Controlling Reactivity and Selectivity in the Mizoroki-Heck Reaction: High Throughput Evaluation of 1,5-Diaza-3,7-diphosphacyclooctane Ligands.

We report a high throughput evaluation of the Mizoroki-Heck reaction of diverse olefin coupling partners. Comparison of different ligands revealed the 1,5-diaza-3,7-diphosphacyclooctane (P2N2) scaffold to be more broadly applicable than common "gold standard" ligands, demonstrating that this family of readily accessible diphosphines has unrecognized potential in organic synthesis. In particular, two structurally related P2N2 ligands were identified to enable the regiodivergent arylation of styrenes. By simply altering the phosphorus substituent from a phenyl to tert-butyl group, both the linear and branched Mizoroki-Heck products can be obtained in high regioisomeric ratios. Experimental and computational mechanistic studies were performed to further probe the origin of selectivity, which suggests that both ligands coordinate to the metal in a similar manner but that rigid positioning of the phosphorus substituent forces contact with the incoming olefin in a π-π interaction (for P-Ph ligands) or with steric clash (for P-tBu ligands), dictating the regiocontrol.

Multicomponent Synthesis of Poly(α-aminophosphine chalcogenide)s and Subsequent Depolymerization.

Multicomponent reactions of primary phosphines (R-PH2), diimines (R'-N═C(H)-R-(H)C═N-R'), and chalcogens (O2, S8) generate poly(α-aminophosphine chalcogenide)s (4-7) through step-growth polymerization. Characterization of the linear polymers using 31P{1H} diffusion-ordered NMR spectroscopy (DOSY) experiments aided in determining the molecular weight (Mw) of the material. Subjecting the polyphosphine oxide or sulfide to reducing conditions in the presence of a Lewis acid resulted in complete depolymerization of the polymers, quantitatively releasing the 1° phosphine and diimine (2) starting materials, with concomitant reduction of diimine to diamine (9).

Reactivity of Primary Phosphines and Primary Phosphine Sulfides towards Imines.

Reactivity of primary phosphines with two stoichiometric equivalents of imine results in the formation of bis-α-aminophosphines (2 a-e), which can be subsequently oxidized in the presence of S8 or H2 O2 to generate air stable bis-α-aminophosphine sulfides (2 b-m(S/O)). To elucidate the mechanism of this three-component reaction, Hammett analysis, kinetic isotope effect (KIE), and trapping experiments were performed. Ultimately a P(V)-P(III) tautomerization is invoked, followed by nucleophilic attack by the P(III) species to generate the desired products.

Synthesis and reactivity of tripodal complexes containing pendant bases.

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris(amido)amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt(II) and zinc(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes [Co(L)](-) with dioxygen reversibly generates a small amount of a cobalt(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn[N{CH2CH2NC(O)CH2N(CH2Ph)2}3)](-) ([Zn(TN(Bn))](-)) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes [M(L)](-) results in complete proteolysis and formation of the ligands H3L. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative and acidic conditions.

Hydroxopalladium(IV) complexes prepared using oxygen or hydrogen peroxide as oxidants.

The cycloneophylpalladium(II) complexes [Pd(CH2CMe2C6H4)(κ2-N,N'-L)], 1 or 2, with L = RO(CH2)3N(CH2-2-C5H4N)2, with R = H or Me, respectively, react with either dioxygen or hydrogen peroxide in the presence of NH4[PF6] to give rare examples of the corresponding hydroxopalladium(IV) complexes [Pd(OH)(CH2CMe2C6H4)(κ3-N,N',N''-L)][PF6], 3 or 4. The complexes 3 and 4 are stable at room temperature and have been structurally characterized. On heating a solution of 3 or 4 in moist dimethylsulphoxide, selective reductive elimination with C(sp2)-O bond formation is observed, followed by hydrolysis, to give the corresponding pincer complex [Pd(OH)(κ3-N,N',N''-L)][PF6] and 2-t-butylphenol as major products. A more complex reaction occurs in chloroform solution. The mechanisms of reaction are discussed, supported by DFT calculations.

