Platinum(IV)-mediated nitrile-sulfimide coupling: a route to heterodiazadienes

Pt(IV)-mediated addition of the sulfimide Ph2S = NH and the mixed sulfide/sulfimides o- and p-[PhS(=NH)](PhS)-C6H4 by the S=NH group to the metal-bound nitriles in the platinum(IV) complexes [PtCl4(RCN)2] proceeds smoothly at room temperature in CH2Cl2 and results in the formation of the heterodiazadiene compounds [PtCl4[NH=C(R)N=SR'Ph]2] (R' = Ph, R = Me, Et, CH2Ph, Ph; R' = o- and p-(PhS)C6H4; R = Et). While trans-[PtCl4(RCN)2] (R = Et, CH2Ph, Ph) reacting with Ph2S=NH leads exclusively to trans-[PtCl4[NH=C(R)N=SPh2]2], cis/trans-[PtCl4(MeCN)2] leads to cis/trans mixtures of [PtCl4[NH=C(Me)N=SPh2]2] and the latter have been separated by column chromatography. Theoretical calculations at both HF/HF and MP2//HF levels for the cis and trans isomers of [PtCl4[NH=C(Me)N=SMe2]2] indicate a higher stability for the latter. Compounds trans-[PtCl4[E-NH=C(R)N=SPh2]2] (R = Me, Et) and cis-[PtCl4[E-NH=C(Me)N=SPh2][Z-NH=C(Me)N=SPh2]] have been characterized by X-ray crystallography. The complexes [PtCl4[NH=C(R)N=SPh2]2] undergo hydrolysis when treated with HCl in nondried CH2Cl2 to achieve the amidines [PtCl4[NH=C(NH2)R]2] the compound with R = Et has been structurally characterized) and Ph2SO. The heterodiazadiene ligands, formed upon Pt(IV)-mediated RCN/sulfimide coupling, can be liberated from their platinum(IV) complexes [PtCl4[NH=C(R)N=SR'Ph]2] by reaction with Ph2PCH2CH2PPh2 (dppe) giving free NH=C(R)=SR'Ph and the dppe oxides, which constitutes a novel route for such rare types of heterodiazadienes whose number has also been extended. The hybrid sulfide/sulfimide species o- and p-[PhS(=NH)](PhS)C6H4 also react with the Pt(II) nitrile complex [PtCl2(MeCN)2] but the coupling--in contrast to the Pt(IV) species--gives the chelates [PtCl2[M-I=C(Me)N=S(Ph)C6H4SPh]]. The X-ray crystal structure of [PtCl2[M-I=C(Me)N=S(Ph)C6H4SPh-o]] reveals the bond parameters within the metallacycle and shows an unusual close interaction of the sulfide sulfur atom with the platinum.     

Cp2TiCl2-catalyzed hydromagnation of disubstituted acetylenes as a stereo-specific route to Z-alkenes

Conclusions The hydromagnation reaction of disubstituted acetylenes has been systematically studied in the presence of catalytic quantities of Cp_{cc 2}TiCl₂. It was found that the conditions used to form the catalytic complex exerted the decisive effect on the course of the reaction, and that the most active hydromagnation reagents were n-Pr₂Mg and i-Bu₂Mg.The hydrotitanation of disubstituted acetylenes using Cp₂TiH was also demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2156–2159, September, 1988.     

Oxidative coupling as a biomimetic approach to the synthesis of scytonemin

The first total synthesis of the dimeric alkaloid pigment scytonemin is described. The key transformations in its synthesis from 3-indole acetic acid are a Heck carbocyclization and a Suzuki-Miyaura cross-coupling, orchestrated in a stereospecific tandem fashion, followed by a biosynthetically inspired oxidative dimerization. The tandem sequence generates a tetracyclic (E)-3-(arylidene)-3,4-dihydrocyclopenta[b]indol-2(1H)-one that is subsequently dimerized into the unique homodimeric core structure of scytonemin.     

