Highly active bifunctional salenTi(Ⅳ) catalysts for asymmetric cyanosilylation of aldehydes and TMSCN

A novel enantiopure salen ligand bearing a diphenylphosphine oxide on the 3-position of one aromatic ring was synthesized and combined with Ti(Oi-Pr)_4 as a monometallic bifunctional catalyst for asymmetric cyanosilylation reaction of aldehydes with tnmethylsilyl cyanide(TMSCN).The catalyst system exhibited excellent activity and moderate enantioselectivity.The addition of TMSCN to 4-nitrobenzaldehyde in the presence of 1 mol%catalyst loading could complete within 10 min at ambient temperature. An intram...     

Unsymmetric salen ligands bearing a Lewis base: intramolecularly cooperative catalysis for cyanosilylation of aldehydes

A series of unsymmetric salen ligands derived from 1,2-diaminocyclohexane bearing an appended Lewis base on the three-position of one aromatic ring were synthesized by the reaction of various functional salicyaldehydes with the condensation product of 1,2-diaminocyclohexane mono(hydrogen chloride) and 3,5-di-tert-butylsalicylaldehyde. These ligands in conjunction with Ti(O(i)Pr)(4) exhibited excellent activity in catalyzing the cyanosilylation of aldehydes with trimethylsilyl cyanide (TMSCN) at mild conditions. The highest activity was observed in the catalyst system with regard to the salen ligand bearing a diethylamino group, which proved to be active even at a high [aldehyde]/[catalyst] ratio up to 50000. In a low catalyst loading of 0.05 mol%, the quantitative conversion of benzaldehyde to the corresponding cyanosilylation product was found within 10 min. at ambient temperature. An intramolecularly cooperative catalysis was proposed wherein the central metal Ti(IV) is suggested to play a role of Lewis acid to activate aldehydes while the appended Lewis base to activate TMSCN.     

Investigation of Lewis Acid versus Lewis Base Catalysis in Asymmetric Cyanohydrin Synthesis

The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}₂] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO₃ or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}₂O] and triphenylphosphine oxide, a non‐linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron‐deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case.     

An efficient catalytic asymmetric addition of trimethylsilyl cyanide to aldehydes at room temperature

The BINOL-salen compound (S)-5c in combination with Ti(OⁱPr)₄ is found to catalyze the addition of TMSCN to aldehydes to form chiral cyanohydrins with very good enantioselectivity (75-85% ee). The reactions are carried out in one-pot at room temperature without the need to isolate the chiral Lewis acid catalyst.     

Germylene Cyanide Complex: A Reagent for the Activation of Aldehydes with Catalytic Significance

The first example of a germanium(II) cyanide complex [GeCN(L)] ( 2 ) (L=aminotroponiminate (ATI)) has been synthesized through a novel and relatively benign route that involves the reaction of a digermylene oxide [(L)Ge?O?Ge(L)] ( 1 ) with trimethylsilylcyanide (TMSCN). Interestingly, compound 2 activates several aldehydes (RCHO) at room temperature and results in the corresponding cyanogermylated products [RC{OGe(L)}(CN)H] (R=H 3 , iPr 4 , tBu 5 , CH(Ph)Me 6 ). Reaction of one of the cyanogermylated products ( 4 ) with TMSCN affords the cyanosilylated product [(iPr)C(OSiMe₃)(CN)H] ( 7 ) along with [GeCN(L)] quantitatively, and insinuates the possible utility of [GeCN(L)] as a catalyst for the cyanosilylation reactions of aldehydes with TMSCN. Accordingly, the quantitative formation of several cyanosilylated products [RC(OSiMe₃)(CN)H] ( 7 – 9 ) in the reaction between RCHO and TMSCN by using 1 mol % of [GeCN(L)] as a catalyst is also reported for the first time.     

A catalytic asymmetric strecker-type reaction promoted by Lewis acid-Lewis base bifunctional catalyst

A general asymmetric Strecker-type reaction is reported, catalyzed by the Lewis acid-Lewis base bifunctional catalyst 1. The reaction of trimethylsilyl cyanide (TMSCN) with various fluorenyl imines, including n-aldimines and alpha,beta-unsaturated imines, proceeds with good to excellent enantioselectivities in the presence of a catalytic amount of phenol as additive (20 mol%) (catalytic system 1). The products were successfully converted to the corresponding amino acid derivatives in high yields without loss of enantiomeric purity. Furthermore, hydrogenation or dihydroxylation of the products from alpha,beta-unsaturated imines afforded saturated or functionalized aminonitriles also without loss of enantiomeric purity. The absolute configuration of the products and a control experiment using catalyst 2 supported the proposed dual activation of the imine and TMSCN by the Lewis acid (Al) and the Lewis base moiety (phosphine oxide) of 1. From the mechanistic studies including kinetic and NMR experiments of the catalytic species, the role of PhOH seems to be a proton source to protonate the anionic nitrogen of the intermediate. Specifically, we have found that TMSCN is more reactive than HCN in this catalytic system, probably due to the activation ability of the phosphine oxide moiety of 1 toward TMSCN. This fact prompted us to develop the novel catalytic system 2, consisting of 1 (9 mol%), TMSCN (20 mol%) and HCN (1.2 mol eq). This new system afforded comparable results with obtained by system 1 (1 (9 mol%)-TMSCN (2 mol eq)-PhOH (20 mol%)).     

