Electrophilic Process Research Articles

Formation of Thioamide Derivatives from Reactions of Isothiocyanates with Oxazol-2-amines

4-Substituted oxazol-2-amines react with isothiocyanates to give products having a thioamide function at C5. The reaction is considered to be an electrophilic process in competition with the usual reaction of the amino group with the isothiocyanate . The nature of the isothiocyanate and the type of substituent at C4 affect the amount of the thioamide product formed. Some chemical properties of the thioamides are also investigated.

Singlet oxygen oxidation of substituted thiobenzamides

The relative rates of oxidation by singlet oxygen of a series of p-substituted thiobenzamides follow a Hammett ϱσ correlation with ϱ = −0.39 indicating that the oxidation proceeds via an electrophilic process. Initial product determinations and comparison with other examples of oxidation of thiocarbonyl compounds by singlet oxygen suggest that the reaction proceeds by attack of singlet oxygen at the sulfur atom.

Effect of substituents on the homo-1,4 addition of difluorocarbene to norbornadienes

Difluorocarbene reacts with 2-methoxy and 2-carbomethoxy-7,7-dimethylnorbornadiene to give homo-1,4 adducts only. The relative rates of addition are compared with that for 7,7-dimethylnorbornadiene and are found to be 2.63, 0.045 and 1.0 respectively. Consequently, an electrophilic process is operating. Difluoro, dichloro and dibromocarbenes react with 7,7-dimethylnorbornadiene exclusively on the endo face to give 1,2 and homo-1,4 adducts in ratios of ≈0.1, 0.5 and 0.7 respectively. The homo-cheletropic reaction, compared to the competing cyclopropanation, is increasingly sensitive to the bulk of the carbene partner.

Gas-phase alkylation and halogenomethylation by free dialkylhalogenonium and halogenomethylium ions

Dimethylhalogenonium ions from the γ-radiolysis of methyl halides were allowed to react in the gas phase with selected substrates, containing either a π- or an n-type nucleophilic centre. The reactions were studied at CH3X pressures ranging from 100 to 760 Torr, in the presence of a thermal radical scavenger (O2) and variable concentrations of an added base (NH3). Dimethylfluoronium ions readily react with all the substrates investigated, yielding methylated products, whose isomeric composition appreciably depends on the pressure of the gaseous system. Their attack on toluene leads to an intramolecular distribution (para/0.5 meta= 2.5) typical of a gas-phase electrophilic displacement process, whereas their substrate selectivity reveals the influence of steric hindrance for the most encumbered substrates. On the other hand, dimethylchloronium and dimethylbromonium ions sharply discriminate between n- and π-type basic centres, showing much greater selectivity than dimethylfluoronium ions. The reactivity and selectivity features of other electrophiles, formed in minor concentrations from the radiolysis of the halides, i.e. the halogenomethylium ions, were also examined. The mechanisms of the gas-phase alkylations are discussed, and compared with those of different gaseous electrophiles and with those of Friedel–Crafts alkylating agents. It is concluded that the behaviour of condensed-phase methylating species and gas-phase dimethyl-fluoronium ions display significant analogies, traced to their common mode of attack on aromatic substrates.

CARBON-PHOSPHORUS HETEROCYCLES. SYNTHESIS OF SUBSTITUTED 2,3,4,5-TETRAHYDRO-5-OXO-2,2-DIPHENYL-1H-2-BENZOPHOSPHEPINIUM SALTS VIA INTRAMOLECULAR ACYLATION WITH 115% POLYPHOSPHORIC ACID

Abstract Intramolecular acylation involving three ω-carboxyalkylphosphonium salts 1a-c has been achieved in the presence of 115% polyphosphoric acid (PPA) to yield the heretofore unknown 2,3,4,5-tetrahydro-5-oxo-2,2-diphenyl-1H-2-benzo-phosphepinium salts 2a-c. The open-chain precursors were easily prepared by quaternization of the corresponding tertiary arylmethyl-substituted phosphines with 3-chloropropanoic acid. Cyclization of the carboxy-substituted salts was accomplished at 200°C with PPA in modest yields (37–64%). A mechanism reminiscent of a classic electrophilic acylation process was tentatively proposed. An x-ray crystallographic analysis of a single crystal of 2a showed the phosphepin ring to exist in a twisted-chair conformation.

Aluminum chloride-catalyzed reactions of ferrocene with phosphorus(III) amides. Novel coordination of the PN system

In aluminum chloride-catalyzed reactions with ferrocene, P III compounds show reactivities in the order, PCl 3 ⪡ R 2NPCl 2 > (R 2N) 2PCl > (R 2N) 3P, revealing that active intermediate species are formed by the coordination of aluminum chloride not to phosphorus but to nitrogen. Evidence is presented which supports this, and indicates that aluminum chloride cleaves the PN bond via bonding to nitrogen, generating PCl. HCl, formed as by-product of the Friedel-Crafts reactions, is not a factor in the cleavage of the PN bond. Catalysis and inhibition of the reactions are discussed in terms of a mechanism based on the concept of a dimolar aluminum chloride complex of a donor species such as R 2NPCl 2 or R 3N. The reactions are seen to generate (Fc) 3P both by a stepwise electrophilic process and by disproportionation of (Fc) 2PCl. The coordination of AlCl 3 to nitrogen of PN is rationalized on the basis of bond energetics and the unavailability of electrons for back-bonding to phosphorus. Several new phosphorus amides of ferrocene are reported.