Mononuclear Four-Coordinate Bis-Fluoride Bis-NHC Complexes of Chromium(II), Iron(II), and Cobalt(II).

The reaction between silylamido complexes of Cr(II), Fe(II), and Co(II) and IMes·2HF salt in the presence of IMes (IMes = 1,3-dimesitylimidazol-2-ylidene) led to isolation of Cr(IMes)2F2 (2-Cr), Fe(IMes)2F2 (2-Fe), and Co(IMes)2F2 (2-Co). X-ray structural studies revealed that 2-Cr adopts square planar geometry, while 2-Fe and 2-Co have distorted tetrahedral geometry. Magnetic susceptibility studies of 2-Cr, 2-Fe, and 2-Co were consistent with high-spin complexes, S = 2 for 2-Cr/2-Fe and S = 3/2 for 2-Co. We demonstrated that fluoride can be successfully exchanged for cyanide and azide using trimethylsilyl cyanide and trimethylsilyl azide (3-Fe and 4-Fe). DFT studies suggest that the preference of 2-Cr to adopt square planar geometry over tetrahedral is due to its d4 metal center, where four electrons fill the lower-lying d-orbitals.

DNA Aptamer-Cyanine Complexes as Generic Colorimetric Small-Molecule Sensors.

Aptamers are promising biorecognition elements for sensors. However, aptamer-based assays often lack the requisite levels of sensitivity and/or selectivity because they typically employ structure-switching aptamers with attenuated affinity and/or utilize reporters that require aptamer labeling or which are susceptible to false positives. Dye-displacement assays offer a label-free, sensitive means for overcoming these issues, wherein target binding liberates a dye that is complexed with the aptamer, producing an optical readout. However, broad utilization of these assays has been limited. Here, we demonstrate a rational approach to develop colorimetric cyanine dye-displacement assays that can be broadly applied to DNA aptamers regardless of their structure, sequence, affinity, or the physicochemical properties of their targets. Our approach should accelerate the development of mix-and-measure assays that could be applied for diverse analytical applications.

Olefin Metathesis by First-Row Transition Metals.

Catalytic olefin metathesis based on the second- and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to first-row metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known.

Stereospecific Ring-Opening Metathesis Polymerization of Norbornene Catalyzed by Iron Complexes.

Developing well-defined iron-based catalysts for olefin metathesis would be a breakthrough achievement in the field not only to replace existing catalysts by inexpensive metals but also to attain a new reactivity taking advantage of the unique electronic structure of the base metals. Here, we present a two-coordinate homoleptic iron complex, Fe(HMTO)2 [HMTO=O-2,6-(2,4,6-Me3 C6 H2 )2 C6 H3 ], that is capable of performing ring-opening metathesis polymerization of norbornene to produce highly stereoregular polynorbornene (99 % cis, syndiotactic). The use of heteroleptic Fe(HMTO)(RO) [RO=(CH3 )2 CF3 CO, CH3 (CF3 )2 CO, or Ph(CF3 )2 CO] prepared in situ significantly increases the polymerization rate while preserving selectivity. The resulting polymers were characterized by 1 H and 13 C NMR spectroscopy and gel-permeation chromatography.

Ring-Closing Olefin Metathesis Catalyzed by Well-Defined Vanadium Alkylidene Complexes.

Vanadium-based catalysts have shown activity and selectivity in ring-opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring-closing olefin metathesis catalyzed by well-defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron-donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both ß-hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation.

Syntheses of Molybdenum Oxo Benzylidene Complexes.

The reaction between Mo(O)(CHAro)(ORF6)2(PMe3) (Aro = ortho-methoxyphenyl, ORF6 = OCMe(CF3)2) and 2 equiv of LiOHMT (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) leads to Mo(O)(CHAro)(OHMT)2, an X-ray structure of which shows it to be a trigonal bipyramidal anti benzylidene complex in which the o-methoxy oxygen is coordinated to the metal trans to the apical oxo ligand. Addition of 1 equiv of water (in THF) to the benzylidyne complex, Mo(CArp)(OR)3(THF)2 (Arp = para-methoxyphenyl, OR = ORF6 or OC(CF3)3 (ORF9)) leads to formation of {Mo(CArp)(OR)2(µ-OH)(THF)}2(µ-THF) complexes. Addition of 1 equiv of a phosphine (L) to Mo(CArp)(ORF9)3(THF)2 in THF, followed by addition of 1 equiv of water, all at room temperature, yields Mo(O)(CHArp)(ORF9)2(L) complexes in good yields for several phosphines (e.g., PMe2Ph (69% by NMR), PMePh2 (59%), PEt3 (69%), or P( i-Pr)3 (65%)). The reaction between Mo(O)(CHArp)(ORF9)2(PEt3) and 2 equiv of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo(O)(CHArp)(OHMT)2, a four-coordinate syn alkylidene complex. Mo(O)(CHArp)(OHMT)2 reacts with ethylene (1 atm in C6D6) to give (in solution) a mixture of Mo(O)(CHArp)(OHMT)2, Mo(O)(CH2)(OHMT)2, and an unsubstituted square pyramidal metallacyclobutane complex, Mo(O)(CH2CH2CH2)(OHMT)2, along with ethylene and ArpCH═CH2. Mo(O)(CHArp)(OHMT)2 also reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the "first-insertion" products that contain a cis C═C bond.

