Page 385 - Advanced Organic Chemistry Part B - Reactions & Synthesis

in synthesis is bis-(cyclopentadienido)hydridozirconium(IV) chloride. Reduction of 357
Cp ZrCl generates a reactive species that can add to alkenes and alkynes. 231 Various
2
2
reductants such as LiAlH 4 232 and LiEt BH 233 can be used. Alkynes readily undergo SECTION 4.6
3
hydrozirconation. With internal alkynes, this reagent initially gives a regioisomeric Hydroalumination,
Carboalumination,
mixture but isomerization occurs to give the less sterically hindered isomer. Hydrozirconation,
and Related Reactions
(Cp) ZrHCl CH 3 CH CH(CH ) CH 3 CH CH(CH )
3 2
2
2
3 2
2
CH C CCH CH(CH )
3
3 2
2
Zr H Zr H
+ CH 3 CH CH(CH )
2
3 2
55:45 95:5
H Zr
The adducts react with electrophiles such as NCS, NBS, and I to give vinyl halides.
2
PMBO PMBO Br
PMBO CH 3 1) (Cp) HZrCl Br
2
C H + C H
5 11
C H 2) NBS 5 11
5 11
CH 3 CH 3
94:6; 78% yield
Ref. 234
(CH ) CO CNH
2
3 3
1) (Cp) HZrCl (CH ) CO CNH
2
2
3 3
(CH ) CHCC CH
3 2
2) I 2 (CH ) CH I
3 2
H
51%
Ref. 235
Alkenes are less reactive and reactivity decreases with increasing substitution.
The adducts from internal alkenes undergo isomerization to terminal derivatives. 236
C H (Cp) ZrHCl t-BuOOH
3 7
C H 2 CH 3 (CH ) Zr(Cp) Cl CH 3 (CH ) OH
2
2 7
3 7
2 7
24 h
Carbon-carbon bond formation from alkyl and alkenyl zirconium reagents usually
involves transmetallation reactions and are discussed in Chapter 8.
231
D. W. Hart, T. F. Blackburn, and J. Schwartz, J. Am. Chem. Soc., 97, 679 (1975); J. Schwartz and
J. A. Labinger, Angew. Chem. Int. Ed. Engl., 15, 333 (1976).
232 S. L. Buchwald, S. J. La Maire, R. B. Nielsen, B. T. Watson, and S. M. King, Tetrahedron Lett., 28,
3895 (1987).
233
B. H. Lipshutz, R. Kell, and E. L. Ellsworth, Tetrahedron Lett., 31, 7257 (1990).
234 A. B. Smith, III, S. S.-Y. Chen, F. C. Nelson, J. M. Reichert, and B. A. Salvatore, J. Am. Chem. Soc.,
119, 10935 (1997).
235 J. R. Hauske, P. Dorff, S. Julin, J. Di Brino, R. Spencer, and R. Williams, J. Med. Chem., 35, 4284
(1992).
236
D. W. Hart and J. Schwartz, J. Am. Chem. Soc., 96, 8115 (1974); T. Gibson, Tetrahedron Lett., 23, 157
(1982).