Elimination Of Ethane Research Articles

A photoionization study of the ion-neutral complexes CH 3CH +CH 3 CH 2CH 3l and CH 3CH 2CH +CH 3.CH 3 in the gas phase: Formation, H-transfer and C-C bond formation between partners, and channeling of energy into dissociation

Photoionization mass spectrometry was used to investigate the dynamics of ion-neutral complex-mediated dissociations of the n-pentane ion (1). Reinterpretation of previous data demonstrates that a fraction of ions 1 isomerizes to the 2-methylbutane ion (2) through the complex CH3CH(+)CH 3 (·) CH2CH3 (3), but not through CH3CH(+)CH2CH 3 (·) CH3 (4). The appearance energy for C3Hin 7 (+) formation from 1 is 66 kJ mol(-1) below that expected for the formation of n-C3H 7 (+) and just above that expected for formation of i-C3H 7 (+) . This demonstrates that the H shift that isomerizes C3H 7 (+) is synchronized with bond cleavage at the threshold for dissociation to that product. It is suggested that ions that contain n-alkyl chains generally dissociate directly to more stable rearranged carbenium ions. Ethane elimination from 3 is estimated to be about seven times more frequent than is C-C bond formation between the partners in that complex to form 2, which demonstrates a substantial preference in 3 for H abstraction over C-C bond formation. In 1 → CH3CH(+)CH2CH3 + CH3 by direct cleavage of the C1-C2 bond, the fragments part rapidly enough to prevent any reaction between them. However, 1 → 2 → 4 → C4H 8 (+) + CH4 occurs in this same energy range. Thus some of the potential energy made available by the isomerization of n-C4H9 in 1 is specifically channeled into the coordinate for dissociation. In contrast, analogous formation of 3 by 1 → 3 is predominantly followed by reaction between the electrostatically bound partners.

Coordination Chemistry of Dimethylgold Halides with Bidentate Phosphorus and Arsenic Ligands – Revisited

AbstractDimethylgold(III) chloride and bromide, (Me2AuX)2, react with bis(diphenylphosphanyl)methane (dppm, molar ratio 1:2) to give the P‐monohapto complexes cis‐Me2AuX(dppm) (1a, b) in quantitative yields. The products are readily oxidized by air to yield the P‐hapto P′‐monoxides 2a, b. Treatment of 1a, b with AgNO3 affords an ionic nitrate [Me2Au(dppm)]2 [NO3−]2 (1c), which contains a dinuclear cation. A mixed bromide/nitrate (from 1b and half the equivalent amount of AgNO3) is thermally unstable and undergoes elimination of ethane to give an ionic tetranuclear gold(I) complex [(dppm)4Au4Br2]2+ [NO3−]2 (3b). – Treatment of (Me2AuCl)2 with CH2(AsPh2)2 (dpam, molar ratio 1:2) gives cis‐Me2‐AuCl(dpam) (1d), the arsenic analog of 1a, which is not sensitive to oxygen. – The reaction of bis(diphenylphosphanyl)‐amine (dppa) yields 1:1 P‐monohapto adducts Me2. AuX(dppa), which are in equilibrium with ionic chelated systems Me2Au(dppa)+ X− (1e, f). Complex 1e is readily oxidized by air to the P‐hapto P′‐monoxide cis complex 2e. Treatment of 1e with AgNO3 gives the ionic nitrate Me2. Au(dppa)+ NO3− (1g). – All compounds were identified by standard analytical and spectroscopic techniques. The crystal structures of 2a, 3b, 1d, and 2e were determined by single‐crystal X‐ray diffraction studies.

Heats of formation for the thienopyridine isomers and their 4-oxygenated derivatives

The electronic energies from HF 3–2lg*. HF/6-31gM and HF/6–3 lg* ab initio geometry optimization calculations are used with tabulated thermochemical data to obtain values for the gas-phase heats of formation of the quinoline, benzothiophene and thienopyridine isomers. Our approach is based on ring fusion reactions in which pairs of singlering aromatic molecules are fused with elimination of ethene. and on reactions in which the isolobal fragments CH and N exchange places in the fused two-ring species. For benzo[ b]thiophene and naphthalene our calculated values are about 8 kJ mol −1 greater than the accepted experimental values: it is reasonable to suppose that the accuracy of the whole set of calculations is uniform so our estimates for the species for which experimental data is lacking are probably good to better than 10kJ mol −1. Similar calculations for the tautomers of the three isomers of 4-hydroxythienopyridine correctly predict the preference of the [2,3- b)] isomer for the enol form, and of the [3,2- b] and [3,4- b] isomers for the keto form.

