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Asymmetric Epoxidation Of Dihydronaphthalene With A Synthesized Jacobs

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Asymmetric Epoxidation of Dihydronaphthalene with a Synthesized Jacobsen's
Catalyst



Abstract. 1,2 diaminocyclohexane was reacted with L-(+)-tartaric acid to yield
(R,R)-1,2-diaminocyclohexane mono-(+)-tartrate salt. The tartrate salt was then
reacted with potassium carbonate and 3,5-di-tert-butylsalicylaldehyde to yield
(R,R)-N,N'-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine, which was
then reacted with Mn(OAc)2*4H2O and LiCl to form Jacobsen's catalyst. The
synthesized Jacobsen's catalyst was used to catalyze the epoxidation of
dihydronaphthalene. The products of this reaction were isolated, and it was
found that the product yielded 1,2-epoxydihydronaphthalene as well as
naphthalene.

Introduction

     In 1990, professor E.N. Jacobsen reported that chiral manganese
complexes had the ability to catalyze the asymmetric epoxidation of
unfunctionalized alkenes, providing enantiomeric excesses that regularly
reaching 90% and sometimes exceeding 98% . The chiral manganese complex
Jacobsen utilized was [(R,R)-N,N'-Bis(3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediaminato-(2-)]-manganese (III) chloride (Jacobsen's Catalyst).

(R,R) Jacobsen's Catalyst Jacobsen's catalyst opens up short pathways to
enantiomerically pure pharmacological and industrial products via the
synthetically versatile epoxy function .
     In this paper, a synthesis of Jacobsen's catalyst is performed (Scheme
1). The synthesized catalyst is then reacted with an unfunctional alkene
(dihydronaphthalene) to form an epoxide that is highly enantiomerically enriched,
as well as an oxidized byproduct.
     Jacobsen's work is important because it presents both a reagent and a
method to selectively guide an enantiomeric catalytic reaction of industrial
and pharmacological importance. Very few reagents, let alone methods, are
known to be able to perform such a function, which indicates the truly
groundbreaking importance of Jacobsen's work.

Experimental Section

     General Protocol. 99% L-(+)- Tartaric Acid, ethanol,
dihydronaphthalene and glacial acetic acid were obtained from the Aldrich
Chemical Company. 1,2 diaminocyclohexane (98% mix of cis/trans isomers) and
heptane were obtained from the Acros Chemical Company. Dichloromethane and
potassium carbonate were obtained from the EM Science division of EM Industries,
Inc. Manganese acetate was obtained from the Matheson, Coleman and Bell
Manufacturing Chemists. Lithium chloride was obtained form the JT Baker
Chemical Co. Refluxes were carried out using a 100 V heating mantle (Glas-Col
Apparatus Co. 100 mL, 90 V) and 130 V Variac (General Radio Company). Vacuum
filtrations were performed using a Cole Parmer Instrument Co. Model 7049-00
aspirator pump with a Büchner funnel. For Thin Layer Chromatography (TLC)
analysis, precoated Kodak chromatogram sheets (silica gel 13181 with
fluorescent indicator) were used in an ethyl acetate/hexane (1:4) eluent.
TLC's were visualized using a UVP Inc. Model UVG-11 Mineralight Lamp (Short-wave
UV-254 nm, 15 V, 60 Hz, 0.16 A). Masses were taken on a Mettler AE 100. Rotary
evaporations were performed on a Büchi Rotovapor-R. Melting points were
determined using a Mel-Temp (Laboratory Devices, USA) equipped with a Fluke 51
digital thermometer (John Fluke Manufacturing Company, Inc.). Optical rotations
([a]D) were measured on a Dr. Steeg and Renter 6mbH, Engel/VTG 10 polarimeter.
Solid IR's were run on a Bio-Rad (DigiLab Division) Model FTS-7 (KBr:Sample
10:1, Res. 8 cm-1, 16 scans standard method, 500cm-1 - 4000cm-1). Flash
Chromatography was carried out in a 20 mm column with an eluant of ethyl acetate
(25%) in hexane.

