Highly Efficient and Regioselective Cyclization Catalyzed by TS-1

Chapter 5

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 15, 2018 | https://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch005

Highly Efficient and Regioselective Cyclization Catalyzed by TS-1 Asim Bhaumik and Takashi Tatsumi Engineering Research Institute, The University of Tokyo, Yayoi, Tokyo 113, Japan

Unsaturated alcohols 1, having general formula R CH=CH(CH ) CHR OH (where R , R = H or C H and n = 1 - 3) have been efficiently cyclized to the corresponding hydroxytetrahydrofuran or hydroxytetrahydropyran over medium pore titanium silicate molecular sieve, TS-1, in one pot at mild liquid phase reaction conditions using dilute hydrogen peroxide as oxidant. When there is a choice of the attack of the hydroxy nucleophile to either of the activated carbon atoms to lead to tetrahydrofuranol and tetrahydropyranol, the former exclusively formed regioselectively. When R / or R = CH , between the diasteroisomeric products trans predominates over cis. In this cyclization reaction TS-1 epoxidizes the olefin and successively catalyzes the opening of the oxirane ring via intramolecular attack of hydroxy oxygen, being bifunctional in nature. 1

1

2 n

2

2

1

3

2

3

Widespread occurrence of the substituted tetrahydrofuran and tetrahydropyran rings in many classes of natural products made them valuable in the building blocks for the synthesis of various biologically active organic target molecules (1). Thus, a new method for the synthesis of these oxacyclic compounds is an important area of research. The convenient route for the stereoselective synthesis of these compounds involves an electrophilic activation of the double bond (2,3) in 1 followed by intramolecular nucleophilic attack of the oxygen atom of the terminal hydroxy 1 group. Ring closure of substituted 4-penten-l-oxy and 5-hexen1-oxy radicals (4,5) is also a useful tool for the preparation of these compounds. Titanium silicate molecular sieve, TS-1 (6) having the medium pore MFI topology

© 2000 American Chemical Society

Song et al.; Shape-Selective Catalysis ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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has been used as an efficient, clean and selective oxidation catalyst over a decade under liquid phase heterogeneous reaction conditions in the presence of dilute hydrogen peroxide (7). Until this date various TS-1 / H 0 catalyzed organic transformations (8-13) have been reported, some of which have been commercialized or tested in a large pilot plant and the aim of the recent research is to find out its applicability to other transformations. Here, we report a highly efficient regioselective cyclization of such olefinic alcohols over TS-1, under mild reaction conditions using dilute hydrogen peroxide as oxidant. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 15, 2018 | https://pubs.acs.org Publication Date: November 2, 1999 | doi: 10.1021/bk-2000-0738.ch005

2

2

Synthesis and Characterization of TS-1 TS-1 used in the present study was synthesized by modifying the standard literature procedure (6) and thoroughly characterized through X R D and FT IR and UV-Vis spectroscopies. The liquid phase reaction was carried out in a two-necked glass reactor fitted with water condenser under inert N atmosphere at the required temperature (298 Κ and 333 K) with vigorous stirring. In a typical reaction the following constituents were employed : 0.02 mole substrate, 0.02 mole H 0 (30 wt % aqueous), catalyst (TS-1, Si / Ti = 29) 20 wt % with respect to the substrate, 10 g acetone, 2-butanol or H 0 (in the three phase system). At various reaction times products were analyzed by a capillary gas chromatograph (Shimatzu 14 A , OV-1 and Chiraldex G-TA with Flame Ionization Detector). Products were identified through G C retention times and GC-MS splitting patterns of the authentic samples. When authentic samples were unavailable identification was done through H N M R spectroscopy. 2

2

2

2

X

Cyclization of Unsaturated Alcohols 3-buten-l-ol. Cyclization of the simplest molecule of this series, 3-buten-l-ol, occurs at room temperature over the TS-1 / H O system. In 2-butanol solvent the reaction rate is slow and it takes 18 h to reach yield of 82 %, 3-hydroxytetrahydrofuran 2, being the sole product (Scheme 1). However, in the presence of water as the dispersion medium (solid catalyst, aqueous H 0 , organic substrate initially forms three distinct three phases) (14) the reaction proceeds at a faster rate (93.6 % conversion after 6h) and selectivity towards 2 decreases to 75.5 %. In this case the oxirane ring opening via attack of external H 0 molecules of the medium competes with intramolecular cyclization process leading to dihydroxylation (1,2,4-butanetriol, selectivity 24.5 %). Interestingly, increasing the reaction temperature to 333 Κ in the latter case decreases the yield of 2 to 2.5 % with selective dihydroxylation (15). Unlike phenylsulfenyl chloride system (3) the cyclization of 3-butene-l-ol is quite efficient over the present TS-1 / H 0 system (16). Another important aspect of the TS-1 catalyzed cyclization is that different from radical addition reaction, the products are hydroxy- substituted oxacyclic compounds. 2

