Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: java on January 03, 2006, 12:42:13 PM
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It's well known in that the best way to remove "OH from alcohols at least make it a better leaving group is to protenate it or convert it to a sulfonic ester. The sulfonic ester group tosylate, brosylate, and mesylate are better leaving groups."..March's Advanced Organic Chemistry 5th ed. pg.446
Now that the alcohol has been converted to a sulfonic ester, the removal of the ester to reduce it to the respective hydrocarbon . It would be good to list the different methods to tosylate and also most important how to reduce them .
So please post your favorite methods to both convert to sulfonic esters and methods of removal...try to give references....java
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Preparation and Reaction of Tosylates
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Reaction type: Nucleophilic Substitution (usually SN2)
Summary
Alcohols can be converted into tosylates using tosyl chloride and a base to "mop-up" the HCl by-product.
Tosylates are good substrates for substitution reactions, reacting with nucleophiles in much the same way as alkyl halides.
The advantage of this method is that the substitutions reactions are not under the strongly acidic conditions.
Used mostly for 1o and 2o ROH (hence SN2 reactions).
The -OH reacts first as a nucleophile, attacking the electrophilic center of tosylate, displacing a chloride ion, Cl-.
Tosylates have a much better leaving group than the original alcohol : the conjugate base of tosic acid, pKa = -2.8 compared to hydroxide, the conjugate base of water, pKa = 15.7.
Alternatives to tosylates are mesylates (using CH3SO2Cl) and triflates (using CF3SO2Cl)
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This is the reagent used to prepare the tosylate ester. It maybe referred to by any of the terms shown.
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The tosylate ester is shown. Note that the oxygen atom from the original alcohol is retained.
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In the reactions of tosylates, the displaced group is the resonance stabilised anion shown, which is a good leaving group.
Note that the preparation of the tosylate is similar to the reaction of an alcohol with thionyl chloride, SOCl2.
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-10-1.html
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Last year i became ome work on PEG-tosylates (polyethylene glycol). For this derivatizations i used 2 methods:
First type reaction of PEG with TosCl using Pyridine as base and CH2Cl2 as solvent. Heating 50C. Theres some PEG ether cleavege occur this reaction i dont know why ???. When using NEt3 reaction proceed smoothly and no such cleavage occurs.
Second procedure i used is reaction PEG-O:Na(alcoxyde) with TosCl then -NaClsolid. For R-O:Na formation i used Naphtalene-Na+ which have green colour and when all the OH groups react - you see this is self-indicating reaction ;)
This is my experience by now...
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The most common way to convert tosylates to alkanes is to reduce directly with LiAlH4. You could, however, also do a two-step procedure where you eliminate with a base like potassium t-butoxide and then hydrogenate with H2 and Pd/C.
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movies.. do you have any references for the two step method you described.....and is there another base that can ber used that isn't so volatile
Stability of potassium_t-butoxide
Stable, but reacts violently with water and acids, possibly leading to fire. Incompatible with water, acids, halogenated hydrocarbons, alcohols, strong oxidizing agents, ketones, carbon dioxide.
that can be used in it's place , as for the hydrogenation I have no problem with...java
Ref:... http://ptcl.chem.ox.ac.uk/MSDS/PO/potassium_t-butoxide.html
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An important point to remember is that tosylates will cause and inversion of your compound, so if youare looking for a particular isomer , make sure you start right to end right...java
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You typically run a potassium t-butoxide reaction with t-butanol as solvent. It's really not that bad, just don't toss the KOtBu into water or acid or anything.
You might be able to use an amine base like pyridine, but I'm not sure.