Gas‐phase nucleophilic reactions in tetraalkoxysilanes

AbstractThe gas‐phase reactions of F−, OH−·, OH−, EtO−, NH and EtNH− with Si(OEt)4 were investigated by Fourier transform ion cyclotron resonance in order to probe the intrinsic reactivity of substrates used in plasma chemical vapour deposition. Isotopic data and the nature of the product ions suggest that reactions proceed primarily by attack of the nucleophile on the silicon centre. The ensuing chemistry can be explained by the fragmentation pathways available to the [XSi(OR)4]− intermediate, which includes alkoxide displacement, internal nucleophilic displacement and cyclic elimination of ethylene in the ethoxy case. Pentacoordinated adducts are observed in the reactions with F− and in the symmetric alkoxide/tetraalkoxysilane reactions. The reaction of F− with Si(OMe)4 is also shown to produce a perplexing ion of unknown structure and identified as [FSi(OMe)2(OCH2)]−.

The effect of α-chloro substitution on the reactivity of β-functional silanes

Some compounds with the general formula ClCH 2Me 2SiCH 2CH 2Y have been prepared and their solvolysis rates determined. The compounds with YOH or OCH 3 underwent the normal solvolysis reaction involving elimination of ethene. The compound with YCl reacted but did not produce alkene.

A Convenient, Short Synthesis of Desethylchloroquine [7-Chloro-4-(4’-ethylamino-1’-methyl-butylamino)quinoline

A short, efficient 2-step synthesis of desethylchloroquine is achieved by generating an internal amide ion from the secondary nitrogen in chloroquine, followed by in situ reaction with 2,2,2-trichloroethyl chloroformate giving rapid elimination of ethylene. The carbamate thus produced easily undergoes deprotection to the target compound at room temperature.

A simple method for differentiating Leu and Ile in peptides. The negative‐ion mass spectra of [M  h]− ions of phenylthiohydantoin Leu and Ile

Phenylthiohydantoin (PTH) amino acid derivatives are formed during sequential Edman degradation of peptides and proteins. Isomeric Leu and Ile (which differ only in respectively having iso-butyl and sec-butyl chains, and are often difficult to distinguish by conventional mass spectrometric techniques) may be readily identified by the characteristic decompositions of the [M-H]- ions of their PTH derivatives. The Leu spectrum shows major loss of propane, while that of the Ile derivative shows elimination of both methane and ethane. This method may be used routinely with 10 microgram quantities of peptide material.

Ion–neutral complex formation and hydrogen‐shifts in the alkene loss from ionized alkyl phenyl thioethers in the gas phase

AbstractThe mechanistic aspects of alkene loss from ionized ethyl‐and n‐propyl phenyl thioethers have been studied by use of deuterium labelling and tandem mass spectrometry. Loss of ethene from the molecular ion of ethyl phenyl thioether proceeds predominantly by a specific H‐shift from the β‐position of the ethyl group to the remaining part of the ion. In contrast, deuterium labelling reveals that elimination of propene from the molecular ions of the n‐propyl phenyl and n‐propyl 2,6‐dimethylphenyl thioethers involves three competing reactions: (i) a 1,2‐hydride shift assisted cleavage of the bond between the propyl group and the sulfur atom yielding an ion–neutral complex of a thiophenyoxy radical and a secondary propyl carbenium ion, which reacts further by proton transfer prior to dissociation; (ii) a partially reversible 1,5‐H shift from the β‐position to the 2‐ or 6‐position of the ring; and (iii) a 1,3‐H shift from the β‐position to the sulfur atom. The preference for transfer of a hydrogen atom from the β‐position is particularly pronounce for the reactions of the metastable molecular ions of the n‐propyl phenyl and n‐propyl 2,6‐dimethylphenyl thioethers. This indicates that the critical energies of the processes involving H‐shifts are lower than the critical energy for propene loss with intermediate formation of an ion–neutral complex. In addition to ethene elimination, ionized ethyl phenyl thioether expels a HS+· radical. This reaction is preceded by an interchange between the hydrogen atoms at the α‐methylene group and the hydrogen atoms at the 2‐ and 6‐positions of the ring as observed also for ionized methyl phenyl thioether.

