Synlett 2014; 25(11): 1529-1533
DOI: 10.1055/s-0033-1341071
cluster
© Georg Thieme Verlag Stuttgart · New York

Formation of Allenes by 1,4-Addition of Intermolecular Phosphane/Borane Frustrated Lewis Pairs to a Conjugated Enyne

Philipp Feldhaus
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany   Fax: +49(251)8336503   Email: erker@uni-muenster.de
,
Birgit Wibbeling
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany   Fax: +49(251)8336503   Email: erker@uni-muenster.de
,
Roland Fröhlich
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany   Fax: +49(251)8336503   Email: erker@uni-muenster.de
,
Constantin G. Daniliuc
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany   Fax: +49(251)8336503   Email: erker@uni-muenster.de
,
Gerald Kehr
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany   Fax: +49(251)8336503   Email: erker@uni-muenster.de
,
Gerhard Erker*
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany   Fax: +49(251)8336503   Email: erker@uni-muenster.de
› Author Affiliations
Further Information

Publication History

Received: 04 February 2014

Accepted: 04 March 2014

Publication Date:
11 April 2014 (online)


Abstract

The t-Bu3P/B(C6F5)3 frustrated Lewis pair (6a) undergoes competing acetylene deprotonation (to give the phosphonium–alkenylborate salt 8) and 1,4-P/B FLP addition to the conjugated enyne 2-methylbutenyne to yield the zwitterionic allene derivative 9a. The less basic (o-tolyl)3P/B(C6F5)3 system (6b) avoids the acetylene deprotonation pathway. The zwitterionic allene derivative 9b formed by 1,4-P/B FLP addition to the enyne is again a prominent reaction product; here competing 1,2-addition is observed to give the olefinic product 10. The allene derivatives 9a and 9b and their competing products 8 and 10 were characterized by X-ray diffraction.

