Question

Chiral catalysts can be designed to favor the formation of one enantiomer over another in reactions where a new chiral center is formed. The example shown below was recently reported in the chemical literature (J. Am. Chem. Soc. 2012. 134, 8054 8057) where a new type of chiral copper (Cu) catalyst was developed. When the Cul catalyst was used, the (S)-enantiomer of the product predominated, and the (R)-enantiomer of the product was selectively formed when Cu" catalyst was used instead. 0 EtoY OEt + O + NO, Eto V Catalyst (Cu' or Cu" form) Base, Solvent 0 oEt NO2

Answer 1

a.   When Cu+ is used S-enantiomer is major product

Cu catalyst TOEt Eto y Base, solvent NO2

When Cu+2 is used R-enantiomer is major product

OET NO2 Cu-2 catalyst POET Eto1 Hi OET Base, solvent NO2

b.   
S-R percent ee of (S) - product = S+R x 100

When Toluene is used as a solvent, given that %ee for S form is 24

24 = S-R S+R x 100; and S + R= 100

::S-R= 24; S+R = 100 → S = 62, R = 38

Solvent	% ee of (S)-product	% yield of total product	%S	%R
Toluene	24	55	62	38
THF	48	33	74	26
Acetonitrile	72	55	86	14
Chloroform	30	40	65	35
Dichloromethane	46	44	73	27
Hexane	51	30	75.5	24.5

%yield is high in Toluene and acetonitrile. But selectivity in high in Acetonitrile. So, Acetonitrile is better solvent for enantioselectivity.

In hexane yeild is low.