Transition metal,p-toluenesulfonates

Submitted by STEPHEN M. HOLMES, SCOTT G. MCKINLEY, and GREGORY S. GIROLAMI  

In the present contribution, we describe the preparation of binary transition metal p-toluenesulfonates (tosylates), which have been known since the 1870s but have been little used as synthetic starting materials. These salts are relatively inexpensive to prepare, can be isolated in high crystalline yields, and are soluble in many polar solvents. Tosylate salts are typically less soluble than the corresponding triflates, but more soluble than corresponding halide salts. 

School of Chemical Sciences, University of Illinois at Urbana—Champaign, 600 South Mathews Ave., Urbana, IL 61801. 

Transition metal tosylates have been described for Ti , Cr ,  

Ybiii 20 metal tosylates have also been briefly described for Li, Na, K, 

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Besides Wacker oxidation, other transition-metal catalyzed oxidations have also been carried out in aqueous medium. For example, methyl groups can be selectively hydroxylated by platinum salts in water.88 In this way, p-toluenesulfonic acid was oxidized to benzy-lic alcohol, which was subsequently oxidized into the aldehyde (Eq. 3.19).89... 

More synthetic interest is generated by the potentially very useful hydration of dienes. As shown on Scheme 9.6, methylethylketone (MEK) can be produced from the relatively cheap and easily available 1,3-butadiene with combined catalysis by an acid and a transition metal catalyst. Ruthenium complexes of several N-N chelating Hgands (mostly of the phenanthroline and bipyridine type) were found active for this transformation in the presence of Bronsted acids with weakly coordinating anions, typically p-toluenesulfonic acid, TsOH [18,19]. In favourable cases 90 % yield of MEK, based on butadiene, could be obtained. 

Acid-catalysed addition of primary, secondary, and tertiary alcohols to 3,4-dihy-dro-2//-pyran in dichloromethane at room temperature is the only general method currently in use for preparing THP ethers and the variations cited below concern the choice of acid. The reaction proceeds by protonation of the enol ether carbon to generate a highly electrophilic oxonium ion which is then attacked by the alcohol. Yields are generally good. Favoured acid catalysts include p-toluenesulfonic acid or camphorsulfonic acid. To protect tertiary allylic alcohols and sensitive functional groups such as epoxides, the milder acid pyridinium p-toluenesulfonate has been employed (Scheme 4.316]. A variety of other acid catalysts have been used such as phosphorus oxychloride, iodotrimethylsilane- and bis(trimethylsilyl)sulfate. but one cannot help but suspect that in all of these cases, the real catalyst is a proton derived from reaction of the putative catalysts with adventitious water. Scheme 4.317 illustrates the use of bis(trimethylsilyl)sulfate in circumstances where other traditional methods failed. - For the protection of tertiary benzylic alcohols, a transition metal catalyst, [Ru(MeCN)2(triphos)](OTf)2 (0.05 mol%) in dichloromethane at room temperature is effective. ... 

The earliest work on polyester synthesis used no catalyst or a simple acid catalyst such as p-toluenesulfonic acid, but use of weakly basic metallic salt catalysts is now almost universal. Many salts have been claimed to be useful in this context, but the best known examples are alkaline earth and transition metal acetates, tin compounds and titanium alkoxides [21-23]. Care must be exercised in selecting ester-interchange catalysts because some may cause degradation/ discoloration in the polymer during the subsequent polymerisation reaction [24], especially for PET and PEN. To prevent this occurrence, catalysts are often sequestered/complexed at the end of the ester-interchange phase by addition of phosphorus compounds such as phosphites, phosphates or polyphosphoric acid [25]. Titanium and tin compounds operate as catalysts for ester-interchange and polymerisation reactions, and in general do not require such procedures. 

In the present contribution, we describe the preparations of several binary transition metal tosylates directly from the metal and p-toluenesulfonic acid under an inert atmosphere. This method is easy to carry out, and affords products that are completely free of contaminating counterions. The presence of excess metal provides a reducing environment, so that divalent products are obtained for all first-row transition metals except for Ti and V, which form trivalent products under these reaction conditions. The Cr° salt may be converted to Cr (OTs)3 by air oxidation in the presence of excess p-toluenesulfonic acid. Tosylate salts of Ti, V, Fe, and Cs can also be prepared by treatment of the corresponding metal chloride with p-toluenesulfonic acid, and a vanadyl salt has been prepared by similar treatment of vanadyl acetylacetonate. 

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