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GATE 2021 CY – Question 43

Organic Chemistry · Organic Synthesis: Synthesis, reactions, mechanisms and selectivity involving the following classes of compounds – alkenes, alkynes, arenes, alcohols, phenols, aldehydes, ketones, carboxylic acids, esters, nitriles, halides, nitro compounds, amines and amides. Uses of Mg, Li, Cu, B, Zn, P, S, Sn and Si based reagents in organic synthesis. Carbon-carbon bond formation through coupling reactions – Heck, Suzuki, Stille, Sonogashira, Negishi, Kumada, Hiyama, Tsuji-Trost reactions; olefin metathesis, McMurry coupling and Buchwald-Hartwig amination reactions. Baylis-Hillman, Henry, Ritter, Sakurai, Tebbe olefination, Pauson-Khand and Nazarov cyclization reactions. Concepts of multistep synthesis – retrosynthetic analysis, strategic disconnections, synthons and synthetic equivalents. Atom economy and green chemistry, Umpolung reactivity – formyl and acyl anion equivalents. Selectivity in organic synthesis – chemo-, regio- and stereoselectivity. Protection and deprotection of functional groups. Concepts of asymmetric synthesis – resolution (including enzymatic), desymmetrization and use of chiral auxiliaries, organocatalysis. Carbon-carbon and carbonheteroatom bond forming reactions through enolates (including boron enolates), enamines and silyl enol ethers. Stereoselective addition to C=O groups (Cram, Prelog and Felkin-Anh models). Asymmetric aldol reactions – Evans reaction and proline catalyzed reaction. · 2 marks · Multiple choice

In the following reaction sequence the major products P and Q, respectively, are:

Diagram for GATE 2021 CY question 43
  1. Diagram A
  2. Diagram B
  3. Diagram C
  4. Diagram D

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Correct answer: (B) Diagram B

Explanation

The methoxymethyl ether directs sec-BuLi to the free ortho aromatic position through coordination. ZnCl2 transmetalates the aryllithium to an arylzinc chloride at that same carbon. Pd(PPh3)4 then couples this carbon with the brominated carbon of 3-bromopyridine in a Negishi reaction. Neither the protecting group nor the saturated ring is displaced. The pair is **B**.