Operationally unsaturated ruthenium complex stabilized by a phosphine 1-azaallyl ligand.

Coordinatively unsaturated transition-metal compounds stabilized by supplemental electron donation from π-basic ligands are described as "operationally unsaturated". Such complexes are useful analogues of active catalyst structures that readily react with substrate molecules. We report that [Ph2P(C6H4)NCHC(CH3)2]- (L1) effectively stabilizes Ru(II) in an operationally unsaturated form. In the absence of Lewis bases, the 1-azaallyl group of L1 dominantly coordinates through a κ1-N mode, but can readily and reversibly isomerize to an η3-NCC coordination mode to stabilize the metal. As an operationally unsaturated complex, Ru(Cp*)(L1) dimerizes at low temperature. At ambient temperature it rapidly reacts with pyridine or PPh3 to form an adduct. These findings with L1 demonstrate that changes in the hapticity of a 1-azaallyl fragment offer an alternative means to stabilize low-coordinate metals.

Changes in ligand coordination mode induce bimetallic C-C coupling pathways.

Carbon-carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd0/PdII catalytic mechanisms to give new C-C bonds. Bimetallic C-C coupling mechanisms that involve a PdI/PdII redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a PdII-Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a PdI dimer, a PdII monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C-C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C-C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C-H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C-C coupling channel through pathways that are unattainable with statically-coordinated ligands.

A reactive Ru-binaphtholate building block with self-tuning hapticity.

A versatile Ru-BINO building block is reported, which offers a straightforward entry point into the chemistry of atropisomeric binaphtholate complexes of ruthenium. Reaction of RuCl(2)(PPh(3))(3)6a with Tl(2)((S)-BINO) affords Ru((S)-BINO)(PPh(3))(2)7 as a mixture of isomers: in 7', the BINO ligand is bound via η(3)-CCO,η(1)-O' donors, and in symmetrical 7″, via η(3)-CCO,η(3)-O'C'C' interactions. The bis(enolate) BINO bonding mode in the latter, not previously observed for any metal, underscores the remarkable geometric and electronic flexibility of the binaphtholate moiety. The BINO ligand proves able to stabilize complexes containing as few as two, and as many as four, additional ligands in 7 and its derivatives, enabling a synthetic versatility that contrasts with that of the superficially similar o-catecholate complex Ru(o-cat)(PPh(3))(3). As with the important achiral Ru precursor 6a, complex 7 undergoes facile transformation into a range of products under mild conditions, including acetonitrile, pyridine, and vinylidene derivatives. Single-crystal X-ray structures are reported for three of these complexes: Ru(η(3),η(3)-(S)-BINO)(PPh(3))(2)7″, Ru(η(3),η(1)-(S)-BINO)(PPh(3))(2)(MeCN) 9, and Ru(η(3),η(1)-(S)-BINO)(PPh(3))(py)(2)11. (13)C{(1)H} NMR signatures are proposed for new and known BINO coordination modes (η(1)-O,η(1)-O'; η(1)-C1,η(1)-O'; η(3)-CCO,η(3)-O'C'C'; η(3)-CCO,η(1)-O'; η(6)-C(6),η(1)-O'), as a potential aid to further developments in late-metal BINO chemistry.

"Lignophines": lignin-based tertiary phosphines with metal-scavenging ability.

Methacrylated lignin was reacted with PH3(g) to prepare a phosphorus rich bio-based polymer containing PH/PH2 functional groups, which were converted to tertiary phosphine units via the phosphane-ene reaction. This represents a straightforward method for the upconversion of low-value biomass waste to useful inorganic polymer with potential utility in metal scavenging applications.

Optimizing ligand structure for low-loading and fast catalysis for alkynyl-alcohol and -amine cyclization.