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex.

Addition of one equiv of water to Mo(CAr)[OCMe(CF3)2]3(1,2-dimethoxyethane) (2, Ar = o-(OMe)C6H4) in the presence of PPhMe2 leads to formation of Mo(O)(CHAr)[OCMe(CF3)2]2(PPhMe2) (3(PPhMe2)) in 34% yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, {Mo(CAr)[OCMe(CF3)2]2(µ-OH)}2(dme) (4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group. Addition of PMe3 to 4(dme) gives the alkylidene oxo complex, (3(PMe3)), an analogue of 3(PPhMe2) (95% conversion, 66% isolated). Treatment of 3(PMe3) with two equiv of HCl gave Mo(O)(CHAr)Cl2(PMe3) (5), which upon addition of LiO-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe3) (6). Compound 6 in the presence of B(C6F5)3 will initiate the ring-opening metathesis polymerization of cyclooctene, 5,6-dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocoupling of 1-decene to 9-octadecene. The poly(DCMNBD) has a cis,syndiotactic structure, whereas poly(DCMNBE) has a cis,syndiotactic,alt structure. X-ray structures were obtained for 3(PPhMe2), 4(dme), and 6.

Thiols make for better catalysts: Au nanoparticles supported on functional SBA-15 for catalysis of Ullmann-type homocouplings.

A strategy for arraying small gold nanoparticles on a mesoporous support modified with single-component or mixed self-assembled monolayers is described. The use of mixed surface modifiers allows easy access to a range of surface chemistries and modalities of interaction between nanoparticles and supports. A combination of thiol groups and linear semifluorinated chains effectively stabilized the nanoparticles against aggregation, while preserving their catalytic activity. The thiol-fluorous-supported catalyst was found active in Ullmann-type homocoupling of aryl halides and showed exceptional selectivity in this reaction.

Synthesis of 2,6-Hexa-tert-butylterphenyl Derivatives, 2,6-(2,4,6-t-Bu3C6H2)2C6H3X, where X = I, Li, OH, SH, N3, or NH2.

A "double benzyne" reaction between 1,3-dichloro-2-iodobenzene and 2,4,6-t-Bu3C6H2MgBr followed by the addition of iodine led to 2,6-(2,4,6-t-Bu3C6H2)2C6H3I (HTBTI) in 65% yield. Lithiation of HTBTI with Li-t-Bu gave Li(Et2O)2HTBT from which HTBTSH, HTBTN3, HTBTNH2, and HTBTOH were prepared. An X-ray structure of W(OHTBT)2Cl4 shows that the two HTBTO ligands are trans to one another with the t-Bu3C6H2 groups on one HTBTO interdigitated with the t-Bu3C6H2 groups on the other HTBTO.

Syntheses of Molybdenum Adamantylimido and t-Butylimido Alkylidene Chloride Complexes Using HCI and Diphenylmethylphosphine.

Reactions between Mo(N-t-Bu)2(CH2-t-Bu)2 or Mo(NAdamantyl)2(CH2CMe2Ph)2 and 3 equiv of HCl in the presence of 1 equiv of PPh2Me yield Mo(NR)(CHR')(PPh2Me)Cl2 complexes, from which Mo(NR)(CHR')(PPh2Me)(OAr)Cl complexes (OAr = a 2,6-terphenoxide) can be prepared. The Mo(NR)(CHR')(PPh2Me)(OAr)Cl complexes were evaluated as cross-metathesis catalysts between cyclooctene and Z-1,2-dichloroethylene. The efficiencies of the test reaction for complexes in which OAr = OTPP, OHMT, OHIPT, or OHTBT (where OTPP is 2,3,5,6-tetraphenylphenoxide, OHMT is hexamethylterphenoxide, OHIPT is hexaisopropylterphenoxide, and OHTBT is hexa-t-butylterphenoxide) maximize when OAr is OHMT or OHIPT. Mo(N-t-Bu)(CH-t-Bu)(PPh2Me)Cl2 is essentially inactive for the reaction between cyclooctene and Z-1,2-dichloroethylene. X-ray structural studies were carried out on Mo(NAd)(CHCMe2Ph)(PPh2Me)Cl2, Mo(N-t-Bu)(CH-t-Bu)(PPh2Me)(OHMT)Cl, Mo(NAd)(CHCMe2Ph)(Cl)(OHTBT)(PMe3), and [Mo(NAd)(CHCMe2Ph)(PMe3)(Cl)]2(µ-O), the product of the reaction between Mo(NAd)(CHCMe2Ph)(Cl)(OHTBT)(PMe3) and 0.5 equiv of water.

Synthesis and Evaluation of Molybdenum and Tungsten Monoaryloxide Halide Alkylidene Complexes for Z-Selective Cross-Metathesis of Cyclooctene and Z-1,2-Dichloroethylene.

