C(arenium)-C(alkyl) bond making and breaking: key process in the platinum-mediated C(aryl)-C(alkyl) bond formation. Analogies to organic electrophilic aromatic substitution

The reaction of cationic platinum aqua complexes 2 [Pt(C(6)H(2)[CH(2)NMe(2)](2)-E-4)(OH(2))](X') (X' = SO(3)CF(3), BF(4)) with alkyl halides RX gave various air-stable arenium complexes 3-5 containing a new C-C bond (R = Me, 3; Et, 4; Bn, 5). Electron-releasing oxo-substituents on the aromatic ligand (E = e.g., OH, b; OMe, c) enhance the reactivity of the aqua complex 2 and were essential for arenium formation from alkyl halides different from MeX. This process is initiated by oxidative addition of alkyl halides to the platinum(II) center of 2, which affords (alkyl)(aryl) platinum(IV) complexes (e.g., 9, alkyl = benzyl) as intermediates. Spectroscopic analyses provided direct evidence for a subsequent reversible 1,2-sigmatropic shift of the alkyl group along the Pt-C(aryl) bond, which is identical to repetitive C(arenium)-C(alkyl) bond making and breaking and concerted metal reduction and oxidation. Temperature-dependent NMR spectroscopy revealed DeltaH degrees = -1.3 (+/- 0.1) kJ mol(-1), DeltaS degrees = +3.8 (+/- 0.2) J mol(-1) K(-1), and DeltaG degrees (298) = -2.4 (+/- 0.1) kJ mol(-1) for the formation of the arenium complex 5b from 9 involving the migration of a benzyl group. The arenium complexes were transformed to cyclohexadiene-type addition products 7 or to demetalated alkyl-substituted arenes, 8, thus completing the platinum-mediated formation of a sp(2)-sp(3) C-C bond which is analogous to the aromatic substitution of a [PtX](+) unit by an alkyl cation R(+). The formation of related trimethylsilyl arenium complexes 6 suggests arenium complexes as key intermediates, not only in (metal-mediated) sp(2)-sp(3) C-C bond making and breaking but also in silyl-directed cyclometalation.     

Cu(ClO(4))(2)-mediated arene C-H bond halogenations of azacalixaromatics using alkali metal halides as halogen sources

Regiospecific halogenation of azacalix[1]arene[3]pyridines at the lower rim position of the benzene ring was achieved from their cross-coupling reaction with cost-effective alkali metal halides through the Cu(ClO(4))(2)-mediated aerobic aryl C-H activation, which gave structurally well-defined aryl-Cu(III) intermediates, and a subsequent C-X bond formation reaction under very mild conditions.     

Efficient 2-aryl benzothiazole formation from acetophenones,anilines, and elemental sulfur by iodine-catalyzed oxidative C(CO)-C(alkyl) bond cleavage

A novel and efficient iodine-catalyzed one-pot reaction of aromatic amines, acetophenones, and elemental sulfur for the synthesis of 2-aryl benzothiazoles is described. The process involves sequential C?S and C-N bond formation followed by C(CO)?C bond cleavage from readily accessible starting materials. A wide range of functional groups is tolerated under oxidant and metal-free condition and moderate to good product yields are obtained.     

A density functional study of C1-C4 alkyl adsorption on Cu(111)

To better understand the nature of alkyl intermediates often invoked in reactions involving hydrocarbon reactants and products, the adsorption of linear and branched C(1)-C(4) alkyls on Cu(111) at 1/4 ML and 1/9 ML coverages was studied using density functional theory. The adsorption energy and site preference are found to be coverage-dependent, and both direct alkyl-alkyl interactions and changes in the Cu electronic structure play a role in these trends. It was found that methyl strongly prefers the hollow sites, the branched alkyls strongly prefer the top site, and the linear C(2)-C(4) alkyls have weak site preferences that change with coverage. To explain these differences, rationalize alkyl adsorption trends, and predict the binding energy of other alkyls, a simple model was developed in which the binding energy is fit as a linear function of the number of C-Cu and C-H-Cu interactions as well as the C-H bond energy in the corresponding alkane. Site preference can be understood as a compromise between C-Cu interactions and C-H-Cu interactions. Density of states analysis was used to gain a molecular-orbital understanding of the bonding of alkyls to Cu(111).     

C(sp)-C(sp3) bond formation through Cu-catalyzed cross-coupling of N-tosylhydrazones and trialkylsilylethynes

Copper-catalyzed cross-coupling of N-tosylhydrazones with trialkylsilylethynes leads to the formation of C(sp)-C(sp(3)) bonds. Cu carbene migratory insertion is proposed to play the key role in this transformation.     

