Aprotic Conditions Research Articles

Mononuclear, Dinuclear, and Pentanuclear [{N,S(thiolate)}Iron(II)] Complexes: Nuclearity Control, Incorporation of Hydroxide Bridging Ligands, and Magnetic Behavior

The mixed N3S(thiolate) ligand 1-[bis[2-(pyridin-2-yl)ethyl]amino]-2-methylpropane-2-thiol (Py2SH) was used in the synthesis of four iron(II) complexes: [(Py2S)FeCl] (1), [(Py2S)FeBr] (2), [(Py2S)4Fe5II(mu-OH)2](BF4)4 (3), and [(Py2S)2Fe2II(mu-OH)]BF4 (4). The X-ray structures of 1 and 2 revealed monomeric iron(II)-alkylthiolate complexes with distorted trigonal-bipyramidal geometries. The paramagnetic 1H NMR spectra of 1 and 2 display resonances from delta = -25 ppm to +100 ppm, consistent with a high-spin iron(II) ion (S = 2). Spectral assignments were made on the basis of chemical shift information and T1 measurements and show the monomeric structures are intact in solution. To provide entry into hydroxide-containing complexes, a novel synthetic method was developed involving strict aprotic conditions and limiting amounts of H2O. Reaction of Py2SH with NaH and Fe(BF4)2.6 H2O under aprotic conditions led to the isolation of the pentanuclear, mu-OH complex 3, which has a novel dimer-of-dimers type structure connected by a central iron atom. Conductivity data on 3 show this structure is retained in CH2Cl2. Rational modification of the ligand-to-metal ratio allows control over the nuclearity of the product, yielding the dinuclear complex 4. The X-ray structure of 4 reveals an unprecedented face-sharing, biooctahedral complex with an [S2O] bridging arrangement. The magnetic properties of 3 and 4 in the range 1.9-300 K were successfully modeled. Dinuclear 4 is antiferromagnetically coupled [J = -18.8(2) cm(-1)]. Pentanuclear 3 exhibits ferrimagnetic behavior, with a high-spin ground state of S(T) = 6, and was best modeled with three different exchange parameters [J = -15.3(2), J' = -24.7(3), and J'' = -5.36(7) cm(-1)]. DFT calculations provided good support for the interpretation of the magnetic properties.

Facile Desulfurization of Thioamides and Thioureas with Tetrabutylammonium Periodate under Mild Conditions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

N‐Bromosuccinimide (NBS) Catalyzed Highly Chemoselective Acetalization of Carbonyl Compounds Using Silylated Diols and Pentaerythritol under Neutral Aprotic Conditions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis of Anhydrides from Acyl Halides and Zinc Carboxylates under Aprotic Conditions.

Synthesis of symmetric as well as unsymmetrical carboxylic acid anhydrides from zinc carboxylates and acid chlorides under aprotic conditions in high yields is reported.

Facile Desulfurization of Thioamides and Thioureas with Tetrabutylammonium Periodate under Mild Conditions

Thioamides and thioureas were reacted with tetrabutylammonium periodate at room temperature to afford the corresponding amides and ureas, respectively, under aprotic conditions.

Zinc‐Diamine]‐Catalyzed Hydrosilylation of Ketones in Methanol. New Developments and Mechanistic Insights

AbstractThe Zn‐promoted direct reduction of various ketones, including alkyl aryl ketones, α‐ and β‐keto esters, α‐ and β‐keto amides, into the corresponding alcohols with polymethylhydrosiloxane (PMHS) in protic conditions is reported. The reaction proceeds chemoselectively under smooth conditions, with simple catalyst combinations based on a zinc precursor [ZnEt2, Zn(OMe)2, Zn(OH)2] and a 1,2‐diamine ligand, e.g., N,N′‐dibenzylethylenediamine (dbea). The reaction rates are significantly faster than under aprotic conditions. For β‐keto esters and amides, the use of an excess of PMHS is required. Moderate enantioselectivities (ee up to 55%) have been obtained using a variety of enantiopure diamine ligands. Two mechanisms are proposed for the new catalytic reaction, on the basis of the synthesis of [(diamine)Zn(alkoxide)2] complexes which are models of the possible reaction intermediates and a study of their activity.