Supporting Information

 
  • References and Notes

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  • 16 Compounds 8 and 9a Tri-tert-butylphosphane (102 mg, 0.50 mmol) in CH2Cl2 (3 mL) was added to a solution of tris(pentafluorophenyl)-borane (256 mg, 0.50 mmol) in CH2Cl2 (10 mL). After addition of 2-methyl-1,3-butenyne (33 mg, 47.5 μL, 0.50 mmol) the reaction mixture was stirred for 24 h at r.t. Subsequently, all volatiles were removed under reduced pressure and the obtained residue was dried in vacuo to give a ca. 1:1 mixture of compounds 8 and 9a (302 mg). The both products were separated by column chromatography (silica gel; eluent: n-pentane–CH2Cl2, 3:5). Drying of the respective fraction in vacuo gave compound 8 (131.9 mg, 33%) and compound 9a (81.6 mg, 21%). Crystals suitable for the X-ray crystal structure analysis for both compound 8 and compound 9a were obtained from a solution of the respective compound in CH2Cl2 layered by n-pentane at –40 °C. Analytical Data of Compound 8 Anal. Calcd for C35H33BF15P: C, 53.87; H, 4.26. Found: C, 53.04; H, 4.22. Mp 141 °C (DSC). 1H NMR (600 MHz, 299 K, CD2Cl2): δ = 5.05 (d, 1 J PH = 428.6 Hz, 1 H, PH), 4.97 (dq, 2 J PH = 2.7 Hz, 4 J HH = 1.0 Hz, 1 H, =CH2 Z), 4.92 (dq, 2 J PH = 2.7 Hz, 4 J HH = 1.5 Hz, 1 H, =CH2 E), 1.82 (dd, 4 J HH = 1.5 Hz, 4 J HH = 1.0 Hz, 3 H, CH3), 1.63 (d, 3 J PH = 15.9 Hz, 27 H, t-Bu). 13C{1H} NMR (151 MHz, 299 K, CD2Cl2): δ = 148.6 (dm, 1 J FC = ca. 240 Hz, C6F5), 138.5 (dm, 1 J FC = ca. 250 Hz, C6F5), 136.9 (dm, 1 J FC = ca. 240 Hz, C6F5), 130.8 (=C), 124.8 (br, ipso-C6F5), 116.8 (=CH2), 95.4 (br, ≡C), 38.1 (d, 1 J PC = 26.8 Hz, t-Bu), 30.4 (t-Bu), 24.4 (CH3); resonance for ≡CB was not observed. 31P NMR (243 MHz, 299 K, CD2Cl2): δ = 60.4 (dm, 1 J PH = 428.6 Hz). 11B{1H} NMR (192 MHz, 299 K, CD2Cl2): δ = –21.0 (ν1/2 = ca. 25 Hz). 19F NMR (564 MHz, 299 K, CD2Cl2): δ = –132.6 (m, 2 F, o-C6F5), –164.0 (t, 3 J FF = 20.3 Hz, 1 F, p-C6F5), –167.4 (m, 2 F, m-C6F5), [Δδ19F m,p  = 3.4]. Analytical Data of Compound 9a HRMS: m/z calcd for C35H33BF15PNa+: 803.2072; found: 803.2031. Decomposition: 247 °C (DSC). 1H NMR (600 MHz, 298 K, CD2Cl2): δ = 5.94 (br m, 1 H, =CH), 3.13 (ddd, 2 J PH = 16.2 Hz, 2 J HH = 14.8 Hz, 5 J HH = 3.1 Hz, 1 H, CH2), 2.26 (ddd, 2 J PH = 15.5 Hz, 2 J HH = 14.8 Hz, 5 J HH = 1.3 Hz, 1 H, CH2), 1.66 (br d, 4 J PH = 3.1 Hz, 3 H, CH3), 1.57 (d, 3 J PH = 13.9 Hz, 27 H, t-Bu). 13C{1H} NMR (151 MHz, 298 K, CD2Cl2): δ = 207.1 (d, 3 J PC = 6.7 Hz, =C=), 148.4 (dm, 1 J FC = ca. 238 Hz, C6F5), 138.4 (dm, 1 J FC = ca. 231 Hz, C6F5), 136.8 (dm, 1 J FC = ca. 233 Hz, C6F5), 125.1 (br, i-C6F5), 98.5 [br q (1:1:1:1), 1 J BC = ca. 50 Hz, =CH], 80.0 (br m, =C), 39.7 (d, 1 J PC = 27.4 Hz, t-Bu), 30.4 (t-Bu), 23.5 (d, 1 J PC = 31.4 Hz, CH2), 23.1 (d, 3 J PC = 3.2 Hz, CH3). 31P{1H} NMR (243 MHz, 298 K, CD2Cl2): δ = 49.2 (ν1/2 = ca. 10 Hz). 11B{1H} NMR (192 MHz, 298 K, CD2Cl2): δ = –15.6 (ν1/2 = ca. 15 Hz). 19F NMR (564 MHz, 298 K, CD2Cl2): δ = –132.2 (m, 2 F o-C6F5), –163.6 (t, 3 J FF = 20.4 Hz, 1 F, p-C6F5), –167.4 (m, 2 F, m-C6F5), [Δδ19F m,p  = 3.8].
