Electron spin resonance studies of the anaerobic and aerobic photolysis of alkylcobaloximes

Aerobic and anaerobic photolysis of methyl(pyridine)cobaloxime, benzyl(pyridine)cobaloxime and analogous compounds in CHCl₃ results only in an electron transfer reaction from an equatorial ligand producing photo-reduction of Co^III to Co^II, the complex retaining its axial ligands.If after the anaerobic photolysis of benzyl(pyridine)cobaloxime the oxygen is introduced without any further photolysis we obtain an ESR spectrum of nitroxide, arising from the attack of a benzyl radical on the dimethylglyoxime equatorial ligand.For the other complexes, homolytic cleavage of the CoC bond occurs and in the presence of oxygen gives rise to the superoxide cobalt complex adduct Py(Co^IIIO₂^?.During photolysis of methyl(pyridine)cobaloxime in isopropanol homolytic cleavage of the CoC bond occurs in preference to electron transfer reaction from the equatorial ligands.The anaerobic photolysis of benzyl(pyridine)cobaloxime in isopropanol or in water at 113–133 K results in an electron transfer reaction. However, at 170 K we observe the formation of the Co^II complex arising from CoC bond cleavage.A mechanism for photo-induced insertion of oxygen in the CoC bond is proposed.     

Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides

The reaction of some organometallic complexes with the surfaces of inorganic oxides leads to the formation of surface organometallic complexes, chemically bound to the surface yet retaining many features of their molecular structure. These surface organometallic complexes can therefore be considered to belong to both the molecular and solid states. In cases where such complexes have been structurally characterised, their reactivity can be interpreted with molecular concepts. In this review article, the stoichiometric and catalytic reactivity of some relatively well-defined surface organometallic fragments is surveyed. Many elementary steps which have precedent in molecular organometallic chemistry and homogeneous catalysis have now been demonstrated with surface organometallic fragments, including reversible ligand binding, oxidative addition, reductive elimination, protonation, heterolytic metal—carbon bond cleavage, electrophilic CH bond activation and insertion into metal—carbon bonds. In some cases, the supported organometallic complexes are highly effective low temperature catalysts, a phenomenon which is not always observed with molecular analogues nor with conventionally prepared heterogeneous catalysts. Applications of surface organometallic chemistry to catalytic alkane hydrogenolysis, olefin isomerisation and hydrogenation, the Fischer—Tropsch synthesis and the water—gas shift reaction are discussed. Proposed mechanisms for several representative catalytic cycles are presented.     

Novel carboncarbon bond formation induced by depalladation of organometallic compounds

Two internal alkynes undergo insertion at 20°C into the PdC bond of the cyclopalladated derivative of dimethylaminomethylferrocene to give new organometallic compounds. When the reaction with diphenylacetylene is performed at higher temperatures, depalladation occurs readily to give six- and seven-membered ortho-fused rings through new annulation reactions of phenyl groups, formation of one of these involving also the cleavage of a CN bond.     

Ring-opening reactions of spiro[2.4]hepta-4,6-diene and spiro[4.4]nona-1,3-diene induced by tungsten(0) carbonyls

Tungsten(0) carbonyls react with the strained spiro[2.4]hepta-4,6-diene and the less strained spiro[4.4]nona-1,3-diene with CC bond cleavage and formation of stable alkylene bridged π-cyclopentadienyl-σ-alkyl complexes. The product containing a two carbon bridge has the same unusual spectroscopic properties as the analogous molybdenum complex.     

Halogenation of thenyl and furyl cobaloximes

The reactions of thenyl and furyl cobaloximes with halogens (bromine and chlorine) in acetic acid and chloroform indicate that, CoC bond cleavage occurs in 2-thenyl and 2-furyl case whereas the ring substitution is faster than CoC band cleavage in 3-thenyl and 3-furyl cobaloximes.     

