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Synlett 2008(10): 1491-1494  
DOI: 10.1055/s-2008-1078418
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Facile One-Pot Synthesis of α-Halo-α-allyl-aldehydes from α,α-Dihaloketones Using Allylsamarium Bromide and DMF

Jucai Di, Songlin Zhang*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry and Chemical Engineering, , Suzhou University, Dushu Lake Campus, Suzhou, 215123, P. R. of China
Fax: +86(512)65880089; e-Mail: zhangsl@suda.edu.cn;
Further Information

Publication History

Received 13 December 2007
Publication Date:
16 May 2008 (online)

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Abstract

A convenient, one-pot synthesis of a range of α-halo-α-allyl aldehydes is described. The protocol involves the reaction of allylsamarium bromide with various α,α-dihalo ketones. A possible mechanism of the transformation is proposed.

Key words

samarium - rearrangements - halides - aldehydes - synthesis

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1,1-Dibromo-2-phenylpent-4-en-2-ol (3a)
1H NMR (400 MHz, CDCl3): δ = 7.46-7.30 (m, 5 H), 5.98 (s, 1 H), 5.56-5.46 (m, 1 H), 5.15-5.05 (m, 2 H), 2.92 (d, 2 H, J = 6.8 Hz), 2.83 (s, 1 H). 13C NMR (100 MHz, CDCl3): δ = 140.55, 132.39, 128.59, 128.34, 126.58, 120.55, 78.99, 57.68, 43.89. HRMS (EI+): m/z calcd for C8H7Br79Br81O [M+ - C3H5]: 278.8843; found: 278.8813.

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1,1-Dimethoxy-2-phenylpent-4-en-2-ol (4a)
1H NMR (400 MHz, CDCl3): δ = 7.53-7.23 (m, 5 H), 5.69-5.55 (m, 1 H), 5.11-5.00 (m, 2 H), 4.26 (s, 1 H), 3.41 (d, 6 H, J = 14.4 Hz), 2.83-2.63 (m, 2 H), 2.66 (s, 1 H). 13C NMR (100 MHz, CDCl3): δ = 142.51, 133.82 128.31, 127.39, 126.83, 118.99, 110.86, 78.06, 58.48 (d, J = 9.7 Hz), 41.26. HRMS (EI+): m/z calcd for C13H16O2 [M+ - H2O]: 204.1150; found: 204.1192.

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General Procedure for the Preparation of α,α-Dihalo- ketones Ketone (10 mmol) in glacial AcOH (10 mL) was heated to 50 °C with stirring, and bromine (22 mmol) in glacial AcOH (5 mL) was added dropwise. When the bromine ceased decoloration, NaOAc (7 g) was added portionwise. The reaction mixture was stirred for 15 min after the addition of bromine, and cooled in refrigerator for 2 h, and then poured into cold H2O. Finally, the crystals were filtered off, and recrystallized from 95% MeOH if necessary.