z

2

2

2

2


2

75

(±) 4-penten-2-ol. The tetrahydrofuran derivative 2-methyl-4-hydroxytetrahydrofiiran 3 (trans : cis ratio 70 : 30) was formed from (±) 4-penten-2-ol using acetone solvent (yield 80 % at room temperature after 18 h reaction time) over TS-1 / H~0 system. In water medium at room temperature selectivity for 3 drops to 70 9t with trans : cis ratio 67 : 33 after 12 h reaction time. At higher temperature sing water dispersion medium the dihydroxylation product predominates m a similar manner to 3-buten-l-oL Interestingly, here also the intermediate epoxide is highly reactive and undergoes very rapid oxirane ring opening either via intramolecular cyclization or dihydroxylation. However, using 2-butanol as solvent 3 forms as the sole product in 84 % yield (trans : cis ratio 72 : 28). High trans selectivity among the diasteriomers of 3 may be due to the higher stability of the transition state at the active site. In Scheme 2 the reaction sequences at 298 Κ and 333 Κ are shown for (±) 4-penten-2-ol using water as dispersion medium.


2

4- penten-l-ol. The cyclization of 4-penten-l-ol occurred regioselectively to the 5exo product tetrahydro-2-furanmethanol 4. Attack of the hydroxyl nucleophile absolutely does not take place on the carbon atom at the 5-position of the intermediate oxirane ring. Reaction medium has no effect, in 2-butanol, acetone and water, in all cases 4 was obtained in 98-99 % yield. This is quite interesting, since, 2,4,4,6-tetrabromo-l,5-cyclohexadienone (2) induced cyclization leads to a mixture of tetrahydopyran to tetrahydrofuran at a mole ratio of 3 : 1. In water medium at high temperature (333 K) also no dihydroxylation product is formed. Cis-4-hexen-l-oL In the case of ds-4-hexen-l-ol 6 (Scheme 3) between the two possibilities of the intermediate oxirane (7) ring opening the 5-exo product, tetrahydro-2-furan-l-ethanol 8, forms exclusively in 92 % yield in acetone at 333 K. The 6-endo product 2-methyl-3-hydroxy-tetrahydropyran, 9 does not form at all although it would have given more stable carbocation intermediate. As shown in Scheme 3, the titanium hydroperoxo species 5 protonates the oxirane 7 and thus activates it for the nucleophilic attack of the O H groups at C . One possible explanation for the 5-exo product formation from 4-penten-l-ol would be of the acidic nature of TS-1 / H 0 system ; if the oxirane ring opening follows S 1 pathways (involving initial protonation), more preferential attack would be on to the more substituted carbon atom. The hydroxyl group would attack preferentially at the more substituted carbon atom due to higher stability of the corresponding carbocation. On the contrary, alkyl group substitution at C does not cause any change in the regioselectivity of cyclization. 4

2

2

N

5

5- hexen-l-ol. In the case of 5-hexen-l-ol, where two products are possible, either tetrahydropyran or its 7-membered regioisomer tetrahydrohomopyran derivative, tetrahydro-2-pyranmethanol 10 only forms in 90 % yield in acetone at 333 Κ (Scheme 1).


OH -(CH ) 2 N

OH

OH

V


"CH

Ο

CH OH

"O

2

4

3

CHOH-CH3

8

CHoOH 10

Scheme 1. Unsaturated alcohols and their cyclized products.

OH

TS-1/H 0 2

:OH

2

H 0at298K 12 h 2

4-penten-2-ol

OH

OH « OH

Ç^CH, Major

V ^ C H I

OH

Minor 70 % Sel.

30 % Sel.

Scheme 2. Reaction sequences for 4-penten-2-ol at 298 Κ and 333 K.



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Scheme 3. Generation of titanium hydroperoxo species in TS-1 / H 0 system and the proposed reaction scheme for the cyclization of cw-4-hexen-l-ol. 2


2

78

Mechanistic aspects Although theoretical calculation on the transition state energies for 4-penten-l-oxyl radical (4) indicates the 5-exo product is strongly favored, in the TS-1 / H 0 system oxidation is believed to occur through titanium hydroperoxo species 5 (17) (Scheme 3) and thus essentially ionic in nature. Restricted geometry inside the TS1 channel (the MFT topology with intersecting 10-member rings of 5.3 X 5.6 A and 5.1 X 5.5 A pore diameters and 0.10 cc/g internal void volume helps in bending the chain) might play a crucial role in the regioselective cyclization. The decreasing trend of the ratio of tetrahydropyrans to tetrahydrofurans from mesoporous MCM-41 (internal void volume very high 0.95 cc/g) to large pore Beta (18) followed by exclusive formation of tetrahydrofiiran rings over medium pore TS-1 supports the above proposition. However, preferential formation of tetrahydrofiiran derivatives is in line with the greater stability of 5-membered heterocycles.


