Asymmetric Epoxidation of Dihydronaphthalene with a Synthesized Jacobsen's Catalyst

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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