Page 475 - Advanced Organic Chemistry Part B - Reactions & Synthesis

448 of alkene involves a second reduction step, which can occur at elevated temperature
in the presence of excess reactant.
CHAPTER 5
II
II
O Cl ClTi O OTi Cl 0 ClTi O OTi Cl R R
III
III
Reduction of Ti Zn
Carbon-Carbon Multiple R C O + TiCl 2 R C Cl R R R R
2
2
Bonds, Carbonyl R R R R R R
Groups, and Other R 2 C O
Functional Groups
HO OH
R R
R R
According to a DFT computational study, this mechanism is plausible. 256
Samarium diiodide is another powerful one-electron reducing agent that can effect
carbon-carbon bond formation under appropriate conditions. 257 Aromatic aldehydes
and aliphatic aldehydes and ketones undergo pinacol-type coupling with SmI or
2
SmBr .
2
R′ R′ SmI
RCR′ SmI 2 RC CR ArCH O 2 ArCH CHAr
O OH OH
OH OH
-Ketoaldehydes and 1,5-diketones are reduced to cis-cyclopentanediols. 258 1,6-Diketo
compounds can be cyclized to cyclohexanediols, again with a preference for cis-
diols. 259 These reactions are believed to occur through successive one-electron transfer,
radical cyclization, and a second electron transfer with Sm 2+ serving as a tether and
Lewis acid, as well as being the reductant.
Sm 2+ Sm 3+ Sm 3+ Sm 3+
O O O – O O – O . O – O –
e –
.
R R R R
Many of the compounds used have additional functional groups, including ester, amide,
ether, and acetal. These groups may be involved in coordination to samarium and
thereby influence the stereoselectivity of the reaction.
The ketyl intermediates in SmI reductions can be trapped by carbon-carbon
2
double bonds, leading, for example, to cyclization of , -enones to cyclopentanols.
O
CCH 3
SmI 2 HO CH 3
C H
CH 2 CH(CH ) CCO C H CH 3 CO 2 2 5
2 2 5
2 2
R R
Ref. 260
256 M. Stahl, U. Pidun, and G. Frenking, Angew. Chem. Int. Ed. Engl., 36, 2234 (1997).
257
G.A. Molander, Org. React., 46, 211 (1994); J. L. Namy, J. Souppe, and H. B. Kagan, Tetrahedron
Lett., 24, 765 (1983); A. Lebrun, J.-L. Namy, and H. B. Kagan, Tetrahedron Lett., 34, 2311 (1993); H.
Akane, T. Hatano, H. Kusui, Y. Nishiyama, and Y. Ishii, J. Org. Chem., 59, 7902 (1994).
258 G. A. Molander and C. Kemp, J. Am. Chem. Soc., 111, 8236 (1989); J. Uenishi, S. Masuda, and
S. Wakabashi, Tetrahedron Lett., 32, 5097 (1991).
259 J. L. Chiara, W. Cabri, and S. Hanessian, Tetrahedron Lett., 32, 1125 (1991); J. P. Guidot, T. Le Gall,
and C. Mioskowski, Tetrahedron Lett., 35, 6671 (1994).
260
G. Molander and C. Kenny, J. Am. Chem. Soc., 111, 8236 (1989).