Reversible double insertion of aryl isocyanates into the Ti-O bond of titanium(IV) isopropoxide

The insertion of phenyl isocyanate into titanium isopropoxide leads to the formation of a dimeric complex [Ti(OⁱPr)₂(μ-OⁱPr){C₆H₅N(OⁱPr)CO}]₂ (1) which has been structurally characterized. Reaction of titanium isopropoxide with two and more than 2 equiv. of phenyl isocyanate is complicated by competitive, reversible insertion between the titanium carbamate and titanium isopropoxide. The ligand formed by insertion of phenyl isocyanate into the titanium carbamate has been structurally characterized in its protonated form C₆H₅N{C(OⁱPr)O}C(O)N(H)C₆H₅ (3aH). Insertion into the carbamate is kinetically favored whereas insertion into isopropoxide gives the thermodynamically favored product.     

Enantioselective addition of diethylzinc to aromatic aldehydes catalyzed by Ti(IV) complexes of C2-symmetrical chiral BINOL derivatives

Enantioselective addition of diethylzinc to a series of aromatic aldehydes is developed using new chiral C₂-symmetric ligand (S)-2,2′-(1,1′-binaphthyl-2,2′-diylbis(oxy))bis(methylene)bis(4-nitrophenol) (S)-2b. The catalytic system employing 10 mol % of (S)-2b and 120 mol % of Ti(OiPr)₄ was found to promote the addition of diethylzinc to a wide range of aromatic aldehydes with electron-donating and electron-withdrawing substituents, giving up to 89% ee and up to 95% yield of the corresponding secondary alcohol under mild conditions.     

An efficient synthesis of benzyl bromides from aromatic aldehydes using polymethylhydrosiloxane and (bromodimethyl)sulfonium bromide or N-bromosuccinimide

Polymethylhydrosiloxane (PMHS) in combination with (bromodimethyl)sulfonium bromide or NBS has been utilized for the first time for reductive bromination of aromatic aldehydes at room temperature to afford the corresponding benzyl bromides in excellent yields.     

Asymmetric addition of KCN and Ac2O to aldehydes catalyzed by recyclable polymeric salen-Ti(IV) complexes

Polymeric salen-Ti(IV) complexes were employed in the enantioselective O-acetyl cyanation of aldehydes with KCN and Ac₂O. The polymeric catalysts with appropriate crosslinking degree exhibited good activities and enantioselectivities, up to 94% yield and 91% ee were obtained at −20 °C with 1 mol % of catalyst (based on bimetallic catalytic unit). Moreover, the crosslinked polymeric catalyst could be easily recovered and reused for six consecutive runs without obvious decrease in activity and enantioselectivity.     

Synthesis of some new chiral bifunctional o-hydroxyarylphosphonodiamides and their application as ligands in Ti(IV) complex catalyzed asymmetric silylcyanation of aromatic aldehydes

Some new chiral bifunctional o-hydroxyarylphosphonodiamides were synthesized starting from (+)-cis-1,2,2-trimethylcyclopentane-1,3-diamine and the absolute configuration of the phosphorus atom was determined by X-ray diffraction analysis. Excellent enantioselectivity (up to 98% ee) was achieved in asymmetric silylcyanation of aromatic aldehydes using a chiral titanium complex formed in situ from Ti(OⁱPr)₄ and o-hydroxyarylphosphonodiamide as the catalyst.     

One-pot synthesis of aromatic homoallylic alcohols from aldehydes and Grignard reagents promoted by Titanium(IV) tetraisopropanolate

An efficient method of Titanium(IV) mediated one-pot reaction approaching aromatic homoallylic alcohols from aldehydes and dual Grignard reagents was developed with good isolated yields. This method could use aromatic-, heteroaromatic-, or aliphatic-aldehydes as starting materials. However, electron-riched benzaldehyde or cinnamaldehyde tended to precede further hydroxyl eliminations to give all conjugated structures as demonstrated.     

Mannich-type reaction of (1-methoxy-2-methylpropenyloxy)trimethylsilane with arylaldehydes and aromatic amines catalyzed by perfluorinated rare earth metal salts in fluorous phase

In this paper, we describe a useful Mannich-type reaction in fluorous phase. By use of perfluorodecalin (C₁₀F₁₈, cis- and trans-mixture) as a fluorous solvent and perfluorinated rare earth metal salts such as Sc(OSO₂C₈F₁₇)₃ or Yb(OSO₂C₈F₁₇)₃ (2.0 mol%) as a catalyst, the Mannich-type reaction of arylaldehydes with aromatic amines and (1-methoxy-2-methylpropenyloxy)trimethylsilane can be performed for many times without reloading the catalyst and the fluorous solvent.     

