SN2-type Reactions Research Articles

Bis(pentamethylcyclopentadienyl)keton und ‐thioketon: Kohlenstoff‐Verbindungen mit präformierter Diels‐Alder‐Geometrie

Bis(pentamethylcyclopentadienyl) Ketone and Thioketone: Carbon Compounds with Performed Diels‐Alder Geometry1,2,3,4,5‐Pentamethyl‐1,3‐cyclopentadien‐5‐carbonyl chloride (2) is formed in good yields by the reaction of pentamethylcyclopentadienyllithium (1) with phosgene. The corresponding carbothioyl chloride 3 is synthesized by treatment of 1 with thiophosgene. Both acyl chlorides are stable against air and moisture and difficult to attack in SN2‐type reactions. Treatment of 2 and 3 with trimethyl(pentamethylcyclopentadienyl)stannane in the presence of boron trifluoride — ether leads to bis(1,2,3,4,5‐pentamethyl‐1,3‐cyclopentadien‐5‐yl) ketone (5) and thioketone (6), respectively. Even at room temperature, 5 and 6 tend to intramolecular [4 + 2] cycloaddition reactions. X‐ray crystal structure investigations of 2, 5, and 6 show the steric demand of the pentamethylcyclopentadienyl ligand and explain the untypical chemical behavior of 2 and the easy [2 + 4] cycloaddition reactions of 5 and 6.

Communication: The Reductive Coupling of Carbohydrate Radicals with Diazonium Ions: a New Access to 2-Amino-2-Deoxy Glycosides

The oxidative coupling of glycals with alcohols is an efficient procedure for creating an α-glycosidic bond. It can be triggered by an “I+”-reagent (iodonium di-sym-collidine perchlorate1 or N-iodosuccinimide2) and occurs in a 1,2-diaxial fashion through a presumed 1,2-iodonium ion intermediate leading to 2-deoxy-2-iodo-α-glycosides. This iodoglycosylation allows the preparation of 2-deoxy-α-glycosides by reductive deiodination with tributylstannane3 or nickel boride.4 Considering the behavior of good leaving groups at C-2 of α-D-mannopyranosides,5 the displacement of iodine by oxygen or nitrogen functions through SN2-type reactions would be difficult or impossible, unless performed with internal nucleophiles.6 In contrast, a C-2 radical can be easily trapped by electron-deficient olefins7 or nitrogen monoxide8 giving access to C-2 branched carbohydrates and oximes, respectively. Nucleophilic radicals have also been trapped by aryldiazonium ions in a reversible manner to give azo radical cations.9 In the presence of a reducing agent (Ti3+, Fe2+, Cr2+, V2+), the equilibrium is shifted towards the azo compound:

Role of single electron transfer in SN2-type substitution reactions of anions with alkyl halides

Les constantes de vitesse des reactions d'electrophiles avec differents carbanions (comme les fluorenures substitues en 9, ou d'autres ions comme les nitranions, oxanions, thianions) ont ete comparees avec les constantes de vitesse de transfert d'un seul electron (SET) calculees grâce a une equation de type Marcus

The absolute configuration of benproperinium dihydrogen phosphate, an antitussive drug.

The (+)- and (-)-enantiomers of benproperinium dihydrogen phosphate, an antitussive drug, have been assigned the R- and S-configurations, respectively, by syntheses of both enantiomers using (S)-2-hydroxypropanoic acid (L-lactic acid) as chiral synthon. The key intermediate, (S)-1-methyl-2-[2-(phenylmethyl)phenoxy]ethyl p-toluenesulfonate, was subjected to an SN2-type reaction with piperidine furnishing (+)-(R)-benproperinium dihydrogen phosphate. (-)-(S)-Benproperinium dihydrogen phosphate was obtained by submitting the same tosylate to two consecutive SN2-type reactions with Br- and piperidine, respectively, acting as nucleophiles.

Geminal bis[(triphenylphosphoranylidene)amino]cyclotriphosphazenes synthesis: substitution reactions, and nuclear magnetic resonance spectra

Cyclotriphosphazenes bearing an amino ligand are readily converted into the corresponding (triphenylphosphoranylidene)amino (–NPPh3) derivatives by means of an excess of triphenylphosphine–tetrachloromethane in an acetonitrile–triethylamine medium (Appel reaction). Thus prepared, N3P3Cl4(NPPh3)2 reacts with dimethylamine in various solvents according to a geminal substitution pattern; this is an indication of SN1-type reactions, as a result of the strong electron-donating character of the ligands. The ultimate substitution product N3P3(NMe2)4(NPPh3)2, also prepared from N3P3(NMe2)4(NH2)2, is isolated as its HCl or 2HCl adduct, depending on the method of preparation. In both adducts the protons are attached to ring nitrogen atoms, and variable-temperature 1H and 31P n.m.r. measurements show prototropic behaviour at elevated temperatures. Trends within the 31P n.m.r. parameters of the derivatives are discussed in terms of the electron-donating ability of the ligands.

ChemInform Abstract: THE REACTION OF ALKYL HALIDES WITH MERCURY(II) AND POTASSIUM THIOACETATES AND SELENOCYANATES

AbstractHg‐ bzw. K‐thioacetat (I) reagieren mit Alkylhalogeniden in Hexan bzw. DMF zu Alkylthioacetaten (II).

The Reaction of Alkyl Halides with Mercury(II) and Potassium Thioacetates and Selenocyanates

Abstract The reactions of alkyl halides with mercury(II) and potassium thioacetates afford alkyl thioacetates (RSCOMe), SN1 and SN2 type reactions taking place. In neither case is ROCS Me formed. Reactions with mercury(II) selenocyanate usually give only small amounts of alkyl selenocyanates and/or isoselenocyanates as isolated products except the case of benzyl bromide from which benzyl selenocyanate was obtained almost quantitatively. The ambident reactivity of this salt was revealed to be not so strong as that of mercury(II) thiocyanate. The reaction with potassium selenocyanate is also reported for comparison.