     (R,R)-1,2 Diaminocyclohexane mono-(+)-tartrate salt. 99% L-(+)-Tartaric
Acid (7.53g, 0.051mol) was added in one portion to a 150 mL beaker equipped
with distilled H2O (25 mL) magnetic stir bar, and thermometer. Once the
temperature had dropped to 17.8 °C, 1,2 diaminocyclohexane (11.89 g, 12.5 mL,
0.104 mol) was added with stirring in one portion. To the resultant amber
solution was added glacial acetic acid (5.0 mL, 0.057 mol). The frothy orange
product was cooled in an ice water bath for 30 minutes. The product was washed
with 5 °C distilled H2O (5.0 mL) and ambient temperature methanol (5.0 mL) and
isolated by vacuum filtration. 8.37 grams of an orange slush were obtained.
The product was further purified by recrystallization of the salt from H2O (1:10
w/v, 84 mL of H2O) and again isolated by vacuum filtration, yielding an off-
white crystalline product (1.2015g; 0.00415 mol; 8.9 % yield; mp=270.4-273.8 °
C Lit. Value mp=273 °C )

     (R,R)-N,N'-Bis (3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine.
Distilled H2O (6.0 mL), (R,R)-1,2 diaminocyclohexane mono-(+)-tartrate salt
(1.1087 g, 0.0042 mol) and K2CO3 granules (1.16 g, 0.0084 mol) were added to a
100 mL RB flask equipped with a magnetic stir bar. The mixture was stirred
until complete dissolution occurred, and then ethanol (22 mL, 0.376 mol) was
added. The solution was then brought to reflux, and then a solution of 3,5-di-
tert-butylsalicylaldehyde (2.0g, 0.0037 mol) dissolved in ethanol (10 mL,
0.1713 mol) was added with a Pasteur pipette. The solution refluxed for 45
minutes. H2O (6.0 mL) was added to the yellow solution, and the mixture cooled
in an ice bath for 30 minutes. The resultant yellow solid was collected by
vacuum filtration and washed with ethanol (5 mL, 0.856 mol). The yellow solid
was dissolved in CH2Cl2 (25 mL, 0.4 mol) and washed with H2O (2 x 5.0 mL) and
saturated aqueous NaCl (5.0 mL) The organic layer was dried (Na2SO4) and then
decanted into an RB flask. Methanol was removed in vacuo, yielding a yellow
crystalline powder (1.56g; 0.00285 mol; 77 % yield; mp=202.9-205.4 °C, Lit.
Value mp=205-207 °CIII; IR (KBr) 2800, 2100, 1631.7, 1506.7, 1173.5, 828,
545cm-1; [a]D20 =-314°, Lit. Value [a]D20 =-315°III)

     [(R,R)-N,N'-Bis (3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato
(2-)]-manganese (III) Chloride. Absolute ethanol (25 mL, 0.429 mol) was added
to (R,R)-N,N'-bis (3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine (1.01 g,
0.001788 mol) in a 50 mL RB equipped with a magnetic stirrer, mantle, claisen
adapter and reflux condenser. The pale yellow mixture was brought to reflux,
and MnO(OAc)2*4H2O (2.0 equivalents, 0.881 g, 0.0036 mol) was added. The
orange mixture refluxed for 30 minutes, and then the reaction flask was equipped
with a glass bleed tube allowing air to bubble through at a slow rate. The
progress of the reaction was monitored by TLC until the starting material
((R,R)-N,N'-bis (3,5-di-tert-butylsalicylidene)-1,2 cyclohexane diamine)) faded
from the TLC readings (Rf=0). At this point, the air was discontinued and
granular LiCl (3 equivalents, 0.24 g, 0.0054 mol) was added to the caramel brown
mixture. The mixture was refluxed for an additional 39 minutes, and the ethanol
was removed in vacuo. The brown solid was redissolved in CH2Cl2 (25 mL), washed
with H2O (2x 10 mL) and saturated aqueous NaCl (15 mL). The organic phase was
dried (Na2SO4) and redissolved in heptane (30 mL, 0.205 mol). The CH2Cl2 was
removed in vacuo, and the brown slurry was cooled in an ice bath for 57 minutes.
The brown solid (0.22g; 0.000354 mol; 19% yield; mp=331.4 -333.6 °C, Lit.
Value mp=324-326 °CIII) was collected by vacuum filtration, and left to air dry
for 1 week.