Efficient preparation of Ellman’s imines from trifluoromethyl ketones promoted by zirconium(IV) tert-butoxide

An efficient synthesis of chiral N-tert-butanesulfinyl trifluoromethyl ketimines by using zirconium(IV) tert-butoxide is described. Compared to conventional titanium(IV) ethoxide, this protocol gave better result especially for sterically hindered ketones. In addition to the high reactivity, a high degree of enantiopurity of the N-tert-butanesulfinyl group is retained.     

Nickel(II) chloride hexahydrate catalyzed reaction of aromatic aldehydes with 2-mercaptoethanol: formation of supramolecular helical assemblage of the product

Various aromatic aldehydes on reaction with 2-mercaptoethanol provided an unanticipated product, bis(2-hydroxyethyl)dithioacetals (3) as the major product along with the expected product 1,3-oxathiolanes (4) in the presence of 0.05 equiv amount of nickel(II) chloride hexahydrate (NiCl₂·6H₂O) under solvent-free conditions. Products 3c and 3e exhibit an interesting hydrogen-bonded infinite supra-molecular helical structure.     

Synthesis and optical activity of aryl(N-trifluoromethylsulfonylimino)thiodifluoroacetic acids and their derivatives

Synthesis and NMR studies of a new type of optically active sulfur(IV) compounds—aryl(N-trifluoromethylsulfonylimino)thiodifluoroacetic acids and their derivatives are described.     

Hydrated ferric sulfate catalyzed synthesis of 3-[(alkyl/arylthio)(aryl)methyl]-1H-indole derivatives through one-pot reaction

A wide variety of 3-[(alkyl/arylthio)(aryl)methyl]-1H-indole derivatives (4a–z) were synthesized through a one-pot three-component reaction from indoles, aromatic aldehydes, and thiols at room temperature using hydrated ferric sulfate as a Lewis acid catalyst. The key features of the present protocol are mild and simple reaction procedure, moderate to good yields, requirement of inexpensive and reusable catalyst, no formation of dithioacetals derivatives from the corresponding aldehydes as well as bis-(indolyl) methane derivatives.     

A new method for the synthesis of N-substituted 1,3,5-dithiazinanes via the catalytic recyclization of 1,3,5-trithiane with aryl(benzyl) hydrazines and aryl amines

An efficient method for the synthesis of N-substituted 1,3,5-dithiazinanes based on the amination reaction of 1,3,5-trithiane with aryl(benzyl) hydrazines and N-aryl amines in the presence of Ti and Fe catalysts has been developed.     

New three-component condensation reaction: synthesis of 1-[(alkylthio)(phenyl)methyl]-naphthalene-2-ol catalyzed by bromodimethylsulfonium bromide (BDMS)

Bromodimethylsulfonium bromide acts as an efficient catalyst for one pot three component condensation reactions of aldehydes, 2-naphthol, and thiols in acetonitrile at room temperature. Various aliphatic and aromatic thiols undergo conjugate addition with in situ generated enone in acetonitrile and provide good yields. The main features of this procedure are mild reaction conditions, good yields, and operational simplicity.     

H-Ge bond activation by tungsten carbonyls: An experimental and theoretical study

The activation of the Ge-H bond and the formation of several hydride complexes, characterized by high-field resonances, have been detected during the ¹H NMR spectroscopy monitoring of the photochemical reaction of Et₃GeH and Et₂GeH₂ with W(CO)₆ and the norbornadiene complex [W(CO)₄(η⁴-nbd)]. The activation of the Ge-H bond of triethylgermane in the photochemical reactions of tungsten(0) complexes has been applied in the hydrogermylation of norbornadiene (nbd), which leads to the formation of endo-triethylgermylnorbornene as the major product. The complex [{W(μ-η²-H-GeEt₂)(CO)₄}₂] has been fully characterized by NMR spectroscopy and by a single-crystal X-ray diffraction study. Evidence for the hydride ligand of the W(μ-η²-H-GeEt₂) group has been provided by ¹H NMR spectroscopy (δ = −9.02, ¹J_H-W = 31 Hz) and by DFT calculations. A DFT study of the structural properties and ¹H NMR chemical shifts of several possible intermediate σ and hydride complexes formed during the photochemical reaction of W(CO)₆ and Et₂GeH₂ has been performed.     

Experimental and computational study of intermolecular migration of N,N-dimethylcarbamoyl group from N(7) to N(1) on a 7-azaindoline derivative

N,N-Dimethylcarbamoylation of the anilinic nitrogen atom N(1) on the spiro 7-azaindoline consists of two steps. The first step is N,N-dimethylcarbamoylation of the pyridyl nitrogen atom N(7), leading to the formation of an isolable intermediate. The second step is intermolecular migration of the N,N-dimethylcarbamoyl group from the pyridyl nitrogen atom N(7) to the anilinic nitrogen atom N(1). We accomplished optimization of the reaction conditions based on the revealed reaction mechanism and a large scale synthesis of compound 3 in quantitative yield.     