Asymmetric Cyanation of Aldehydes,Ketones, Aldimines,and Ketimines Catalyzed by a Versatile Catalyst Generated from Cinchona Alkaloid,Achiral Substituted 2,2′‐Biphenol and Tetraisopropyl Titanate

Full investigation of cyanation of aldehydes, ketones, aldimines and ketimines with trimethylsilyl cyanide (TMSCN) or ethyl cyanoformate (CNCOOEt) as the cyanide source has been accomplished by employing an in situ generated catalyst from cinchona alkaloid, tetraisopropyl titanate [Ti(OiPr)₄] and an achiral modified biphenol. With TMSCN as the cyanide source, good to excellent results have been achieved for the Strecker reaction of N‐Ts (Ts=p‐toluenesulfonyl) aldimines and ketimines (up to >99 % yield and >99 % ee) as well as for the cyanation of ketones (up to 99 % yield and 98 % ee). By using CNCOOEt as the alternative cyanide source, cyanation of aldehyde was accomplished and various enantioenriched cyanohydrin carbonates were prepared in up to 99 % yield and 96 % ee. Noteworthy, CNCOOEt was successfully employed for the first time in the asymmetric Strecker reaction of aldimines and ketimines, affording various α‐amino nitriles with excellent yields and ee values (up to >99 % yield and >99 % ee). The merits of current protocol involved facile availability of ligand components, operational simplicity and mild reaction conditions, which made it convenient to prepare synthetically important chiral cyanohydrins and α‐amino nitriles. Furthermore, control experiments and NMR analyses were performed to shed light on the catalyst structure. It is indicated that all the hydroxyl groups in cinchona alkaloid and biphenol complex with Ti^IV, forming the catalyst with the structure of (biphenoxide)Ti(OR*)(OiPr). The absolute configuration adopted by biphenol 4 m in the catalyst was identified as S configuration according to the evidence from control experiments and NMR analyses. Moreover, the roles of the protonic additive (iPrOH) and the tertiary amine in the cinchona alkaloid were studied in detail, and the real cyanide reagent in the catalytic cycle was found to be hydrogen cyanide (HCN). Finally, two plausible catalytic cycles were proposed to elucidate the reaction mechanisms.     

Synthesis of α-aryl nitriles through B(C6F5)3-catalyzed direct cyanation of α-aryl alcohols and thiols

Various α-aryl nitriles have been prepared in excellent yield from the corresponding α-aryl alcohols employing 3 mol % of B(C₆F₅)₃ (1) as Lewis acid catalyst and (CH₃)₃SiCN (TMSCN) as cyanide source. Cyano transfer from TMSCN to alcohol proceeds within short reaction time at rt. α-Aryl thiols also produce corresponding nitriles in good to excellent yield at reflux condition.     

Chemistry of iron complexes. Part V. Electron spin resonance spectra of some iron(III) complexes and X-Ray diffraction (powder) data of iron(II) complexes employing tertiary phosphine/arsine oxides

Summary Electron spin resonance spectra (X-band, 9.3 GHz) of iron(III) chloride complexes with tri-p-tolylarsine oxide (T₃AO), methylenebis(diphenylphosphine oxide) (mdpo) and 1,4-tetramethylenebis(diphenylphosphine oxide) (tmdpo) support their structures as [Fe(L-L)₂Cl₂][FeCl₄] (L-L=mdpo or tmdpo) and [Fe(T₃AO)₂Cl₂(OH₂)₂][FeCl₄]2H₂O. The x-ray powder diffraction patterns of some iron(II) iodide complexes with mdpo, tmdpo, dmdpo, dmdao and tmdao [dmdpo-1,2-dimethylenebis(diphenylphosphine oxide); dmdao and tmdao are the arsine analogs of dmdpo and tmdpo] show that the complexes are crystalline but not isomorphous.     

