[563-68-8] · C2H3O2Tl · Thallium(I) Acetate-Iodine · (MW 263.43)
[7553-56-2] · I2 · Thallium(I) Acetate-Iodine · (MW 253.80)
(preparation of vic-iodoacetates for Prevost (dry) and Woodward-Prevost (wet) conversion of alkenes into trans- and cis-diols;1 for iodolactonizations,2 iodination of phenols,3 formation of a-iodo ketones4)
Physical Data: TlOAc: mp 131 °C; d 3.76 g cm-3. I2: mp 113.5 °C; bp 184.35 °C; d 4.93 g cm-3.
Solubility: TlOAc: v sol cold H2O, EtOH, CHCl3; insol acetone. I2: sparingly sol hot H2O (0.078 g/100 mL); sol ether (20.6 g/100 mL), EtOH (20.5 g/100 mL).
Preparative Methods: prepared in situ by heating Thallium(I) Acetate (2 equiv) with dry acetic acid under reflux for 1 h, and then treating the cooled mixture with the alkene (1 equiv) and Iodine (1 equiv). The resulting suspension is stirred and heated at reflux, filtered from yellow thallium(I) iodide, and worked up1 to give the vic-diol ester. Alternatively, a dichloromethane solution of iodine (1 equiv) is added dropwise to a stirred suspension of thallium(I) acetate (1 equiv) in dichloromethane containing the alkene (1 equiv) and the mixture worked up to give the vic-iodocarboxylate.
Handling, Storage, and Precautions: thallium salts are extremely toxic and thus great care must be taken in their use and disposal. All operations should be carried out in an efficient fume hood, and rubber gloves should be worn. Dispose of thallium wastes carefully.5-8
The reaction of an alkene with iodine and thallium(I) acetate in the ratio 1:1:1.2 in acetic acid at room temperature gives the trans-a-iodoacetate, generally in yields of 85-89% (eq 1). Thallium(I) benzoate in benzene can also be used.1,2
Since the iodocarboxylates can be transformed into appropriate derivatives of either syn- or anti-diols by suitable choice of conditions for solvolysis, the reaction provides an inexpensive alternative to the Prevost reaction,9 which uses silver(I) carboxylates. Thus solvolysis of a trans-iodoacetate in wet acetic acid gives the corresponding cis-hydroxyacetate; trans-diacetates are obtained by solvolysis in dry acetic acid with added sodium acetate. Unlike silver(I) carboxylates, thallium(I) carboxylates are readily prepared in high yield as crystalline solids and they do not induce solvolysis at room temperature.2,10 With the exception of 3-phenylpropene, reactions with unsymmetrical alicyclic alkenes are regioselective, addition occurring in the Markovnikov orientation (eq 2).
Marked differences exist between the modes of reaction of thallium(I) carboxylates and silver(I) carboxylates. No reaction occurs between the thallium(I) carboxylate, iodine, and an alkene unless all three are present, in contrast to the silver(I)-mediated systems where prior formation of the Simonini complex between the acyl hypoiodite, the silver carboxylate, and iodine is the case. Thus it has been suggested that an iodonium ion is formed from the interaction of a thallium(I) species and an alkene-iodine p-complex (eq 3), which gives the iodocarboxylate that then reacts by a normal Woodward or Woodward-Prevost mechanism.1
Reaction of thallium(I) acetate-iodine with 1-methylcyclohexene in wet acetic acid at 20 °C gives a trans-iodoacetate as the major product, but also affords a significant amount of regioisomer (eq 4), indicating that the high regioselectivity observed for acyclic compounds does not apply to substituted carbocycles.11 Likewise, 3-t-butylcyclohexene gives a mixture (ca. 3:1) of stereoisomeric iodoacetates (eq 5).12
These results can be explained based on the assumption that two stereoisomeric iodonium ions are formed in a reversible pre-equilibrium, which then react at rates dependent on steric and conformational factors.
Substituted cyclopropanes (e.g. phenylcyclopropane) react with thallium(I) acetate-iodine less readily than alkenes, but give ring opened 1,3-disubstituted propanes as the major products, and in some cases products from solvolysis of intermediate iodoacetates (eq 6).13
Neighboring groups influence the course of reactions by taking part in rearrangements which typically involve carbocationic intermediates.14
The trans vic-iodoesters formed from alkenes are readily converted into cis-hydroxyesters by oxidative displacement with m-Chloroperbenzoic Acid (eq 7).15
Unlike iodine(I) acetate (prepared from iodine(I) chloride and Silver(I) Acetate) which oxidatively cleaves vic-diols rapidly and in high yield at 20 °C, thallium(I) acetate and iodine(I) chloride has no effect.16
Addition of iodine in ether to a suspension of an unsaturated thallium(I) carboxylate in ether at room temperature using a stoichiometric ratio of 1:1 gives high yields of iodolactones.17 Addition to b,g-unsaturated acids can in appropriate cases give b-lactones as the products of reaction under kinetic control. Rearrangement then gives the isomeric g-lactone (eq 8). Using thallium(I) acetate and iodine in a nonpolar solvent at 0 °C gives even better yields of the lactones (eq 8).18 This reaction has been extended to bromolactonization using Thallium(I) Carbonate and bromine.19
Iodination of activated phenols with thallium(I) acetate-iodine using a ratio of 1:1:1.2 gives selective ortho-iodination (eq 9).3 Use of dichloromethane as the solvent rather than acetic acid results in increased solubility of the iodine and facilitates workup since neutralization of the acetic acid is no longer necessary.
Treatment of enol acetates with thallium(I) acetate-iodine affords high yields of a-iodo ketones (eq 10).4 The a-iodo ketones are probably formed via an acetoxyiodonium ion by a pathway similar to that for iodocarboxylations of alkenes.
Richard C. Cambie, Peter S. Rutledge & Paul D. Woodgate
University of Auckland, New Zealand