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Consider the compound $\ce{Alkyl-(4 oxocyclopent-2-ene)carboxylate (A)}$. For example consider- enter image description here Now consider the operations:

$\ce{1. (A) + (i)BH3/THF (ii) H+}$

$\ce{2. (A) + H2/Pt}$

$\ce{3. (A) + H2/Pd-C}$

Products

In $1.$ ester group reduced to an alcohol.

In $2.$ double bond reduced to a single bond, and

In $3.$ double bond reduced to a single bond, ester group reduced to an alcohol.

However, on the next page addition of $\ce{(i)BH3/THF (ii) H+}$ selectively reduced carboxylic acid when both ester and carboxylic acid were present.

My questions:

$(i)$ I know that $\ce{(i)BH3/THF (ii) H+}$ can reduce carboxylic acid, but I am unaware if it can reduce ester.

$(ii)$ I only knew that, the reduction of ester in presence of carbonyl group could be done by first protecting the carbonyl group (converting it to cyclic ketal by reacting with ethylene glycol) and then using $\ce{LiAlH4}$. Is reaction $3.$ correct?

$(iii)$ Why aren't product $2.$ and $3.$ same? I've always seen the two work same.

Note:

My chemistry teacher gave me few notes for Chemistry Olympiad preparation, so I can not cite source. I don't know how to make compounds, so will completely reside on IUPAC nomenclature.

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    $\begingroup$ Man reduction is highly specific and its trends are difficult. However, I am sure there should be a reason to this. All I can add is that Pd-C is stronger than Pt as an adsorbent. Nice question! $\endgroup$
    – Haha Hahaha
    Jan 11, 2020 at 16:34
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    $\begingroup$ I'm not sure that differences in Pt vs Pd hydrogenation can be easily explained. I'm also not entirely convinced that H2 with Pd/C reduces esters. Maybe in a few isolated cases, or maybe with higher pressures, but I doubt it's anywhere near as facile as hydrogenation of C=C bonds. As far as I know, this doesn't occur sufficiently regularly to use it in a predictive sense. Also, your IUPAC name is slightly ambiguous: is this a CO2R group sticking off carbon-1 of your cyclopentene? $\endgroup$
    – orthocresol
    Jan 11, 2020 at 16:53
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    $\begingroup$ Yes, it's CO2R group. What's the correct IUPAC name? $\endgroup$
    – Zenix
    Jan 11, 2020 at 17:01
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    $\begingroup$ @Zenix: from which book is this question? I feel that these questions are interesting! $\endgroup$
    – Rahul Verma
    Jan 17, 2020 at 12:54
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    $\begingroup$ For (i), it is correct that borane/THF will not reduce an ester. One explanation is here: ch.imperial.ac.uk/rzepa/blog $\endgroup$
    – Andrew
    Mar 30, 2020 at 22:25

1 Answer 1

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1.The selective reduction of the carboxylic group is explained in this article. Link:https://www.ch.imperial.ac.uk/rzepa/blog/?p=5114. It could reduce esters but is not favourable at all and difficult.So if u only have an ester grp and its treated with bh3, then you can reduce it. However if both carboxylic and ester grps are present, carboxylic acid gets reduced .

  1. Regarding reduction with h2/pt and h2/pd-c not much can be said. The thing is at higher temperatures and pressures(extreme conditions)both could reduce esters. Generally esters are NOT reduced via cataytic hydrogenation (it is just not favourable).However reduction has been performed in mild conditions via other complexes like ruthenium.(https://pubs.acs.org/doi/pdfplus/10.1021/op4003278
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