Coordination Chemistry in Water of a Free and a Lipase-Embedded Cationic NCN-Pincer Platinum Center with Neutral and Ionic Triarylphosphines

Oxidative Addition of a Hypervalent Iodine Compound to Cycloplatinated(II) Complexes for the C-O Bond Construction: Effect of Cyclometalated Ligands

The complex [PtMe(Obpy)(OAc)₂(H₂O)], 2a, Obpy = 2,2'-bipyridine N-oxide, is prepared through the reaction of [PtMe(Obpy)(SMe₂)], 1a, by 1 equiv of PhI(OAc)₂ via an oxidative addition (OA) reaction. Pt(IV) complex 2a attends the process of C-O bond reductive elimination (RE) reaction to form methyl acetate and corresponding Pt(II) complex [Pt(Obpy)(OAc)(H₂O)], 3a. The kinetic of OA and RE reactions are investigated by means of different spectroscopies. The obtained results show that the reaction rates of OA step of 1a are faster than its analogous complex [PtMe(ppy)(SMe₂)], 1b, ppy = 2-phenylpyridine. The density functional theory (DFT) calculations signify that the OA reaction initiated by a nucleophilic attack of the platinum(II) central atom of 1b on the iodine(III) atom while it had commenced by a nucleophilic substitution reaction of coordinated SMe₂ in 1a with a carbonyl oxygen atom of PhI(OAc)₂. Our calculation revealed that the key step for 1a is an acetate transfer from the I(III) to Pt(II) through a formation of square pyramidal iodonium complex. This can be attributed to the more electron-withdrawing character of Obpy ligand than to ppy which reduces the nucleophilicity of Pt atom in 1a. Furthermore, 2a with electron-withdrawing Obpy ligand prone to C-O bond formation faster than complex [PtMe(ppy)(OAc)₂(H₂O)], 2b, with an electron-rich ppy ligand which conforms to the anticipation that REs occur faster on electron-poor metal centers.

Replacing the Z-phenyl Ring in Tamoxifen® with a para-Connected NCN Pincer-Pt-Cl Grouping by Post-Modification †

Post-modification of a series of NCN-pincer platinum(II) complexes [PtX(NCN-R-4)] (NCN = [C₆H₂(CH₂NMe₂)₂-2,6]^–, R = C(O)H, C(O)Me and C(O)Et), X = Cl^– or Br^–) at the para-position using the McMurry reaction was studied. The synthetic route towards two new [PtCl(NCN-R-4)] (R = C(O)Me and C(O)Et) complexes used above is likewise described. The utility and limitations of the McMurry reaction involving these pincer complexes was systematically evaluated. The predicted “homo-coupling” reaction of [PtBr(NCN-C(O)H-4)] led to the unexpected formation of 3,3′,5,5′-tetra[(dimethylamino)methyl]-4,4′-bis(platinum halide)-benzophenone (halide = Br or Cl), referred to hereafter as the bispincer-benzophenone complex 13. This material was further characterized using X-ray crystal structure determination. The applicability of the pincer complexes in the McMurry reaction is shown to open a route towards the synthesis of tamoxifen-type derivatives of which one phenyl ring of Tamoxifen^® itself is replaced by an NCN arylplatinum pincer fragment. The newly synthesized derivatives can be used as potential candidates in anti-cancer drug screening protocols. Two NCN-arylpincer platinum tamoxifen type derivatives, 5 and 6, were successfully synthesized and of 5 the separation of the diastereomeric E-/Z-forms was achieved. Compound 6, which is the pivaloyl protected NCN pincer platinum hydroxy-Tamoxifen^® derivative, was obtained as a mixture of E-/Z-isomers. The new derivatives were further analyzed and characterized with ¹H-, ¹³C{¹H}- and ¹⁹⁵Pt{¹H}-NMR, IR, exact mass MS and elemental analysis.

Direct Aryloxylation/Alkyloxylation of Dialkyl Phosphonates for the Synthesis of Mixed Phosphonates

A strategy for the direct functionalization strategy of inertial dialkyl phosphonates with hydroxy compounds to afford diverse mixed phosphonates with good yields and functional-group tolerance has been developed. Mechanistic investigations involving both NMR studies and DFT studies suggest that an unprecedented highly reactive P^V species (phosphoryl pyridin-1-ium salt), a key intermediate for this new synthetic transformation, is generated in situ from dialkyl phosphonate in the presence of Tf₂ O/pyridine.

Artificial Metalloenzymes: Reaction Scope and Optimization Strategies

The incorporation of a synthetic, catalytically competent metallocofactor into a protein scaffold to generate an artificial metalloenzyme (ArM) has been explored since the late 1970's. Progress in the ensuing years was limited by the tools available for both organometallic synthesis and protein engineering. Advances in both of these areas, combined with increased appreciation of the potential benefits of combining attractive features of both homogeneous catalysis and enzymatic catalysis, led to a resurgence of interest in ArMs starting in the early 2000's. Perhaps the most intriguing of potential ArM properties is their ability to endow homogeneous catalysts with a genetic memory. Indeed, incorporating a homogeneous catalyst into a genetically encoded scaffold offers the opportunity to improve ArM performance by directed evolution. This capability could, in turn, lead to improvements in ArM efficiency similar to those obtained for natural enzymes, providing systems suitable for practical applications and greater insight into the role of second coordination sphere interactions in organometallic catalysis. Since its renaissance in the early 2000's, different aspects of artificial metalloenzymes have been extensively reviewed and highlighted. Our intent is to provide a comprehensive overview of all work in the field up to December 2016, organized according to reaction class. Because of the wide range of non-natural reactions catalyzed by ArMs, this was done using a functional-group transformation classification. The review begins with a summary of the proteins and the anchoring strategies used to date for the creation of ArMs, followed by a historical perspective. Then follows a summary of the reactions catalyzed by ArMs and a concluding critical outlook. This analysis allows for comparison of similar reactions catalyzed by ArMs constructed using different metallocofactor anchoring strategies, cofactors, protein scaffolds, and mutagenesis strategies. These data will be used to construct a searchable Web site on ArMs that will be updated regularly by the authors.

Lipase active site covalent anchoring of Rh(NHC) catalysts: towards chemoselective artificial metalloenzymes

Artificial metallo-enzymes derived from active site-inhibited lipases show chemoselective reactivity in catalytic hydrogenations. Embedding of a non-natural metallic center leads to full and competitive selectivity.