Premium
1,3‐Dithiolanes from Cycloadditions of Alicyclic and Aliphatic Thiocarbonyl Ylides with Thiones: Regioselectivity
Author(s) -
Huisgen Rolf,
Mloston Grzegorz,
Polborn Kurt,
Sustmann Reiner
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200204659
Subject(s) - regioselectivity , alicyclic compound , chemistry , steric effects , diazomethane , cycloaddition , ylide , aryl , medicinal chemistry , computational chemistry , stereochemistry , organic chemistry , catalysis , alkyl
Abstract The regiochemistry of 1,3‐dithiolanes obtained from thiocarbonyl ylides 9 and thiones 10 shows a striking dependence on substituents. Previously and newly performed experiments indicate that sterically hindered cycloalkanethione S ‐methylides and dialkylthioketone S ‐methylides react with alicyclic and aliphatic thiones to give the 2,2,4,4‐tetrasubstituted 1,3‐dithiolanes 11 exclusively. Aryl groups in one or both reactants lead to a preference for, or even complete formation of, 4,4,5,5‐tetrasubstituted 1,3‐dithiolanes 12 . Several mechanisms appear to be involved, but the paucity of experimental criteria is troubling. Quantum‐chemical calculations (see preceding paper) on the cycloaddition between thioacetone S ‐methylide and thioacetone furnish lower activation energies for the concerted process than for the two‐step pathways via C , S ‐ or C , C ‐biradicals; the favoring of the 2,4‐substituted 1,3‐dithiolanes over the 4,5‐substituted type would be expected to increase with growing bulk of substituents. Aryl groups stabilize intermediate biradicals. Experimental criteria for the differentiation of regioisomeric dithiolanes are discussed. Thiocarbonyl ylides 9 are prepared by 1,3‐cycloadditions between diazomethane and thioketones and subsequent N 2 elimination from the usually isolable 2,5‐dihydro‐1,3,4‐thiadiazoles 17 ; different ratios of the two rate constants lead to divergent product formation scenarios.