A new reaction manifold for the Barton radical intermediates: synthesis of N-heterocyclic furanosides and pyranosides via the formation of the C(1)-C(2) bond

The first radical intermediate in the thiourethane-mediated deoxygenation of an alcohol (Barton-McCombie reaction) can participate in an exo-hex-5-enyl- or exo-hept-6-enyl-type radical cyclization when a suitable radical acceptor (e.g., alpha,beta-unsaturated ester, oxime ether, or hydrazone) is appropriately placed. Carbohydrate-derived imidazolyl and triazolyl thiourethanes with such acceptors, upon addition to excess of a good hydride donor (reverse addition), undergo efficient cyclization reactions to give N-heterocyclic furanosides, and, surprisingly even N-pyranosides. Depending on the acceptor, glycosides with either a C(2)()-amino or a C(2)()-carbon substituent are formed.     

Synthesis of alkyl dithiocarbamates by using thiuram disulfide reagents: A metal-free C(sp3)-S bond formation

An efficient and environmentally benign protocol for the synthesis of S-alkyl dithiocarbamates has been achieved. By reacting tetraalkylthiuram disulfides with alkyl halides, the desired products could be obtained in good to excellent yields. The methodology features efficiency, transition-metal-free, nice yields and easy performance, illustrating it practical synthetic value for the convenient preparation of some potential biologically active molecules.     

Construction of C(aryl)-C(alkynyl) bond from copper-mediated arene-alkyne and aryl iodide-alkyne cross-coupling reactions: a common aryl-Cu(III) intermediate in arene C-H activation and Castro-Stephens reaction

Both copper(II)-mediated oxidative C-H bond activation and oxidative addition of copper(I) into a C-I bond produced an identical and structurally well-defined aryl-Cu(III) intermediate. The cross-coupling reaction of an aryl-Cu(III) intermediate with both terminal alkynes at an elevated temperature and alkynyllithium reagents under mild conditions led effectively to the formation of a C(aryl)-C(alkynyl) bond. An alternative mechanism has been proposed for the Castro-Stephens reaction.     

C(10)-C(19) bond cleavage reaction of 19-oxygenated androst-4-ene-3,6-dione steroids under various conditions

C(10)-C(19) bond cleavage reaction of 19-hydroxy- and 19-oxoandrost-4-ene-3,6,17-triones (5, 6) was explored under various conditions. Treatment of steroids 5 and 6 with KOH in MeOH gave the A-ring aromatized product 6-oxoestrone (11) in a fair yield, respectively, in contrast, the treatment with a weak base yielded 4-methyl steroid 17 (20%) in the case of 19-alcohol 5 or 19-nor-Delta(5(10))-steroid 9 (12-67%) along with compound 11 (6-27%) in the case of 19-aldehyde 6. Reaction of compound 6 with HCl in MeOH produced 3-methyl ethers of 6-oxoestrone and Delta(6)-estrone, compounds 12 and 14 (ca. 20% each). Thus, 6-oxosteroids 5 and 6 showed unique C(10)-C(19) bond cleavage reactions with a base or acid.     

Heats of formation of C1?C4 alkyl nitrites (RONO) and their RO-NO bond dissociation energies

The heats of formation of C₃ and C₄ alkyl nitrites (RONO) have been determined via their heats of combustion by bomb calorimetry, thereby providing a complete set of values of ΔHº_f for C₁-C₄ alkyl nitrites. The experimental values are in excellent agreement with values derived from group additivity rules. For branched compounds these calculations involve corrections for gauche interactions. In these cases, the gauche interactions are reflected in the activation energies E₁ determined by recent kinetic studies, required for breaking the RO-NO bond. The heats of formation of the alkoxy radicals involved together with ΔHº_f(NO) = 21.6 kcal/mole leads to the result D(RO-NO) = 41.5 ± 1 kcal/mole. The concordance between D(thermochemical) and D(kinetic), unlike previous kinetic studies, implies that E₂ = 0 ± 1 kcal/mole.     

(CF(3))(4)Au(2)(C(5)H(5)N)(2)] - a new alkyl gold(ii) derivative with a very short Au-Au bond

A new gold(ii) species [(CF(3))(4)Au(2)(C(5)H(5)N)(2)] with a very short unsupported Au-Au bond (250.62(9) pm) was generated by photo irradiation of a silver aurate, [Ag(Py)(2)][Au(CF(3))(2)], unambiguously characterized by (19)F and (109)Ag NMR studies.     

