Reduction of acids with borane

What is the mechanism of the reduction of organic acids with borane? Borane reductions don't work well with electron deficient carbonyl groups. Why is that so, and how isn't the carboxyl group an electron deficient group? 





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> What is the mechanism of the reduction of organic acids with borane?
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![enter image description here](https://i.stack.imgur.com/75NWz.png)


Source:  

Henry Rzepa. Chemistry with a twist. Mechanism of the reduction of a carboxylic acid by borane: revisited and revised. October 16th, 2011. <http://www.ch.imperial.ac.uk/rzepa/blog/?p=5114>  

DOI: [10.15200/winn.142795.57029](https://dx.doi.org/10.15200/winn.142795.57029) provided by The Winnower, a DIY scholarly publishing platform.


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