Entry to γ-Amino-α-bromo-alkenylphosphonates by a Bromination-­Elimination Sequence

Abstract

γ-Amino-α-bromo-1-alkenylphosphonates were prepared in a convenient way upon bromination of γ-amino-α,β-unsaturated phosphonates followed by elimination of the β-bromine by basic treatment.

Key words

alkenes - amines - bromine - eliminations - phosphorus

References

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9

The description of a typical experiment is given below:
The oxalic salt of the corresponding γ-amino-α,β-unsaturated phosphonates was dissolved in dry CH₂Cl₂. One equivalent of pyridinium hydrobromide perbromide (PHP) was added. After 10 min stirring at r.t., a second equivalent of PHP was added. After 10 min stirring at r.t., the mixture was brought to reflux. The reaction was monitored by ¹H NMR and ³¹P NMR. After completion of the bromination (see Table [1] ), the mixture was cooled to r.t. and 4 equiv of Et₃N in dry CH₂Cl₂ were added drop by drop. Further stirring for 20 min at r.t. was followed by gentle reflux during 1.5 h. The mixture was cooled, the precipitated salts were filtered off and the filtrate was washed with a mixture of CH₂Cl₂-Et₂O (1:1, v/v). The filtration was repeated until the salts stopped precipitating. The residual red-brown oil was then purified on a silica gel column with ethyl acetate and a gradient (10-50%) of EtOH. The analytical data for 7 with R = benzyl: ¹H NMR (300 MHz, CDCl₃): δ = 1.35 (6 H, t, J = 6.16 Hz, 2 × OCH₂CH₃), 4.03 (2 H, d × d, J _H-P = 6.88 Hz, J = 2.48 Hz, CH₂CH=C_q), 4.07 (2 H, s, CH₂C_ar), 4.15 (4 H, m, 2 × OCH₂CH₃), 7.26 (1 H, d × t, J _H-P = 39.35 Hz, J = 7.02 Hz, CH=C_q), 7.36 (3 H, m, ArH), 7.55 (2 H, m, ArH). ¹³C NMR (75 MHz, CDCl₃): δ = 16.26 (J _C-P = 5.77 Hz, 2 × OCH₂CH₃), 46.03 (J _C-P = 3.46 Hz, CH₂CH=C_q), 51.34 (CH₂C_ar), 64.14 (J _C-P = 4.62 Hz, 2 × OCH₂CH₃), 115.89 (J _C-P = 193.84 Hz, CH=C_q), 129.16 (C_ar), 129.35 (C_ar), 129.57 (C_ar), 130.00 (C_ar), 130.19 (C_ar), 131.56 (C_{q ar}), 144.14 (J _C-P = 13.85 Hz, CH=C_q). ³¹P NMR (109 MHz, CDCl₃): δ = 6.94. IR (NaCl): 3412 (ν_N-H), 1239 (ν_P=O), 1608 (ν_C=C), 978 (ν_CH=CBr) cm^-1. MS (positive mode): m/z = 362/364 (100) [M⁺]. Anal. Calcd for C₁₄H₂₁BrNO₃P: C, 46.42; H, 5.84; N, 3.87. Found: C, 46.38; H, 5.87; N, 3.81.