# How does chelation control result in rate acceleration?

In the chapter on diastereoselectivity in Clayden, Warren & Greeves (2012), the Cram-Chelate model is discussed. Most significantly, under chelation control, the diastereoselectivity of the addition to the carbonyl group is reversed. Additionally, chelation control achieves high levels of diastereoslectivity, often greater than $$\ce {95:5}$$. Lastly, it also mentions that chelation control leads to rate acceleration. The above are discussed on p. 864.

It is not surprising that Felkin-Ahn control leads to rate acceleration, especially when both the steric and orbital factors are both satisfied. More details of the model are explained in this post. However, chelation control does not seem to provide any cause for rate acceleration. In fact, as chelation control goes against the steric and orbital considerations, we may even expect it to slow down the addition reaction, rather than speeding it up.

Reference

Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.

• I could go into my usual refrain of how organometallic reactions are complicated, but more helpfully, when you add a chelating metal ion which binds to a carbonyl, is that not effectively the same as a Lewis acid? – orthocresol Mar 19 '19 at 13:34
• @orthocresol I understand that organometallic reactions are often not straightforward... However, in this case, the results of chelation control are very obvious and has been shown to give consistent rate accelerations with high diastereoselectivity, as mentioned in Clayden's. However, I understand that the effect at play for the rate acceleration may not be straightforward. If anyone has any idea, please feel free to make a suggestion. – Tan Yong Boon Mar 19 '19 at 14:32
• I thought I made a suggestion. – orthocresol Mar 19 '19 at 16:10
• @orthocresol Oh yes... I didn't read your comment carefully. Yes, it does make sense. – Tan Yong Boon Mar 19 '19 at 16:31