solo - 3-1-2006 at 09:37
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........solo
solo - 3-1-2006 at 10:40
Preparation and Reaction of Tosylates
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)
This is the reagent used to prepare the tosylate ester. It maybe referred to by any of the terms shown.
The tosylate ester is shown. Note that the oxygen atom from the original alcohol is retained.
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...
[Edited on 3-1-2006 by solo]
solo - 4-1-2006 at 06:49
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..........solo
[Edited on 4-1-2006 by solo]
Drunkguy - 7-1-2006 at 03:38
This is an interesting reaction for me also. Whereas piperidine can be used the alkylate phenethylbromide for some reason the same authors used the
mesylate when performing the corresponding rxn on the phenyl-isopropyl compound. I'm not sure if this was to underscore the sterochemical consequences
of doing the reaction on a chiral sterocentre or not. But on a secondary RX the reaction mechanism will be a SN2/SN1 mix. I think solvent media will
have some impedus on this also.
Now my question is this. Is there any reason why PhCH2CHBrCH3 would be a bad choice for doing the reaction? Apparently this can be made from 48% HBr
and the corresponding alcohol. Suppose the reaction does occur through an SN1 mechanism then will carbocation rearrangement be likely to affect the
product of this reaction?
Eg Ph.CH2CH(+).Me <-> Ph.CH(+).CH2CH3 [1,2-Hydride Shift]
Remember that resonance stabilization of the benzylic position is possible even if it means rearrangement from a secondary carbocation to a primary
carbocation intermediate.