Acid-catalysed hydrolysis of N-sulphonyl sulphilimines—II

The basicity and the acid-catalysed hydrolysis of ph(R)SNTs and o-HC₆H₄(Me)SNTs sulphilimines have been studied by UV spectrophotometric and kinetic methods, respectively, in aqueous HClO₄ (1–10 M) and 1:1 (v/v) EtOH/H₂O-HClO₄ (0.5–6 M). Depending on the constitution of the substrates, sulphilimine hydrolysis can follow three different courses, according to rate-acidity profiles, Bunnett-Olsen's treatment, activation parameters and product analysis. Most typical for sulphilimines is S_N2 hydrolysis with S^IV-N bond cleavage. In this case the reaction starts with the nucleophilic addition of water and is promoted by acid-base catalysis. If a relatively stable carbenium ion can be formed from R group, an S_N1 reaction with S^IVC bond cleavage takes place. Sulphilimine with X = o-CO₂H due to neighbouring-group participation hydrolyses very rapidly via acyloxy-sulphurane and acyloxy-sulphonium ion intermediates with five-memembered ring (S_Ni reaction involving S^IVN bond cleavage).     

Cleavage of silicon- and germanium-transition metal bonds. Dependences of the stereochemistry on the nature of the ligands and the geometry of complexes

The cleavages of some new optically active complexes containing CoSi (orGe), MnSi (orGe), ReGe and WGe bonds are described. Electrophiles cleave the CoSi bond with good retention of configuration at silicon, while the MnSi bond is not cleaved under the same conditions. The M′Si and M′Ge bonds (where M'  transition metal) are cleaved by nucleophiles with retention or inversion of configuration. In the case of triginal bipyramidal geometry (cobalt complexes) the stereochemical outcome of the reaction is strongly dependent upon electronic effects, the size of the ligand trans to the CoSi (orGe) bond, and the nature of the nucleophilic reagant, in accord with the general rules for nucleophilic substitution at silicon. In contrast the transition metalsilicon orgermanium bonds in the octahedral complexes of manganese, rhenium and tungsten are always cleaved with poor retention of configuration regardless of the nature of the ligands or the nucleophilic reagent. The results provide the first cases in which the stereochemistry of nucleophilic displacement at silicon is independent of the electronic features of both the leaving group and the nucleophile.     

Organometallic compounds in organic synthesis—part 17: Reactions of tricarbonylcyclohexadienyliron salts with o-silylated enolates,allyl silanes and aspects of their synthetic equivalents

Efficient CC bond formation results during reactions of O-silylated enolates and allytrialkysilanes with a range of tricarbonylcyclohexadienyliron salts to give tricarbonylcyclohexadieneiron complexes in good to excellent yields. This represents a new and efficient type of conversion of aldehydes, ketones, esters and lactones, through enol TMS ethers, into synthetically useful products. An advantage of the allylsilane process is that CC formation occurs at the end of the double bond remote from the silane group. The cations employed can be defined as synthetic equivalents of the 5-cation of substituted cyclohexadienes, or of the 4-cation of cyclohex-2-enones, or alternatively of specifically substituted aryl cations, dependent on their structures and the subsequent treatment of the reaction product.     

Synthesis and characterisation of chelated (1-cyanoethyl)ruthenium(II) carbonyl complexes

The reaction between acrylonitrile and the RuH bond in HRu(CO)Cl(PPh₃)₃ results in the formation of a binuclear ruthenium(II) complex having chlorine bridges which are easily broken by sodio-derivatives of bidentate chelating ligands giving mononuclear hexacoordinated ruthenium(II) compounds. The RuC bond in these new complexes has been found to be stable towards nucleophilic reagents. The stereochemistry for these complexes has been suggested on the basis of IR, ¹H and ³¹P NMR spectra.     

Spin trapping identification of radical intermediates during photolysis of organoselenium compounds

Radical intermediates formed during irradiation of selenides and diselenides have been trapped by using nitrosodurene. ESR spectra show that, depending upon the organoselenium compound used, the radical species results from the cleavage of either a CSe bond or a SeSe bond. The hyperfine coupling with ⁷⁷Se has been detected for RSeN(O)R', and indicates a low spin density on the selenium.     

Radical-induced cleavage of ketoepoxides

Radical-induced cleavage of ketoepoxides leads to products of a novel fragmentation featuring epoxide CO bond cleavage.     

Cycloaddition of group VIII metal-dioxygen complexes to electrophilic olefins

The dioxygen complexes (Ph₃P)₂MO₂ (M  Pd, Pt) readily add to electrophilic olefins, such as 1,1-dicyano-olefins, at room temperature, to give cyclic peroxy-adducts in high yield. The adducts undergo thermal decomposition in solution to carbonyl compounds, the reaction proceeding via carboncarbon bond cleavage.     

