Page 943 - Advanced Organic Chemistry Part B - Reactions & Synthesis

CH I 919
2 2
(C H ) Zn + CF CO H CF CO ZnC H CF CO ZnCH I
2
2 5 2
2
3
3
2
2 5
2
3
SECTION 10.2
Iodomethylzinc phenoxides can be prepared in a similar fashion. The best phenols are Reactions Involving
Carbenes and Related
the 2,4,6-trihalophenols and the readily available 2,4,6-trichlorophenol was examined Intermediates
most thoroughly. 180
CH I
2 2
(C H ) Zn + ArOH ArOZnC H ArOZnCH I
2 5 2
2 5
2
This reagent can achieve better than 90% yields for a variety of unactivated alkenes.
CH
CH 3 3
CH I
2 2
+ ArOZnC H
Ph 2 5 Ph
The reactivity of the oxy anions is in the order CF CO > ArO >> RO .
−
−
−
3 2
In molecules containing hydroxy groups, the CH unit is selectively introduced on
2
the side of the double bond syn to the hydroxy group in the Simmons-Smith reaction
and related cyclopropanations. This indicates that the reagent is complexed to the
hydroxy group and that the complexation facilitates the addition. Entries 3 and 4 in
Scheme 10.9 illustrate the stereodirective effect of the hydroxy group. It is evidently
the Lewis base character of the group that is important, in contrast to the hydrogen
bonding that is involved in epoxidation. The lithium salts of allylic alcohols are also
strongly activated, even more so than the alcohols. This reactivity has been used to
advantage in the preparation of relatively unstable products. 181
-
+ Li O CH 3 1) Zn-Cu HO CH 3
CH I
2 2
2) H 2 O
CH(CH ) CH(CH )
3 2
3 2
While amino groups alone are not effective directing groups, both ephedrine and
pseudoephedrine derivatives give high diastereoselectivity. This is evidently due to
chelation by the hydroxy group, as both auxiliaries give the same facial selectivity
despite differing in configuration at the nitrogen position. 182
Ph OH Ph OH
Ph OH
Zn(CH I)
Zn(CH I) 2 2
2 2
CH 3 95 % yield CH 3 N
N Ph 95% yield CH 3 N >98:2 dr Ph
>98:2 dr Ph
CH 3 CH 3
CH 3
Dioxolanyl oxygens are also effective directing groups. 183
180
A. B. Charette, S. Francouer, J. Martel, and N. Wilb, Angew. Chem. Int. Ed. Engl., 39, 4539 (2000).
181
D. Chang, T. Kreethadumrongdat, and T. Cohen, Org. Lett., 3, 2121 (2001).
182 V. K. Aggarwal, G. Y. Fang, and G. Meek, Org. Lett., 5, 4417 (2003).
183
A. G. M. Barrett, K. Kasdorf, and D. J. Williams, J. Chem. Soc., Chem. Commun., 1781 (1994).