A novel bifunctional Ti(IV) complex catalyzed asymmetric silylcyanation of aldehydes

A novel Lewis acid and Lewis base bifunctional Ti(IV) complex was formed in situ upon the treatment of ligand (R)‐3,3′‐bis(diphenylphosphinoyl)‐BINOL with Ti(PrO‐i)₄, which is proven to be an efficient catalyst in the asymmetric trimethylsilylcyanation of aldehydes. The corresponding cyanohydrins were obtained in high to excellent chemical yield (83–93%) albeit with moderate enantioselectivities (up to 51% enantiomeric excess). © 2010 Wiley Periodicals, Inc. Heteroatom Chem 22:31–35, 2011; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.20652     

Polymerization of vinyl chloride by the Ti(O-n-Bu)4–AlEt3–epichlorohydrin catalyst system

The polymerization of vinyl chloride was carried out by using a catalyst system consisting of Ti(O-n-Bu)₄, AlEt₃, and epichlorohydrin. The polymerization rate and the reduced viscosity of polymer were influenced by the polymerization temperature, AlEt₃/Ti(O-n-Bu)₄ molar ratios, and epichlorohydrin/Ti(O-n-Bu)₄ molar ratios. The reduced viscosity of polymer obtained in the virtual absence of n-heptane as solvent was two to three times as high as that of polymer obtained in the presence of n-heptane. The crystallinity of poly(vinyl chloride) thus obtained was similar to that of poly(vinyl chloride) produced by a radical catalyst. It was concluded that the polymerization of vinyl chloride by the present catalyst system obeys a radical mechanism rather than a coordinated anionic mechanism.     

[2-(6,6-Dimethylbicyclo[3.3.1]hept-2-enyl)ethyl]-diphenylphosphine and phosphine oxide. Coordination properties and application in catalysis

[2-(6,6-Dimethylbicyclo[3.1.1]hept-2-enyl)ethyl]diphenylphosphine and the corresponding phosphine oxide, that hold promise as ligands in metal complex catalysis, were synthesized on the basis of (1R)-(-)-nopol. A Pd(II) bisphosphine complex is obtained on the basis of the synthesized phosphine. When the system [PdCl₂(COD)]-[2-(6,6-dimethylbicyclo[3.1.1]hept-2-enyl)ethyl]diphenylphosphine oxide is used as catalyst in the reaction of cyclohexa-1,3-diene with trichlorosilane, asymmetric induction occurs.     

New entries in Lewis acid-Lewis base bifunctional asymmetric catalyst: catalytic enantioselective Reissert reaction of pyridine derivatives

The first catalytic enantioselective Reissert reaction of pyridine derivatives that affords products with excellent regio- and enantioselectivity is described. The key for success is the development of new Lewis acid-Lewis base bifunctional asymmetric catalysts containing an aluminum as a Lewis acid and sulfoxides or phosphine sulfides as a Lewis base. These reactions are useful for the synthesis of a variety of chiral piperidine subunits, and catalytic enantioselective formal synthesis of CP-293,019, a selective D4 receptor antagonist, was achieved. Preliminary mechanistic studies indicated that both sulfoxides and phosphine sulfides can activate TMSCN as a Lewis base. In addition, the sulfoxides with appropriate stereochemistry might stabilize a highly enantioselective bimetallic complex (a presumed active catalyst) through internal coordination to aluminum.     

Asymmetric cyanosilylation of ketones catalyzed by recyclable polymer-supported copper(II) salen complexes

Various ketones have undergone asymmetric trimethylsilylcyanation at room temperature with (CH₃)₃SiCN (TMSCN) in the presence of a chiral-supported Cu(salen) complex and Ph₃PO as the catalyst. Aromatic, aliphatic, and heterocyclic ketones have been converted into the corresponding cyanohydrin trimethylsilyl ethers in 83?96% yields with 52?84% ee. Several factors concerning the reactivity and enantioselectivity have been discussed. A double activation where Cu(salen) plays the role of Lewis acid and Ph₃PO acts as a Lewis base is reported. Poly(ethylene glycol) monomethyl ether (MeO-PEG) has been used as a soluble support while JandaJel (JJ) and Merrifield (MF) resins served as insoluble supports. Each polymer is linked to the salen catalyst through a glutarate spacer. The soluble catalysts were recovered by precipitation with a suitable solvent while the insoluble catalysts were simply filtered from the reaction mixture. The JandaJel-attached Cu(salen) catalyst could be used for five cycles with the retention of efficiency and the Merrifield-bound Cu(salen) catalyst was found to loose activity with each use.     

