranuclear palladacycle with bridging Ph2PCH2PPh2 ligands

The reaction of the tetranuclear halide-bridged complexes 1?2(a?d) with Ph₂PCH₂PPh₂ (dppm) or Ph₂PC(CH₂)PPh₂ (vdpp) in 1:2 molar ratio and NH₄PF₆ afforded the novel tetarnuclear palladacycles 3?6 (a, c, d) as 1:2 electrolytes with bridging diphosphine and halogen ligands. The structure of 4a has been determined by X-ray diffraction analysis, and represents the first example of a tetranuclear palladacycle with bridging dppm and halogen ligands. Reaction of 1?2(a?d) with (Ph₂PCH₂CH₂)₂PPh (triphos) in 1:2 molar ratio gave 7(a?d) bearing two pentacoordinated palladium atoms. The structure of 7a, as determined by X-ray diffraction analysis, shows the distorted square pyramidal geometry around the metal centers. Treatment of 1?2(a?d) with dppm, vdpp or Ph₂PN(Me)PPh₂ (dppma) in 1:4 molar ratio gave the dinuclear palladacycles 8?10(a?d) with a chelating diphosphine ligand at each metal center; further treatment of 9(a?c) with the nucleophiles pyrrolidine, piperidine, morpholine or 4-methyl-piperidine gave the Michael addition derivatives 11?12(a?c), 13b, 13c and 14c, promoted by the withdrawing effect of the palladacycle which activates the CCH₂ double bond.     

Synthesis and characterization of insoluble cobalt(II), nickel(II), zinc(II) and palladium(II) Schiff base complexes: Heterogeneous catalysts for oxidation of sulfides with hydrogen peroxide

The condensation reaction of 1,2-bis(2′-aminophenoxy)benzene with 2-pyridinecarbaldehyde in a mole ratio of 1:2 gives a new Schiff base ligand (L). Four Schiff base complexes, CoL(NO₃)₂ (1), NiLCl₂ (2), ZnL(NO₃)₂ (3) and Pd₂LCl₄ (4) have been prepared by direct reaction of the ligand (L) and appropriate metal salts. The Schiff base ligand (L) has been characterized by IR, ¹H NMR and ¹³C NMR spectroscopy and elemental analysis. Also, all complexes have been characterized by IR and XRD spectroscopy techniques and elemental analysis. The synthesized complexes have very poor solubility in all polar and non-polar solvents such as: H₂O, MeOH, EtOH, CH₃CN, DMSO, DMF, CHCl₃, CH₂Cl₂, THF, etc; therefore, they have been used as heterogeneous catalysts. Catalytic performance of the complexes was studied in oxidation of thioanisole using hydrogen peroxide (H₂O₂) as the oxidant. Various factors including the reaction temperature, amount of oxidant and catalyst amount were optimized. The palladium Schiff base complex, Pd₂LCl₄ (4), shows better catalytic activity than other complexes. Therefore, the Pd(II) Schiff base complex has been used as a catalyst for oxidation of different sulfides to their corresponding sulfones in acetonitrile with hydrogen peroxide as the oxidant. The palladium Schiff base complex, Pd₂LCl₄ (4), has shown a very good recyclability, up to five times, without any appreciable decreases in catalytic activity and selectivity.     

A new approach to synthesis of α-nitronyl nitroxide carboxylates. First metal complexes with α-imino nitroxide carboxylate

An effective procedure has been developed for the synthesis of the potassium salt of 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl-3-oxide-2-carboxylic acid K(1). The salts of 4,4,5,5-tetramethyl-1-oxyl-4,5-dihydro-1H-imidazole-2-carboxylic acid X(2) are kinetically much more stable than K(1). Due to this, a series of heterospin complexes Cu(2)₂(CH₃OH)₂, Cu(2)₂(H₂O)₂, Cu(2)₂, Co(2)₂(CH₃OH)₂, Ni(2)₂(H₂O)₂, Ni(2)₂(py)₂, and Ni(2)₂(dipy) · H₂O were isolated. For K(2), Cu(2)₂(CH₃OH)₂, and Ni(2)₂(py)₂, crystal and molecular structure has been determined. For Cu(2)₂ and Ni(2)₂(bipy), cooperative magnetic ordering has been recorded at 16.6 and 4.6 K, respectively.     

The diazo route to 2-vinylcyclopropylidenes

2-Vinylcyclopropylidene(2),3-methyl-2-vinylcyclopropylidenes(79,81)and2-(1-propenyl)cyclopropylidenes (95,97) were generated from the corresponding nitrosoureas in methanol at room temperature. The diazo route is initiated by the formation of 2-vinylcyclopropanediazonium ions (e.g.43) which do not undergo 1,3-carbon shifts. No cyclopentenyl products were found in weakly basic methanol where the diazonium ions prevail. Ring opening of the diazonium ions gives pentadienyl cations and products derived therefrom. Delocalisation of the pentadienyl cations was demonstrated by the distribution of deuterium and methyl labels. In the presence of strong base, 1-diazo-2-vinylcyclopropanes (e.g.48) arise by deprotonation of the diazonium ions. Rearrangement of 48 was excluded by independent generation of the potential product, 4-diazocyclopentene (103). Substantial quantities of 3-methoxycyclopentene (108) were obtained from 103, but not from 48. The 2-vinylcyclopropylidenes 2,79 and 95, arising by loss of nitrogen from the corresponding diazo compounds, undergo allene formation and Skattebøl rearrangement competitively. Cis-oriented methyl groups at either C-2(81) or C-2'(97) prevent the Skattebøl rearrangement. The cyclopentenylidenes 3 and 83 yield 4-methoxycyclopentenes (52,86) in excess over cyclopentadiens (4,84). In the presence of methyl vinyl ether, cycloaddition of 3 and electrophilic addition of 3-cyclopentenyl cation (51) occurred in a 1:14 ratio. Stereospecific formation of 52 indicates protonation of a ‘foiled carbene’ (3a) to give a bishomocyclopropenyl ion (51a). Our studies confirm that the various routes to 2-vinylcyclopropylidenes converge at the carbene stage.