(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).     

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.     

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.     

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.     

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.     

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.     

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),

Full-size image (3K)

(≡ 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.     

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.     

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.     

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.     

Homochiral (R)- and (S)-1-heteroaryl- and 1-aryl-2-propanols via microbial redox

Preparation of various heteroaryl propanols 2a–g and of the corresponding propanones 3a–g as starting materials for microbial redox is described. The kinetic resolution of the racemic propanols 2a–g is obtained via oxidation with Pseudomonas paucimobilis and Bacillus stearothermophilus [(R)-alcohols, ee 74–100%]. Similar results are achieved with 3-(2-hydroxypropyl)trifluoromethylbenzene 7 (44%, ee 100% of the (R)-alcohol 6). The reduction of the propanones 3a–d and 3g with baker's yeast and other fungi gives the (S)-alcohols (ee 100%). The pure (S)-alcohols are also obtained by reduction of 1-[3-(trifluoromethyl)phenyl]-2-propanone 7. 1-[(4,4-Dimethyl)-2-(Δ²)oxazolinyl]-2-propanone 3e and 1[2-(Δ²)-thiazolinyl)-2-propanone 3f are not reduced. The heterocyclic rings of (S)-5-(2-hydroxypropyl)-3-methylisoxazole 2d and of (S)-2-(2-hydroxypropyl)-4-methylthiazole 2g are deblocked to the homochiral enamino ketone 8 (78%) and to the protected β-hydroxy aldehyde 9 (73%), respectively. The (R)-3-(2-hydroxypropyl)trifluoromethylbenzene 6 is transfomed into the homochiral precursor of (S)-fenfluramine 10 (overall yield 65%).     

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.     

Chiral chelate phosphanes: XI. Application of cyclopentane-based C2 chiral bis(phosphane) ligands C5H8(PR2)2 to Pt---Sn-catalyzed styrene hydroformylation

Treatment of [(1,5-C₈H₁₂)PtCl(X)] (X=Cl, CH₃, CH₂CMe₃) with C₂ chiral cyclopentane-1,2-diyl-bis(phosphanes) C₅H₈(PR₂)₂, either as racemic mixtures or as resolved enantiomers, afforded the chelate complexes [C₅H₈(PR₂)₂Pt(Cl)(X)] (X=Cl: R=Ph (1), N-pip (2), OPh (3); X=CH₃: R=Ph (4), N-pip (5), OPh (6); X=CH₂CMe₃: R=Ph (7), N-pip (8), OPh (9); ‘N-pip’=N(CH₂)₅), (+)-[(1R,2R)-C₅H₈{P(OPh)₂}₂PtCl₂] [(R,R)-3], (−)-[(1S,2S)-C₅H₈{P(OPh)₂}₂PtCl₂] [(S,S)-3], (−)-[(1R,2R)-C₅H₈(PPh₂)₂Pt(Cl)(X)], and (+)-[(1S,2S)-C₅H₈(PPh₂)₂Pt(Cl)(X)] (X=CH₃: (R,R)-4, (S,S)-4; X=CH₂CMe₃: (R,R)-7, (S,S)-7). Reacting 4, 6, and 7 with AgO₃SCF₃ led to triflate derivatives [C₅H₈(PR₂)₂Pt(X)(OSO₂CF₃)] [X=CH₃: R=Ph (11), OPh (12); X=CH₂CMe₃: R=Ph (13)] with covalently bonded OSO₂CF₃ ligands. The unusual Pt₂ complex [μ-Cl{C₅H₈(PPh₂)₂PtCH₃}₂]O₃SCF₃ (14) containing an unsupported single Pt---Cl---Pt bridge was also isolated. In the presence of SnCl₂, complexes 1, 3, 4, 6, 7, and 9 are catalysts for the hydroformylation of styrene forming 2- and 3-phenylpropanal together with ethylbenzene. Except for 1, they also catalyze the consecutive hydrogenation of the primary propanals to alcohols. High regioselectivities towards 2-phenylpropanal (branched-to-normal ratios ≥91:9) were obtained in hydroformylations catalyzed by 3 and 4, for which the influence of varied CO/H₂ partial pressures, catalyst-to-substrate ratios and different reaction temperatures and times on the outcome of the catalytic reaction was also studied. When tin-modified complexes (R,R)-3, (S,S)-3, and (S,S)-4 were used as optically active Pt(II) catalysts, an only low stereoselectivity for asymmetric hydroformylation (e.e.<18%) was observed. The Pt---Sn complexes [C₅H₈(PR₂)₂Pt(CH₃)(SnCl₃)] [R=Ph (15), OPh (17)], resulting from SnCl₂ insertion into the Pt---Cl bonds of 4 or 6, undergo rapid degradation in solution, forming mixtures composed of [C₅H₈(PR₂)₂Pt(X)(Y)] with R=Ph or OPh and X/Y=Cl/SnCl₃ (16, 18), Cl/Cl (1, 3), and SnCl₃/SnCl₃ (19, 20), respectively. In the presence of SnCl₂, triflate complex 11 also becomes a catalyst for styrene hydroformylation and consecutive hydrogenation of the aldehydes to alcohols. The crystal structures of 11 complexes — 2, 5, 7, 8, 9, 10 (the previously prepared [C₅H₈{P(N-pip)₂}₂Pt(CH₂CMe₃)₂]), 13, 14, 16, (R,R)-3, and (S,S)-3 — were determined by X-ray diffraction.     

