Vinylidene Cation Research Articles

Cyclopropenylmethyl Cation: A Concealed Intermediate in Gold(I)-Catalyzed Reactions.

The last years have witnessed many gold‐catalyzed reactions of alkynes. One of the most prominent species in the reaction of two alkyne units is the vinyl‐substituted gold vinylidene intermediate. Here, we were able to show that the reaction of a haloacetylene and an alkyne proceeds via a hitherto overlooked intermediate, namely the cyclopropenylmethyl cation. The existence and relative stability of this concealed intermediate is verified by quantum chemical calculations and 13C‐labeling experiments. A comparison between the cyclopropenylmethyl cation and the well‐known vinylidene intermediate reveals that the latter is more stable only for smaller cycles. However, this stability reverses in larger cycles. In the case of the smallest representative of both species, the vinylidene cation is the transition state en route to the cyclopropenylmethyl cation. The discovery of this intermediate should help to get a deeper understanding for gold‐catalyzed carbon–carbon bond‐forming reactions of alkynes. Furthermore, since enynes can be formed from the cyclopropenylmethyl cation, the inclusion of this intermediate should enable the development of new synthetic methods for the construction of larger cyclic halogenated and non‐halogenated conjugated enyne systems.

Ruthenium-Catalyzed Oxidative Amidation of Alkynes to Amides.

Complex CpRuCl(PPh3)2 catalyzes reactions of terminal alkynes with 4-picoline N-oxide and primary and secondary amines to afford the corresponding amides. The reactions occur in chlorinated solvent and aqueous medium, showing applications in peptide chemistry. Stoichiometric studies reveal that the true catalysts of the processes are the vinylidene cations [CpRu(═C═CHR)(PPh3)2]+ which are oxidized to the Ru(η2-CO)-ketenes by the N-oxide. Finally, nucleophilic additions of primary and secondary amines to the free ketenes yield the corresponding amides.

Ruthenium(II) pentamethylcyclopentadienyl half-sandwich carbene complexes with polypyridyl ligands

Polypyridyl complexes [RuCl(η5-C5Me5)(N–N)] (1–4) [N–N 2,2′-bipyridine (bpy) (1), 4,4′-dimethyl-2,2′-bipyridine (4,4′-Me2bpy) (2), 1,10-phenanthroline (phen) (3) and 4,7-dimethyl-1,10-phenanthroline (4,7-Me2phen) (4)] were prepared by reacting [RuCl2(η5-C5Me5)]2 with Zn dust in the presence of polypyridyl ligands. Unlike the related [RuCl(η5-C5Me5)(L1)(L2)]+ [L1, L2 = P(OR)3, PPh3] derivatives with P-donor ligands, treatment of chloro compounds 1–4 with diazoalkanes Ar1Ar2CN2 afforded polypyridyl carbene complexes [Ru(η5-C5Me5)(CAr1Ar2)(N–N)]BPh4 (5–8) [Ar1 = Ar2 = Ph (a), Ar1 = Ph, Ar2 = p-tolyl (b)]. Comparative DFT studies of the formation of metallacarbenes from aromatic diazoalkanes are reported. Phosphite complex [Ru(η5-C5Me5)(bpy){P(OEt)3}]BPh4 (9) and acetylide derivatives [Ru(η5-C5Me5)(CCAr)(bpy)] (10, 11) [Ar = Ph (10), p-tolyl (11)] were also obtained by substitution of the carbene in 5–8. Unstable vinylidene cations [Ru(η5-C5Me5){CC(H)Ar](bpy)]+ (12, 13) were formed by protonation with HBF4•Et2O of acetylide species 10, 11. The complexes were characterized spectroscopically (IR and 1H, 13C, 31P NMR) and by X-ray crystal structure determination of [Ru(η5-C5Me5){C(Ph)(p-tolyl)}(bpy)]BPh4 (5b).

Rapid Markovnikov addition of HCl to a pendant alkyne: evidence for a quinoidal cumulene

Reaction of cis-[RuCl2(dppm)2]BF4 with TlBF4 and 1,4-diethynyl-benzenes results in the formation of the vinylidene cations trans-[Ru([double bond, length as m-dash]C[double bond, length as m-dash]CH-C6H2-2,5-R2-4-C[triple bond, length as m-dash]CH)Cl(dppm)2](+) (R = H, Me). Subsequent reaction with [N(n)Bu4]Cl results in nucleophilic attack at the coordinated organic ligand, but not at the expected metal-bound carbon atom. Instead, trans-[Ru(C[triple bond, length as m-dash]C-C6H2-2,5-R2-4-CCl[double bond, length as m-dash]CH2)Cl(dppm)2] was generated which, when coupled with DFT calculations, provides evidence for an intermediate quinoidal cumulene complex.

