Rhodium 2 , axial substitution

Kinetics of reactions of binuclear rhodium(II) complexes are perhaps rather fast for this chapter, but mention may be made of a kinetic determination of the rhodium-rhodium bond strength in an octaethylporphyrinato species and of kinetics of axial substitution in the tetraacetato complex [Rh2(OAc)4(OH2)2]. ... 

Related substitution patterns are observed in tetranuclear cobalt and rhodium clusters. Thus, the small ligand, P(OMe) (L), occupies axial sites in [Co,(CO)1 L ] (x = 1,2) (17) whereas steric effects become important witfi (OPh) and the isomers shown in Fig. 2 are obtained with tetrarhodium derivatives. 

Metal salen complexes can adopt non-planar conformations as a result of the conformations of the ethane-1,2-diyl bridge. The conformations may have Cs or C2 symmetry, but the mixtures are racemic. Replacement of the ethylenediamine linker by chiral 1,2-diamines leads to chiral distortions and a C2 chiral symmetry of the complex due to the half-chair conformation of the 5-membered ring of the chelate. Depending on substitution at the axial positions of the salen complex, the symmetry may be reduced to Q, but as we have seen before in diphosphine complexes of rhodium (Chapter 4) and bisindenyl complexes of Group 4 metals (Chapter 10) substitution at either side leads to the same chiral complex. Figure 14.10 sketches the view from above the complex and a front view. 

Biaryls with axial chirahty were prepared by pyridine- or isoquinoline-directed rhodium-catalyzed coupling with alkynes (Scheme 65). A number of diaryl-substituted alkynes coupled with l-(naphthyl)isoquinoline in the presence of [RhCp Cl2]2 and AgSbF with a copper (II) acetate oxidant in excellent yields (90-98%). The coupling proceeded smoothly with 2-aryl substituted pyridines (89-99%). A number of substituents were tolerated on the pyridines at C-3, including methyl, chloride, and methoxy. The isoquinoline-based biaryls were easily oxidized with MCPBA to provide... 

The cationic rhodium(l)/axially chiral biaryl bisphosphine complex-catalyzed [2+2+2] cycloaddition of biaryl-linked tetraynes with dialkynylketones or dialkynylphosphine oxides afforded helically chiral l,l -bitriphenylenes, containing adensely substituted fluorenone or phosphafluorene core (Scheme 21.23) [27]. 

The intramolecular [2+2+2] cycloaddition of triynes affords tricyclic compounds, which are not readily accessible by other methods. The double [2+2+2] cycloaddition of a diphenylphosphinoyl-substituted hexayne proceeded in the presence of the cationic rhodium(I)/tol-BINAP catalyst to give the corresponding Cj-symmetric axially chiral biaryl bisphosphine oxide with high enantioselectivity (Scheme 21.24) [28]. 

In 2011, Nadeau and coworkers [27] developed the first rhodium-catalyzed asymmetric arylation of N-benzyl nicotinate stJts (other C=N double bond substrates) with arylboronic acids. The use of an axial chiral bisphosphane ligand (i )-CTH-P-Phos gave a variety of six-substituted dihydropy-ridines with excellent enantioselectivities (Scheme 6.22). The usefulness of the method lies on the facile conversion of the dihydropyridines into the synthetically useful piperidines. 

Continuous, flash, and laser flash photolyses of phthalocyanine complexes [Rh(pc)(MeOH)X], with X = Cl, Br, or I, show that replacement of halide by methanol is a redox-induced process, via rhodium(III)-pc and rhodium(II) species/ This may be compared with copper(II) analogues, where redox-induced substitution involves copper(III)-pc complexes/ Axial ligand substitution processes must also be involved in photoinduced hydrogen abstraction from coordinated solvent in rhodium(III)-phthalocyanine complexes/ ... 

The enantioselective desymmetrization of substituted malononitriles via cationic rhodium(I)/axially chiral biaryl bisphosphine-catalyzed [2 + 2 + 2] cycloaddition with a 1,6-diyne proceeded at room temperature to give enantio-enriched bicycUc pyridines in good yields with moderate ee values (Scheme 4.66) [55],... 

The enantioselective synthesis of axially chiral P—N ligands was also accomplished by rhodium-catalyzed [2 + 2+-2] cycloaddition. The reactions of 1,6-diynes 75 with diphenylphosphinoyl-substituted isoquinolinyl acetylenes 76 furnished diphenylphosphinoyl-substituted axially chiral 1-arylisoquinolines 77 with high yields and ee values (Scheme 9.28) [23], The new diphenylphosphinoyl-substituted axially chiral 1-arylisoquinoline 77 (Z = NTs, R = Me) was derivatized to the corresponding axially chiral P—N ligand 78 and isoquinoline A-oxide 79 without racemization, which could be used in the rhodium-catalyzed hydroboration and Lewis base-catalyzed allylation, respectively [23],... 

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