- UNEXPECTED REGIOSELECTIVE DIELS-ALDER CYCLOADDITION REACTIONS BETWEEN 3-FLUOROBENZYNE AND 2-ALKYLFURANS
-
The Diels-Alder reaction between 3-fluorobenzyne (1) and 2-alkylfurans (2) gives a mixture of syn adduct 3 and anti adduct 4.The syn cycloadduct invariably predominates and the regioselectivity increases in the order: R= Me Et i-Pr t-Bu, to a maximum of 90percent syn adduct 3d for 2-tert-butylfuran.These results are rationalized in terms of a concerted nonsynchronous transition state that reflects an alkyl steric effect and a polarized aryne 1.
- Gribble, Gordon W.,Keavy, Daniel J.,Branz, Stephen E.,Kelly, William J.,Pals, Mark A.
-
p. 6227 - 6230
(2007/10/02)
-
- VICINAL ALKYLATION OF ALKYNES. A SHORT ROUTE TOWARD Δα,β BUTENOLIDES, FURANS AND CYCLOPENTENONES.
-
Cyclobutenones 1 which are readily prepared from alkynes and keteniminium salts 2 were regiospecifically converted into Δα,β butenolides 4 or cyclopentenones 7.Reaction of 4 with diisobutylaluminium hydride yielded the corresponding substituted
- Schmit, C.,Sahraoui-Taleb, S.,Differding, E.,Dehasse-De Lombaert, C. G.,Ghosez, L.
-
p. 5043 - 5046
(2007/10/02)
-
- Gas-phase Heteroaromatic Substitution. Part 4. Electrophilic Attack of t-Butyl Cation on Pyrrole, N-Methylpyrrole, Furan, and Thiophene
-
t-Butyl cation, obtained in the gas state from the γ-radiolysis of neopentane has been allowed to react with pyrrole (1), N-methylpyrrole (2), furan (3), and thiophene (4).Experiments have been carried out in the pressure range 50-760 Torr and in the presence of variable concentrations of a gaseous base (NMe3).The reactivity of simple heteroaromatic compounds relative to toluene in competition experiments is found to depend markedly upon the composition of the gaseous reaction environment.The apparent kS/kT ratios decrease slightly with increasing pressure.At atmospheric pressure, these ratios increase with the NMe3 concentration levelling off to a value of 1.0 (1), 2.2 (2), 2.6 (3), and 0.6 (4), at the highest base concentration (PNMe3=10 Torr).Under the same conditions, predominant β substitution occurs in the pyrroles , whereas α-attack is favoured in the case of furan (β:α=9:91) and thiophene (β:α=20:80).The gaseous elecrophile appears rather unselective between the n- and the ?-type centres of furan (O:ring=50:50) and thiophene (S:ring=40:60), while it displays no or very limited substitution on the heteroatom of pyrroles.A mechanism based upon reversible addition of the t-butyl cation to the heteroaromatic nucleus, involving premilinary formation of an electrostatic adduct, is used to account for the formation of the t-butylated products and their isomeric composition.It is concluded that the gas-phase t-butyl alkylation of simple five-membered heteroaromatic compounds is regulated by electrostatic interactions established within the encounter pair.A close correspondence exists between this rationalization of the present gas-phase results and recent theoretical and experimental evidence concerning related alkylation processes.
- Margonelli, Andrea,Speranza, Maurizio
-
p. 1491 - 1498
(2007/10/02)
-