2

2

In the oxidative cyclization (afunctional behavior, epoxidation followed by acid catalyzed cyclization) observed in the oxidation of linalool (18) over Al-TiBeta and Al-Ti-MCM-41 the acidity at the A l sites was responsible for the oxirane ring opening leading to cyclized product. Since TS-1 employed here contains no A l , protonic character of the titanium hydroperoxo species 5 seems to promote the cyclization. In the transition state of the acid-catalyzed S 2 cleavage of the oxirane ring, bond-breaking proceeds faster than bond-making, and the carbon has acquired a considerable positive charge. Thus the reaction has considerable S 1 character and the nucleophilic attack is easy at the crowded carbon atom that can best accommodate the positive charge. However, for 3-buten-l-ol and (±) 4penten-2-ol, the attack of the O H group on such a corbon leading to the four membered ring is unfavorable. The attack on the less crowded carbon occurs instead. Thus at high temperatures the attack of water predominates over the attack of the intramolecular O H group. In contrast, for 4-penten-l-ol the attack of the OH group on the crowded carbon to produce 4 is favorable, excluding the occurrence of dihydroxylation even at high temperature. N

N

In the case of products 2 and 3 where the dihydroxylation product predominates at higher temperature in water medium this can be accounted by higher probability of the formation of titanium hydroperoxo species 5, which protonates the oxirane oxygen fast. This promotes the hydrolysis due to easy access of water molecules of the medium. Interestingly, no intermediate epoxide is detected either while studying the kinetics of various constituents of the reaction mixture by GC, indicating that TS-1 catalyzed the present cyclization process at a very fast rate and ring closure takes place inside the cages of the zeolite immediately after the epoxidation.


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Conchisioiis In the presence of aqueous HjO* TS-1 generates titanium hydroperoxo species 5, which not only efficiently epoxydizes the double bond of the unsaturated alcohols of type 1, but catalyzes the oxirane ring opening via intramolecular attack of hydroxy nucleophile leading to oxacyclicringformation.The reaction sequence indicates that when there is a choice of hydroxy tetrahydrofuran and tetrahydropyran, the former exclusively produced under the present reaction conditions. However, when there is no possibility of smaller oxacyles other than 6membered one, hydroxy-tetrahydropyranformsexclusively. Acknowledgments. A.B. thanks Japan Society for the Promotion of Science for a postdoctoral fellowship. Literature Cited (1) Lord, M.D., Negri, J.T.; Paquette, L.A. J. Org. Chem. 1995, 60, 191. (2) Ting, P.C.; Bartlett, P.A. J. Am. Chem. Soc. 1984, 106, 2668. (3) Tuladhar, S.M.; Fallis, A.G. Tetrahedron Lett. 1987, 28, 523. (4) Hartung, J., Stowasser, R., Vitt, D.; Bringmann, G. Angew. Chem.Int.Ed. Engl. 1996, 35, 2820. (5) Trost, B.M.; Li, C.J. J. Am. Chem. Soc. 1994, 116, 10819. (6) Taramaso, M., Perego, G.; Notari, B. U. S. Patent 1983, 4410501. (7) Tatsumi, T., Nakamura, M., Negishi, S.; Tominaga, H. J. Chem. Soc. Chem. Commun. 1990, 476. (8) Huybrechts, D.R.C., DeBruycker,L.;Jacobs,P.A.Nature 1990, 345, 240. (9) Tatsumi, T., Yako, M., Nakamura, M., Yuhara, Y .; Tominaga, H. J. Mol. Catal. 1993, 78, L41. (10) Clerici, M.G.; Ingallina, P. J. Catal. 1993, 140, 71. (11) Tatsumi, T.; Jappar, N. J. Catal. 1996, 161, 570. (12) Reddy, J.S.; Jacobs,P.A.J. Chem. Soc. ParkinTrans.1 1993, 2665. (13) Reddy, R., Reddy, J.S., Kumar, R.; Kumar, P. J. Chem. Soc. Chem. Commun. 1992, 84. (14) Bhaumik, Α.; Kumar, R. J. Chem. Soc. Chem. Commun. 1995, 349. (15) Bhaumik, Α.; Tatsumi, T. J. Catal. 1998, 176, 305. (16) Bhaumik, Α.; Tatsumi, T. J. Chem. Soc. Chem. Commun. 1998, 463. (17) Bellussi, G., Carati, Α., Clerici, M.G., Maddinelli, G.; Millini, R. J. Catal. 1992, 133, 220. (18) A. Corma, Iglesias, M.; Sanchez, F. J. Chem. Soc. Chem. Commun. 1995, 1635.


Highly Efficient and Regioselective Cyclization Catalyzed by TS-1

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