[ashmus]

Hello everyone. I'm a new member stuck with some questions and thought I might give this forum a try.

See attached image for reaction scheme.

When regarding dialkyl or diaryl H-phosphonates, why is that only one of the O-R groups is displaced in the presence of excess ammonium hydroxide (reaction scheme 1)? And if you rationalize some sort of electronic effect with the H-phosphonate salt, could you explain why in activated phosphates the leaving group can be displaced since there seems to be a similar electronic effect (reaction scheme 2)?

Any help or suggestions for me to think about would be appreciated.

[discodermolide]

Well is the first reaction not just ester hydrolysis and the second a nucleophillic displacement?

[orgopete]

I cannot answer this question, but it does raise another question. It appears as though the reactions are nucleophilic attack on the phosphorous. If so, why doesn't ammonia attack to give a phosphoramidate in the first example?

[ashmus]

@orgopete
Thanks for bringing up the question on ammonia not attacking, it is an interesting point to think about.

[ashmus]

I came up with some plausible explanations for my questions that some friends could accept. For those interested, I think it is an electronic effect in the first reaction scheme preventing the second displacement and regarding the second reaction scheme, I came across a thesis and some papers that propose a tricoordinated pentavalent phosphorus as a transition state before being attacked by a nucleophile. Thanks to those that took a look and replied.

@orgopete - after some thought, I actually think the ammonia attacks the phosphorus center first and is then replaced with water followed by 2 deprotonation steps.

[Babcock_Hall]

Tertiary amines can remove one benzyl group (but not an ethyl group) from phosphorus acids (Baddiley, Clark, Michalski, and Todd, J. Chem. Soc.(?) 1949, p. 815).  From what I can gather, a second benzyl group is not removed.  I assume that this is a nucleophilic attack on the benzylic carbon.  With respect to why only one benzyl group, I would explain it as a repulsion of the nucleophile by the negative charge on the newly deprotected oxygen atom.  There is also a paper from the Todd group that deals with attack on P versus attack on C:  VM Clark and AR Todd, J. Chem. Soc. (1950) p. 2023.