kilomentor

Wolf & Lamb reactions are reactions or reaction sequences wherein at least two mutually reactive agents are kept in the same reactor isolated from each other by being attached to separate solid phases, which cannot interpenetrate each other. For example one polymer may have an oxidant attached to it while another solid in the same reactor has a reducing agent attached to it but they cannot react with each other because each is held on a separate resin or porous solid. Alternately a strong base containing for example triphenyl methylide anions may be on one resin and the second resin may have acidic groups bound to it.

What is the characteristic of a transformation or set of consecutive reactions that can be performed more efficiently in a medium providing this site isolation possibility?

We can imagine a mental cartoon in which a Substrate (S) moves to an immobilized reaction site and a reaction happens there because

i) there is a reagent tethered there

ii) the environment there is different (pH, solvent composition, ionic strength)

iii) there is a catalyst immobilized there

iv) there is a trapping agent for a functional group there (this last possibility applies more to the product of a reaction on another immobilizing solid).

Intuitively some transformations seem more addaptable to site isolation reaction:

• unimolecular isomerizations with no change in molecular formula
• oxidations (loss of electrons, loss of hydrogens, addition of oxygen)
• reductions (addition of electrons, addition of hydrogen, loss of oxygen)
• base abstractions using a polymer supported base
• dehydrations
• dehydrohalogenations
• sulfonation using pyridine sulfur trioxide
• halogenation
• transfer metal carbonylation reactions

Why would this situation be advantageous?

• the reagent could attack another reagent present in situ if both were not immobilized
• there are two competing reactive functionalities in the same substrate and the less reactive functionality will compete with the more reactive unless the desired product is trapped out on a separate resin.
• the product of a reaction can react with the starting material if it is not trapped out on a separate resin to keep it away from the residual starting material.

An excellent paper to give you a better idea about some of the possibilities is Wolf and Lamb Reactions: Equilibrium and Kinetic Effectts in Multipolymer Systems, B.J. Cohen, M.A. Kraus and A. Patchornik, J. Am. Chem.. Soc. 103(25), 7620, (1981).

Insoluble reagents which are not polymers can also be classified as site isolation reagent:. an example would be activated manganese dioxide.

One can imagine the use of manganese dioxide with a strong base bound to a resin also combined with an epoxidation peracid bound to a second resin combined with semicarbazide adsorbed on silica gel. This combination might be expected to convert an olefin to an epoxide using the peracid; the epoxide could be isomerised to an alllylic alcohol by the tethered strong hindered base; the allylic alcohol could be oxidized to an apha- beta unsaturated ketone by manganese dioxide and the ketone could be trapped and immobilized on the silica by the semicarbazide carbonyl derrivatizer.

I’m not saying this would work! It illustrates the concept.