Asymmetric Halogen-Metal Exchange (AHME) of Geminal Dihalides with Planar Chiral Organometallic Reagents Dept. of Chemistry, Oregon State University, Corvallis, Oregon.
Paul R. Blakemore
Our goal was to realize an AHME process using a recyclable planar chiral iodoferrocene reagent. Addition of putative scalemic lithium carbenoid reagents generated via the illustrated AHME cycle to aldehydes resulted in no enantioselectivity. It is suspected that the ate-complex intermediate reacted directly with the aldehyde. Further experiments are in progress to verify this hypothesis.
• •
ferrocenyl lithium incorporates iodine from diiodides (or chloroiodides), and returns to starting state without racemization
X = I prochiral diiodomethane derivative X = Cl racemic chiral chloroiodomethane derivative
I
optimal syntheses of silyl and alkyl substituted dihalides have been developed
R2
X
• O Fe
H i-Pr
X
Fe
H
Li O N i-Pr
AHME cycle
i-Pr2N
I O
Li
R3
H
R2
ate-complex reacts directly with aldehyde; SET character of addition precludes enantioselectivity ?
? R3 R2
R1–I
•
•
O
0% ee for a range of aldehydes
O
Li/I exchange from iodoferrocene is readily achieved with standard alkyllithium reagents
?
– LiX
Fe
H R1–Li
O i-Pr2N ≥ 95% ee
I
Li R2 * X
R3
H
