Allylic rhodium

Scheme 1.5 Silica, alumina and titania surface oxygens behaving as ligands in the M.L.H. Green formalism [9] after reaction of r -tris(allyl)rhodium with a partially dehydroxylated surface [39].

Scheme 1.7 Elementary steps occurring in the reaction of CO on a silica surface with supported ri-allyl rhodium.

Figure 7.3 Proposed structures of 7C-allyl rhodium siloxide complexes immobilized on silica.

Scheme 7.2 Reactions of the surface 7t-allyl rhodium siloxide complex with trimethylphosphine.

Scheme 7.3 Reaction of surface Jt-allyl rhodium siloxide complex with carbon monoxide.

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

It seems logical that these reactions are mechanistically related. In every case reported to date, the relative stereochemistry between the nucleophile and alcohol moiety is trans, indicating anti addition in the ring-opening event. Lautens and coworkers suggest a syn insertion of the rhodium to form the allyl rhodium alkoxide. Anti addition of the soft nucleophile, with possible initial protonation of the... 

The first example involving a rhodium catalyst in an ene reaction was reported by Schmitz in 1976. An intramolecular cyclization of a diene occurred to give a pyrrole when exposed to rhodium trichloride in isobutanol (Eq. 2) [15]. Subsequently to this work, Grigg utilized Wilkinson s catalyst to effect a similar cycloisomerization reaction (Eq. 3) [16]. Opplozer and Eurstner showed that a n -allyl-rhodium species could be formed from an allyl carbonate or acetate and intercepted intramolecularly by an alkene to afford 1,4-dienes (Eq. 4). Hydridotetrakis(triphenylphosphine)rhodium(l) proved to be the most efficient catalyst for this particular transformation. A direct comparison was made between this catalyst and palladium bis(dibenzylidene) acetone, in which it was determined that rhodium might offer an additional stereochemical perspective. In the latter case, this type of reaction is typically referred to as a metallo-ene reaction [17]. 

Substantial evidence from investigation of the formation of this product suggests that the mechanism is different from that of the typical Alder-ene reaction. It is thought to involve the formation of a j-allyl-rhodium complex species rather than a metallocyclo-pentene intermediate (Scheme 8.10). 

The co-product of the addition of water to the 7r-oxo-allyl rhodium complex, 5, in THF at 25°C was suggested to be an (unobserved) hydroxy rhodium species which rapidly dimerises to give complex 7, a square planar /z-hydroxy dimer. This complex was easily identified by the high field shifts of the /z-hydroxy protons ( — 1.9 ppm) and its symmetry (a single Rh-coupled doublet is observed at 55 ppm in the 31P 1H NMR spectrum). The assignment was confirmed by independent synthesis [(S)-BINAP)Rh(/x-Cl)]2 reacted cleanly with KOH in aqueous THF to give 7, as did addition of (S) -BINAP to the conveniently prepared complex [Rh(/z-OH)(cod]2 (cod = 1,5-cyclo-octadiene). 

A particularly interesting synthetic route to complexes of the type [RhX(PF3)2] (X = Cl, Br) is via the oxidative addition of HC1 (or BuBr) to the i/3-allylic rhodium(I) complex [Rh(C3Hs)(PF3)3] (method I) (Scheme 1), followed by alkene elimination. 

Tris(allyl)rhodium is made by treatment of C3H5MgCl with [(> -C3H5)2RhCl]2 prepared from CjHjCl and H2O with [RhCl(CO)2]2A related, mixed crotyl-allyl complex is prepared ... 

Changing the transition-metal from palladium to rhodium (equation 62) makes possible, in addition to the straight-chain alkylation product (243), the regio- and stereoselective synthesis of amino acid derivatives with a terminal double bond (242), starting from optically active branched allylic substrates 241 (Table 21)" . Remarkably, the substitution products were obtained with high enantiomeric excesses, what might result from a slow isomerization of the intermediary formed allyl rhodium complexes ". 

