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|Title:||One Electron Reduction of Sulfonium Salts: A Mechanistic Study|
|Author(s):||Sullivan, Thomas Allen|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Twenty-three phenylsulfonium salts have been reduced to sulfides using the heterogeneous intercalation compound potassium/graphite (KC(,8)). The products, consisting of sulfides and hydrocarbons, have been identified by glc, ('1)H NMR and mass spectroscopic comparison to authentic material, and/or elemental analysis. The systems studied were acyclic (1), cyclic (2), and benzofused cyclic (3) sulfonium salts.
The acyclic salts, 1, included R(,1) = Ph with R(,2), R(,3) = Me, Et; Me,n-Bu; Me, 3-butenyl; n-Bu, i-Pr; 3-butenyl, i-Pr; Me, 2-pentyl; Me,i-Pr; Me, 1-phenyl-2-propyl; Me, PhCH(,2); Me, (CH(,2))(,4)S(Me)Ph;(' )PhCH(,2)CH(,2), (CH(,2))(,4)S(Ph)CH(,2)CH(,2)Ph; PhCH(,2)CH(,2), PhCH(,2)CH(,2); Ph,(' )Me; Ph, Et; R(,1) = R(,2) = R(,3) = 4-MeC(,6)H(,4). No reduction occurredwhen R(,1) = t-Bu and R(,2) = R(,3) = Me.
The acyclic systems (2) included R(,1) = Ph with R(,2), n = H, 1; H, 2; Me, 1. When R(,1) = Me, R(,2) = H, and n = 2 no reduction occurred.
Benzofused cyclic sulfoniums salts (3) included R(,1), R(,2), n = Me, H, 1; Me, H, 2; Me, Me, 1; and Ph, H, 1. S-methylthioxanthenium fluoborate was also reduced. In most cases the anion X('-) = PF(,6)('-), however, in a few cases X('-) = BF(,4)('-) or BPh(,4)('-).
Comparison of the product ratios from suitably substituted salts with models of a radical and anionic nature shows that a one electron reduction, followed by radical cleavage, is involved in this reduction. Specifically, the cleavage ratio of benzyl:2(DEGREES):1(DEGREES):Me:Ph groups in acyclic, hence unperturbed, sulfonium salts is 28:(6.0 (+OR-) 0.3):1.0:(0.53 (+OR-) 0.09):(< 0.05). No phenyl cleavage was detected in the monophenyl acyclic salts. These ratios appear to be independent of the electron transfer agent, as demonstrated by a 2(DEGREES):1(DEGREES) ratio of 5.7 (+OR-) 1.2 when the homogeneous reducing agent K('+)Nap('-(' ))was used.
The inability of KC(,8) to reduce two trialkylsulfonium salts suggests that the initial electron transfer is to the LUMO of the phenyl ring to provide a ligand-(pi) radical intermediate. Cleavage of a radical directly from this tetrahedral conformer is apparently controlled by overlap of the appropriate (sigma)('*) orbital with the LUMO of the aromatic ring. This overlap can be inhibited by stereoelectronic control due to the geometry of certain systems, specifically the thiaindan and thiachroman rings.
Alternately, the ligand-(pi) intermediate may collapse to a TBP conformer, whereupon cleavage from an apical position dominates. This appears to be competitive only when at least two phenyl groups are present in an acyclic salt. In the benzofused salts, both modes of cleavage are conceivable when a 5-membered ring and/or a phenyl group is present to stabilize the TBP conformer. Product analysis does not allow for determining which mechanism is correct.
The fate of the cleaved radical is dependent on the nature of the starting sulfonium cation as well as the reducing agent. With KC(,8) the radicals, once cleaved, are in a relatively high local concentration and bimolecular processes dominate. These same radicals, when generated by use of K('+)Nap('-(' ))under more dilute conditions, follow unimolecular routes of decomposition. Thus, cyclic and benzofused cyclic sulfonium salts may give products that are not indicative of the initial cleavage ratio. Specifically, intramolecular S(,H)i displacement of an exocyclic phenyl group can occur following cleavage of the endocyclic primary group of 1-phenylthiaindan hexafluorophosphate upon reduction, but the product yields suggest that this path is followed to less than 10% with KC(,8). This ring opening-ring closing is not observed when groups other than phenyl are present on sulfur.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.
|Date Available in IDEALS:||2014-12-15|