Carbon-nitrogen bond formation in the reaction of the reaction of diphenyl diazomethane Ph2 CN2 with diiron-carbene complexes (Fe2(CO)7x03BD;-C(R)C(NEt2)x03BD;-C(R)C(NEtx03BC;-C(R)C(NEt2)N(NCPh2)x03BC;-C(R)C(NEtx03BC;-C(R)C(NEt2)NN(CPh2)x03BC;-C(R)C(NEt)]

The diiron ynamine complex [Fe₂(CO)₇{μ-CR)C(NEt₂)}] (1:R=Me,2:R = C₃H₅.3:R=SiMe₃.4:R = Ph) reacts at room temperature with diphenyldiazomethane Ph₂CN₂, in hexane to yield complexes [Fe₂(CO)₆{C(R)C(NEt₂)N (NCPh₂)] (5a:R=Me,6a:R=C₃H₅.7a R=SiMe₃.8a:R=Ph) resulting from the insertion of the terminal nitrogen atom into the Fe=C carbene bond. Insertion the second nitrogen atom and formation of compounds [Fe₂(CO)₆zμ-C(R)C(NEt₂)NN(CPh₂)}] (5b:R=Me,6b:R=C₃H₅,7b:R=SiMe₃,8b:R=Ph) is observed when compounds5a-5a are treated in refluxing hexane. Transformation of compoundsa tob is also obtained at room temperature within a few days. All compounds were identified by their¹H NMR spectra. Compounds6a, 7a, 8a, and8b were characterized by single crystal X-ray diffraction analyses. Crystal data: for6a: space group = P2₁/n,a=12.853(1) A,b=24.800(7) A,c=8.947(6) A,β=99.29(3)°,Z=4, 2227 rellectionsR=0,038; for7a: space group=Pl,a=ll.483(4) A,b=14.975(4) A,c = 17.890(8) A,α = 82.80(3)°,β=94.29(7)°,γ=85.42(2),Z = 4, 5888 reflectionR = 0.035: for8a: space group = Pcab.a = 31.023(8) A.b=20.137(1) A.c=9.686(2) A.Z=8. 1651 reflections,R=0.071; for8b: space group=P2₁/n,a=21.459(4),b=10,100(3) A,c=28,439(8) A,ß=103.86(4)°,Z=8. 2431 reflections.R=0.057.     

Linear metal-metal-bonded tetranuclear M-Mo-Mo-M complexes (M = Ir and Rh): oxidative metal-metal bond formation in a tetrametallic system and 1,4-addition reaction of alkyl halides

Reaction of Mo2(pyphos)4 (1) with [MCl(CO)2]2 (M = Ir and Rh) afforded linear tetranuclear complexes of a formula Mo2M2(CO)2(Cl)2(pyphos)4 (2, M = Ir; 3, M = Rh). X-ray diffraction studies confirmed that two "MCl(CO)" fragments are introduced into both axial sites of the Mo2 core in 1 and coordinated by two PPh2 groups in a trans fashion, thereby forming a square-planar geometry around each M(I) metal. Treatment of 2 and 3 with an excess amount of tBuNC and XylNC induced dissociation of the carbonyl and chloride ligands to yield the corresponding dicationic complexes [Mo2M2(pyphos)4(tBuNC)4](Cl)2 (5a, M = Ir; 6a, M = Rh) and [Mo2M2(pyphos)4(XylNC)4](Cl)2 (7, M = Ir; 8, M = Rh). Their molecular structures were characterized by spectroscopic data as well as X-ray diffraction studies of BPh4 derivatives [Mo2M2(pyphos)4(tBuNC)4](BPh4)2 (5b, M = Ir; 6c, M = Rh), which confirmed that there is no direct sigma-bonding interaction between the M(I) atom and the Mo2 core. The M(I) atom in 5 and 6 can be oxidized by either 2 equiv of [Cp2Fe][PF6] or an equimolar amount of I2 to afford Mo(II)2M(II)2 complexes, [Mo2M2(X)2(tBuNC)4(pyphos)4]2+ in which two Mo-M(II) single bonds are formed and the bond order of the Mo-Mo moiety has been decreased to three. The Ir(I) complex 5a reacted not only with methyl iodide but also with dichloromethane to afford the 1,4-oxidative addition products [Mo2Ir2(CH3)(I)(tBuNC)4(pyphos)4](Cl)2 (13) and [Mo2Ir2(CH2Cl)(Cl)(tBuNC)4(pyphos)4](Cl)2 (15), respectively, although the corresponding reactions using the Rh(I) analogue 6 did not proceed. Kinetic analysis of the reaction with CH3I suggested that the 1,4-oxidative addition to the Ir(I) complex occurs in an SN2 reaction mechanism.     