Preparation of half-sandwich ethylene complexes of Osmium(II)

Ethylene complexes [OsCl(η6-p-cymene)(η2-CH2CH2)L]BPh4 (1, 2) [L = P(OMe)3, P(OEt)3, PPh(OEt)2, tBuNC) were prepared by allowing the dichloro compounds OsCl2(η6-p-cymene)L to react with CH2CH2 in mild conditions (1 atm). Treatment of 1 with amine or hydrazine yielded [OsCl(p-tolyl-NH2)(η6-p-cymene){PPh(OEt)2}]BPh4 (3) or [OsCl(η6-p-cymene)(CH3NHNH2){PPh(OEt)2}]BPh4 (4) derivatives. The vinylidene cation [OsCl{CC(H)Ph}(η6-p-cymene){PPh(OEt)2}]+ (5+) was also prepared from 1 and PhCCH. The complexes were characterized spectroscopically and by X-ray crystal structure determination of [OsCl(η6-p-cymene)(η2-CH2CH2){P(OMe)3}]BPh4 (1a). The nature of the Os–ethylene bond was investigated from a computational point of view.

Substitution of Tetracyanoethene by Ethynyl–Metal Complexes Gives Tricyanovinylethynyl (Tricyanobutenynyl) Derivatives: Syntheses, Protonation, and Addition of Metal–Ligand Fragments

A series of complexes containing the new tricyanovinylethynyl (3,4,4-tricyanobut-3-en-1-ynyl) ligand have been obtained by substitution of a CN group in tetracyanoethene upon reaction with the ethynyl complexes M(C≡CH)(PP)Cp′ (M = Ru, Os, (PP)Cp′ = (PPh3)2Cp; M = Ru, PP = dppe, Cp′ = Cp, Cp*). The reactions proceed in higher yield as the metal environment becomes more sterically hindered, the normal [2 + 2]-cycloaddition/ring-opened product M{C[═C(CN)2]CH═C(CN)2}(PP)Cp′ also being formed in some cases. The diynyl complex Ru(C≡CC≡CH)(dppe)Cp* reacts with tcne to give only the ring-opened adduct Ru{C≡CC[═C(CN)2]CH═C(CN)2}(dppe)Cp*. Protonation (HBF4 or HPF6) of Ru{C≡CC(CN)═C(CN)2}(dppe)Cp* afforded the vinylidene cation [Ru{═C═CHC(CN)═C(CN)2}(dppe)Cp*]+. A second transition-metal fragment MLn (MLn = Ru(PPh3)2Cp, M′(dppe)Cp* (M′ = Ru, Os), RuCl(dppe)2) can be added to the CN group trans to the metal center; electrochemical, spectroscopic, and computational studies indicate that there is little ground-state d...

Preparation of Half-Sandwich Alkoxycarbene Complexes of Osmium(II)

Alkoxy-alkylcarbene complexes [OsCl{═C(OR′)CH2R′′}(η6-p-cymene)L]BPh4 (1−4) [R′ = Me, Et; R′′ = Ph, p-tolyl, But; L = P(OMe)3, P(OEt)3, PPh(OEt)2, PPh2OEt] were prepared by allowing dichloro compounds OsCl2(η6-p-cymene)L to react with terminal alkyne R′′C≡CH in alcohol. Ethoxy-methylcarbene [OsCl{═C(OEt)CH3}(η6-p-cymene)L]BPh4 (5) was also prepared from reaction with trimethylsilyl acetylene. A reaction path for the formation of compounds 1−5, involving the nucleophilic attack of alcohol on intermediate vinylidene complexes, is discussed. Acetylide derivatives OsCl(C≡CAr)(η6-p-cymene)L (6, 7) [Ar = Ph, p-tolyl; L = P(OMe)3, P(OEt)3, PPh(OEt)2] were prepared by reacting dichloro compounds OsCl2(η6-p-cymene)L with lithium acetylide (Li+[ArC≡C]−) in thf. Protonation reaction with Brønsted acids of 6 and 7 led to vinylidene cations [OsCl{═C═C(H)Ar}(η6-p-cymene)L]+. Complexes OsCl2(η6-p-cymene)L also reacted with both PhC≡CH and (CH3)3SiC≡CH in the presence of H2O to give alkyl-carbonyl derivatives [Os(η1-CH2Ph)(CO)(η6-p-cymene)L]BPh4 (8) and [Os(η1-CH3)(CO)(η6-p-cymene)L]BPh4 (9). The complexes were characterized spectroscopically (IR and 1H, 13C, 31P NMR) and by X-ray crystal structure determinations of [OsCl{═C(OEt)CH2Ph}(η6-p-cymene){PPh(OEt)2}]BPh4 (1c), [OsCl{═C(OEt)CH2Ph}(η6-p-cymene)(PPh2OEt)]BPh4 (1d), and [Os(η1-CH2Ph)(CO)(η6-p-cymene){PPh(OEt)2}]BPh4 (8c).