Bis-ureidodeazapterin (Bis-DeAP) as a general route to supramolecular star polymers

Supramolecular recognition unit Bis-DeAP, 1, containing two high affinity hydrogen-bonding acceptor-acceptor-donor-donor (AADD) arrays was designed to self-assemble into cyclic assemblies. It was prepared through a highly scalable synthesis and was further functionalized with 2-bromo-2-methylpropanoyl bromide and used to initiate the polymerization of methylmethacrylate (MMA). Bis-DeAP-PMMA polymers quantitatively self-assembled into star polymers in toluene. In DMF Bis-DeAP-PMMA forms a mixture of star polymers and unassembled polymers. Bis-DeAP was also functionalized with polyethylene glycol (PEG) polymers. The Bis-DeAP-PEG polymers formed star polymers in DMF; however, higher molecular weight polymeric assembles that varied with concentration were observed in water. Mixing studies in toluene indicated that the self-assembled star polymers are kinetically stable and resist mixing even at elevated temperatures. In DMF, kinetically controlled structures are initially observed, however, mixing occurs at a faster rate and assembled star polymers show a decrease in polydispersity index (PDI) over time. In addition, Bis-DeAP functionalized PS and PMMA were mixed in DMF to generate a star copolymer through self-assembly.     

Vinyl oxidative coupling as a synthetic route to catalytically active monovalent chromium

Reaction of the deprotonated form of cis-{(t-Bu)N(H)P[μ-N(t-Bu)](2)PN(H)(t-Bu)} with CrCl(3)(THF)(3) afforded the trivalent cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li (THF)])CrCl(2) (1). Subsequent reaction with 2 equiv of vinyl Grignard (CH(2)=CH)Mg Cl gave the butadiene derivative (cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li(THF)])Cr(cis-η(4)-butadiene) (3) formally containing the metal in its monovalent state. The presence of the monovalent state was thereafter confirmed by DFT calculations. The coordination of the butadiene unit appears to be rather robust since reaction with Me(3)P afforded cleavage of the dimeric ligand core but not its displacement. The reaction formed the new butadiene complex [(t-Bu)N-P-N(t-Bu)]Cr(cis-η(4)-butadiene)PMe(3) (4) containing a regular NPN monoanion. In agreement with the presence of monovalent chromium, complexes 3 and 4 act as single-component self-activating catalysts for selective ethylene trimerization and dimerization, respectively.     

Nickel-promoted coupling of orthomanganated aryl ketones with alkenes as a route to indanols

Orthomanganated aryl ketones undergo NiBr₂(PPh₃)₂-promoted reactions with activated alkenes to give indanol products with higher specificity than similar Pd(II)- or Me₃NO-promoted reactions.     

Oxidative cycloamination of olefins with aziridines as a versatile route to saturated nitrogen-containing heterocycles

Highly reactive [5,3] and [6,3] bicyclic aziridines can be readily prepared from the corresponding NH aziridines and N-bromosuccinimide by intramolecular oxidative cycloamination of olefins. These compounds, including surprisingly stable exo-methylene bicyclic aziridines, provide versatile synthetic entries into a wide range of pyrrolidine- and piperidine-containing heterocycles.     

Oxidative acetylenic coupling reactions as a surface chemistry tool

A novel method to prepare redox monolayers on silicon electrodes has been developed that employs Cu(I)-catalyzed oxidative acetylenic coupling reactions for molecular electronic type applications. As the first case study, ethynylferrocene was covalently immobilized onto an acetylene-terminated monolayer on a Si(100) surface to give a 1,3-diyne (C≡C-C≡C-) linked redox assembly. The derivatization process requires no protection/de-protection steps, nor activation procedures. The effect of the conjugated diyne linkage on the rate of electron transfer between tethered ferrocenyl units and the silicon electrode is benchmarked against well-established "click" products (i.e. 1,2,3-triazole linkage). The surfaces, after each step, are characterized thoroughly using X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The coupling chemistry provides a useful strategy for functionalizing silicon surfaces and contributes to an expanding repertoire of wet chemistry routes for the functionalization of solid substrates.     

Dehydrogenative coupling as an efficient route to ferrocene functionalized siloxanes

Ferrocene grafted siloxanes were prepared in high yields (∼79-97%) via Rh-catalyzed dehydrogenative coupling of a series of monomeric, polymeric, and cyclic hydrosiloxanes with ferrocenemethanol. Wilkinson’s catalyst was the most efficient of those surveyed (Karstedt’s catalyst, H₂PtCl₆, Co₂(CO)₈, 10% Pd/C, 10% Pt/C, 5% Rh/C) with respect to yield and selectivity. Benzene and toluene were better solvents than tetrahydrofuran and methylene chloride.     

Oxidative transformations of 6-trifluoromethyl-2H-thiopyran as a route to fluoro-containing thiopyranosides

The synthesis of cis- and trans-6-(trifluoromethyl)-3,4-dihydro-2H-thiopyran-3,4-diols from 6-(trifluoromethyl)-2H-thiopyran via an OsO₄-catalysed dihydroxylation and bromohydroxylation-alkaline hydrolysis sequence is described. Acetylation of the diols followed by S-oxidation affords the corresponding cis- and trans-3,4-diacetoxy-6-(trifluoromethyl)-3,4-dihydro-2H-thiopyran S-oxides which reacted with acetic anhydride and boron trifluoride diethyl ether complex by an additive Pummerer pathway giving tetraacetyl derivatives of trifluoromethyl-containing thiopyranoses.     

