Norbornadiene complexes with rhodium

Chiral methyl chiral lactic acid (5). This labeled molecule, useful for study of stereospecificity of enzymic reactions, has been prepared in a way that allows for synthesis of all 12 possible isomers. One key step is the stereospecific debromination of 1, accomplished by conversion to the vinyl-palladium cr-complex 2 followed by cleavage with CF3COOT to give the tritium-labeled 3. The next step is the catalytic deuteration of 3, accomplished with a rhodium(I) catalyst complexed with the ligands norbornadiene and (R)-l,2-bis(diphenylphosphino)propane. This reaction gives 4 with an optical purity of 81%. The product is hydolyzed to 5, which is obtained optically pure by cr3rstallization. 

The first bisphosphine calixarenes that have been used in catalysis are di(amide)-phosphine hybrids calix[4]ar-ene. Reaction of [RhCl(norbornadiene)]2 with these calixarene derivatives gave an organometallic complex whose norbornadiene-rhodium moiety lies above the cavity defined by the four substituents of the calixarene and between the two amide functionalities. This complex was applied in the hydroformylation reaction of styrene. The rather low reaction rate observed (7.5 turnovers per Rh per hour) has been attributed to a partial encapsulation of the metal center preventing the approach of the substrate. Indeed, the metal center may be viewed as located in a hemispherical ligand environment. 

Reactions with propene and allyl alcohol occur in a similar way. Analogous complexes containing cycloheptene, cyclooctene, 1,5-COD, norbornene, and norbornadiene are also formed from rhodium trichloride in aqueous ethyl alcohol however, the reaction stoichiometry for these olefins was not established. It is possible that the oxidation of alcohol occurs. For the preparation of rhodium and iridium complexes with cycloolefins of the formula [M2Cl2(olefin)4], the best results are obtained if aqueous ethyl or isopropyl alcohol is used as the solvent ... 

Rhodium complexes with the ferrocenyl ligand (i ,5)-Cy2PF-PPh2 (109) have been shown to catalyse asymmetric hydroalkynylation of norbornadienes with <99.9% ee A hydroxorhodium complex with (R)-Segphos (110) has been shown to catalyse the hydroarylation of 3-pyrrolines (111) with arylboroxines (112) under neutral conditions to give 3-arylpyrrolidines (113) (<96% ee) ... 

When [Rh(Cl)(Nbd)(bipy)] (Nbd = norbornadiene) 92 prepared in situ was treated with Ph2P(o-C6H4CHO) 93 in methanol, oxidative addition of aldehyde to rhodium followed by insertion of norbornadiene into the Rh-H bond occurred (Equation 29) <2005EJI1671 >. NMR spectroscopy, including two-dimensional (2-D) experiments, allowed complete characterization of complex 94. 

The reaction can be explained by the transmetalation of the borate with PdCl2 giving phenylpalladium intermediate and the subsequent insertion of a C=C double bond of norbornadiene into the Pd-Ph bond. Cationic 7t-allyl-palladium(II) and -rhodium(I) complexes having a BPhJ counter anion induce phenyl group migration from B to the transition metals at elevated temperature to form phenyl complexes [54,55] (Eq. 5.17). 

A fortunate feature of asymmetric catalysis is that chiral biphosphine rhodium diene complexes, especially with norbornadiene, crystallize well... 

More polar functional groups such as ketones are hydrogenated effectively with homogeneous cationic rhodium and ruthenium complexes. Polymer catalysts active for both olefin and ketone hydrogenation activity were synthesized from ionic precursors such as Rh(norbornadiene)(acac) + HCIO4 or Rh(norbornadiene)(PE 3)2 + C104. Two types of coordination were observed, depending on the method of attachment " ... 

In contrast to the behavior with platinum, a number of rhodium(I) and iridium(III) complexes react with cycloheptatriene to form complexes of the isomeric bicyclic chelate diolefin, norbornadiene (15,137). 

In contrast with the Osborn rhodium catalysts, in which the exclusive formation of the product arises from a cis-addition to the alkyne due to the difference of coordination power of the substrates, in these osmium complexes, the strong tendency of the alkyne to undergo insertion to yield the observable styryl intermediate is the step that determines the selectivity (83-85). Complex Os-HCl(CO)(P Prs)2 is also active in tandem ROMP-hydrogenation of norbomene and 2,5-norbornadiene (86). 

In case of the AT-Me substituted compound, the reaction led to an enete-tramine as a stable yellow solid. In contrast, with the sterically more bulky A/ -diphenylmethyl precursor, base treatment and subsequent addition of [Rh(NBD)2]BF (NBD = norbornadiene) afforded the desired rhodium complex as an air- and moisture-stable golden-yellow powder. Reaction of the enetetramine and [Rh(NBD)2]Bp4 produced a mononuclear rhodium complex. Upon treatment with CO, a dicarbonyl complex was formed. Alternatively, a crystalline dinuclear rhodium complex was prepared by reaction with 2 equiv of [RhCl(COD)]2 in a solution of THF [64]. 

In view of these previous developments, we directly coupled various N-substituted imidazoles, which display nucleophilic reactivity, with 2-bromo-oxazolines to give the imidazolium precursors (Scheme 15.15) [55]. This direct condensation of an oxazoline and an imidazole salt provided a straightforward and modular route to the development of a new family of stereodirecting ligands. NHC - rhodium complexes could be obtained by reaction of the imidazolium salt with [ Rh(p-OtBu)(nbd))2] (nbd, norbornadiene) generated iw sifn [56]. 

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