Molybdenum complexes with the general formula Mo(NR)(CHR')(OR″)(Cl)(MeCN) (R = t-Bu or 1-adamantyl; OR″ = a 2,6-terphenoxide) recently have been found to be highly active catalysts for cross-metathesis reactions between Z-internal olefins and Z-1,2-dichloroethylene or Z-(CF3)CH═CH(CF3). In this paper we report methods of synthesizing new potential catalysts with the general formula M(NR)(CHR')(OR″)(Cl)(L) in which M = Mo or W, NR = N-2,6-diisopropylphenyl or NC6F5, and L is a phosphine, a pyridine, or a nitrile. We also test and compare all catalysts in the cross-metathesis of Z-1,2-dichloroethylene and cyclooctene. Our investigations indicate that tungsten complexes are inactive in the test reaction either because the donor is bound too strongly or because acetonitrile inserts into a W═C bond. The acetonitrile or pivalonitrile Mo(NR)(CHR')(OR″)(Cl)(L) complexes are found to be especially reactive because the 14e Mo(NR)(CHR')(OR″)Cl core is accessible through dissociation of the nitrile to a significant extent. Pivalonitrile can be removed (>95%) from Mo(NAr)(CHCMe2Ph)(OHMT)(Cl)(t-BuCN) (Ar = 2,6-diisopropylphenyl; OHMT = 2,6-dimesitylphenoxide) to give 14e Mo(NAr)(CHCMe2Ph)(OHMT)Cl in solution as a mixture of syn and anti (60:40 at 0.015 M) nitrile-free isomers, but these 14e complexes have not yet been isolated in pure form. The syn isomer of Mo(NAr)(CHCMe2Ph)(OHMT)Cl binds pivalonitrile most strongly. Other Mo(NR)(CHR')(OR″)(Cl)(L) complexes can be activated through addition of B(C6F5)3. High stereoselectivities (>98% Z,Z) of ClCH═CH(CH2)6CH═CHCl are not restricted to tert-butylimido or adamantylimido complexes; 96.2% Z selectivity is observed with boron-activated Mo(NC6F5)(CHR')(OHIPT)(Cl)(PPhMe2). So far no Mo═CHCl complexes, which are required intermediates in the test reaction, have been observed in NMR studies at room temperature.

Cooperative organocatalysis of Mukaiyama-type aldol reactions by thioureas and nitro compounds.

A unique organocatalytic system for Mukaiyama-type aldol reactions based on the cooperative action of nitro compounds and thioureas has been identified. This system is compatible with a wide range of substrates and does not require low temperatures, inert atmospheres, or an aqueous workup. A catalytic mechanism based on nitro group-mediated silyl cation transfer has been proposed.

Palladium N-Heterocyclic Carbene Precatalyst Site Isolated in the Core of a Star Polymer.

An approach for supporting a Pd-NHC complex on a soluble star polymer with nanoscale dimensions is described. The resulting star polymer catalyst exhibits excellent activity in cross-coupling reactions, is stable in air and moisture, and is easily recoverable and recyclable. These properties are distinct and unattainable with the small-molecule version of the same catalyst.

Amplification of chirality through self-replication of micellar aggregates in water.

We describe a system in which the self-replication of micellar aggregates results in a spontaneous amplification of chirality in the reaction products. In this system, amphiphiles are synthesized from two "clickable" fragments: a water-soluble "head" and a hydrophobic "tail". Under biphasic conditions, the reaction is autocatalytic, as aggregates facilitate the transfer of hydrophobic molecules to the aqueous phase. When chiral, partially enantioenriched surfactant heads are used, a strong nonlinear induction of chirality in the reaction products is observed. Preseeding the reaction mixture with an amphiphile of one chirality results in the amplification of this product and therefore information transfer between generations of self-replicating aggregates. Because our amphiphiles are capable of catalysis, information transfer, and self-assembly into bounded structures, they present a plausible model for prenucleic acid "lipid world" entities.

Cooperative catalysis with block copolymer micelles: a combinatorial approach.

A rapid approach to identifying complementary catalytic groups using combinations of functional polymers is presented. Amphiphilic polymers with "clickable" hydrophobic blocks were used to create a library of functional polymers, each bearing a single functionality. The polymers were combined in water, yielding mixed micelles. As the functional groups were colocalized in the hydrophobic microphase, they could act cooperatively, giving rise to new modes of catalysis. The multipolymer "clumps" were screened for catalytic activity, both in the presence and absence of metal ions. A number of catalyst candidates were identified across a wide range of model reaction types. One of the catalytic systems discovered was used to perform a number of preparative-scale syntheses. Our approach provides easy access to a range of enzyme-inspired cooperative catalysts.

Star block-copolymers: enzyme-inspired catalysts for oxidation of alcohols in water.

A number of fluorous amphiphilic star block-copolymers containing a tris(benzyltriazolylmethyl)amine motif have been prepared. These polymers assembled into well-defined nanostructures in water, and their mode of assembly could be controlled by changing the composition of the polymer. The polymers were used for enzyme-inspired catalysis of alcohol oxidation.