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General Procedure for the Synthesis of α-Halo,α-allyl-aldehyde
Allyl bromide (1.2 mmol) and Sm (1.1 mmol) in dry THF (10 mL) were mixed under a nitrogen atmosphere at r.t. The mixture was stirred for about 5 min, and a color changed to purple. The stirring was continued until the Sm powder disappeared (1 h). Then, the solution of substrates a,a-dihaloketones(1 mmol) was added dropwise to the reaction mixture (the reaction was monitored by TLC). The reaction mixture was stirred for 10 min and then dry DMF (10 mL) was added. The reaction mixture was stirred for another 15 min (when X = Cl, the reaction mixture was stirred for 10 h) and then was poured into cold H2O and 5% HCl (10 mL) was added. The resulting mixture was extracted with Et2O (4 × 15 mL), the Et2O solution was dried over anhyd MgSO4. The solvent was removed by evaporation under reduced pressure. Purification by column chromatography on silica gel afforded the product (300-400 mesh, PE as eluent).
Analytical Data
2-Bromo-2-phenylpent-4-enal (2a) 1H NMR (400 MHz, CDCl3): δ = 9.47 (s, 1 H), 7.44-7.34 (m, 5 H), 5.74-5.60 (m, 1 H), 5.11-5.05 (m, 2 H), 3.11 (d, 2 H, J = 9.2 Hz). 13C NMR (100 MHz, CDCl3): δ = 190.37, 135.63, 132.61, 129.23, 129.29, 128.58, 120.23, 74.06, 43.39. HRMS (EI+): m/z calcd for C11H11Br81O [M+]: 239.9973; found: 240.0038; m/z calcd for C11H11Br79O [M+]: 237.9993; found: 238.0006.
2-Bromo-2- p -tolylpent-4-enal 1H NMR (400 MHz, CDCl3): δ = 9.44 (s, 1 H), 7.32-7.17 (m, 4 H), 5.73-5.63 (m, 1 H), 5.10-5.05 (m, 2 H), 3.10 (d, 2 H, J = 6.8 Hz), 2.35 (s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 190.41, 139.27, 132.77, 132.70, 129.92, 128.43, 120.00, 74.06, 43.22, 21.44. HRMS (EI+): m/z calcd for C12H13Br81O [M+]: 254.0129; found: 254.0130; m/z calcd for C12H13Br79O [M+]: 252.0150; found: 252.0162.
2-Bromo-2-(4-fluorophenyl)pent-4-enal 1H NMR (400 MHz, CDCl3): δ = 9.46 (s, 1 H), 7.44-7.06 (m, 4 H), 5.72-5.63 (m, 1 H), 5.13-5.06 (m, 2 H), 3.10 (d, 2 H, J = 3.2 Hz). 13C NMR (100 MHz, CDCl3): δ = 190.23, 163.14 (1 J CF = 248.2 Hz), 132.36, 131.66, 130.58 (3 J CF = 8.4 Hz), 120.53, 116.26 (2 J CF = 21.7 Hz), 73.11, 43.47. HRMS (EI+): m/z calcd for C11H10OFBr81 [M+]: 257.9879; found: 257.9884; m/z calcd for C11H10OFBr79 [M+]: 255.9899; found: 255.9874.
2-Bromo-2-(4-chlorophenyl)pent-4-enal 1H NMR (400 MHz, CDCl3): δ = 9.45 (s, 1 H), 7.37 (m, 4 H), 5.71-5.61 (m, 1 H), 5.13-5.06 (m, 2 H), 3.09 (d, 2 H, J = 3.2 Hz). 13C NMR (100 MHz, CDCl3): δ = 190.06, 135.48, 134.33, 132.21, 130.06, 129.44, 120.66, 73.06, 43.36. HRMS (EI+): m/z calcd for C11H10OCl35Br79 [M+]: 271.9604; found: 271.9599; m/z calcd for C11H10OCl35Br81 [M+]: 273.9583; found: 273.9567.

2-Chloro-2-(4-chlorophenyl)pent-4-enal 1H NMR (400 MHz, CDCl3): δ = 9.39 (s, 1 H), 7.38 (m, 4 H), 5.70-5.61 (m, 1 H), 5.13-5.08 (m, 2 H), 3.01 (d, 2 H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3): δ = 191.92, 135.47, 134.09, 131.26, 129.48, 129.08, 120.83, 77.18, 42.91. HRMS (EI+): m/z calcd for C11H10Cl35 2O [M+]: 228.0109; found: 228.0112; m/z calcd for C11H10Cl35Cl37O [M+]: 230.0079; found: 230.0141.
2-Allyl-2-bromooctanal
1H NMR (400 MHz, CDCl3): δ = 9.39 (s, 1 H), 5.84-5.74 (m, 1 H), 5.21-5.17 (m, 2 H), 2.77 (t, 2 H, J = 6.8 Hz), 2.01-1.91 (m, 2 H), 1.29-1.45 (m, 8 H), 0.88 (t, 3 H). 13C NMR (100 MHz, CDCl3): δ = 193.98, 132.50, 120.29, 73.39, 40.77, 36.72, 31.92, 29.68, 25.61, 22.96, 14.48. HRMS (EI+): m/z calcd for C11H19Br81O [M+]: 248.0599; found: 248.0952; m/z calcd for C11H19Br79O [M+]: 246.0619; found: 246.0674.
3-Bromo-3-phenylhex-5-en-2-one
1H NMR (400 MHz, CDCl3): δ = 7.42-7.29 (m, 5 H), 5.64-5.54 (m, 1 H), 5.03-4.97 (m, 2 H), 3.15-3.01 (m, 2 H), 2.21 (s, 3 H). 13C NMR (100 MHz, CDCl3): d = 201.48, 137.83, 133.26, 129.08, 128.72, 127.77, 119.61, 75.79, 46.10, 26.32. HRMS (EI+): m/z calcd for C12H13Br79O [M+]: 252.0150; found: 252.0138; m/z calcd for C12H13Br81O [M+]: 254.0129; found: 254.0159.