Organic Reactions Volume 4 - Sciencemadness Dot Org

276 ORGANIC REACTIONS
Cinnamaldehyde does not undergo the benzoin condensation. 27 2-
Furaldehyde, 48 2-picolinaldehyde, 49 and quinaldehyde, 60 3,4-methylenedioxybenzaldehyde,
61 a-naphthaldehyde, 62 and 4-phenylbenzaldehyde, 52
condense in 38, (?), 66, 80, 13, and 95% yields respectively.
Unsymmetrical Benzoins
The benzoin condensation is more generally applicable to the preparation
of unsymmetrical benzoins than to the preparation of symmetrical
benzoins. No prediction can be made with respect to the aldehydes that
will react with other aldehydes. Benzaldehyde and its mono- and dialkoxy
and methylenedioxy derivatives give unsymmetrical benzoins.
Among the substituted benzaldehydes that do not undergo symmetrical
benzoin condensation but react readily with other aldehydes are 4dimethylamino-,
16 3~meth.oxy~4-eth.oxy-, 40 and 3~ethoxy-4-methoxy-benzaldehyde.
40 Other substituted aldehydes that give unsatisfactory yields
of symmetrical benzoins give good yields of unsymmetrical benzoins;
2-chlorobenzaldehyde, which is reported to form a symmetrical benzoin
in 20-40% yield, condenses with 4-dimethylamino-, 53 4-methoxy-, 18
3-methoxy~4~ethoxy~, 40 3,4-diethoxy-, 40 3,4-dimethoxy-, 18 and 3-ethoxy-
4-methoxy»benzaldehyde 40 to form unsymmetrical benzoins in yields of
36, 60, 60, 63, 70, and 81%, respectively. 2~Furaldehyde (furfural) 48 - 53
and 3,4-methylenedioxybenzaldehyde 18 ' 19 ' 82 ' 40 ' 83 ' 54 also condense with a
variety of substituted aldehydes.
Two different aldehydes might be expected to yield a mixture of two
symmetrical benzoins and two unsymmetrical benzoins, but only a single
unsymmetrical benzoin usually is isolable. The second unsymmetrical
benzoin may be formed when the reactivity of the two aldehydes is
similar. 19 ' 40 ' 63 In the products examined, the carbinol group usually is
adjacent to the unsubstituted phenyl group or to the ortho, metch, or
para-halogen substituted phenyl group. The carbonyl group generally
is next to the methylenedioxy-, 4~dimethylamino~, methoxy-, dimethoxy-,
methoxyethoxy-, or diethoxy-substituted ring or to the furan nucleus.
However, in the following products (I-IV) the carbinol group is adja-
48
Fischer, Ber., 13, 1334 (1880); Hartman and Dickey, J. Am. Chem. Soc, 55, 1228
(1933); Fischer, Ann,, 211, 214 (1882).
49
Harries and Lenart, Ann., 410, 95 (1915).
50
Kaplan, J. Am. Chem. Soc, 63, 2654 (1941).
B1
Toirey and Sumner, /. Am. Chem. Soc, 32, 1492 (1910).
62
Gomberg and Bachmann, /. Am. Chem. Soc , 49, 236, 2584 (1927); Gomberg and
Van Natta, ibid., 51, 2238 (1929).
53
Buck and Ide, /. Am. Chem. Soc, 52, 220 (1930).
54
Buck and Ide, J. Am. Chem. Soc, 53, 2350 (1931).