Gurjar, M K

Rao, A V Rama

Sarma, BVNBS

NOPR

Indian Journal of ChemistryVol. 36B, March 1997, pp. 213-215Rapid CominunicationEnantioselective epoxidation of 2,2-Dial-kyl-2H-chromenes with Oxone" and Ja-cobsen's (S,Sj-Mn(m)(Ph-salen)CI complex: Solvent effect!M K Gurjar*, BVNBS Sarma & A V Rama RaoIndian Institute of Chemical Technology,Hyderabad 500 007, IndiaReceived 17 October-1996; accepted 26 November 1996Eriantioselective epoxidations of 2,2-dialkyl-2H-chromenes 2a- 2c to 3a-c with oxone in the presenceof (S-S)-Mn(ill}(Ph-salen}CI as a catalyst, in varioussolvents are described.Chiral manganese salen complexes are effective inenantioselective epoxidation of alkenes'< Recentdevelopment in this area includes designing ofnew ligands! and discovering new nucleophilic ox-idants. After the pioneering research by Jacobsenwith NaOCF, several other oxidants such as iodo-sobenzene/, molecular oxygen", H2025, Bu4NI046,dimethyldioxirane 7 and mCPBN were explored.Oxone'' has found a great deal of importance inorganic chemistry as a cheap oxidising agent".However, its utility in enantioselective epoxidationhas not been fully" explored. In this report, wewish to present oxone as a suitable and alterna-tive oxidant in combination with Jacobsen'sMn(III}-salen catalyst 1 for enantioselective epoxi-dation of 2,2-dialkyl-2H-chromenes 2a-2c. Theenantioselectivity markedly depends upon the sol-vent used and the solvent CH3CN was distinctlysuperior.We first studied (cf, Table I} the epoxidation of2a with 2.5 equiv. of oxone under essentially thesimilar conditions 10.03 equiv. of {S,S}-l, and 0.3equiv. of 4-phenylpyridine-N-oxide (4-PPNO)] re-ported'? by others using CH2C12 as a solvent atO°c. Although the reaction went to completion in12 hr, the enantiomeric excess of 19.8% (chiralHPLC), observed for the epoxide 3a, was ratherdisappointing. We reasoned that the low enan-tioselectivity was perhaps due to the slow rate ofthe reaction in CH2C12•The effect of solvents" on chiral Mn-salencomplex catalysing the enantioselective epoxida-tion of alkenes has not been fully investigated. WetIlCT Communication No. 3580Ph Ph,---.~-Q:HN~N9-HHe ? ~ o~i(0 . ¥ '\ He- CI -But tau(S, S) • ( Ph - SQ I.n ) CI (1)Il) R1 ,. He I R 2: CNR 1 •• He R2 = HIRl ••-ICH2)5-" Rl , R 2: Hb)c)observed that when compound 2a was treatedwith oxone (2.5 equiv.), {S,S}-l and 4-PPNO, butwith CH3CN as a solvent, the optical purity of 3adramatically improved to 85% ee (chiral HPLC)(entry 2). In addition the epoxidation reaction wascompleted in just 15 minutes. We could not ex-plain the dramatic enhancement in rate of thereaction and the enantioselectivity with CH3CNas a solvent. However, the result was indeed ap-pealing and therefore deserved further investiga-tion. Table I indicates the number of reactionswith different solvents carried out with modelsubstrates 2a-2c.We have also studied (cf. Table II) the effect ofadditives 12 on the course of the epoxidation keep-ing CH3CN as a common solvent. Entries 1-4 in-dicate that there was no dramatic effect on theenantioselectivity of the epoxidation of 2a eitherby changing the ratio of 4-PPNO or by usingother additives. Entry 5, where no additive" wasadded, the enantiomeric excess of 95% was notedfor the epoxide 3a (chiral HPLC and optical rota-tion). Entries 6 and 7 represent reactions carriedout without the additive.In conclusion we have described oxone as acheap and complimentary nucleophilic oxidant inthe enantioselective epoxidation with chiralMn{III}-salen catalyst. The effect of solvent in theepoxidation was indeed interesting. Further re-214 INDIAN J. CHEM. SEC B, MARCH 1997Table I-Enantioselective epoxidation With Oxone (2.5 equiv.), Mn(III)-salen catalyst 1 (10 mol%) and4-PPNO (0.3 equiv.)Entry Substrate Solvent (Reaction period) Product ee(%)(a) Yield (%)'b)1 2a CH2Cl2 (12 