Page 464 - Advanced Organic Chemistry Part B - Reactions & Synthesis

The isolated double bonds in the dihydro product are much less easily reduced than the 437
conjugated ring, so the reduction stops at the dihydro stage. Alkyl and alkoxy aromatics,
phenols, and benzoate anions are the most useful reactants for Birch reduction. In SECTION 5.6
aromatic ketones and nitro compounds, the substituents are reduced in preference to the Dissolving-Metal
Reductions
aromatic ring. Substituents also govern the position of protonation. Alkyl and alkoxy
aromatics normally give the 2,5-dihydro derivative. Benzoate anions give 1,4-dihydro
derivatives.

OCH 3 OCH 3 CO 2 – CO 2 –
Li, NH 3 Li, NH 3
C H OH
C H OH 2 5
2 5
The structure of the products is determined by the site of protonation of the
radical anion intermediate formed after the first electron transfer step. In general,
ERG substituents favor protonation at the ortho position, whereas EWGs favor proto-
nation at the para position. 215 Addition of a second electron gives a pentadienyl
anion, which is protonated at the center carbon. As a result, 2,5-dihydro products
are formed with alkyl or alkoxy substituents and 1,4-products are formed from EWG
substituents. The preference for protonation of the central carbon of the pentadienyl
anion is believed to be the result of the greater 1,2 and 4,5 bond order and a higher
concentration of negative charge at C(3). 216 The reduction of methoxybenzenes is
of importance in the synthesis of cyclohexenones via hydrolysis of the intermediate
enol ethers.
OCH 3 Li, NH 3 OCH 3 H + O
ROH
H O
2
The anionic intermediates formed in Birch reductions can be used in tandem alkylation
reactions.

1) Li, NH 3
CO H
2
2) CO 2 H
Br 71%
Ref. 217

O CH OCH 3 O CH OCH
2
H C 2 3
1) K, NH , 5 2
C 3 C
N t-BuOH, 1 equiv N
2) LiBr, C H I
Si(CH ) 2 5 Si(CH ) 97%
3 3
3 3
Ref. 218
215
A. J. Birch, A. L. Hinde, and L. Radom, J. Am. Chem. Soc., 102, 2370 (1980); H. E. Zimmerman and
P. A. Wang, J. Am. Chem. Soc., 112, 1280 (1990).
216
P. W. Rabideau and D. L. Huser, J. Org. Chem., 48, 4266 (1983); H. E. Zimmerman and P. A. Wang,
J. Am. Chem. Soc., 115, 2205 (1993).
217 P. A. Baguley and J. C. Walton, J. Chem. Soc., Perkin Trans. 1, 2073 (1998).
218
A. G. Schultz and L. Pettus, J. Org. Chem., 62, 6855 (1997).