Hello Rashid, could you elaborate on the problem you are having? Why do you need to protect the boronic acid while attaching it to polyesters with multiple hydroxylase. What is the goal of your reaction? If you want the boronic acid to condense then you should not protect it. Are you attempting to reduce the reactivity of the boronic acid so that it will selectively condense with a particular pair of hydroxyl groups?

Thank you Mr. Lovinger for your reply.

I want to graft PBA (Phenyl boronic acid) with some polymers having pendant hydroxyl groups. From Previous literature and my own observation, boronic acid forms a complex with polyols and forms a gel. I want to fabricate a gel system through chemical crosslinking at the end by adding a crosslinker, but i get a gel due to this complex with polymer prior to chemical crosslinking. I want to use these boronics in further application, e.g. glucose sensing, as i said, it forms complex with diols. Could you suggest or share his experience that how can this complex formation b/w boronic and hydroxyls be overcomed?

Kind Regards

I think that you should be able to keep the boronic acid tied up as a cyclic boronate, with ethylene glycol or tetramethylethylene glycol, and have it stably protected through the polymerization and cross-linking steps.

There are a variety of protection strategies you can use. The first is to protect your boronic acids with chelating diols such as (in descending order of stability): pinacol, neopentyl glycol, 1,3-propane diol, or ethylene glycol. I would not recommend using labile diols such as catechol, ethylene glycol, or 1,3-propane diol as they are likely to be displaced by the hydroxyl groups of your substrate and it sounds like you don't want that. It is very easy to protect boronic acids with diols, simply suspend your boronic acid and diol in pentane and after several hours at room temperature you should have a quantitative yield of your desired boronic esters (the water byproduct will partition out of the pentane similarly to the strategy used in a Dean Stark apparatus). You can find a procedure for this in the supporting information of this paper: http://science.sciencemag.org/content/351/6268/70 I have a copy on my profile as well. If you want to make sure your boronic acid is protected you can also convert it to the corresponding MIDA ester. Just look up a procedure for the preparation of boronic acid MIDA esters (reaxys or scifinder are good search tools). The chelating diols are easy to install and, depending on which one you use, are not too hard to remove later if you want to; MIDA is probably the best Boronic acid protecting group but it is harder to get back off later. Please let me know if I should explain more or if I can offer any further advice, I'm more than happy to help if I can.

Thank you for your reply Gabriel. I want to attach protected boronics with polyesters, after that i want a crosslinked network. But the important thing here is, I want to deprotect these protected boronics after or before formation of crosslinked network. I want the hydroxyls of boronics free at the end. What can be the better option in your suggestion?

Kind Regards

Hello Rashid, here is a really good paper on the stability of boronic esters with various diol ligands and the rates of transesterification from one diol ligand to another.

https://www.google.com/amp/s/www.researchgate.net/publication/244056927_Stability_of_boronic_esters_-_Structural_effects_on_the_relative_rates_of_transesterification_of_2-phenyl-132-dioxaborolane/amp

The paper is on research gate and sciencdirect.

In general mixing a boronic esters and a diol in pentane for several hours, removing the water layer that forms, reducing the solvent volume and passing the concentrated reaction solution through a plug of silica gel with DCM as the eluent is a very effective way of esterefying and purifying a boronic ester. This is the method we often use in our lab. You can find the actual procedure in the SI of the Science paper on my researchgate profile. Here is the link to the paper http://science.sciencemag.org/content/351/6268/70

If you don't have access to Science Journal just let me know and I can upload the SI to my profile. Another common method that is used involves mixing boronic acids and diols in dichloromethane for several hours with a desiccants such as 4 angstrom molecular sieves (the method used in the first paper I mentioned).