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Heck Reaction Tetrahedron Letters 51 (2010) 6227–6231 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet One-pot synthesis of stilbenes by dehydrohalogenation–Heck olefination and multicomponent Wittig–Heck reaction Akeel S. Saiyed, Ashutosh V. Bedekar ⇑ Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India article info a b s t r a c t Article history: A variant of olefination reaction involving in situ generation of styrene by either one-pot dehydrohalo- Received 2 August 2010 genation–Heck or one-pot multicomponent Wittig–Heck reaction is developed. Revised 7 September 2010 Ó 2010 Elsevier Ltd. All rights reserved. Accepted 10 September 2010 Available online 17 September 2010 Keywords: Heck reaction In situ generation of styrene One-pot multicomponent reactions Dehydrohalogenation–Heck Wittig–Heck The concept of performing more than one transformation in a the olefin is somehow synthesized in situ during the Mizoroki–Heck single vessel, or developing one-pot processes, can generate less reaction. There are a few reported examples of one-pot combination waste, reduce work-up procedures, avoid separation of unstable of synthesis of substituted olefins.6–13 These include synthesis of ole- intermediates, save time and energy, and increase the efficiency fins via Hunsdiecker reaction,6 Knoevenagel-decarboxylation se- of the desired conversion.1 Since its discovery the Mizoroki–Heck quence,9 Mizoroki–Heck, Wittig reaction,11 or four component reaction,2 which involves palladium catalyzed coupling of aryl ha- Ugi–Heck sequence.12 lide and styrene, remains the best method for preparation of stil- In this letter we present two one-pot methods consisting of bene derivatives. This method has been a subject of extensive olefination processes and the Mizoroki–Heck reaction. The prere- research, while it has been used for the synthesis of a large number quisite of planning the one-pot multi-step reactions is the compat- of natural and artificial molecules.3 In most cases the reaction is ibility of the reagent system and reaction conditions. In the present done with suitable aryl halide and a styrene or similar olefin in effort the second reaction, the Mizoroki–Heck is carried out with the presence of small quantity of Pd-catalyst and stoichiometric Pd-catalyst and a suitable base. Taking this into consideration the amount of a base. Usually the aryl halide or in some cases aryl tri- required olefin can be either synthesized by base mediated dehy- flate4 and aryl diazonium salt5 are easily obtained either from a drohalogenation of suitable alkyl halide or by Wittig reaction from commercial source or prepared by known methods. The other com- aldehyde and appropriate phosphonium salt. The two approaches ponent is usually a styrene or a derivative of acrylic acid/acryloni- are outlined in Scheme 2 taking the example of the in situ synthe- trile etc. Scheme 1. sis of styrene 1 and then subjected to Mizoroki–Heck conditions to Although, the Mizoroki–Heck reaction has proved its effective- form stilbene 2. In the approach A-1 the styrene is prepared by ness, the synthetic utility purely depends on the availability of these dehydrohalogenation of (2-bromoethyl)benzene 3, and in A-2 from components. Usually ArBr, ArI, ArOTf, and ArN2þXÀ are relatively (1-bromoethyl)benzene 4 using the same base which also facili- easy to prepare or are readily available. On the other hand, in many tates subsequent Mizoroki–Heck reaction. Reaction of 3 may be cases the other components, alkenes, are not readily available; either more favorable compared to 4 if it follows the E1cB mechanism14 they are difficult to prepare or tricky to purify due to their tendency and hence, the availability of styrene will be more for further to polymerize during distillation or storage. Even under usual high Mizoroki–Heck reaction. reaction temperature conditions there is a possibility of polymeriza- tion and hence, it is used in excess. The latter problem can be solved if Ar Ar R R + Y + - R = aryl, CN, COOR' etc Y = halogen/OTf/N2 X Corresponding author. Tel.: +91 0265 2795552. ⇑ E-mail address: [email protected] (A.V. Bedekar). Scheme 1. Disconnection approach for stilbene or similar molecules. 