Influence of the solvent on the rates of SN1-type solvolysis reactions of metal complexes in water + co-solvent mixtures

The relationships developed for the variation of rate with dielectric constants have been extended to include effects derived from changes in solvent structure in order to interpret the linear plots for log (rate constant) against reciprocal of dielectric constant which show slope invariant with the identity of the co-solvent for some metal complexes in water + co-solvent mixtures. In order to apply free energy cycles to such reactions, the free energies of transfer of halide ions from water into water + co-solvent have been determined for water + ethanol and water + dioxan mixtures. The application of these cycles to SN1-type solvolysis shows that, not only do changes in solvation of the transition state have a dominant effect on the rate for a wide range of metal complexes with a range of co-solvents, but that the same conclusion can be drawn for some organic halides.

Reactions of 2,2,2-triphenylethyl p-toluenesulfonate in alcohol solutions in the presence of base under high pressure. Effects of pressure on SN1- and SN2-type reactions

Reactions of 2,2,2-triphenylethyl p-toluenesulfonate in alcohol solutions in the presence of base under high pressure. Effects of pressure on SN1- and SN2-type reactions

Chemistry of tropan-3-yl ethers. Part III. Kinetic properties of tropan-3α-yl methanesulphonate

The reactions of tropan-3α-yl methanesulphonate with various nucleophiles have been investigated. Rate constants of the displacement reactions have been correlated with the configuration at C-3 of the reaction products. For stronger nucleophiles SN2 type reactions predominate over SNI type, and more β-substituted tropanes are produced than α-isomers.

Reactivity of vinyl sulphonic esters. Part VI. SN1-type reactivity of 1,2-diphenyl-2-phenylthiovinyl and triphenylvinyl derivatives: anchimeric assistance due to the β-sulphur atom

The kinetic responses of trans-1,2-diphenyl-2-phenylthiovinyl sulphonates (I) and of the analogous triphenylvinyl derivatives (II) in SN1 processes are directly compared and anchimeric assistance effects in the case of the former esters are evaluated. Rate factors, k(I)/k(II), ranging between 16 and 33 are observed in a variety of conditions and estimated as the lower limits of rate enhancements due to β-sulphur anchimeric assistance. The rather small kinetic effect due to β-sulphur assistance in, as well as the absence of β-aryl assistance from, SN1-type reactions of derivatives (I) and (II) are probably due to the ‘levelling’ effect of the α-aryl residue but other factors, such as the inherent difficulty of an intramolecular nucleophilic attack in the plane of the ethylenic system, may be involved.

Anchimeric assistance effects due to the β-sulphur atom in solvolytic reactions of dialkyl-β-thiovinyl sulphonates

Large rate enhancements (104–105) due to anchimeric assistance by the β-sulphur have been observed in SN1-type reactions of 1,2-dialkyl-2-alkylthiovinyl sulphonates.

Stereochemistry and Mechanism of Catalytic Hydrogenation and Hydrogenolysis. III. Catalytic Hydrogenolysis of Benzyl-type Alcohols and Their Derivatives

Abstract In order to obtain evidence for the mechanism we proposed of the catalytic hydrogenolysis of benzyl-type alcohols and their methyl ethers, optically active 2-phenyl-2-butanol (Ia), 2-phenyl-2-methoxybutane (IIa), ethyl atrolactate (Ib), and ethyl 2-phenyl-2-methoxypropionate (IIb) were hydrogenolyzed over Raney nickel and Pd(H) catalysts under hydrogen or helium atmosphere in an ethanol solution. In all cases, the hydrogenolysis proceeded stereoselectively with retention of configuration over Raney nickel catalyst but with inversion of configuration over palladium catalyst. A reverse stereoselectivity was observed when the benzyl alcohols (Ia and 2-phenyl-1,2-dihydroxypropane (Ic)) and methyl ethers (IIa) and (IIb) were hydrogenolyzed in the presence or absence of various additives over nickel catalysts. It was also found that the hydrogenolysis of benzyl alcohols always proceeds with retention of configuration over nickel catalyst containing sodium hydroxide. Various binary mixtures of Ia, IIa, Ib and IIb were hydrogenolyzed competitively over Raney nickel and Pd(H) catalysts. The order of relative rates was found to be Ia\gtrsimIb\gtrsimIIa\gtrsimIIb over Raney nickel, and IIa\gtrsimIa>>IIb\gtrsimIb over Pd(H) catalyst. The results support our mechanism. The catalytic hydrogenolysis is a competitive reaction of two courses (SNi type and SN2 type reactions), and the stereoselectivity reflects the difference of the free energy levels of the transition states to form π-benzylic complexes. The free energy levels of the corresponding transition states depend on stereoelectronic factor, affinity of substituents for catalyst metal, catalyst hindrance and electronic effects of substituents. The acetates (IVa) and (IVb) of Ia and IIa were hydrogenolyzed with inversion of configuration over nickel and palladium catalysts, whereas the stereoselectivity of the hydrogenolysis of the benzoate (V) of IIa depended on the kind of catalyst. The results are also explained on the basis of our mechanism.

SECONDARY DEUTERIUM ISOTOPE EFFECTS IN SOLVOLYSES OF SMALL-RING COMPOUNDS

The importance of nonclassical ions as relatively stable intermediates in SN1- type reactions is a matter of considerable current controversy. A very important question in this controversy is the extent to which the observed rate accelerations are due to nonclassical electron delocalization in the transition states as compared to high initial state free energies because of steric strain.