Polymeric salen-Ti(IV) or V(V) complex catalyzed asymmetric synthesis of O-acetylcyanohydrins from KCN, Ac2O and aldehydes

Polymeric salen-Ti(IV) and V(V) complexes were employed in the enantioselective O-acetyl cyanation of aldehydes with potassium cyanide and acetic anhydride. The crosslinked polymeric salen-Ti(IV) catalyst exhibited good activities and enantioselectivities, up to 91% ee with 99% conversion was obtained at −20 °C with 1 mol% of catalyst (based on bimetallic catalytic unit). Moreover, six consecutive recyclings with the easily recovered crosslinked polymeric catalyst showed no obvious decrease in either activity or enantioselectivity. Linear polymeric salen-V(V) catalyst showed good catalytic efficiency too, up to 94% ee with 99% conversion was obtained at −42 °C with 5 mol% of catalyst.     

Reactivity and regioselectivity in the synthesis of spiroindoles via indole o-quinodimethanes. An experimental and computational study

The 1,3-dipolar cycloaddition reactions of stable nitrile oxides with indole o-quinodimethanes have been examined. In all cases the ‘exo-anti’ addition products, dispiroisoxazolines, were isolated in moderate to good yields (25-47%). In addition, from the reaction of one of the indole quinodimethanes with mesitonitrile oxide the ‘exo-syn’ addition product was isolated in 7% yield along with the remarkable indole quinodimethane dimerization and cycloaddition product, which was isolated in 13% yield. An analogous dimerization and cycloaddition product was isolated in 18% yield from the reaction of the N-acetyl-indole quinodimethane with mesitonitrile oxide. In the case of the reaction of the N-benzoylindole quinodimethane with the 2,6-dichlorobenzonitrile oxide an oxime was also isolated in 13% yield. The proposed reaction mechanism is supported by semiempirical (AM1) MO calculations via FMO interactions. The observed selectivity was explained by an investigation of the transition states carried out also for analogous dispiroisoxazolines.     

Catalytic activity of Mn(III) and Fe(III) complexes of meso-tetra(n-propyl)porphyrin in oxidation of olefins: Meso-alkyl substituent in comparison with the alkenyl and aryl ones

Catalytic activity of Mn(III) and Fe(III) complexes of meso-tetra(n-propyl)porphyrin, MnT(n-Pr)P(X) and FeT(n-Pr)P(X) (X = Cl, SCN, OAc) in oxidation of olefins with tetra-n-butylammonium periodate at room temperature has been studied. The influence of different parameters including the molar ratio of catalyst to imidazole, type of counter ion (X) and oxidative stability of metalloporphyrins on the efficiency of the catalysts was investigated. The results of competitive oxidation of cis- and trans-stilbene suggest the presence of a high-valent Mn-oxo as the predominant oxidant species in equilibrium with a six coordinate complex, MnT(n-Pr)P(ImH)(IO₄) in the case of MnT(n-Pr)P(OAc). An unusual preference for trans-stilbene over cis-stilbene was observed in the reaction catalyzed by FeT(n-Pr)P(OAc). Control reaction indicated a significant cis- to trans-isomerization (81%) in oxidation of cis-stilbene catalyzed by FeT(n-Pr)P(OAc) which may explain the observed unusual cis to trans-stilbene oxide ratio. While oxidation of cyclooctene and styrene led to the exclusive formation of the corresponding epoxides, oxidation of cyclohexene gave 2-cyclohexe-1-ol and cyclohexene oxide as the products. However, the results of this study clearly demonstrate the key role played by the group substituted at the meso positions of metalloporphyrins on their catalytic activity, apart from the electron-donating or electron-withdrawing properties of the substituents.     

Titanium (IV) as an essential promoter in the asymmetric addition of diethylzinc to aldehydes catalyzed by aminonaphthol and imine ligands based on 3-substituted binaphthol

By using indirect reductive amination and condensation, two types of aminonaphthol and imine ligands based on 3-substituted binaphthol have been synthesized, respectively. When their catalytic effectiveness was tested by the ethylation of aldehydes with diethylzinc, titanium tetraisopropoxide was found essential to get good results with ee up to 90%.     

Protecting group directed ring-closing metathesis (RCM): the first total synthesis of an anti-malarial nonenolide

The first synthesis of a newly found naturally occurring anti-malarial nonenolide is described. A pivotal step in the synthesis is the ring-closing metathesis of a dienoic ester prepared by coupling an acid and alcohol that were stereoselectively synthesized from (S)-α-hydroxy-γ-butyrolactone and 1,2-O-isopropylidene d-glyceraldehyde, respectively.