Discussion
In the following reaction sequence
the major products P and Q, respectively, are:
- A.
- B.
- C.
- D.
Answer: B
The answer is (B): metalation, and then the pyridyl group, ortho to the OMOM group.
The methoxymethyl ether is a strong directing group for ortho-lithiation: its oxygens coordinate sec-BuLi and deliver it to the adjacent ring C-H. Of the two positions ortho to the OMOM group in 4-(methoxymethoxy)indane, one is the ring-fusion carbon, which has no hydrogen, so lithiation occurs at C5. ZnCl2 exchanges Li for ZnCl, giving the arylzinc P.
A Negishi coupling with 3-bromopyridine under Pd(PPh3)4 then joins the pyridin-3-yl group to C5 with retention of position, giving Q.
(A) and (C) metalate positions meta and para to the directing group, which sec-BuLi does not reach, and (D) deprotonates a benzylic CH2, which the directing group does not favour.
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