Strain-promoted azide-alkyne cycloaddition polymerization as a route toward tailored functional polymers

The preparation of polymers under mild conditions, using highly efficient reactions, allows the incorporation of new, functional monomer units. Here, we explore the use of strain-promoted azide-alkyne cycloaddition (SPAAC) as a polymerization tool. We show that a polymer library with diverse thermal and stimulus-responsive properties can be rapidly prepared in minutes. This polymerization proceeds without heating or the addition of catalyst, which enables the preservation of sensitive moieties, such as stimulus-responsive spiropyran structures. Solid- and solution-state stimulus-responsiveness from these sensitive groups are demonstrated within the SPAAC-polymerized products.     

P-Triazinylphosphonium chlorides as a new group of coupling reagents

Tertiary phosphines in reaction with 2-chloro-4,6-dialkoxy-1,3,5-triazines gave unstable quaternary P-triazinylphosphonium chlorides, which readily decomposed with a departure of the alkyl group of the triazine ring substituent. Stable quaternary P-triazinylphosphonium chlorides were obtained only in reaction of 2-chloro-4,6-diphenoxy-1,3,5-triazine. Both, stable quaternary P-triazinylphosphonium chlorides as well unstable analogues prepared in situ activated carboxylic acid yielding “superactive” triazine esters, useful as highly efficient reagents in peptide bond synthesis.      

Oxidative alkylamination of azinones as a direct route to aminoazinones: study of some condensed diazinones

Oxidative alkylamination of azinones is a promising method for the preparation of a great variety of alkylaminoazinones. Treatment of 6,8-dimethyl-2-R-pyrimido[4,5-c]pyridazin-3,5,7(2H,6H,8H)-triones 7, 1,3-dimethyl-5-R-pteridin-2,4,6(1H,3H,6H)-triones 8 and 1,3,6-trimethylpyrimido[4,5-d]pyrimidin-2,4,7(1H,3H,6H)-trione 9 with alkylamine/AgPy₂MnO₄ or alkylamine/KMnO₄ has been shown to produce their 4-, 7- and 5-alkylamino derivatives, respectively, in good yields. While 1-methylquinoxalin-2(1H)-one 10 is smoothly alkylaminated under the above conditions giving 3-alkylamino derivatives, quinoxaline itself does not take part in this reaction. Factors influencing oxidative alkylamination of azinones and a regioselectivity of the process are discussed.     

Regioselective coupling of pentafluorophenyl substituted alkynes: mechanistic insight into the zirconocene coupling of alkynes and a facile route to conjugated polymers bearing electron-withdrawing pentafluorophenyl substituents

The reaction of Cp(2)ZrCl(2) with 2 equiv of BuLi at -78 degrees C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)C[triple bond]CAr(f) (R(1), Ar(f) = (CH(2))(4)Me, p-C(6)F(4)H; Me, p-C(6)F(4)H; Ph, C(6)F(5)), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar(f) substituents preferentially adopt the 3,4-positions (beta beta) of the zirconacyclopentadiene ring. With Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (alpha beta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alpha alpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp(2)ZrC(4)-2,5-Ph(2)-3,4-(C(6)F(5))(2), which resulted from the coupling of PhC[triple bond]C(C(6)F(5)), was converted to the corresponding thiophene by reaction with S(2)Cl(2), and to an arene by reaction with MeO(2)CC[triple bond]CCO(2)Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF(3)C(6)H(4))C[triple bond]C(p-MeC(6)H(4)) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C(6)F(5))C[triple bond]CCH(2)](2)CH(2) with Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alpha alpha). Zirconocene coupling of the diyne (C(6)F(5))C[triple bond]C-1,4-C(6)H(4)-C[triple bond]C(C(6)F(5)) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.     

Conformation as a protecting group: a regioselective aromatic bromination en route to complex pi-electron systems

A new strategy to achieve regioselective functionalization of a sterically congested aromatic system driven by conformational demands is described. Electrophilic substitution occurs at the more planarizable subunit without undesired chemistry at mutually reactive sites and without the need for protecting or masking groups that must be manipulated later. Model studies are described to understand this selectivity, and possibilities for the construction of orthogonal, differentially substituted pi-systems of relevance for molecular electronics are demonstrated.