Thermolysis of the P-coordinated KITPHOS monophosphine
complex
[(p-cymene)RuCl2(KITPHOS)] in chlorobenzene
at 120 °C resulted in displacement of the p-cymene
to afford [{κ(P)-η6-KITPHOS}RuCl2], the first example of a constrained-geometry complex in
which the κ(P)-bonded diphenylphosphino group and the η6-coordinated proximal phenyl ring are connected by an unsaturated
two-carbon tether; both complexes form efficient catalysts for the
direct ortho arylation of 2-phenylpyridine and N-phenylpyrazole
with a range of aryl chlorides.
We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)2Ru(n,n′‐dhbp)]Cl2 with n = 6 and 4 in 1A and 2A, respectively). Full characterization data are reported for 1A and 2A and single crystal X‐ray diffraction for 1A. Both 1A and 2A are diprotic acids. We have studied 1A, 1B, 2A, and 2B (B = deprotonated forms) by UV‐vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy 3MLCT states relative to the acidic forms. Complexes 1A and 2A produce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC50 light values as low as 0.50 μM with PI values as high as >200 vs. MCF7. Computational studies were used to predict the energies of the 3MLCT and 3MC states. An inaccessible 3MC state for 2B suggests a rationale for why photodissociation does not occur with the 4,4′‐dhbp ligand. Low dark toxicity combined with an accessible 3MLCT state for 1O2 generation explains the excellent photocytotoxicity of 2.
Electrophilic gold(I) complexes of diphenyl-and dicyclohexylphosphino-based KITPHOS monophosphines catalyze the 5-exo-dig cycloisomerization of a range of propargyl amides to afford the corresponding alkylidene oxazolines; in all cases catalysts formed from diphenylphosphino-substituted KITPHOS monophosphines outperformed their dicyclohexylphosphino counterparts as well as that based on triphenylphosphine, an indication that the biaryl/biaryl-like framework may be responsible for imparting high catalyst efficiency.
A homologous series
of triaryl-like KITPHOS-type monophosphines
containing one, two, or three bulky 12-phenyl-9,10-dihydro-9,10-ethenoanthracene
(KITPHOS) units have been developed, and the influence of increasing
steric bulk on their efficacy as ligands in gold(I)-catalyzed carbon–heteroatom
bond-forming cyclizations has been investigated. Detailed solution
NMR studies on Ph-TRISKITPHOS, its oxide, and the corresponding gold(I)
chloride adduct identified a conformational exchange process involving
a concerted librational motion of the individual anthracene-derived
organic substituents about their P–C bonds. The cessation of
this motion at reduced temperatures lowers the molecular symmetry
such that the two C6H4 rings in each of the
KITPHOS units become inequivalent; a lower energy process involving
restricted rotation of the biaryl-like phenyl ring has also been identified.
Electrophilic gold(I) complexes of these triaryl-like KITPHOS monophosphines
catalyze the 5-exo-dig cycloisomerization of propargyl
amides to afford the corresponding methylene oxazolines, which were
used in a subsequent tandem carbonyl-ene reaction to afford functionalized
2-substituted oxazolines. A comparative survey revealed that catalyst
efficiency for cycloisomerization decreases in the order MONOKITPHOS
= BISKITPHOS > PPh3 > TRISKITPHOS. The optimum system
also
catalyzes the selective 6-endo-dig cyclization of
2-alkynylbenzyl alcohols, 2-alkynylbenzoic acid, and 2-phenylethynyl
benzamides; gratifyingly, in several cases the yields obtained are
markedly higher and/or reaction times significantly shorter than those
previously reported for related gold catalysts. Moreover, these are
the first examples of gold(I)-catalyzed 6-endo-dig
cycloisomerizations involving 2-phenylethynyl benzamides and, reassuringly,
the optimum gold(I)/MONOKITPHOS systems either rivaled or outperformed
existing silver or palladium-based catalysts. The steric parameters
of this homologous series of phosphines have been quantified and compared
with selected triarylphosphines using a combination of Solid-G calculations,
to determine the percentage of the metal coordination sphere shielded
by the phosphine (the G parameter), and Salerno molecular
buried volume calculations (SambVca) to determine
the percent buried volume (%V
bur); the
corresponding Tolman cone angles have also been determined from correlations.
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