A series of [Ru(Cp/Cp*)(PR2NR'2)(MeCN)]PF6 complexes were prepared, in which the steric and electronic properties of the primary coordination sphere were varied (R = Ph, t-Bu, Bn; and Cp vs. Cp*). These complexes were catalytically active in the cyclization of alkyne substrates with an intramolecular nucleophile (amine or alcohol) to produce 5- and 6-membered heterocycles. The effect of the 1° coordination sphere structure on catalyst performance was evaluated. Steric bulk around the metal centre was a key feature to achieve rapid catalysis at low temperatures. The catalyst [Ru(Cp)(Pt-Bu2NPh2)(MeCN)]PF6 gave a turnover number that was >1 order of magnitude more active than previous catalysts in the cyclization of the benchmark substrate 2-ethynylaniline. This catalyst is tolerant of a diversity of functional groups and is competent at the formation of various substituted indoles.

Mild and selective Pd-Ar protonolysis and C-H activation promoted by a ligand aryloxide group.

A bidentate nitrogen-donor ligand with an appended phenol group, C5H4NCH[double bond, length as m-dash]N-2-C6H4OH, H(L1) was treated with a palladium cycloneophyl complex [Pd(CH2CMe2C6H4)(COD)], with both Pd-aryl and Pd-alkyl bonds, to give a Pd-alkyl complex, [Pd(CH2CMe2C6H5)(κ3-N,N',O-OC6H4N[double bond, length as m-dash]CH(2-C5H4N))], 1. The cleavage of the Pd-aryl bond and the deprotonation of the ligand phenol to afford a bound aryloxide, indicates facile Pd-aryl bond protonolysis. Deuterium labelling experiments confirmed that the ligand phenol promotes protonolysis and that the reverse, aryl C-H activation, occurs under very mild reaction conditions (within 10 min at room temperature). An unusual isomerization of the Pd-alkyl complex 1 to a Pd-aryl complex, [Pd(C6H4(2-t-Bu))(κ3-N,N',O-OC6H4N[double bond, length as m-dash]CH(2-C5H4N))], 2, was observed to give an equilibrium with [2]/[1] = 9 after 5 days in methanol. The isomerization requires that both aryl C-H activation and Pd-alkyl protonolysis steps occur. The very large KIE value (kH/kD = ca. 40) for isomerization of 1 to 2, suggests a concerted SE2-type mechanism for the Pd-alkyl protonolysis step.

A Mass-Producible and Versatile Sensing System: Localized Surface Plasmon Resonance Excited by Individual Waveguide Modes.

A plasmonic sensing system that allows the excitation of localized surface plasmon resonance (LSPR) by individual waveguide modes is presented conceptually and experimentally. Any change in the local environment of the gold nanoparticles (AuNPs) alters the degree of coupling between LSPR and a polymer slab waveguide, which then modulates the transmission-output signal. In comparison to conventional LSPR sensors, this system is less susceptible to optical noise and positional variation of signals. Moreover, it enables more freedom in the exploitation of plasmonic hot spots with both transverse electric (TE) and transverse magnetic (TM) modes. Through real-time measurement, it is demonstrated that the current sensing system is more sensitive than comparable optical fiber plasmonic sensors. The highest normalized bulk sensitivity (7.744 RIU-1) is found in the TM1 mode. Biosensing with the biotin-streptavidin system shows that the detection limit is on the order of 10-14 M of streptavidin. With further optimization, this sensing system can easily be mass-produced and incorporated into high throughput screening devices, detecting a variety of chemical and biological analytes via immobilization of the appropriate recognition sites.

Pincer-plus-one ligands in self-assembly with palladium(ii): a molecular square and a molecular tetrahedron.

The combination of a palladium(ii) precursor with a diimine-phenol ligand and an oxidant (H2O2 or O2) under different conditions has, serendipitously, given both a molecular square and a molecular tetrahedron by self-assembly of building blocks comprising palladium(ii) centres coordinated to the oxidised forms of the ligand.