Preparation and study of bi-supported Ziegler-Natta catalyst with nano graphene oxide and magnesium ethoxide supports for polymerization of polyethylene

In this study, we have reported the preparation of bi-supported Ziegler-Natta catalysts using magnesium ethoxide and graphene oxide as support. The polymerization process was carried out in slurry phase using triisobutylaluminum as a co-catalyst.The XRD analysis of TiCl₄/graphene oxide/Mg(OEt)₂ catalyst demonstrated that the space between the layers of graphene oxide had increased to 0.2 nm.The catalyst was characterized by XPS, BET, BJH, SEM, and TGA. The catalyst activity was studied for various Al/Ti molar ratios, and the catalyst activity was optimum at Al/Ti molar ratio of 315.     

Synthesis of α-aminonitriles under mild catalytic,metal-free conditions

α-Aminonitriles have been synthesized by a Strecker synthesis from aldehydes and ketones under mild catalytic, metal-free conditions. Aromatic aldehydes (1 equiv) were reacted with aromatic and 1° or 2° aliphatic amines (1 equiv) in EtOH containing 3 mol % of NH₄Cl to give high yields of α-aminonitriles. An alternative to adding NH₄Cl as a catalyst involved the use of excess TMSCN (1 equiv) and to promote the process. The reaction was also successful under microwave conditions using excess TMSCN with no solvent. Ketones similarly reacted with aromatic amines and excess TMSCN under conventional and microwave heating, but 30 mol % of added NH₄Cl was required for optimum conversion.     

[NiCl2(dppp)]‐Catalyzed Cross‐Coupling of Aryl Halides with Dialkyl Phosphite,Diphenylphosphine Oxide,and Diphenylphosphine

We present a general approach to C? P bond formation through the cross‐coupling of aryl halides with a dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphane by using [NiCl₂(dppp)] as catalyst (dppp=1,3‐bis(diphenylphosphino)propane). This catalyst system displays a broad applicability that is capable of catalyzing the cross‐coupling of aryl bromides, particularly a range of unreactive aryl chlorides, with various types of phosphorus substrates, such as a dialkyl phosphite, diphenylphosphine oxide, and diphenylphosphane. Consequently, the synthesis of valuable phosphonates, phosphine oxides, and phosphanes can be achieved with one catalyst system. Moreover, the reaction proceeds not only at a much lower temperature (100–120 °C) relative to the classic Arbuzov reaction (ca. 160–220 °C), but also without the need of external reductants and supporting ligands. In addition, owing to the relatively mild reaction conditions, a range of labile groups, such as ether, ester, ketone, and cyano groups, are tolerated. Finally, a brief mechanistic study revealed that by using [NiCl₂(dppp)] as a catalyst, the Ni^II center could be readily reduced in situ to Ni⁰ by the phosphorus substrates due to the influence of the dppp ligand, thereby facilitating the oxidative addition of aryl halides to a Ni⁰ center. This step is the key to bringing the reaction into the catalytic cycle.     

Ionic Liquid [bmim]BF4 as an Efficient and Recyclable Reaction Medium for the Synthesis of O-Acetyl Cyanohydrin via One-Pot Condensation of Aldehyde,TMSCN, and Ac2O

Ionic liquid [bmim]BF₄ has been demonstrated to be an efficient and environmentally friendly reaction medium as well as reaction promoter for the synthesis of O-acetyl cyanohydrin via one-pot condensation of aldehyde, TMSCN, and Ac₂O without Lewis acid or any special activation. In addition, the recovered ionic liquid could be reused for subsequent runs without the loss of activity.     

Enantioselective cyanosilylation of ketones by a catalytic double-activation method employing chiral Lewis acid and achiral N-oxide catalysts

[reaction: see text] Enantioselective addition of TMSCN to ketones is achieved by a catalytic double-activation method using 1a-Ti(IV) complex as the Lewis acid and achiral N-oxide 2 as the Lewis base to activate ketones and TMSCN, respectively.     

Group-transfer polymerization: Recent advances and applications

Abstract: Catalysis of GTP of MMA with nucleophilic anions on cross-linked polystyrene supports was studied. With anion-bound supported catalyst, evidence is presented for formation in solution of ester enolates as reaction intermediates. Study of the cyanide-catalyzed initiation of GTP of MMA by TMSCN, has provided quantitative data for the association constant K_a for the complexation of cyanide by TMSCN and, by inference, an upper limit for the K_a for the association of this nucleophilic anion with silyl ketene acetals. The effects of i-propyl- and t-butoxy-silyl analogs of TMSCN on anion-complexation and on initiation and propagation of GTP are discussed. Coordination by hydrogen-bonding of nucleophilic anions to acetonitrile is shown to be the mechanism for “livingness-enhancement” of anion-catalyzed GTP at low concentrations of acetonitrile. GTP was used to prepare an ABC triblock dispersant, poly(methacrylic acid)-block-poly(2-phenylethyl methacrylate)-block-poly(ethoxytriethylene glycol methacrylate), and the surface activity of an aqueous solution of the potassum salt was compared with that of other polymer architectures.