Competing dissociation of and bond formation between CH 3CHOH + and .CH 2CH 3 formed by bimolecular processes

The C 4H 10O .+ potential energy surface was accessed at several energies through different ion/molecule reactions. Reaction of CH 3CH .+ 3 with CH 3CHO and CH 3CHO .+ with CH 3CH 3 gave predominantly CH 3CHOH + + . CH 2CH 3 and small amounts of CH 3CH 2CHOH + + . CH 3. CH 3CH .+ 3 also produced a small amount of CH 3CHO +CH 3 + . CH 3 upon reaction with CH 3CHO. CH 2 = CHOH . + did not react with CH 3CH 3. CH 3CH 2OH . + reacted CH 2 = CH 2 and CH 2 = CH .+ 2 with CH 3CH 2OH to produce CH 3CH 2OH + 2 and CH 3CHOH +, but only the second pair of reactants produced detectable C 3H 7O + ions. CH 3CH 2CHO.+ +CH 4 produced only CH 3CH 2CHOH +. In all of the reactions examined, initial proton or H-transfer was much more often followed by simple dissociation than by C C bond formation or multiple H-transfers. This contrasts with the metastable decompositions of ionized 2-butanol, in which elimination of ethane and methane through the complexes [CH 3CHOH + .CH 2CH 3] and [CH 3CH 2CHOH + .CH 3] are important processes. This contrast is attributed to the ion/molecule reactions taking place in a higher energy regime than the metastable decompositions.

Gas‐phase reactivity of ambident thio‐enolate and oximate anions

AbstractThe gas‐phase chemical behaviour of ambident thio‐enolate anions from thioacetaldehyde and oximate anions from acetaldoxime and acetone oxime has been studied using the method of Fourier‐transform ion cyclotron resonance mass spectrometry. It appears that the thio‐enolate anions, generated by an efficient base‐induced E2 elimination of ethene from ethyl vinyl sulfide, exhibit a strong preference for addition via the sulfur rather than via the carbon nucleophilic center in the reactions with a series of unsaturated polyfluorocarbon compounds. This preferred addition via the sulfur nubleophilic center is considered to be promoted both by specific frontier orbital and by electgrostatic interactions between the reactants. Further, acetaldoxime and acetone oximate anions exclusively react via the oxygen nucleophilic center with the series of unsaturated polyfluorocarbon compounds. Electrostatic interactions are held responsible for the reaction selectivity of the oximate anions. This is in contrast with the corresponding enolate anions, for which it is considered that the previously studied ambident chemical behaviour is governed by specific frontier orbital interactions.

Efficient laser-induced generation and polymerization of the highly unsaturated compound diethynylsilene in the gas phase

The CO 2 laser photosensitized (SF 6) decomposition of 1,1-diethynyl-1-silacyclobutane in the gas-phase involves a clean elimination of ethene to give a transient highly unsaturated diethynylsilene, which undergoes very efficient polymerization to yield saturated macromolecules with a cross-linked structure. The results reveal the very large enhancement of the ease of triple CC bond polymerization for molecules containing proximal CC and SiCH 2 bonds.

The synthesis of mycophenolic acid

A new synthesis of mycophenolic acid 1, has been accomplished using silyloxy-1,3-cyclohexadiene 9 which undergoes cycloaddition to dimethyl acetylenedicarboxylate and subsequent elimination of ethylene (Alder-Rickert reaction) to give the trisubstituted dimethyl phthalate 11. After conversion of 11 to phthalide 16, the (E)-4-methyl-4-hexenoic acid side-chain was constructed via an orthoester Claisen rearrangement using allylic alcohol 19 and triethyl orthoacetate.