0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2010.09.050 6228 A. S. Saiyed, A. V. Bedekar / Tetrahedron Letters 51 (2010) 6227–6231 H Br Ph Ph O 3 A-1 B-1 5 Ph3PCH3I dehydrohalogenation Ph Wittig 1 Br A-2 B-2 Ph X HCHO Heck Ph PPh Br Ph Pd-L 3 4 6 Ph Ph 2 Scheme 2. Proposed routes of in situ synthesis of styrene for subsequent one-pot Heck reaction. Alternatively the intermediate styrene may be prepared by components, that is, an aldehyde and a phosphonium salt, synthe- Wittig reaction of an aldehyde and suitable phosphonium salt sis of styrene can be achieved either by approach B-1 using benz- (approach B of Scheme 2). Since the Wittig reaction involves two aldehyde 5 and phosphonium salt obtained from methyl iodide or Table 1 Synthesis of stilbenes with one-pot two component procedure of dehydrohalogenation-Heck reaction Entry Alkyl halide (1.0 equiv) Aryl halide (1.2 equiv) Olefin obtained Yielda (%) 1 3 Iodobenzene (13) trans-Stilbene (14) 80 (66)b (54)c 2 4 Iodobenzene (13) trans-Stilbene (14) 67 3 3 4-Iodoanisole (15) trans-4-Methoxystilbene (16) 54 4 3 1-Bromo-4-nitrobenzene (17) trans-4-Nitrostilbene (18) 67 5 3 1-Bromo-3-nitrobenzene (19) trans-3-Nitrostilbene (20) 78 Ph 6 3 Br 69 (21) (22) OMe OMe I I Ph 7 3 Ph 74 MeO (23) MeO (24) 8 4 4-Iodoanisole (15) trans-4-Methoxystilbene (16) 84 9 4 1-Bromo-4-nitrobenzene (17) trans-4-Nitrostilbene (18) 80 Ar 10 11 Iodobenzene (13) 64 (25, Ar = C6H5) Ar 11 11 4-Iodoanisole (15) 86 (26, Ar = 4-MeOC6H4) Ar 12 11 4-Iodoacetanilide (27) 54 (28, Ar = 4-AcNHC6H4) Ar 13 11 4-Bromo chlorobenzene (29) 88 (30, Ar = 4-ClC6H4 14 12 Iodobenzene (13) trans-4-Nitrostilbene (18) 76 15 12 4-Iodo anisole (15) trans-1-(4-Methoxystyryl)-4-nitrobenzene (31) 77 All reactions run with aryl halide (1 equiv), alkyl halide (1.2 equiv), K2CO3 (3 equiv), Pd(OAc)2 (0.5%), 7 or 8 (0.55%) in DMA at 140 °C for 40 h. For entry 7:3 (3 equiv), K2CO3 (4 equiv), Pd (1.0%)-8 (2.5%). a Isolated Yield. b With L-10. c With L-9. A. S. Saiyed, A. V. Bedekar / Tetrahedron Letters 51 (2010) 6227–6231 6229 O O OHC CHO CHO PPh 1 eq. N 2 50 5 2 eq. N H C PPh H C PPh N i. 2 3 C-1 C-2 i. 2 3 PPh2 NHR 2 eq. 2 eq. O 10 (dppp) ii. 7, R = H 9 ii. I Br 8, R = CH2Ph 49 % 89 % Br Figure 1. Ligands investigated for the present study. 2 eq. 1 eq. 13 51 by B-2 using formaldehyde and phosphonium salt 6 obtained from benzyl bromide and triphenylphosphine. 52 Several ligands were chosen for the present Mizoroki–Heck reaction as shown in Figure 1. There is considerable interest in Scheme 3. One-pot five-component approach for 1,4-distyryl benzene 52. screening phosphine free ligands for Pd mediated Mizoroki–Heck reaction,15 whereas 7 and 8 were synthesized as per the reported procedures.16 The second approach involves the in situ synthesis of styrenes by In order to demonstrate the generality of this approach (A), a the classical Wittig olefination.17 Reaction of aromatic aldehyde series of alkyl bromides are prepared to access a variety of stilb- with phosphonium salts undergoes Wittig reaction even with weak enes and are listed in Figure 2. The mixture of suitable aryl halide, base like potassium carbonate,18 a typical base for Mizoroki–Heck + an alkyl halide, an excess of K2CO3, catalytic quantity of Pd(OAc)2- reaction. Hence an equimolar amount of benzaldehyde, Ph3P CH3IÀ, ligand in dry DMA was heated at 140 °C. Careful TLC analysis indi- iodobenzene, excess of K2CO3, catalytic quantity of Pd(OAc)2 oxaz- Á cated the formation of corresponding stilbene, which was isolated olinyl ligand 7 or 8, TBAB as PTC was heated in DMF and we found and characterized. The results are presented in Table 1. the formation of trans-stilbene in good yield. Further experimenta- In most of the cases the isolated yields of olefin are good with tion demonstrated that this concept is clearly reproducible for a just 1.2 equiv of alkyl halide. As expected the formation of trans- number of aldehydes, three representative phosphonium salts, stilbene 14 from alkyl halide 3 was more favored than from alkyl and another set of aryl halides (Table 2). One carbon aldehyde unit halide 4, due to the more facile elimination process. However, it was accessed by employing para-formaldehyde (0.34 equiv) in the will be more logical and practical to use alkyl halides of the type reaction and hence, the same number of variations was available 4 and 11, as they are more readily accessed from ethyl aryls by for the facile generation of substituted stilbenes. Utilizing the well benzylic bromination condition [NBS-Bz2O2-light], as we did. established, transition metal free, high yielding Wittig reaction for Br Br Br Br O2N 3 4 11 12 Figure 2. Alkyl halides used for in situ generation of olefins by dehydrohalogenation for Mizoroki–Heck reaction. Table 2 Synthesis of stilbenes by one-pot three component procedure of Wittig–Heck reaction Entry Aldehyde (1 equiv) Phosphonium salt Aryl halide (1 equiv) Olefin Yielda (%) (1 equiv) + b 1 Benzaldehyde (5) Ph3P CH3 IÀ Iodobenzene (13) trans-Stilbene (14) 91 + c 2 4-Nitrobenzaldehyde (32) Ph3P CH3 IÀ Iodobenzene (13) trans-4-Nitrostilbene (18) 94 (85) + 3 4-Chloro benzaldehyde (33) Ph3P CH3 IÀ Iodobenzene (13) trans-4-Chlorostilbene (34) 90 + 4 4-Anisaldehyde (35) Ph3P CH3
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