Direct detection of polyynes formation from the reaction of ethynyl radical (C2H) with propyne (CH3-C[triple bond]CH) and allene (CH2=C=CH2)

The reactions of the ethynyl radical (C(2)H) with propyne and allene are studied at room temperature using an apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. The C(2)H radical is prepared by 193-nm photolysis of CF(3)CCH and the mass spectrum of the reacting mixture is monitored in time using synchrotron-photoionization with a dual-sector mass spectrometer. Analysis using photoionization efficiency curves allows the isomer-specific detection of individual polyynes of chemical formula C(5)H(4) produced by both reactions. The product branching ratios are estimated for each isomer. The reaction of propyne with ethynyl gives 50-70% diacetylene (H-C[triple bond]C-C[triple bond]C-H) and 50-30% C(5)H(4), with a C(5)H(4)-isomer distribution of 15-20% ethynylallene (CH(2)=C=CH-C[triple bond]CH) and 85-80% methyldiacetylene (CH(3)-C[triple bond]C-C[triple bond]CH). The reaction of allene with ethynyl gives 35-45% ethynylallene, 20-25% methyldiacetylene and 45-30% 1,4-pentadiyne (HC[triple bond]C-CH(2)-C[triple bond]CH). Diacetylene is most likely not produced by this reaction; an upper limit of 30% on the branching fraction to diacetylene can be derived from the present experiment. The mechanisms of polyynes formation by these reactions as well as the implications for Titan's atmospheric chemistry are discussed.     

Magnetic coupling through the carbon skeleton of malonate in two polymorphs of ([Cu(bpy)(H2O)][Cu(bpy)(mal)(H2O)])(ClO4)2 (H2mal = malonic acid; bpy = 2,2'-bipyridine)

Two polymorphic malonato-bridged copper(II) complexes of formula ([Cu(bpy)(H2O)][Cu(bpy)(mal)(H2O)])-(ClO4)2 (1 and 2) (bpy = 2,2'-bipyridine and mal = malonate dianion) have been prepared and their structures solved by X-ray diffraction methods. Compound 1 crystallizes in the monoclinic space group P2(1)/a, with a = 23.743(3) A, b = 9.7522(5) A, c = 27.731(2) A, beta = 114.580(10) degrees, and Z = 4. Compound 2 crystallizes in the orthorhombic space group Pbcn, with a = 23.700(5) A, b = 25.162(5) A, c = 9.693(5) A, and Z = 4. The structures of 1 and 2 are made up of uncoordinated perchlorate anions and malonate-bridged zigzag copper(II) chains grouped in an isosceles triangle running parallel to the b (1) and c (2) axes. These chains are built by a [Cu(bpy)(mal)(H2O)] unit acting as bis-monodentate ligand toward two [Cu(bpy)(H2O)] adjacent units through its OCCCO skeleton in an anti-anti conformation, whereas the OCO carboxylate bridges exhibit the anti-syn conformation. Compounds 1 and 2 contain four crystallographically independent copper(II) atoms, but the environment of all of them is distorted square pyramidal: the axial position is occupied by a water molecule, whereas the equatorial plane is formed by a chelating bpy and either a bidentate malonate or two carboxylate oxygens from two malonate groups. The equatorial Cu-O(mal) (1.911(4)-1.978(4) (1) and 1.897(5)-1.991(4) A (2)) and Cu-N(bpy) (1.983(4)-2.008(5) (1) and 1.971(6)-2.007(6) A (2)) bonds are somewhat shorter than the axial Cu-O(w) one (2.257(5)-2.524(5) (1) and 2.236(5)-2.505(6) A (2)). The angles subtended at the copper atom by the chelating bpy vary in the ranges 80.9(2)-81.8(2) degrees (1) and 80.4(2)-82.1(2) degrees (2), values which are somewhat smaller than those of the chelating malonate (80.4(2)-82.1(2) degrees (1) and 93.0(2)-93.6(2) degrees (2)). The intrachain copper-copper separations through the OCCCO fragment are 8.227(1) (1) and 8.206(2) A (2), whereas those through the OCO bridging unit are 4.579(1)-5.043(1) (1) and 4.572(2)-5.040(2) A (2). The magnetic behavior of 1 and 2 in the temperature range 2.0-290 K is very close, and it corresponds to an overall ferromagnetic coupling, the chi MT versus T curve exhibiting a maximum at 18 K. The analysis of the magnetic data through a numerical expression derived for the real topology of 1 and 2, that is, chains of isosceles triangles with two intrachain exchange pathways J1 (exchange coupling through the OCO carboxylate) and J2 (exchange coupling through the OCCCO malonate), indicates the occurrence of ferro- (J1 = +4.6 cm-1) and antiferromagnetic couplings (J2 = -4.2 cm-1). The magnetic coupling through these exchange pathways is further analyzed and substantiated by density functional theory calculations on a malonate-bridged trinuclear copper(II) model system.     

