Carbocation-forming reactions in dimethyl sulfoxide

Mesylate derivatives of 3-aryl-3-hydroxy-beta-lactams and thiolactams react in DMSO-d(6) by first-order processes to give alcohol products. Substituent effect studies implicate carbocation intermediates (ion-pairs) that are captured by DMSO-d(6) to give transient oxosulfonium ions. Rapid reaction of the oxosulfonium ions with trace amounts of water leads to the alcohol product and regenerates DMSO-d(6). H(2)(17)O labeling studies show that (17)O is incorporated into the DMSO. The mesylate derivatives of endo- and exo-2-hydroxy-2-phenylbicyclo[2.2.1]heptan-3-one also react in DMSO-d(6) to give the alcohol products. Ion-pair intermediates that capture DMSO giving unstable oxosulfonium ions are again proposed. Exo-2-phenyl-endo-bicyclo[2.2.1]heptyl trifluoroacetate readily eliminates trifluoroacetic acid in DMSO-d(6) via a cationic mechanism involving loss of the endo-trifluoroacetate leaving group as well as an exo-hydrogen. The O-methyl oxime derivative of alpha-chloro-alpha,alpha-diphenylacetophenone reacts in DMSO-d(6) to give 1-methoxy-2,3-diphenylindole, a product derived from cyclization of a cationic intermediate. A common ion rate suppression provides further evidence for a cationic mechanism. The triflate derivative of pivaloin reacts by a cationic mechanism in DMSO-d(6) to give rearranged products. The rate is even faster than in highly ionizing solvents such as trifluoroethanol or trifluoroacetic acid. 1-Adamantyl mesylate reacts in DMSO-d(6) by a first-order process (Y(OMs) = -4.00) to give a long-lived oxosulfonium ion, 1-Ad-OS(CD(3))(2)(+), which can be characterized spectroscopically. This oxosulfonium ion reacts only slowly with water at elevated temperatures to give 1-adamantanol. DMSO is therefore a viable solvent for k(s), k(C), and k(Delta) cationic processes.     

Preparation and characterization of p-toluene sulfonyl ester and amino derivatives of tri- and poly(ethylene glycol)

Methods described in the literature are inadequate for the preparation of pure polyethylene glycol (PEG) tosylate. Therefore an improved method is presented. The hydroxyl groups on PEG can be quantitatively converted into the tosylate and isolated from the reaction medium free from impurities with no chain cleavage or reduction in molecular weight. 1,2-Di(N-phenyl 2-aminoethoxy) ethane, α,ω-di(N-phenyl 2-aminoethyl) poly(oxyethylene), and α,ω-di(N-phenyl, N-benzyl 2-aminoethyl) poly(oxyethylene) were prepared from the tosylates of tri- and poly(ethylene glycol)s and the corresponding primary and secondary aromatic amines.     

Derivative spectrophotometric determination of nickel using Br-PADAP

A major problem with spectrophotometric methods for nickel is cobalt interference, because many of the reagents for nickel also react with cobalt. In this work, the interference of cobalt in the determination of nickel using 2-(5-bromo-2-pyridylaxo)-5-diethylaminophenol (Br-PADAP) was eliminated by the use of derivative spectrophotometry, using the zero-crossing method for evaluation of the derivative signal. Br-PADAP reacts with nickel(II) in the presence of Triton X-100 to form a red complex with absorption maxima at 530 and 562 nm. The reactions parameters and the conditions for the measurements of the first-derivative signal were studied and the results demonstrated that using the derivative technique, Br-PADAP can be used for nickel determination with a selectivity higher than that of ordinary spectrophotometry and with a limit of detection of 0.2 ng ml(-1). The pH should be in the range 5.0-6.0 using an acetate buffer. The determination of nickel in the presence of cobalt was performed with conventional and derivative procedures, and the results demonstrated that only the derivative method should be used and, of the methods used for evaluation of the derivative signal, the zero-crossing method is the best. The proposed procedure was used for nickel determination in steels standards. The results demonstrated that the procedure has satisfactory accuracy and precision. Cobalt interference can be also eliminated by using dual-wave-length spectroscopy.     

Polymer-supported nitroxyl radical catalyst for selective aerobic oxidation of primary alcohols to aldehydes

PS-TEMPO, a polymer-supported 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), was successfully applied as a recyclable, active and selective catalyst for the oxidation of primary aliphatic and benzylic alcohols to aldehydes by molecular oxygen in the presence of Co(NO3)2 and Mn(NO3)2 as co-catalysts.     

