This is a question from Org. Chem. by Solomons, Fryhle & Snyder, adapted for JEE by M.S. Chouhan :

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  1. In the first step, I expected that the reaction would stop at alkene formation via syn addition of $\ce{H_2}$ since Lindlar catalyst is poisobed and heterogeneous. What are some other other poisoned catalysts that stop at alkene?

  2. In the second step, I expected syn addition of $\ce{D_2}$ since $\ce{Pd-C}$ is a heterogeneous catalyst, forming product (a). But the answer given is (c) — does this mean that some heterogeneous catalyts can give anti addition, or is it just an error on the adapting author's part?


1 Answer 1

  1. The "Lindlar catalyst" (Pd/CaCO3) is probably the most common way of effecting stereoselective reduction of alkynes to (Z)-alkenes. It is fairly accessible and the reaction is easy to run. But it is not the only way: there are other catalyst systems. One of the more notable ones is probably the Brown catalyst "P2-Ni" (Wikipedia).

    There is a nice review on alkyne semihydrogenation here: Chem. Rev. 2013, 113 (3), 1313–1350, which covers both heterogeneous and homogeneous methods, as well as methods involving other hydrogen sources apart from H2 gas itself. I recommend it to interested readers and especially anybody considering carrying out such a reduction.

  1. The Lindlar hydrogenation of an alkyne forms (Z)-alkenes stereoselectively. Syn hydrogenation of the (Z)-alkene gives product (c), not product (a).

    Reaction scheme

    Note that in the fully reduced alkane, there is free rotation about the central C–C bond. So, having both D atoms pointing "out of the plane" is irrelevant, as you can easily rotate to a conformation in which one D atom is pointing in the plane. However, the configuration of the product cannot be changed, and that is the difference between options (a) and (c).


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