Conversion of [Pt(SRf)2(PPh2 -n(C6F5)n+ 1)2]( n = 0 or 1, Rf=C6HF4-4) through carbon-fluorine bond activation to [Pt(SRf)2(1,2-C6F4(SRf)-(PPh2))] and chiral [Pt(SRf)2(1,2-C6F4(SRf)(PPh(C6F5)))

Treatment of trans-[PtCl(2)(PPh(2 - n)(C(6)F(5))(n + 1))(2)](n = 0 or 1) with Pb(SC(6)HF(4)-4)(2) yields a mixture of monometallic cis/trans [Pt(SC(6)HF(4)-4)(2)(PPh(2 - n)(C(6)F(5))(n + 1))(2)], thiolate-bridged bimetallic cis/trans [Pt(2)(mu-SC(6)HF(4)-4)(2)(SC(6)HF(4)-4)(2)(PPh(2 - n)(C(6)F(5))(n + 1))(2)] and [Pt(SC(6)HF(4)-4)(2)(1,2-C(6)F(4)(SC(6)HF(4)-4)(PPh(2 - n)(C(6)F(5))(n))].     

Example of highly stereoregulated ruthenium amidine complex formation: synthesis,crystal structures,and spectral and redox properties of the complexes [Ru(II)(trpy)[NC(5)H(4)-CH=N-N(C(6)H(5))C(CH(3))=NH]](ClO(4))(2) (1) and [Ru(II)(trpy)(NC(5)H(4)-CH=N-NH-C(6)H(5))Cl]ClO(4) (2) (trpy = 2,2':6',2"-terpyridine)

The reaction of Ru(trpy)Cl(3) (trpy = 2,2':6',2"-terpyridine) with the pyridine-based imine function N(p)C(5)H(4)-CH=N(i)-NH-C(6)H(5) (L), incorporating an NH spacer between the imine nitrogen (N(i)) and the pendant phenyl ring, in ethanol medium followed by chromatographic work up on a neutral alumina column using CH(3)CN/CH(2)Cl(2) (1:4) as eluent, results in complexes of the types [Ru(trpy)(L')](ClO(4))(2) (1) and [Ru(trpy)(L)Cl]ClO(4) (2). Although the identity of the free ligand (L) has been retained in complex 2, the preformed imine-based potentially bidentate ligand (L) has been selectively transformed into a new class of unusual imine-amidine-based tridentate ligand, N(p)C(5)H(4)-CH=N(i)-N(C(6)H(5))C(CH(3))=N(a)H (L'), in 1. The single-crystal X-ray structures of the free ligand (L) and both complexes 1 and 2 have been determined. In 2, the sixth coordination site, that is, the Cl(-) function, is cis to the pyridine nitrogen (N(p)) of L which in turn places the NH spacer away from the Ru-Cl bond, whereas, in 1, the corresponding sixth position, that is, the Ru-N(a) (amidine) bond, is trans to the pyridine nitrogen (N(p)) of L'. The trans configuration of N(a) with respect to the N(p) of L' in 1 provides the basis for the selective L --> L' transformation in 1. The complexes exhibit strong Ru(II) --> pi* (trpy) MLCT transitions in the visible region and intraligand transitions in the UV region. The lowest energy MLCT band at 510 nm for 2 has been substantially blue-shifted to 478 nm in the case of 1. The reversible Ru(III)-Ru(II) couples for 1 and 2 have been observed at 0.80 and 0.59 V versus SCE, respectively. The complexes are weakly luminescent at 77 K, exhibiting emissions at lambda(max), 598 nm [quantum yield (Phi) = 0.43 x 10(-2)] and 574 nm (Phi = 0.28 x 10(-2)) for 1 and 2, respectively.