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CHEM 264

Exam #1

Spring 2010

ENolen NAME

The exam contains 6 pages with 10 problems for a total of 180 points.

Please be reminded of the Colgate Honor Code.

1. [14pt] For the two general reactions shown in the boxes, complete the energy profiles/diagrams

and for each case label both products on the diagram. Assume these are one step reactions.

O OH OH

HH

S R

E n e r g y

Rxn

O OH OH

H

H

S R

E n e r g y

Rxn

R RR

major

2. [14pt] A small portion of the 1H-NMR spectrum for the two compounds is shown below. The

labeled methylene (CH2) displays a different pattern in these two compounds. Why?

O

CH3

CH3

O

Ha Hb

OCH3

O

Hc Hd

NMR

NMR

4.5 ppm

4.5 ppm

H

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3. [24pt] The following result was observed for an asymmetric hydroboration using (Ipc*)-BH2.

BH

H

(Ipc*)-BH2

B

H

B

H

initial products

9:1

a) What is the percent enantiomeric excess (%ee) for this initial reaction product mixture?

b) The initial product mixture is usually recrystallized. After isolation of the new crystals, what

would you expect the %ee to be? (pick one choice below)

-less than before

-more than before

-the same-no way to tell

c) A typical procedure on the recrystallized product mixture is to react the borane with basic

hydrogen peroxide (NaOH, H2O2) to give the alcohol products, outlined below. The question is…

can you reverse the order and get the same results? As shown along the left side, can you first

convert to the alcohols, and then do the recrystallization to get the same results? For full credit,

draw the alcohols and describe the results.

initial product mixture recrystallize recrystallizedproduct mixture

NaOH, H2O2

NaOH, H2O2

alcohols

alcohols

?

recrystallize

9 : 1

TYPICAL PROTOCOL

2

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4. [48pt] Page O’Reactions. Provide the product(s) of the following transformations. Assume an

appropriate workup was employed in each case. There is no need to draw specific stereochemistry

—use the blank on the far right to indicate stereochemical concerns: if the reaction favors a single

enantiomer (*), a racemic mixture(±), or if chirality is of no concern (no*).

Br

1. Mg

2. O

_____

O

Cl

CC

Na

THF

_____

NH

OMgBr

1 eq

_____

CH3

RhCl(PPh3)3, H2,EtOH

_____

H3CO

CO2H

[(S)-BINAP]Ru(OAc)2,H2, CH3OH

_____

CH3 1. 9-BBN-H

2. Pd(PPh3)4, NaOH,

Br

_____

3

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5. [10pt] Provide an arrow-pushing mechanism for the following transformation.

O

Si(CH3)3

Br

OH

H

Bu4N F

6. [14pt] Part of the mechanism for a Wilkinson hydrogentation is shown below.

ORh

PPh3

Ph3P Cl

H HRh

PPh3

Ph3P Cl

H

OH

H2H

RhPPh3

Ph3P Cl

H

1 2 3

a) For the rhodium in each compound, tabulate the coordination number and the oxidation

number.

Cmpd Coordination # Oxidation #

1

2

3

b) Considering the results of your table, below label the two reactions as oxidative addition,

reductive elimination, migratory insertion, or other.

1 2 3

4

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7. [24pt] Complete the following exercise with reference to (S)-BINAP, shown here.

PPh2

PPh2

a) There are no tetrahedral carbons in (S)-BINAP, yet it is optically active. What is the source of

chirality? Briefly.

b) Draw (R)-BINAP in the space above.

c) The resolution of the (R)- and (S)-enantiomers for the BINAP reagents was accomplished on

the binaphthols, which are have somewhat acidic protons. Describe in words how you would

carry out the resolution using any of the following:

good separation skills, a base, an acid, a chiral base, a chiral acid, luck

OH

OH

R- & S-binaphthol

_____________________________________________________________________________

5

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8. [32pt] Synthesis. Prepare a single enantiomer of compound 1 using bromobenzene, lithium

dimethylcuprate, and cyclohexanone as your source of carbons. A tremendous amount of partial

credit is possible here. Table 16.2 is attached below.

OH

H

CH3 Br Li(CH3)2Cu

1

O

6

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