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)).     

Photobleaching of 5,10,15,20-tetrakis(m-hydroxyphenyl)porphyrin (m-THPP) and the corresponding chlorin (m-THPC) and bacteriochlorin(m-THPBC). A comparative study

The photobleaching of compounds of the 5,10,15,20-tetrakis(m-hydroxyphenyl)porphyrin series at different reduction levels (m-THPP 1, m-THPC 2, m-THPBC 3) has been studied in methanol and in methanol–water (3:2, v/v) using an argon laser (514 nm) by observing the diminution of absorbance of band I (for 2 and 3) and of band IV (for 1) with time. Under the conditions studied here, true photobleaching only occurs for m-THPC (2) and m-THPBC (3), with photomodification being the major process for m-THPP (1). The rates for the photobleaching of 2 and 3 are presented in different solvents. The photobleaching rate of the bacteriochlorin 3 is found to be 90 times higher than that of the chlorin 2 in methanol–water (3:2, v/v). Singlet oxygen appears to be the reactive species responsible for the photobleaching of 2 and 3 and the photomodification of 1.     

An experimental and theoretical investigation on hypervalent organoselenium(II) halides: Case study of [2-(Et2NCH2)C6H4]SeX (X = Cl,Br, I)

Cleavage of the Se–Se bond in [2-(Et₂NCH₂)C₆H₄]₂Se₂ (1) with SO₂Cl₂ (1:1 molar ratio) yielded the organoselenium(II) chloride [2-(Et₂NCH₂)C₆H₄]SeCl (2). Treatment of 2 with excess of KX yielded the organoselenium(II) halides [2-(Et₂NCH₂)C₆H₄]SeX [X = Br (3), I (4)]. The new compounds 2–4 were characterized by solution NMR spectroscopy (¹H, ¹³C, ⁷⁷Se, 2D experiments). The solid-state molecular structures of 2, 2·HCl and 3 were established by single crystal X-ray diffraction. Distorted T-shaped coordination geometries of type (C,N)SeX (X = Cl, Br) and CSeCl₂ were found for the neutral halides 2 and 3, and the zwitterionic species [2-{Et₂N⁺(H)CH₂}C₆H₄]SeCl₂￣ (2·HCl), respectively. DFT calculations were performed on 2–4 and the related tellurium compounds [2-(Et₂NCH₂)C₆H₄]TeX [X = Cl (5), Br (6) and I (7)] in order to elucidate the bond nature and FT-Raman features of this class of organochalcogen(II) derivatives.     

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.