Butadiene is polymerized by rhodium compounds in aqueous or alcoholic solution [178]. It is generally accepted that the active species is a TT-allyl rhodium complex of low valency [28, 179] which is not rapidly terminated by reaction with water or alcohol. No clear kinetic pattern was observed in the earlier papers but a recent investigation [180] has shown the rate and molecular weight data to be accommodated by a scheme involving monomer transfer and physical immobilization of the active centres in precipitated polymer. In the initial stages the polymerization is first order in rhodium and, at constant monomer concentration, is (pseudo) zero order E = 14.8 kcal mole" ). This is followed by a declining rate which is almost independent of temperature. Molecular weights rise slowly to a maximum value with time (ca. 4000 after 22 h at 70°C). 

Silica-supported rhodium hydrides are highly efficient isomerisation catalysts. They are prepared by the reaction of silica suspended in toluene with tris(allyl)rhodium. The intermediate complex reacts with hydrogen to eliminate propene and propane to give a material believed to have neighbouring rhodium sites connected by hydrogen bridges.164... 

Reaction with sulfur dioxide yields a o--allylic rhodium-sulfur dioxide complex (1). 

The 7 -allyl rhodium complex, Rh( -C3H5)3, can be incorporated into an acidic zeolite with the formation and loss of propene, and several species formed in subsequent reactions with dihydrogen, hydrogen chloride, carbon monoxide, and phosphines, have been investigated [270, 287]. The same intrazeolite triallyl complex has been studied as an alkene hydrogenation catalyst [288],... 

Allyl rhodium complexes deposited on silica gel activate methane. The products of the reaction are allyl and hydrido rhodium complexes, as well as propylene and small amounts of butene and butane. 

Halogen-allyl rhodium or iridium complexes react in a similar manner with olefin evolution. 

Myrcene is a very abundant acyclic monoterpene available from the essential oils of various plants including wild thyme and hops. Recently, an excellent overview on the manufacture and transformation of this natural product was given by Behr and Johnen [125]. Commercially, myrcene is produced by the pyrolysis of pinenes [126]. The rhodium-catalyzed hydroformylation of myrcene gives usually a mixture of fragrance aldehydes in more than 90% combined yields (Scheme 6.37) [127, 128]. The main aldehyde, which accounted for 70 - 80% of the mass balance, results from the reaction with the less substituted C=C bond through the formation of a T) -allyl rhodium intermediate complex [127]. The reaction was also performed in a toluene/water biphasic system using the water-soluble TPPTS ligand and a cationic surfactant [84]. 

Krische and co-workers reported an intramolecular version of the tandem conjugate addition-aldol reaction. Enone-ketone 59 reacts with phenylboronic add (2m) in dioxane containing 5 equiv. of water [40] (Scheme 4.26). This cyclization probably proceeds through the (oxa-7t-allyl)rhodium intermediate. Because the intramolecular addition of the intermediate to the ketone is faster than protonolysis with water, the al-dol product is obtained in high yields. Binap is a ligand of choice, leading to the conjugate addition-aldol product in up to 95% ee. 

Rhodium catalyzes retro-allylation of homoallylic alcohols to generate nucleophilic allylic rhodium species. A combination of [RhCl(cod)]2, PMcj, and cesium carbonate catalyzes crotyl transfer from homoallylic alcohol to benzaldehyde in dioxane at 100 "C to afford the corresponding homoallylic alcohol (Scheme 5.37) [26]. Like allylic nickel, allylic rhodium experiences rapid and E-Z interconversions under the reaction conditions, which prevents stereoselectivity of the crotylation. Interestingly, a similar crotylation is followed by isomerization into saturated ketone with the aid of a bulkier catalyst [RhCl(cod)]2/PtBu3 at a higher temperature. 

Tris(allyl)rhodium(in) was reported to react with the hydroxyl-terminated surfaces of silica, titania and alumina to form a surface bis(allyl)rhodium(III) fragment, which on reaction with carbon monoxide afforded hexa-1,5-diene in quantitative fashion by reductive coupling of two allyl ligands. 

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