Peter Bougeard

Reaction of [η5-C5H5)2ZrH(μ-H)]2 with diphenylacetylene: mechanistic and theoretical considerations

The polymeric (Cp2ZrH2)x molecule is shown by 1H NMR studies to adopt a dimeric structure with bridging and terminal hydride ligands in both benzene-d6 and toluene-d8 solution. The reaction of the zirconium dihydride with diphenylacetylene has been re-investigated and produces a zirconacyclopentadiene complex 7, contrary to literature suggestions of a dimetallic zirconabenzene structure. A qualitative molecular orbital analysis of the [Cp2ZrH(μ-H)]2 dimer lends support to a suggested mechanism involving reaction with diphenylacetylene via a dimeric species.

Homolytic displacement at carbon: X. Toluenesulphonyl iodide as a source of toluenesulphonyl radicals for the formation of allyl-, benzyl-, cyclopropylcarbinyl-, spirocyclopropylcycloalkyl-, bicyclo[1.0.3]alkyl-, and bicyclo[1.0.4]Alkyl-4-tolylsulphones from organocobaloximes☆

Abstract 4-Toluenesulphonyl iodide reacts thermally at ⩽ 40°C with a wide range of alkenyl- and benzyl-cobaloximes to give good yields of the organo-4-tolylsulphone. Allylcobaloximes yield predominantly, and in some cases exclusively, the rearranged allylsulphone; alicyclic but-3-enylcobaloximes yield the corresponding cyclopropyl- carbinylsulphone; cycloalkenylethylcobaloximes yield the spiro-1,1-cyclopropyl- cycloalkylsulphone; and cycloalk-2-enylmethylcobaloximes yield the bicyclo- [1.0.N] alkylsulphone. Spiro- and bicyclo-alkyl compounds are also formed with other free radical precursors. The reactions are believed to take place through a chain mechanism in which cobaloxime(II), present adventitiously or formed by partial homolysis of the substrate, abstracts iodine from the toluenesulphonyl iodide to give the toluenesulphonyl radical, which attacks the organic ligand of the cobaloxime, preferably at the terminal olefinic carbon, thereby displacing cobaloxime(II) and giving the observed organic product.

Dicyclopentadienylnickel: A problematical reagent in cluster synthesis

Abstract The room temperature reaction of (C5H5)2Ni with Co3(CO)9CR, where R is CO2CH(CH3)2, gives almost quantitative replacement of a Co(CO)3 group by a (C5H5)Ni fragment. However, the same reagents in refluxing THF do not lead to further replacement of Co by Ni but instead yield the products (C5H5)3-Co3(CO)(H)CR and (C5H5)3Co2Ni(CO)CR in which carbonyl groups have been replaced by cyclopentadienyl ligands. The isolobal nature of the fragments in these complexes is noted.

Homolytic displacement at carbon : VI. Synthesis of trichloroethylarenes from benzylcobaloximes☆

Abstract Benzylbis(dimethylglyoximato)pyridinecobalt(III) reacts with bromotrichloromethane at from 50 to 90°C in chloroform to give good yields of trichloroethylbenzene, which are higher when imidazole is present in the reaction mixture. Methyl- and polymethyl-substituted benzylbis(dimethylglyximato)pyridinecobalt(III) complexes give higher yields of the corresponding trichloroethylarenes (85–90%), whereas 4-chlorobenzylbis(dimethylglyoximato)pyridinecobalt(III) gives a lower yield and 4-nitrobenzylbis(dimethylglyoximato)pyridinecobalt(III) only gives the 4-nitro-trichloroethylarene when imidazole is present during the reaction. Similar reactions were observed with benzylcobaloximes and trichloromethanesulphonyl chloride both thermally and under irradiation by tungsten lamps through all-pyrex apparatus. The reactions are interpreted as a direct attack of the trichloromethyl radical on the α-carbon of the benzyl ligand.

[η5-1,2,3,4,5-pentakis(carbomethoxy)cyclopentadienyl]tricarbonylmanganese(I): Synthesis, spectroscopy and reactivity

Abstract Treatment of the potassium salt of pentakis(carbomethoxy)cyclopentadiene with bromopentacarbonylmanganese(I) gives pentakis(carbomethoxy)cyclopentadienyltricarbonylmanganese(I) in good yield. Photochemical substitution of a metal carbonyl group by tributylphosphine, triphenylphosphine and triethyl phosphite is reported. Attempts to modify the ester function are described; hydrolysis or amination give mixed products but reduction with lithium aluminum hydride yields ultimately the pentamethylcyclopentadienyl complex.