Temperature‐Controlled Regioselectivity in the Reductive Cleavage of p‐Methoxybenzylidene Acetals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Mild Synthesis of Unsymmetrical Bis‐Alkoxysilanes Through Catalyzed Alcoholysis of Hydridosilanes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Some Stability and Stereochemical Considerations of Simple Bicyclo[4.2.0]octanols

Bicyclooctanols 3 and 4 were formed with control of the stereochemistry at C8 by using an aluminium enolate of cyclohexanone in 1,2-dimethoxyethane. Bicyclooctanols 1 and 3 were stable towards aqueous hydrochloric acid for short times (< 3.5 h), and ring open to a diastereomeric mix of monoalkylated 5 under protic or aprotic conditions in the presence of base (NaOH or lithium diisopropylamide). The rate of ring opening appears to be dependent on reaction temperature. Bicyclooctanols 1, 3, and 4 displayed remarkable stability when heated at reflux in benzene, whereas use of ethanol prompted stereoselective ring opening of 1 and 3 to give individual diastereomers of monoalkylated 5 and ring opening of 4 to give a 50 : 50 mix of diastereomers of 5. This unexpected result was attributed to the stereochemistry of the bicyclooctanol and placement of hydrogen-bonding sites.

Selective Oxidation of Methylarenes with Pyridinium Chlorochromate

A simple and selective method for oxidation of methyl­arenes, using pyridinium chlorochromate (PCC) is described. This reagent can efficiently oxidize methylarenes to the corresponding aldehydes under mild aprotic conditions.

Immediate and Efficient Oxidative Deprotection of Dithioacetals to Carbonyl Compounds by Zinc Dichromate Trihydrate (ZnCr2O7 · 3H2O)

Zinc dichromate trihydrate (ZnCr2O7 · 3H2O) is an efficient oxidizing agent for the immediate deprotection of 1,3‐dithianes, 1,3‐dithiolanes, and dibutyl dithioacetals to their corresponding carbonyl compounds in non‐aqueous and aprotic conditions at room temperature.

N-Bromosuccinimide (NBS) Catalyzed Highly Chemoselective Acetalization of Carbonyl Compounds Using Silylated Diols and Pentaerythritol under Neutral Aprotic Conditions

Various types of carbonyl compounds are efficiently converted to their 1,3-dioxanes and pentaerythritol diacetals by the use of either 1,3-bistrimethylsiloxy propane (A) or 1,3-bistrimethylsilanyloxy-2,2-bistrimethylsilanyloxymethyl propane (D) and a catalytic amount of N-bromosuccinimide (3-10 mol%) under essentially neutral aprotic condition, respectively. A variety of functionalities such as both aliphatic and phenolic -OTBDMS, -OMe, -OBz, furan ring, double bonds and more significantly phenolic -OTHP survived under the present reaction condition. The efficient conversion of two α-tertiary ketones to their cyclic acetals was also achieved using the present protocol.

Reduction of Carbonyl Compounds with NaBH4 under Ultrasound Irradiation and Aprotic Condition.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ultrasound‐Promoted Reduction of Conjugated Enones, α‐Diketones and Acyloins with NaBH4 under Aprotic Condition.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Immediate and Efficient Oxidative Deprotection of Dithioacetals to Carbonyl Compounds by Zinc Dichromate Trihydrate (ZnCr2O7×3H2O).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Temperature-controlled regioselectivity in the reductive cleavage of p-methoxybenzylidene acetals.

The regioselective ring opening of pyranosidic 4,6-p-methoxybenzylidene acetals with BH(3)/Bu(2)BOTf in THF can be tuned by adjusting the reaction temperature and reagent concentrations. Reductive cleavage at 0 degrees C resulted in the exclusive formation of 4-O-p-methoxybenzyl (PMB) ethers, whereas reaction at -78 degrees C produced 6-O-PMB ethers in high yields. The latter condition was observed to be compatible with a variety of acid-sensitive functional groups, including allyl and enol ethers. The presence of water does not interfere with reductive ring opening and may contribute toward in situ generation of H(+) as a catalyst for 6-O-PMB ether formation. Reductive cleavage under rigorously aprotic conditions is greatly decelerated, and yields only the 4-O-PMB ether. The temperature-dependent reductive cleavage of the 4,6-acetal can be described in terms of kinetic versus thermodynamic control: Lewis-acid coordination of the more accessible O-6 is favored at higher temperatures, whereas protonation of the more basic but sterically encumbered O-4 predominates at low temperatures.