Calculation of normal vibrations as applied to the study of properties of inorganic compounds

Using the Morse potential and spectroscopic constants it has been shown for diatomic molecules that the force constant of the bond (Kq) is proportional to its dissociation energy (D) so that Kq can be regarded as a measure of the bond strength. An analysis of K_MA of a series of complexes clearly indicates the correlation between the force constants K_MA and the MA bond length, thermodynamic constants of the MA bond dissociation, and reaction rates of the MA bond rupture.     

Reaction of N-alkoxycarbonylaziridines with nitriles

The acid catalysed reaction of acetonitrile or benzonitrile with N-alkoxycarbonylaziridines, 1a and 1b yields the corresponding 1-alkoxycarbonyl-2-imidazolines, 2ax, 2ay, and 2bx. The imidazolines obtained by the reaction of N-ethoxycarbonyl-2,3-tetramethyleneaziridine (1c) with acetonitrileor benzonitrile are labile and are readily hydrolysed to trans-cyclohexane-1,2-diamine derivatives (3cx or 3cy). The nitrile-addition supposedly proceeds S_N2 type CN bond cleavage and CN bond formation.     

Reactivity of main grouptransmittion metal bonds : III. Reactions of halogens with Ph3SiMn(CO)5 and Ph3SnMn(CO)5

The products of the reaction of Ph₃MMn(CO)₅ (M = Si or Sn) with chlorine, bromine and iodine in tetrachloromethane, have been determined.Ph₃SiMn(CO)₅ does not react with iodine, and the SiMn bond is cleaved by chlorine and by bromine.Ph₃SnMn(CO)₅ reacts in a much more complex manner, PhSn bond cleavage occurring with all three halogens to give mono-, di-, and, with the exception of iodine, tri-halogenated derivatives. Bromine, in high concentration, and iodine also cleave the SnMn bond.     

Macrolide ring closuke: “Double activation” mechanism

We reinterpret kinetic results of Wollenberg, Nimitz and Gokcek¹ to indicate substantial SC bond cleavage and NH bond formation in the transition-state of the ratelimiting step for macrolide ring closure from ωi-hydroxy 2-pyridyl thiolesters.     

CC-verknüpfung von alkenen mit isocyanaten am nickel(0

Isocyanates react with alkenes, e.g. ethylene and propene, in a CC bond forming reaction on ligand-nickel(O) systems to yield azanickelacyclopentanones. These metallacycles have been characterized spectroscopically, and their reaction behaviour is discussed. The regioisomeric distribution of the CC coupling reaction is reported for the reaction with propene. The azanickelacyclopentanones obtained from isocyanates and alkenes react further with ethylene to give ring expansion, whereupon β-elimination to unsaturated amides follows.     

Interaction of VC14 with α-olefins. π-complex formation,with concomitant oligomerisation,isomerisation and methathesis reactions

Stable π-complexes of olefins with tervalent vanadium have been isolated from the reaction of olefins with vanadium tetrachloride at ?78°C. The isolation of such complexes is of considerable significance in relation to the frequently proposed formation of intermediate π-complexes in catalytic systems. At higher temperatures concomitant oligomerisation, isomerisation and methathesis reactions were observed, giving rise to a complex array of products whose analysis indicates that insertion into a labile VCl bond is a key step in the above catalytic activity.     

Breakdown of cyclic dithiohemiorthoamide tetrahedral intermediates - RC(SR′)(NR″2)SH - at sub-zero temperatures 1,2

The uslfhydrolyses of thioimidate salts $\text{2a}$–$\text{2d}$, in the presence of anh. NASH in dry 2-butanone at ?82°C, proceed by the preferential cleavage of the CN bond rather than the CS bond.     

Thermische stabilität von Bis(alk-2-enyl)zink-Verbindungen

The dialk-2-enylzinc compounds I–III react slowly at 20 to 50°C by addition of the ZnC bond to the CC bond of an alk-2-enyl group to give oligomers from which the alkenes XIII–XV are released on hydrolysis. For I–III homolytic cleavage of the ZnC_allyl bond, followed by coupling of the allyl radicals to give the alkadienes V–VII, IX and XI predominates above 50°C. IV decomposes mainly homolytically even at 20°C.