Free radical rearrangements of organocobaloximes: alkynyl to cycloalkylidene and hexenyl to cyclopentylmethyl

Abstract Under irradiation by tungsten light in pyridine solution, several substituted alkylcobaloximes undergo rearrangement to more stable substituted alkyl- or alkenyl-cobaloximes. When the same reactions are carried out in the presence of carbon tetrachloride or chloroform, no rearranged organocobaloximes are obtained, but a variety of organic products are obtained derived from the interception of transient organic radicals by the halogenated solvent. The rearrangements are rationalised in terms of a reversible homolysis of the carbon-cobalt bond, rearrangement of the organic radical and recapture by the cobalt(II) fragment to give complexes that are more stable to irradiation than their precursors.

Fluxionality of M2C2 tetrahedral clusters containing cyclopentadienyldicarbonylmetal vertices: Unexpectedly different behaviour of molybdenum and tungsten analogues

Abstract The syntheses of a series of molecules [Cp′M(CO)2]2(RCCR′), where Cp′ = C5Me5, C5H4Me; M = Mo, W; R = Ph, R′= H, Et, CO2Me, SnMe3 are described. The molybdenum complexes possess a semi-bridged carbonyl thus rendering the molecules chiral; their 13C NMR spectra show four carbonyl resonances and two cyclopentadienyl environments. The barrier to racemisation depends on the bulk of the substituents on the ring. In contrast, the NMR spectra of the tungsten analogues show only single carbonyl and cyclopentadienyl environments even at −90°C and 9.4 T.

Homolytic displacement at carbon : VII. Regiospecific synthesis of S-allyl-N,N-dimethylsulphonamides from allylcobaloximes and the addition of N,N-dimethylsulphonyl chloride to terminal olefins☆

Abstract Unsymmetrical allylcobaloximes, e.g. 3-methylbut-3-enylbis(dimethylglyoximato)pyridinecobalt(III), react regiospecifically with N,N -dimethylsulphamoyl chloride to give good yields of the rearranged product S -allyl- N,N -dimethylsulphonamide, e.g. 1,1, N,N -tetramethylallylsulphonamide. Symmetrical allylcobaloximes react similarly to give the expected single allylsulphonamides. Buta-1,2-dienylbis(dimethylglyoximato)pyridinecobalt(III) also reacts regiospecifically with N,N -dimethylsulphamoyl chloride to give 1, N,N -trimethylpropynylsulphonamide. It is proposed that the organic product-forming step of the reaction involves the homolytic displacement of cobaloxime(II) by regiospecific attack of the N,N -dimethylsulphamoyl radical on the γ-carbon of the allyl or propadienyl ligand. This is supported by the observation that N,N -dimethylsulphamoyl chloride adds to terminal olefins under free radical conditions.

Homolytic displacement at carbon: XI. Intramolecular homolytic displacement as a route to cyclopentane and tetrahydrofuran derivatives from hex-5-enyl- andhex-3-oxo-5-enylcobaloximes☆

Abstract 5-Methylhex-5-enylcobaloxime reacts with carbon tetrachloride and with fluorotrichloromethane at 80–100°C to give substantially pure 1-methyl-1-(β,β,β-trichloroethyl)- and 1-methyl-1-β-fluoro-β,β-dichloroethyl)-cyclopentane. Hex-5-enylco-baloxime also gives trichloroethylcyclopentane from carbon tetrachloride, but the yield is dependent on the concentration of carbon tetrachloride. Similar cyclisation to give trichloroethyl- or fluorodichloroethyltetrahydrofuran is observed in the reactions of hex-3-oxo-5-enylcobaloxime with carbon tetrachloride and fluorotrichloromethane. However, no cyclisation was observed in the reactions of the ester, hex-2-one-3-oxo-5-enylcobaloxime, with carbon tetrachloride. These reactions are believed to take place by attack of a polyhalogenomethyl radical at the terminal unsaturated carbon of the organic ligand, followed either by an intramolecular homolytic displacement in which the carbon radical at position-5 attacks carbon-1 with displacement of cobaloxime(II), or by a halogen atom abstraction.

Stopping a chromium carousel: X-ray crystallographic and variable-temperature 13C n. m. r. studies on dicarbonyl(hexaethylbenzene)-thiocarbonylchromium(0) and related complexes

The low temperature 13C n.m.r. spectrum of (C6Et6)Cr(CO)2CS shows the molecule to have Cs symmetry as it does in the solid state; this result provides the first clear evidence of cessation of tripodal rotation in a neutral half-sandwich compound of Cr.