Current Research
Edmond Lam’s Research:
A number of indole-phosphorus ligands have been synthesized and characterized. Direct lithiation
of NH heterocycles gives N-lithio derivatives, but the reactivity of the indole ring can be
controlled with aminoalkyl protecting groups. Our simplest ligand is DPIP
(2-diphenylphosphino-3-methylindole) (1). Coordination of these phosphines to Pd(COD)Cl2 have shown the phosphine to behave exclusively as a monodentate 2-electron P-donor ligand, where the indolyl substituent remains protonated
at the NH position, as shown in the palladium dimer Pd2Cl2(m-Cl)2(DPIP)2 (2). However, upon controlled deprotonation of the amino proton on the indole, the ligand may behaves as a novel 6-electron anionic m3,h2-P,N donor ligand as observed in the palladium tetrameric butterfly cluster Pd4Cl4(m3,h2-DPIP)2(DPIP)2 (3).
Indolylphosphines are readily functionalized at the N1 position of the indolyl substituent by deprotonation of the N1 proton with sodium hydride prior to addition of an alkyl reagent. New unsymmetrical diphosphines such as DPIP-PCy2 (4) can be synthesized through this simple methodology. Cu(I) catalyzed arylation of indoles have led to development of new P,N bidentate indolylphosphines such as DPIP-Py (5). Coordination of 5 to Pd(COD)Cl2 gave the expected square-planar complex PdCl2(DPIP-Py) (6).
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The influence of the steric and electronic properties of these derivatized phosphines in the Pd-catalyzed Heck-Mizoroki and Buchwald-Hartwig amination reactions of alkenes with various alkyl and aryl halides is the subject of an ongoing investigation.
Joanne Yu's Research:
An alternate one-pot synthetic route to our indolylphosphine ligands has been realized with the use of CO2 as a protecting group. The 3-methylindole nitrogen is protected by introduction of a CO2 protecting group, followed by addition of the appropriate phosphine chloride, and subsequent work-up, affords the ligand of interest in moderate yields.
The triindolylphosphine ligand TMIP (2,2',2"-tri(3-methylindolyl)phosphine) (7), was successfully synthesized using the above route. The single crystal x-ray structure of (7) demonstrates the potential of this ligand to behave as a neutral P-donor, as well as an anionic N-donor by deprotonation of the indolyl nitrogen's.
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9
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The deprotonation of TMIP is facile with NaH, and introduction of PPh2Cl forms the ligand (Ph2P)3(TMIP) (8) in moderate yields. Ligand 8 has the potential of coordinating metal ion species as a tetradentate neutral ligand. Addition of Pt(COD)Cl2 and NaBPh4 to 8 results in a novel trigonal bipyramidal coordinated Pt(II) complex, {PtCl[(Ph2P)3(TMIP)]}[BPh4] (9).
The introduction of various phosphines onto the indolyl nitrogen is currently under investigation. Possible candidates include substituents which are chiral, bulky, electron-withdrawing and electron-donating. Their interation with Pt(II) will also be explored, along with possible catalytic activtities in the hydroformylation reaction.
Investigation of the basicity of our indolylphosphine ligands with Ni(CO)4 is also under examination.