hr) 3a 19.8 612 2a CH3CN (15 min.) 3a 85.0 673 2a Acetone (12 hr) 3a 60.8 624 2b CH3CN (15 min.) 3b 65.0 795 2b Acetone (12 hr) 3b 32.9 616 2c CH3CN (15 min.) 3c 85.6 817 2c CH2Cl2 (12 hr) ----------no reaction---------aee determined by chiral HPLC (chiralcel OD with isopropanol-hexane as eluents and at 254 nm) andor by optical rotations."Isolated yieldsTable II-Enantioselecrive epoxidation of 2a-2c with Oxone (2.5 equiv.) and Mn(Ill)-salen catalyst 1studied by varying additives and using CH:iCN as a common solventEntry Substrate (S,5)-1 Additive (equiv.) ee (%)(a) Yield (%)(b)(hplc)1 2a 10 (mol%) 4-PPNO (1.2) 86.5 822 2a 10 (mol%) NMO(5.0) 87.5 853 2a 10 (mol%) Imidazole (1 ) 83.9 834 2a 1.5 mol%) No additive 95.0 865 2b 15 (mol%) No additive 91.0 806 2c 15 (mol%) No additive 90.0 75aeedetermined by optical rotation and HPLe.blsolated yield.search to improve the enantioselectivity withother catalyst in this domain will be forthcoming.Experimental SectionGeneral procedure for epoxidation of 2,2-dial-kylchromene 2a-2c. Oxone (1.35 mmoles) wasdissolved in water (3 mL) and then basified to pH11 by adding 1NKOH solution. This solution wasthen added to a mixture of olefin (2a-2c)14 (0.54mmole) and catalyst (15 mol%) in acetonitrile (5mL) at O°e. The reaction was monitored by TLCThe two layers were separated and the organiclayer was dried over Na2S04 and concentrated.The residue was chromatographed over silica gel(60-120. mesh) using 5% ethyl acetate in n-hexaneas eluent to give the epoxide (3a-3c).1H NMR (CDCl3, Varian Gemini 200 MHz) of:3a: 6 1.28, 1.53 (2s, 6H), 3.46 (d, J= 4.2 Hz,1H), 3.86 (d, J=4.2 Hz, 1H), 6.78 (d, J=8.5 Hz,1l1), 7.42 (dd, J= 8.5, 1.5 Hz, 1H), 7.57 (d,J= 1.5 Hz, 1H).3b: 6 1.25 (s, 3H), 1.57 (s, 3H), 3.42 (d,J=4.25 Hz, 1H), 3.83 (d, J=4.25 Hz, 1H), 6.7-6.95 (m, 2H), 7.15-7.35 (m, 2H).3c: 6 1.2-2.2 (m, 10H), 3.42 (d, J=4.25 Hz,1H), 3.80 (d, J= 4.25 Hz, 1H), 6.75-6.95 (m, 2H),7.15-7.35 (m, 2H).AcknowledgementB V N B S thanks CSIR, New Delhi for finan-cial support in the form of a junior research fel-lowship.References1 Jacobsen E N, in Catalytic asymmetric synthesis, edited byI Ojima (VCH Press, New York), 1993, Chapter 4.2.2 (a) Zhang W, Loebach J L, Wilson S R & Jacobsen E NJ Am Chern Soc, 112, 1990,2801. '(b) Jacobsen E N, Zhang W, Muci A R, Ecker J R &Deng L, JAm Chern Soc, 113,1991,7063.3 (a) Katsuki T, Coordin-Chemi Rev, i40, 1995, 189.(b) Mukaiyama T, Yamada T, Nagata T & Imagawa K,Chern Left, 1993,327.4 Yamda T, Imagawa K, Nagata T & Mukaiyama T, BullChern Soc Ipn; 67, 1994,2248.5 Pietikainen P, Tetrahedron Left, 35,1994,941.6 Pietikainen P, Tetrahedron Left, 36, 1995,319.7 Adam W, Jeko J, Levai A, Nemes C, Patonay T & SebokP, Tetrahedron Left, 36, 1995,3669.8 Paiucki M, McCormick G J & Jacobsen E N, TetrahedronLeu, 36,1995,5457.9 (a) Kenedy R J & Stock A M, J Org Chern, 25, 19601901. 'RAPID COMMUNICATIONS 215(b) Bloch R, Absecassis J & Hassan D, J Org Chern, 50,1985,1544.10 .Lee N H, Muci A R & Jacobsen E N, Tetrahedron Lett32, 1991,5055.11 (a) Murary R W, Singh M, Williams B L & Moncrieff HM, Tetrahedron Lett, 36, 1995,2437.(b) Adam W & Smerz A K, Tetrahedron, 51, 1995,13039.12 Jacobsen E N, Deng L, Furukawa Y & Martinez L E, Te-trahedron, 50, 1994,4323.13 Zhang W & Jacobsen EN, J Org Chern, 56, 1991,2296.14 (a) Evans J M, Fake G S, Hamilton T C, Poyser R H &WattsE A, J Med Chern, 26, 1983, 1582.(b) Bergamn R & Gericke R, J Med Chern, 33, 1990,492.(c)Kabe HJ, Synthesis, 1978,886.

Enantioselective epoxidation of 2,2-Dialkyl-2H-chromenes with Oxone and Jacobsen's (S,S)-Mn(III)(Ph-salen)CI complex : Solvent effect

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Enantioselective epoxidation of 2,2-Dialkyl-2H-chromenes with Oxone and Jacobsen's (S,S)-Mn(III)(Ph-salen)CI complex : Solvent effect

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