Oxidatively induced reductive eliminations. Kinetics and mechanism of the elimination of ethane from the 17-electron cation radical of rhodium complex Cp*Rh(PPh3)(CH3)2

The dimethylrhodium compound Cp[sup *]Rh(PPh[sub 3])(CH[sub 3])[sub 2] (1; Cp[sup *] = [eta][sup 5]-C[sub 5]Me[sub 5]) undergoes an overall 2-electron oxidation at 0.04 V vs the ferrocene/ferrocenium (Fc) couple in 9:1 acetonitrile/dichloromethane, 0.1 M Bu[sub 4]N[sup +]PF[sub 6][sup [minus]]. The oxidation results in the intramolecular reductive elimination of ethane and yields Cp[sup *]Rh(PPh[sub 3])(NCMe)[sub 2][sup 2+] (2) as the Rh-containing product. A mechanistic and kinetic investigation of the reaction by derivative cyclic voltammetry (DCV) showed the reductive elimination to be a first-order reaction with respect to 1[sup [sm bullet]+]. Very small solvent effects (acetonitrile, acetone, THF, dichloromethane) on the reaction rate indicated the nonintervention of 19-electron species in the reductive elimination. The DCV analysis provided kinetic parameters k(20[degrees]C) = 96 [plus minus] 3 s[sup [minus]1], [Delta]H[sup [double dagger]] = 53.1 [plus minus] 0.8 kJ/mol, and [Delta]S[sup [double dagger]] = [minus]25.9 [plus minus] 3.3 J/(K[center dot]mol) in acetonitrile. The data are interpreted in terms of a direct, concerted elimination of ethane from the 17-electron cation radical 1[sup [sm bullet]+]. It is estimated that the rate of reductive elimination has been increased by a factor of at least 3 [times] 10[sup 9] due to the oxidation of 1 to 1[sup [sm bullet]+].more » In dichloromethane, the intramolecular elimination of ethane was initiated by an overall 1-electron process. The major product in this case was Cp[sup *]Rh(PPh[sub 3])(CH[sub 3])X[sup +] (X most likely is coordinated dichloromethane or the counterion PF[sub 6][sup [minus]]) as a result of a secondary intermolecular methyl-transfer process. The methyl transfer was mimicked by a quantitative ([sup 1]H NMR) reaction between 1 and 2 which yielded Cp[sup *]Rh(PPh[sub 3])(NCMe)CH[sub 3][sup +] (3). 18 refs., 4 figs., 1 tab.« less

Methylene radical cation transfer from ionized oxirane to CH3SCH3 and C6H5SCH3 in the gas phase: Formation of sulphur distonic ions and/or insertion in a carbon–sulphur bond

AbstractThe α‐distonic sulphur‐containing ion \documentclass{article}\pagestyle{empty}\begin{document}$ {}^ \cdot {\rm CH}_2 \mathop {\rm S}\limits^ + \left({{\rm CH}_3 } \right)_2 $\end{document} has been generated by transfer of CH from ionized oxirane to dimethyl thioether and distinguished from the molecular ion of ethyl methyl thioether by collision induced dissociation (CID) experiments. In particular, the α‐distonic ion expels CH2 to a minor extent following collision, whereas the molecular ion of ethyl methyl thioether does not undergo this reaction. The metastable C3H8S+˙ ions formed by CH transfer to dimethyl thioether and ionization of ethyl methyl thioether decompose by competing losses of CH3R˙, CH4 and C2H4. The elimination of ethene is taken as evidence for isomerization of the α‐distonic ion to the molecular ion of ethyl methyl thioether prior to spontaneous dissociation. Evidence for the formation of stable α‐distonic sulphur‐containing ions by transfer of CH from ionized oxirane to methyl phenyl thioether has not been obtained. The collision‐induced and spontaneous reactions of the ions formed by CH transfer to methyl phenyl thioether indicate that a mixture of the radical cations of CH3C6H4SCH3, C6H5SCH2CH3 and C6H5CH2SCH3 is generated implying that attack on the phenyl group occurs in addition to a formal insertion of a methylene entity in a CS bond.

An ab initio molecular orbital study of the thermal decomposition mechanisms of ethylarsine and ethylgallane

Ab initio molecular orbital calculations were used to study the pyrolysis of ethylarsine and ethylgallane. Within the computational models selected, the dominant decomposition channel for ethylarsine was found to be the unimolecular elimination of ethane, which requires approximately 55.0 kcal/mol. However, an ethyl free radical mechanism requiring 56.0 kcal/mol was found to be competitive. The dominant decomposition channel for ethylgallane was found to be the β-elimination of ethylene, which has a barrier of only 35.8 kcal/mol

Four-centered concerted ethane elimination in the IR and UV laser photolysis of dimethyldisulfide. Real-time observation of S 2 and CH 3S radicals

