FELKIN-ANH MODEL PREDICTIONS 14) MPV reduction of BENZOIN Consider the sizes of the substituents on the a-carbon of benzoin to be as follows: H = small; OH = medium; Ph = large. In the space below, and in a manner like that shown in the lab lecture notes, analyze the hydride reduction of benzoin using the Felkin-Anh model and clearly indicate the major diol diastereomer predicted to form.
FELKIN-ANH MODEL PREDICTIONS 14) MPV reduction of BENZOIN Consider the sizes of the substituents on the a-carbon of benzoin to be as follows: H = small; OH = medium; Ph = large. In the space below, and in a manner like that shown in the lab lecture notes, analyze the hydride reduction of benzoin using the Felkin-Anh model and clearly indicate the major diol diastereomer predicted to form.
Chemistry
10th Edition
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Chapter1: Chemical Foundations
Section: Chapter Questions
Problem 1RQ: Define and explain the differences between the following terms. a. law and theory b. theory and...
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Transcribed Image Text:Felkin-Anh model - used to predict/explain the asymmetry induced upon nucleophilic attack at a C=O with an a-asymmetric center
bearing substituents of differing size.
M
S
M
L = large
M= medium
S = small
M
S
OH
1) Nu:
2) H+
=0
Nu
R
1) Nu:-
2) H+
Nu
OH
R
Felkin-Anh model - makes three assumptions:
1) the most reactive conformers are 1 & 2, where the large group (L) is perpendicular to the C=O plane
2) nuc. attack occurs at the C=O face opposite the large group (L)
3) most reactive of the two conformers has the SMALLEST group in the trajectory of the Nu:
,a-chiral center makes C=O faces diastereotopic; i.e., nucleophilic attack leads to diastereomers A & B (in unequal amounts
since the chiral center induces asymmetry in the newly forming chiral center)
II
Nu:
M
$
R
rotate
180°
S
1) Nu:
2) H+
Nu
1)
2)
OH
M
Ø
S
R
Nu:
H+
Nu
M
Bürgi-Dunitz trajectory: nucleophiles approach C=O at an oblique angle (~100° w/respect to C=O axis)
OH
B
-Nu
OH
*=*
2
...nucleophile encounters more steric repulsion in conformer 2; thus, B is the major product
Q: Why not simply ask which is the most stable product?
A: Because product stability is only important in reversible (equil.) rxns - the hydride reduction is essentially irreversible (non-equil. rxn).
Thus, product ratio is established by fastest rxn pathway (smallest AG‡ for the SLOW step — kinetic control)
& NOT by the most stable product (smallest AG- thermodynamic control).
Nu
R
Nu
4
OH
5

Transcribed Image Text:FELKIN-ANH MODEL PREDICTIONS
14) MPV reduction of BENZOIN
Consider the sizes of the substituents on the a-carbon of benzoin to be as follows: H = small; OH = medium; Ph = large.
In the space below, and in a manner like that shown in the lab lecture notes, analyze the hydride reduction of benzoin using the Felkin-Anh
model and clearly indicate the major diol diastereomer predicted to form.
15) MPV reduction of BENZOIN PIVALATE
Consider the sizes of the substituents on the a-carbon of benzoin pivalate to be as follows: H = small; Ph = medium; -O₂C-C(CH3)3 = large.
In the space below, and in a manner like that shown in the lab lecture notes, analyze the hydride reduction of benzoin pivalate using the
Felkin-Anh model and clearly indicate the major diastereomer of product predicted to form.
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