Rhodium bonding

The anionic Rh(I) porphyrin [Rh(OEP) induced ring-opening reactions with 4- and 5-membered ring lactones to give organometallic products with the rhodium bonded to the alkoxide carbon rather than the carbonyl carbon. 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

Hayashi et al. proposed a catalytic cycle for the rhodium-catalyzed 1,4-addition of phenylboronic acid to 2-cyclo-hexenone (Scheme 28), which was confirmed by NMR spectroscopic studies.96 The reaction presumably involved three intermediates, phenylrhodium a, oxa-7r-allylrhodium b, and hydroxorhodium c complexes. Complex a reacted with 2-cyclohexenone to give b by insertion of the carbon-carbon double bond of enone into the phenyl-rhodium bond followed by isomerization into the thermodynamically more stable complex. Complex b was converted to c upon addition of water, liberating the phenylation product. Transmetallation of the phenyl group from phenylboronic acid to rhodium took place in the presence of triphenylphosphine to regenerate a. 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

Investigation using P NMR revealed that a direct transmetallation of the phenyl group from boron to rhodium of the (oxa-n -aUyl)rhodium complex is involved in the reaction of B-Ph-9BBN. The catalytic cycle consists of two steps 1) insertion of the en-one into the aryl-rhodium bond and 2) transmetallation to the (oxa-7r-aUyl)rhodium complex forming an arylrhodium species and the boron enolate (Scheme 3.19). Unfor-... 

As predicted by Schoeller et al., coordination of XIIc induces a considerable contraction of the bond angle about the carbene center (from 162° in the free carbene XIIc to 119° in XIIcRh). The carbene-rhodium bond distance [2.096(7) A] is... 

An interesting study of rhodium complexes of 1,3-divinyldisiloxanes and disilazanes has recently been reported in which the nature and geometry of the complex can be altered photochemically196. The rhodium atom binds to both vinyl groups of a single disiloxane or disilazane molecule in either a cis or trans manner, 376 and 377, or the rhodium bonds to two disiloxane or disilazane molecules, again in a cis or trans relationship, 378 and 379, as shown in Scheme 68. 

Replacement of the 2-naphthyl groups by 2-dimethylaminomethylphenyl groups in H2(npOEP) also led to a rhodium porphyrin being able to extract leucine from water, however, the situation is complicated by dimerization of the rhodium porphyrin due to intermolecular amine-rhodium bonding [286]. A rhodium complex of a trifunctional chiral bis(2-hydroxynaphthyl)porphyrin related to the above-mentioned RhCl(npOEP) system was used to separate diastereomers formed via two-point fixation of amino acids [287],... 

The interest in the lower valence states of rhodium also led to the synthesis of the rhodium(II) carboxylates,12 which, although shown to be diamagnetic, excited little interest until the end of that momentous decade when an X-ray crystal structure revealed the source of the diamagnetism to be their rhodium—rhodium bonds.13... 

Despite the vast number of cobalt(IT) complexes known, the dipositive oxidation state is rare for rhodium. The bulk of the rhodium(ri) complexes known are dimeric, the classic examples being the diamagnetic carboxylato complexes that adopt the classic lantern structure (26). However, even the aqua complex is dimeric so the bridging carboxylato ligands are not essential for the formation of the rhodium-rhodium bond. 

A further difference between rhodium(II) and cobalt(II) complexes becomes apparent from a study of the monomeric complexes. Whereas virtually all cobalt(II) complexes are high spin d1 species, with the exception of [Co(CN)5]3 and related species, the rhodium(II) complexes are all low spin complexes. Thus, because of the lack of spin reorientation required in forming a low spin rhodium(III) complex, they are excellent reducing agents. The stability of the rhodium-rhodium bond in the dimers prevents their facile oxidation. 

However, not all bidentate ligands form complexes of this stoichiometry. Although sulfur dioxide displaces CO from a binuclear rhodium(I) complex, the product (97) retains a rhodium-rhodium bond.1048 The complexes of (98) form primarily fac products when allowed to react with rhodium trichloride.1049 The strength of the rhodium-sulfur bond is also shown in the failure of diphos or... 

The reaction is stereoselective, 1,4-hexadiene being mainly obtained in the E configuration the Z-isomer is present only to a small extent and its formation can be further decreased by donor ligands such as Bu3P=0 and (Me2N)3P=0. The reaction is also regioselective the positional isomer CH2=CHCH(CH3)CH=CH2, derived from ethylene insertion into the more substituted carbon-rhodium bond of the allylic system, is present to less than 1 % when ethanol is in large excess. 

A truly hemilabile amino functionalised NHC ligand was introduced by Jimdnez et al. who synthesised a series of rhodium(I) compounds using anunonium functionalised imidazolium salts as starting materials [150] (see Figure 3.52). Interestingly, initially an ionic rhodium(I) compound was obtained that did not contain a carbene-rhodium bond. The rhodium carbene complex could be obtained after further deprotonation and coordination of the amine sidearm to the metal occurred only after chloride abstraction with AgBF. ... 

Apart from the many carbonyls, very few complexes exist in the lowest oxidation state Rh-I, and its chemistry is essentially that of the tetrahedral anion [Rh(PF3)4]. This ligand also gives rise to a rare, authentic example of a noncarbonyl rhodium(O) complex. Orange-red, dimeric [Rh2(PF3)g] contains a rhodium-rhodium bond and has trigonal bipyramidal geometry around each metal atom. It can be... 

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