CO 2 and KrF laser-induced dissociation of dimethyl disulfide opens two major primary dissociation channels. The moleculer elimination channel produces S 2 and C 2H 6 via a four-centered transition state whereas the radical channel generates two CH 3S radicals. The time-resolved UV absorption spectra of the transients CH 3S and S 2 are reported. From the real time formation of CH 3S radical, a unimolecular dissociation rate constant for dimethyl disulfide has been found to be 6.8 × 10 5 s −1 and ⩾ 1 × 10 7 s −1 in the IR and UV laser photolysis, respectively. In SF 6-sensitised dissociation of dimethyl disulfide, a time lag between the laser pulse and the onset of dissociation is observed, which is ascribed to intermolecular energy transfer from vibrationally excited SF 6 to the substrate.

Studien zur C–H‐Aktivierung, VIII, Hydrido(vinyl)rhodium(III)‐Komplexe aus [RhCl(PiPr3)2] und Olefinen

Studies on C–H Activation, VIII[1]. – Hydrido(vinyl)rhodium(III) Complexes from [RhCl(PiPr3)2] and OlefinsThe reaction of [RhCl(PiPr3)2]n (1) with methyl vinyl ketone leads to the formation of the olefin complex trans‐[RhCl(CH2 = CHC(O)Me)(PiPr3)2] (2) which rearranges at ambient temperatures to give the hydrido(vinyl)rhodium(III) isomer magnified image (3). From 1 and the α,β‐unsaturated ketones RCH = CHC(O)Me (R = MeO, Me, Ph) the corresponding hydrido(vinyl) derivatives magnified imagemagnified image (5–7) are directly obtained. Treatment of 1 with acroleine CH2 = CHC(O)H gives the olefin complex trans‐[RhCl(CH2 = CHC(O)H)(PiPr3)2] (8) that does not react by C–H activation but by elimination of ethylene to form the carbonyl compound trans‐[RhCl(CO)(PiPr3)2] (4). The reactions of 1 with CH2 = CHCO2Me, (E)‐ and (Z)‐CH(CO2Me) = CH(CO2Me) afford monomeric (10, 12) or dimeric (14) olefin‐rhodium(I) compounds from which 12 is transformed by UV irradiation to give the hydrido(vinyl) isomer magnified imagemagnified image (16). Structurally related hydrido(vinyl)rhodium(III) complexes magnified imagemagnified image (17) and magnified image(PiPr3)2] (20, 21) are obtained either directly from 1 and the α,β‐unsaturated carbonyl compound or in two steps via the olefinrhodium(I) precursor. The IR and NMR data of the hydrido(vinyl) compounds prove unambiguously that the vinyl ligand is coordinated by the carbon and the carbonyl oxygen atom.

Synthesis of halogeno, pseudohalogeno, and carboxylatopalladium(IV) complexes by halogen exchange. Crystal structure of azido(2,2′-bipyridyl)- benzylpalladium(II), formed on reductive elimination of ethane from Pd(N 3)Me 2(CH 2Ph)(bpy)

The first examples of fluoro, pseudobalogeno, and carboxylatopalladium(IV) complexes have been obtained by the exchange of the bromo ligand in PdBrMe 2(CH 2Ph)bpy) (bpy = 2,2′-bipyridyl) on addition of a silver salt, or silver nitrate and the appropriate anion, in acetonitrile. The syntheses have permitted a study of the effect of groups X on the selectivity in reductive elimination of ethane and ethylbenzene from PdXMe 2(CH 2Ph)bpy). For X  Br, N 3, SCN, or O 2CPh, reductive elimination from the unstable palladium(IV) complexes in acetone gives ethane and PdX(CH 2Ph)(bpy) only, but these products together with ca. 10% of ethylbenzene and PdXMe(bpy) are formed from the complexes PdXMe 2(CH 2Ph) (bpy) (X  F, Cl, I, OCN, SeCN, O 2CCF 3). The binding mode of the ambidentate pseudohalides OCN −, SCN − and SeCN − has not been determined.

