Rhodium catalysts regioselectivity effects

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

The electronic and steric properties of the phosphine ligand(s) can have dramatic effects on the rate and selectivity of the rhodium catalysts. As mentioned above, electron-rich alkylated phosphines generally have a negative effect on the rate and regioselectivity, while more electron deficient phosphines such as PPhs and phosphites (see Electron Deficient Compound) generate more active and regioselective catalysts (Table 3). 

The choice of catalyst can have a significant effect on these ratios. For reaction 26.5, a cobalt carbonyl catalyst (e.g. HCo(CO)4) gives a 80% C4-aldehyde, 10% C4-alcohol and 10% other products, and an h ratio 3 1. For the same reaction, various rhodium catalysts with phosphine cocatalysts can give an n i ratio of between 8 1 and 16 1, whereas ruthenium cluster catalysts show a high chemo-selectivity to aldehydes with the regioselectivity depending on the choice of cluster, e.g. for Ru3(CO)i2, a 2 1, and for [HRu3(CO)ii], 74 1. Where the hydroformylation... 

The reversal in the regioselectivity of the hydroboration of styrenes using a cationic rhodium complex (see Scheme 5.14), to provide the secondary (rather than primary) organoborane, allows the study of the asymmetric hydroboration of mono-Caryl) substituted alkenes (styrenes). A variety of chiral phosphine ligands can be used to good effect, a popular choice being (7 )-2,2 -bis(diphenylphosphino)-l, T-binaphthyl (BINAP). Just 0.02 molar equivalents of the rhodium catalyst and... 

In a similar manner, enamines also reacted with the corresponding 1,2- or 1,3-Af-substituted diamines 1-3 under the effect of a rhodium catalyst with moderate to excellent iso-regioselectivity (Figure 5.22) [94]. 

Noteworthy, also fert-butyl groups in arylphosphines enhance the solubility in scCOj, but they may have simultaneously a detrimental effect on the reaction rate [81]. The rate and regioselectivity of the hydroformylation in SCCO2 are dependent on the CO2 pressure [80]. Occasionally, the degree of enantioselectivity has been affected. Thus, in the asymmetric hydroformylation of styrene using an unmodified (7J,S)-BINAPHOS rhodium catalyst, the enantiomeric excess values dropped at high CO2 densities [82]. 

Alternatively, 3-vinylthiophenes 13 can be prepared through regioselective oxidative coupling of thiophene-2-carboxylic acid derivatives 11 with different alkenes 12 in the presence of a rhodium catalyst ([Cp RhCl2]2) and copper (II) acetate monohydrate as oxidant (Scheme 7) [32]. Thereby, the carboxylic group facilitates ort/io-olefination and is subsequently removed by decarboxylation in a one-pot sequence. Additionally, benzo[Z>]thiophenes can equally be coupled effectively with butyl acrylate giving rise to the vinylated product in a yield of 70%. 

A comparison between experimental and MO data on regioselectivity concerning the hydroformylation of several vinyl substrates (propene, 2-methylpropene, 1-hexene, 3,3-dimethylbutene, fluoroethene, 3,3,3-trifluoropropene, vinyl methyl ether, allyl methyl ether, styrene) with unmodified rhodium catalysts was reported. The activation energies for the alkyl rhodium intermediate formation, computed at either level along the pathways to branched or linear aldehydes, allowed one to predict the regioselectivity ratios. Steric effects may be less important for the unmodified carbonyl complexes and electronic factors dominate, assuming that alkene insertion in Rh-H is irreversible. 

Having established that pure enantiomer ( S,ZR)-77 was capable of undergoing remarkably regioselective and diastereoselective C-H activation, it followed that highly efficient enantiomeric differentiation of rac-77 could be accomplished.199 Hence, the Rh2(5Y-MEPY)4-catalyzed reaction of rac-77 effectively gave close to a 1 1 mixture of enantioenriched (lY)-78 (91% ee) and ( R)-79 (98% ee) (Equation (68)). Other equally spectacular examples of diastereo- and regiocontrol via chiral rhodium carboxamide catalysts in cyclic and acyclic diazoacetate systems have been reported.152 199 200 203-205... 

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