Dimerization mechanisms of heterocyclic carbenes

AbstractThe dimerization reactions of a series of heterocyclic carbenes based on the 1,3‐dimethylimidazol‐2‐ylidene template were studied extensively using molecular orbital calculations. The carbenes studied were the 1,3‐XY five‐membered ring heterocyclic carbenes with all 10 possible combinations of X,Y = NCH3, PCH3, O and S. Two different mechanisms for dimerization were studied: a direct carbene plus carbene dimerization reaction and a proton‐catalysed dimerization. The parent carbene with XY = NN is found to be kinetically and thermodynamically stable under both mechanisms, whereas XY = NS is predicted to be kinetically stable under aprotic conditions. All remaining carbenes were predicted to be not particularly stable towards dimerization. Indirect schemes for calculating the stability of carbenes towards dimerization were found to give good estimates of the enthalpy of dimerization, but do not take into account the activation barriers. Copyright © 2004 John Wiley & Sons, Ltd.

N,N‐Dibromobenzenesulfonamide as a Reagent for Oxidative Cleavage of Oximes to Their Parent Carbonyl Compounds under Non‐Aqueous and Aprotic Conditions.

Deprotection of different oximes to their parent aldehydes and ketones in high yields has been carried out by using N,N-dibromobenzenesulfonamide(dibromoamine-b) under mild conditions.

Homobimetallic anthracene-bridged η 5-cyclopentadienyl derivatives of rhodium(I) and iridium(I): large molecules or supramolecular species?

The reaction of 9,10-bis[(cyclopentadienylmethyl)thallium(I)]anthracene ( 2), obtained from 9,10-bis(cyclopentadienylmethyl)anthracene ( 1), with the chloro derivatives of rhodium(I) of formula [RhClL 2] 2 (L=η 2-C 8H 14 or L 2=η 4-C 8H 12) leads to the corresponding bimetallic complexes [L 2Rh{C 5H 4CH 2(9,10-anthrylene)CH 2C 5H 4}RhL 2] 3 (L=η 2-C 8H 14) and 4 (L 2=η 4-C 8H 12), in 22.8% and 15.0% yields, respectively. Analogously, by reacting 2 with [IrClL 2] 2 (L=η 2-C 8H 14 or L 2=η 4-C 8H 12), the corresponding bimetallic iridium(I) complexes [L 2Ir{C 5H 4CH 2(9,10-anthrylene)CH 2C 5H 4}IrL 2] 5 (L=η 2-C 8H 14) and 6 (L 2=η 4-C 8H 12) were obtained, in 24.5% and 43.0% yields, respectively. All complexes have been characterised by elemental analysis, mass spectrometry, and 1H NMR. The structure of 4 was elucidated also by single crystal X-ray diffraction: it crystallises in the P2 1/ c space group with a=19.932(11), b=6.4417(4), c=12.377(2) Å; α=90°, β=100.90(4)°, γ=90°. V=1560.5(9) Å 3. Z=2, D calc=1.606 g cm −1, R 1=0.0449 [ I> σ( I)], wR 2=0.1121. The UV–Vis spectra (280–530 nm) of 3– 6 are indicative of the existence of strong electronic interactions among the 9,10-anthrylene chromophore and the two cyclopentadienylML 2 moieties. When excited at ca. 370 nm, 1 results to be an efficient light-emitting molecule, while the fluorescence emission of the 9,10-anthrylene chromophore is almost completely quenched in complexes 3– 6. The study of the electrochemical behaviour of 3– 6 in strictly aprotic conditions allows a satisfactory interpretation of the observed electrode processes and gives information about the location of the redox sites along with the thermodynamic characterisation of the corresponding redox processes. These data show that the occurrence of an intramolecular charge-transfer process between the photo-excited 9,10-anthrylene group and the cyclopentadienylML 2 moiety is a possible route for the observed quenching of emission in the compounds 3– 6. The one-electron oxidation of compounds 3– 6 by thallium(III) trifluoroacetate leads to the formation of the corresponding cation radicals. Three of them, i.e., 3 +, 5 + and 6 +, give rise to good X-band EPR spectra that were fully interpreted by computer simulation as well as by semi-empirical calculations (PM3 level) of the spin density distribution.

Reduction of Carbonyl Compounds with NaBH4 under Ultrasound Irradiation and Aprotic Condition

A variety of carbonyl compounds are reduced to their corresponding alcohols with sodium borohydride under ultrasound irradiation and aprotic condition. Reduction reactions are performed in THF at room temperature or under reflux condition. The product alcohols were obtained in good to excellent yields. The chemoselective reduction of aldehydes over ketones was achieved successfully with this system.