     1,2-Epoxydihydronaphthalene. 0.05 M Na2HPO4 (5 mL, 0.037g 2.5*10-4 mol)
was added to household Clorox bleach (12.5 mL), and the resultant clear liquid
was adjusted to pH 11.3 by adding 1M NaOH (1 drop). [(R,R)-N,N'-Bis(3,5-di-
tert-butylsalicylidene)-1,2-cyclohexanediaminato (2-)]-manganese (III) chloride
(0.2 g, 0.00031 mol) was added to a solution of 4-phenylpyridine N-oxide (0.13
g, 0.00076 mol) and dihydronaphthalene (0.51 g, 0.0038 mol) in CH2Cl2 (5 mL,
0.076 mol). The brown liquid was stirred vigorously for 2 hours. The progress
of the reaction was monitored by TLC until the starting material
(dihydronaphthalene, Rf=) faded from the TLC readings (Rf=0). Once the starting
material was gone, the stir bar was removed and dichloromethane (50 mL, 0.76
mol) was added. The brown organic phase was separated, washed twice (NaCl aq)
and dried (Na2SO4). The brown organic layer was then isolated by vacuum
filtration, and then the dichloromethane was removed in vaccuo. The dark brown,
oily solid (0.4 g; 0.0027 mol; 71% yield; IR (NEAT) 2964.0, 2857.1, 1747.0,
1373.0, 1239.0, 1048.7cm-1; GC Retention Times (minutes) 3.75 (70%,
naphthalene), 6.75 (29%, 1,2-epoxydihydronaphthalene)) was stored for one week
and then purified by flash chromatography.

Results and Discussion

     Synthesis of (R,R) Jacobsen's Catalyst (Scheme 1). The first step in
the synthesis of Jacobsen's catalyst was the selective crystallization of one
of three stereoisomers present in 1,2-diaminocyclohexane. The yield from this
reaction was 8.9% (Appendix 1). The reaction produced 1.2015 g of an off-white
crystal (Product 1) with a melting point of 270.4-273.8 °C, which was identified
as (R,R)-1,2-diaminocyclohexane mono-(+)-tartrate salt (Table 1).

Table 1.          Selected Data Utilized in Identification of Product 1
Compound     Product 1      (R,R)-1,2-diaminocyclohexane mono-(+)-tartrate
saltIII Physical Description     Off-white crystals     Off-white to beige
crystalline solid

Melting Point (°C)     270.4-273.8     273

The percent yield was so low (8.9%) largely because of experimental error. An
unknown amount of Product 1 was lost because it was not retrievable from the
reaction flask, and a further unspecified amount was lost when a portion of the
product recrystallized on the filter paper during a vacuum filtration. This
recrystallization occurred because the funnel and filter flask were not heated
properly. The second step of the Jacobsen synthesis involved the reaction of
the isolated diamine salt (Product 1, (R,R)-1,2-diaminocyclohexane mono-(+)-
tartrate salt) with an aldehyde (3,5-di-tert-butylsalicylaldehyde) to produce
the organic backbone of the catalyst. The percent yield from this reaction was
77%. This reaction produced 1.56 g of an oily, yellow powder (Product 2) with a
melting point of 202.9-205.4 °C and an optical rotation ([a]D20) of -314° that
was identified as (R,R)-N,N'-Bis (3,5-di-tert-butylsalicylidene)-1,2-
cyclohexanediamine (Table 2).

Table 2.          Selected Data Used in Identification of Product 2
Compound     Product 2     (R,R)-N,N'-Bis (3,5-di-tert-butylsalicylidene)-
1,2-cyclohexanediamineIII Physical description
     Oily, yellow powder     Yellow powder Melting Point (°C)
     202.9-205.4     205-207 [a]D20     -314°     -315°

Product was lost during transfers between containers and in the separatory
funnel when the reaction material was washed. It is also possible that product
was lost because the reaction was not allowed to reflux to completion and was
cut short by fifteen minutes. The fourth and final step of the Jacobsen
catalyst synthesis involved the insertion of the oxidizing metal (in the form of
Mn(OAc)2*4 H2O followed by 2 equivalents of LiCl) into the organic backbone
(Product 2) of the catalyst. The percent yield for this reaction was 19%. The
reaction produced 0.22 g of a brown, oily solid (Product 3) with a melting point
of 331-333.6 °C that was identified as Jacobsen's catalyst; [(R,R)-N,N'-Bis
(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminato (2-)]-manganese (III)
Chloride (Table 3).