Vinylidene-acetylene cation isomerization investigated by large scale ab initio calculations

The planar isomerization routes of the vinylidene/acetylene cation in the lowest electronic states are accurately examined for the first time, by using large scale MRCI and CCSDT calculations in a complementary way. They are compared with the similar calculations performed for the neutral ground state isomerization. An accurate value of the adiabatic ionization potential of vinylidene (11.26 eV) is predicted. The vinylidene cation lowest state, 1 2A1, follows an almost flat pathway with a shallow secondary minimum on the 1 2A' potential energy surface, before suddenly dropping to the stable acetylene cation ground state, X 2Piu. It is therefore confirmed to be completely unstable with respect to isomerization. The first excited state of the vinylidene cation, 1 2B1, which also correlates with the 2Piu ground state of acetylene cation along a 2A' isomerization route, has been studied at the same level of calculation. This 1 2B1 state is lying only 0.15 eV above the 1 2A1 state, and exhibits a potential energy barrier of 0.55 eV which explains the earlier assignment of this symmetry to the ground state of vinylidene cation. In addition to large scale calculations, a comprehensive description of the important steps of isomerization drawn from a very simple model involving monoconfigurational states is presented. In particular, the behavior of one unique orbital, namely, the 5a1 outer molecular orbital, is shown to completely govern the molecular geometry and energy evolution along the isomerization route of the ground state cation C2H2+.

Bis(alkynyl) and alkynyl–vinylidene iron(II) complexes with monodentate phosphite ligands

The bis(alkynyl) derivatives [Fe(CCR)2L4][R = Ph, p-tolyl or But; L = P(OMe)3, P(OEt)3 or PPh(OEt)2] were prepared and their protonation and methylation reactions with HBF4 and CF3SO3Me afforded alkynyl–vinylidene cations [Fe(CCR){CC(H)R}L4]+ and [Fe(CCR){CC(Me)R}L4]+, respectively. The aryldiazovinylidene [Fe(CCR){CC(NNC6H4Me-p)Ph}{P(OEt)3}4]BPh4 was also prepared. The complexes were characterized by infrared and 1H, 31P and 13C NMR spectra and the crystal structure of [Fe(CCPh){CC(H)Ph}{P(OEt)3}4]BF4 has been determined. The reactivity of the new vinylidene complexes was studied and showed the rearrangement in solution of the [Fe(CCR){CC(H)R}L4]+ cations to enynyl [Fe(η3-RC3CHR)L4]+ derivatives only in the case of LPPh(OEt)2. Deprotonation with base giving [Fe(CCR)2L4] as well as substitution of the vinylidene ligand in [Fe(CCR){CC(H)R}L4]+ cations by CO and CNC6H4Me-p giving [Fe(CCR)-(CO){P(OEt)3}4]+ and [Fe(CCR)(p-MeC6H4NC){P(OEt)3}4]+ derivatives are also discussed.

Theory and experiment in accord: the vinylidene radical cation

Using ab initio calculations based on electronic structure theory, the activation barrier of the 2B 1 ground state of the vinylidene cation radical 3 for rearrangement to the 2Π u acetylene ground state ion 2 is predicted to be 9.4 kcal mol −1 [MP4/6-311G(d,p)//HF/6-31G(d)]. At the same level of theory, the 2A 1 excited state of H 2CC + 4 is calculated to rearrange without a barrier to 2. In contrast to the doublet states, the activation barrier for hydrogen migration of the lowest-lying 4A 2 quartet state of H 2CC + 5 is predicted to be very high (57.2 kcal mol −1). It is concluded that the experimentally observed long-living vinylidene cation radical is probably 5.

Activation of a µ3-ethylidyne ligand through oxidation–deprotonation: X-ray structure of [Ru3(µ3-CMe)(µ-CO)3(η-C5Me5)3][BF4

Oxidation of the µ3-ethylidyne complex [Ru3(µ3-CMe)(µ-CO)3(η-c5Me5)3] yields [Ru3(µ3-CMe)(µ-CO)3(η-C5Me5)3]+(characterized by X-ray diffraction) and [Ru3(µ3-CMe)(µ-CO)3(η-C5Me5)3]2+; the latter deprotonates to give the µ3– vinylidene cation [Ru3(µ3-CCH2)(µ-CO)3(η-C5Me5)3]+ which reacts with methyl-lithium to form [Ru3(µ3-CEt)(µ-CO)3(η-C5Me5)3], completing a µ3-Cme th µ3-CEt homologaion.

The generation and identification of the transient vinylidene cation, [H 2CC] +.

The vinylidene cation, [CH 2C] +., was generated as a transient species by collisionally induced charge reversal of the corresponding anion. The collisionally induced mass spectrum of the transient [CH 2C] +. is characteristically different from that of ionised acetylene.