Cleavage by halogens of carbon—cobalt bonds in organometallic complexes of cobalt(III) : III. Kinetics and mechanism of the reactions of organobis(dimethylglyoximato)cobalt(III) complexes with iodine in chloroform

The kinetics of the reaction between organocobaloximes, RCo(DH)₂H₂O, and iodine have been investigated. They reveal the participation of an RCo(DH)₂H₂O · I₂ intermediate which undergoes intramolecular transalkylation and acts as an electrophile towards a second organocobaloxime molecule. The trend in reactivity as the R group is varied is discussed.     

Synthesis of 4H-3,1-benzoxazines, quinazolin-2-ones, and quinoline-4-ones by palladium-catalyzed oxidative carbonylation of 2-ethynylaniline derivatives

An effective and straightforward approach to the synthesis of 4H-3,1-benzoxazines 3 and 4, quinazolin-2-ones 5, and quinoline-4-one derivatives 6 and 7 is provided by palladium-catalyzed cyclization-alkoxycarbonylation of variously substituted 2-(trimethylsilanyl)ethynylaniline amide or urea derivatives 2. Reactions are carried out in 7:1 MeCN/MeOH at 65 or 75 degrees C in the presence of catalytic amounts of 10% Pd/C in conjunction with Bu(4)NI and KF and under 2.4 MPa of a 3:1 mixture of CO and air. Anti and syn 6-exo-dig cyclization modes account for the formation of the two stereoisomers. Isomerization of the vinylpalladium intermediate may occur as well. Formation of a double carbonylation product 7r and of a gem-dimethoxycarbonylation product 6s, whose structures have been determined by X-ray diffraction analysis, is justified through an unusual type of rearrangement.     

Remarkably facile solvolyses of triflates via carbocationic processes in dimethyl sulfoxide

A number of triflates have been shown to undergo clean pseudo-first-order solvolysis reactions in DMSO-d(6) to give products derived from carbocationic intermediates. Thus, t-BuCH(OTf)CO-t-Bu (5) and t-BuCH(2)OTf (9) react readily in DMSO-d(6) at 25 degrees C to give a rearranged oxosulfonium salts, and subsequent alkene products where methyl migration to the incipient cationic center occurs. t-BuCH(OTf)CO(2)CH(3) (14) gives analogous rearranged products, and 1-methylcyclopropyl triflate (21) gives a ring-opened allylic oxosulfonium salt. These triflates react primarily via k(Delta) pathways. 6-Methylbicyclo[3.1.0]hex-6-yl triflate (23), bicyclo[2.2.1hept-1-yl triflate (24), 1,6-methano[10]annulen-11-yl triflate (25), (CH(3))(2)C(OTf)CO(2)CH(3) (26), and (CH(3))(2)CCN(OTf) (29) all react in DMSO-d(6) to give carbocation-derived products. PhCH(OTf)CF(3) (33) and substituted analogues also react readily in DMSO-d(6), and the Hammett rho(+) value is -3.7. This suggests a "borderline" mechanism where the transition state has substantial charge development. The primary feature of these solvolyses is the high reactivity of all of these triflates in DMSO-d(6). Thus, these triflates are all more reactive in DMSO-d(6) than in HOAc, and for most, rates are faster than in CF(3)CH(2)OH. Triflates 5, 21, 29, and 33 are 10(8)-10(9) times more reactive in DMSO-d(6) than the corresponding mesylates. It is suggested that the decreased need for electrophilic solvation of triflate anion, and the high cation solvating ability of DMSO, are the reasons for the high triflate reactivity in DMSO-d(6).     

A one-pot synthesis to [(Me3Si)3SiSb]4; a potential precursor for Sb42-

The reaction of (Me(3)Si)(3)SiK[middle dot]18-crown-6 with SbCl(3)(3 : 1 equiv.) provides a simple route to the title complex [(Me(3)Si)(3)SiSb](4). The potassium base initially acts as a nucleophile and then as a coupling agent, forming Sb-Sb bonds.     

DABCO-directed self-assembly of bisporphyrins (DABCO=1,4-diazabicyclo[2.2.2]octane)

Three isomeric zinc bisporphyrins have been prepared by covalently linking together two aminoporphyrins with an isophthalic acid derivative. The porphyrins differ in the substitution pattern on the meso phenyl groups, that is, ortho, meta, or para. Titrations carried out by UV-visible and 1H NMR spectroscopy have been used to map out the stabilities and the stoichiometries of the complexes formed with 1,4-diazabicyclo[2.2.2]octane (DABCO) in chloroform. The ortho- and meta-substituted bisporphyrins form 1:1 intramolecular sandwich complexes. The para-substituted bisporphyrin cannot adopt the cofacial conformation required for this type of complex and forms a higher order 2:2 intermolecular assembly, which is stable over a wide range of DABCO concentrations.