Conformational studies in palladium (IV) and platinum (IV) chemistry. Crystal structure of the 1,1-bis(pyrazol-l-yl)- ethane complex fac-PtIMe 3(pz) 2CHMe-N, N ′

The reaction of iodomethane with the 1,1-bis(pyrazol-1-yl)ethane complex PtMe 2 (pz)2CHMe- N,N′ gives the platinum(IV) complex fac-PtIMe 3(pz) 2 CHMe- N,N′, which has been fully characterized, by, among other means, an X-ray structural study. The analogous palladium(IV) complex has also been prepared by a similar reaction in (CD 3) 2CO at — 30°C, and the two complexes exhibit similar 1H NMR spectra. For the palladium(IV) species, however, reductive elimination of ethane occurs above — 30°C to give PdIMe(pz) 2CHMe- N,N′. The 1H NMR spectra of the platinum(IV) and palladium(IV) complexes show that they are present in one configuration in solution, and large downfield shifts of the methine C HCH 3 resonance, compared with the same proton in MMe 2(pz) 2C4H Me- N,N′, are consistent with a conformation of the six-membered chelate ring that places the methine proton adjacent to the iodine atom. The octahedral platinum(IV) complex adopts this configuration in the solid state, with I ⋯ H = 3.1 Å The 1H NMR and structural studies support earlier assignments of solution behaviour for platinum(IV) and palladium(IV) complexes of related ligands, e.g. (pz)(py)CH 2 and (py) 2CHMe (py = pyridin-2-yl), and confirm the influence of hydrogen … halogen interactions on 1H NMR spectra of octahedral complexes of this type.

Reactivity of tervalent titanium compounds (.eta.5-C5Me5)2TiR (R = Me, Et): insertion versus .beta.-hydrogen transfer and olefin extrusion. Preparation of the paramagnetic titanium alkoxide, iminoacyl, acyl, vinyl, and azomethide complexes (.eta.5-C5Me5)2TiX and oxidation of these with PbCl2 to the diamagnetic tetravalent derivatives (.eta.5-C5Me5)2Ti(X)Cl

The paramagnetic, tervalent titanium alkyls Cp*2TiR (1, R = Me; 2, R = Et) were compared in their behavior toward a range of reactive molecules. These 15-electron, d1 systems appear to be weak Lewis acids, reluctant to form adducts. Only for 1 and Me3CC=N could an instable adduct Cp*2TiR.L be isolated. With active-hydrogen-containing substrates HX (X = O2C(H)Me2, OEt, C=CMe), Cp*2TiX and RH were produced. Polar, unsaturated molecules like Me3CN=C, CO, and paraformaldehyde inserted to give Cp*2Ti{eta-2-C(R)=NCMe3}, Cp*2Ti{eta-2-C(O)R}, and Cp*2TiOCH2R for both 1 and 2. Apolar unsaturated substrates did not insert into the Ti-C bond with the exception of MeC=CMe, which reacted with 1 to produce the vinyl compound Cp*2TiC(Me)=CMe2. A striking difference between 1 and 2 was found in their reaction with CO2, Me3CC=N, Me2C=O, and RC=CR1 (R, R1 = Me, Ph). While 1 either gave a normal insertion (CO2 and MeC=CMe) or adducts (Me2CO, Me3CC=N) or did not react (PhC=CPh), 2 lost ethene and gave compounds that appared to be products of insertion into a Ti-H bond, Cp*2TiO2CH, Cp*2TiN=C(H)CMe3, Cp*2TiOC(H)Me2, and Cp*2TiC(R)=C(H)R1. Facile beta-hydrogen transfer from the ethyl ligands was also observed in the reaction of 2 with olefins CH2=CHR (R = Me, Ph) to give ethene and Cp*2TiCH2CH2R. This reaction is reversible and equilibrium constants could be determined. Ground-state differences between 2 and Cp*2TiCH2CH2R were found to be 9 (2) (R = Me) and (2) kJ.mol-1 (R = Ph). Isotope-labeling experiments showed that liberation of ethene and formation of the new insertion products proceed via an intermediate hydride, Cp*2TiH. The kinetic preference for insertion of an unsaturated substrate into the Ti-H bond relative to insertion into the Ti-C bond, in combination with a rapid equilibrium between ethene elimination and reinsertion causes 2 to react in most cases as a hydride, and explains the striking difference in reactivity between 1 and 2. The products of 1 and 2 with various substrates were also characterized as their monochloride derivative Cp*2Ti(R)Cl after quantitative oxidation with PbCl2. Comparison of spectroscopic data gives information about the specific coordination of ligand R in both Cp*2TiR and Cp*2Ti(R)CI and shows that the Lewis acidity of the metal center increases substantially on oxidation to Ti(IV).