Olefin rhodium complexes, proton

Figure 1. Proton NMR studies on olefin rhodium Complexes 2-5. The NOE effect is indicated in square brackets.

Whatever metal is used, homogeneous processes suffer from high cost resulting from the consumption of the catalyst, whether recycled or not. This is why two-phase catalytic processes have been developed such as hydroformylation catalyzed by rhodium complexes, which are dissolved in water thanks to hydrophilic phosphines (cf. Section 3.1.1.1) [17]. Due to the sensitivity of most dimerization catalysts to proton-active or coordinating solvents, the use of non-aqueous ionic liquids (NAILs) as catalyst solvents has been proposed. These media are typically mixtures of quaternary ammonium or phosphonium salts, such as 1,3-dialkylimi-dazolium chloride, with aluminum trichloride (cf. Section 3.1.1.2.2). They prove to be superb solvents for cationic active species such as the cationic nickel complexes which are the active species of olefin dimerization [18, 19]. The dimers. 

The preparation of two cyclo-octatetraene-gold complexes, (ct)AuCl and (cot)-AU2CI4, has been reported. The structures of biscyclo-octatetraenyl complexes of titanium, vanadium, thallium, and uranium, were deduced from their i.r. spectra. Protonation of (p-cyclo-octatetraene) (p-cyclopentadienyl) complexes has been studied. For the ruthenium and osmium complexes protonation occurs on the eight-membered ring to give CgH moiety co-ordinated to the metal atom via both an T -alkyl and an olefin-metal bond. For the cobalt and rhodium complexes a bicyclic cation (287) is produced which undergoes isomerization to the monocyclic (288). ... 

Many transition metal hydrides and low-valent complexes that can generate an M—H bond by protonation catalyze hydrogen migrations in olefins. Rhodium trichloride or rhodium(I) compounds plus HCl rapidly isomerize 1-butene to an equilibrium mixture of butenes in which trans-2-butene is the largest single component. Most of the complexes that catalyze olefin dimerization also catalyze isomerization. The isomerization mechanism postulated by Cramer is similar to his dimerization mechanism except that no insertion step is involved (185). 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

Most of the ground states of complexes seem to have structure XXIV, but XXV reasonably could provide a mechanism for rotation about the metal-olefin bond axis with a low energy barrier. Cramer (II) found that 7r-cyclopentadienylbis(ethylene)rhodium(I), XXVI, gave two broad signals (r = 7.23, 8.88 ppm) for the ethylene protons at —25° and that... 

Olefin rotation was first observed in some ethylene-rhodium(I) complexes using H n.m.r. spectroscopy 139>. At room temperature the complex [Rh(Cp)(C2H4)2] shows two broad peaks at 8.7 and 6.9 r which are due to pairs of non-equivalent protons undergoing rotation at an intermediate rate on the n.m.r. time scale. 

A thermally unstable complex of rhodium(I), [ir-CgH8RhCl]2, prepared from ethanolic rhodium(III) chloride and cyclooctatetraene, shows two proton resonance lines at 5.8 r and 4.3 r in carbon disulfide, and probably also contains the tub form of the olefin (13). 

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

The catalytic hydrocarbonylation and hydrocarboxylation of olefins, alkynes, and other TT-bonded compounds are reactions of important industrial potential.Various transition metal complexes, such as palladium, rhodium, ruthenium, or nickel complexes, have widely been used in combination with phosphines and other types of ligands as catalysts in most carbonylation reactions. The reactions of alkenes, alkynes, and other related substrates with carbon monoxide in the presence of group VIII metals and a source of proton affords various carboxylic acids or carboxylic acid derivatives.f f f f f While many metals have successfully been employed as catalysts in these reactions, they often lead to mixtures of products under drastic experimental conditions.f i f f f In the last twenty years, palladium complexes are the most frequently and successfully used catalysts for regio-, stereo-, and enantioselective hydrocarbonylation and hydrocarboxylation reactions.f ... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

Inability to observe metal-proton coupling satellites in certain silver complexes (mentioned in the discussion in various places above) could be interpreted to denote rapid exchange of olefin between different metal atoms. In view of the small coupling constants for metals in the second transition series however [cf. small coupling constants for rhodium, 7r-(Cs)M discussion above], it may be that the satellites simply are not resolvable from the... 

The formation of a stable 3-rhoda-l,2-dioxolane by dioxygenation of a rhodium-olefin complex supported the involvement of such complexes as intermediates in rhodium-catalyzed oxygenation of olefins to ketones. This 3-rhoda-l,2-dioxolane complexes rearranged to rhodium formylmethyl hydroxycomplexcs upon exposure to light or protons (Scheme 25). ... 

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