Table 3.          Selected Data Used in Identification of Product 3
Compound     Product 3     Jacobsen's Catalyst Physical Description
Brown, oily solid
     Brown Solid Melting Point (°C)     331.4-333.6     324-326

Again, product was lost because the reflux was cut short and not allowed to run
to completion, causing loss of product. Additional product was either lost or
unreacted when the air bleed tube was inserted, causing some product to splash
out of the reaction flask. These experimental errors may very well have led to
a high amount of impurities in Product 3, which would account for the difference
between the experimental melting point and the literature value. The net
percent yield for the synthesis of Jacobsen's catalyst was 1.9% (Appendix 1)

     Asymmetric Epoxidation of Dihydronaphthalene. The synthesized
Jacobsen's catalyst (Product 3) was used to run an enantiomerically guided
epoxidation of an unfunctionalized alkene (dihydronaphthalene). The percent
yield for this reaction was 71%. The reaction yielded a 0.4 g of a dark brown,
oily solid (Product 4) that was purified by flash chromatography, analyzed by
GC/MS and IR (NEAT) (Figure 1, Table 4).

Table 4.          Selected IR Data for Identification of Epoxidaton of
Dihydronaphthalene Products Compound     Product 4***Fig     1,2-
epoxydihydronaphthalene     Naphthalene Prominent IR Peaks     2964.0 (C-H,
alkane) 1747.0 (C=C, alkene) 1239.0 (C-O, ether) 1048.7 (C=C-H, alkene)
     2970-2850 (C-H, alkane) 1750-1620 (C=C, alkene) 1300-1000 (C-O, ether)
1050-675 (C=C-H, alkene)     2970-2850 (C-H, alkane) 1750-1620 (C=C, alkene)
1050-675 (C=C-H, alkene) GC: Retention Times (min.) and Corresponding Mass Spec
(m/z)     3.75 min.: (128)

6.75 min.: (146) Structure, Physical Properties


          Mass=146.18
     Mass=128.17

Product 4 displays properties of both 1,2-epoxydihydronaphthalene and
naphthalene. The peaks seen in the IR (NEAT) of product 4 at 2964.0, 1747, 1239,
and 1048.7 cm-1 (FIG 1) could be interpreted to represent the presence of just
1,2-epoxydihydronaphthalene. The GC that was run on product 4; however,
indicated that naphthalene was also present (FIG 2-4). This leads to the
conclusion final product of this Jacobsen catalyzed epoxidation was a mixture of
1,2-epoxydihydronaphthalene (30%) and naphthalene (70%) (FIG 2-3, Scheme 2).
The presence of an oxidized product (naphthalene) indicates that the solution in
which the reaction took place was probably too basic. Such a situation could be
corrected by either adding less Clorox or by adding NaOH that is less
concentrated than 1M. It is also possible that not all of the epoxidized
product was isolated, and that much of it remained stuck in the silica gel of
the flash chromatography column. In order to remedy this situation, a solvent
that is more polar than the 25% ethyl acetate in hexane that was used for the
flash chromatography in this experiment.

Conclusion

     The synthesized Jacobsen's catalyst did not guide this enantiomeric
epoxidation as was hoped; however, both the reagent and mechanism showed that
it is possible to produce a significant amount of an enantiomerically enriched
epoxide. The problem with the reaction described above was not the reagent or
the mechanism of the reaction, it was the conditions in which the reaction was
carried out. In order for the Jacobsen catalyzed epoxidation to produce highly
enamtiomerically enriched epoxides as was hoped, more care must be taken in the
transferring and washing of products, and reactions must be allowed to run to
completion. If this is successfully done, then the impurities that were
present in the final product will be effectively minimized, and the results that
were obtained by Dr. E.N. Jacobsen may be repeated.

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