  • 17 Compounds 10 and 9b A solution of tri-ortho-tolylphosphane (152 mg, 0.50 mmol) in CD2Cl2 (4 mL) was added to a solution of tris(pentafluoro-phenyl)borane (256 mg, 0.50 mmol) in CD2Cl2 (4 mL) at r.t. Then the reaction mixture was added to 2-methyl-1,3-butenyne (33 mg, 47.5 μL, 0.50 mmol), and the resulting mixture was stirred for 10 min. After one day, an aliquot of the reaction mixture was investigated by NMR experiments at low temperature. A mixture of compound 10 (1,2-addition), compound 9b (1,4-addition) and tri-ortho-tolylphosphane. [10/9b/phosphane = ca. 28:37:35 (1H NMR at 248 K)] was characterized. Then the volume of the reaction mixture was reduced to one half followed by separation of the compounds by column chromatography (silica gel; eluent CH2Cl2n-pentane = 2:3). The first fraction contained compound 10 admixed with tri-ortho-tolylphosphane, which subsequently was purified by crystallization from a solution of compound 10 in CH2Cl2 layered by n-pentane to give compound 10 (89 mg, 20%). The second fraction contained compound 9b (148.9 mg, 33%). Crystals suitable for the X-ray crystal structure analysis for both compounds 10 and 9b, respectively, were obtained from a solution of the respective compound in CH2Cl2 layered with n-pentane at –40 °C. Analytical Data of Compound 10 Anal. Calcd for C44H27BF15P·CH2Cl2: C, 55.87; H, 3.02. Found: C, 55.50; H, 2.55. Mp 98 °C (DSC). 1H NMR (500 MHz, 233 K, CD2Cl2): δ = 8.27 (br d, 3 J PH = 37.0 Hz, 1 H, =CH), 7.86 (o), 7.63 (p), 7.51 (m), 7.33 (m′) (4 × m, 4 × 1 H, o-tola), 7.65 (p), 7.60 (o), 7.43 (m), 7.36 (m′) (4 × m, 4 × 1 H, o-tolb), 7.65 (p), 7.52 (m′), 7.27 (m), 7.27 (o) (4 × m, 4 × 1 H, o-tolc), 4.97 (s, 1 H, =CH2 Z), 4.58 (s, 1 H, =CH2 E), 2.63 (s, 3 H, CH3 of o-tolc), 1.63 (s, 6 H, CH3 of o-tola,b), 0.88 (s, 3 H, CH3). 13C{1H} NMR (126 MHz, 233 K, CD2Cl2): δ = 181.2 (br, =CH), 147.7 (dm, 1 J FC = ca. 242 Hz, C6F5), 145.1 (d, 2 J PC = 8.0 Hz, ortho′), 134.8 (d, 2 J PC = 11.4 Hz, ortho), 134.7 (d, 4 J PC = 2.1 Hz, para), 133.3 (d, 3 J PC = 10.2 Hz, meta′), 126.8 (d, 3 J PC = 12.2 Hz, meta), 116.0 (d, 1 J PC = 80.7 Hz, ipso) (of o-tolc), 144.1 (d, 2 J PC = 7.7 Hz, ortho′), 134.5 (d, 2 J PC = 12.0 Hz, ortho), 134.2 (d, 4 J PC = 2.6 Hz, para), 133.4 (d, 3 J PC = 11.4 Hz, meta′), 126.9 (d, 3 J PC = 12.8 Hz, meta), 118.1 (d, 1 J PC = 83.2 Hz, ipso) (of o-tolo), 143.3 (d, 2 J PC = 6.8 Hz, ortho′), 136.2 (d, 2 J PC = 13.5 Hz, ortho), 134.1 (d, 4 J PC = 2.4 Hz, para), 132.9 (d, 3 J PC = 10.3 Hz, meta′), 126.9 (d, 3 J PC = 12.8 Hz, meta), 120.7 (d, 1 J PC = 80.6 Hz, ipso) (of o-tola), 139.3 (d, 2 J PC = 14.6 Hz, =C), 138.1 (dm, 1 J FC = ca. 240 Hz, C6F5), 136.3 (dm, 1 J FC = ca. 240 Hz, C6F5), 124.0 (br, i-C6F5), 124.8 (d, 1 J PC = 64.7 Hz, =CP), 122.6 (d, 3 J PC = 10.9 Hz, =CH2), 23.1 (br, CH3 of o-tolc), 22.9 (CH3), 22.7 (d, 3 J PC = 1.8 Hz, CH3 of o-tolb), 22.5 (d, 3 J PC = 3.7 Hz, CH3 of o-tola). 31P{1H} NMR (202 MHz, 233 K, CD2Cl2): δ = 23.4 (ν1/2 = ca. 60 Hz). 11B{1H} NMR (160 MHz, 233 K, CD2Cl2): δ = –16.2 (ν1/2 = ca. 60 Hz). 19F NMR (282 MHz, 295 K, CD2Cl2): δ = –131.6 (m, 2 F, o-C6F5), –162.2 (m, 1 F, p-C6F5), –166.6 (m, 2 F, m-C6F5), [Δδ19F m,p  = 4.4]. Analytical Data of Compound 9b HRMS: m/z calcd for C44H27BF15PNa+: 905.1604; found: 905.1602. Mp 110 °C (DSC). 1H NMR (500 MHz, 298 K, CD2Cl2): δ = 7.67 (p), 7.65 (o), 7.45 (m′), 7.37 (m) (4 × m, 4 × 3 H, o-tol), 5.55 (br m, 1 H, =CH), 4.12, 3.24 (2 × m, 2 × 1 H, CH2), 2.15 (s, 9 H, CH3 of o-tol), 1.27 (m, 3 H, CH3). 13C{1H} NMR (126 MHz, 298 K, CD2Cl2): δ = 206.7 (d, 3 J PC = 9.2 Hz, =C=), 148.4 (dm, 1 J FC = ca. 240 Hz, C6F5), 143.7 (d, 2 J PC = 8.6 Hz, o′ of o-tol), 138.4 (dm, 1 J FC = ca. 245 Hz, C6F5), 136.8 (dm, 1 J FC = ca. 250 Hz, C6F5), 135.9 (d, 2 J PC = 11.7 Hz, o of o-tol), 135.3 (d, 4 J PC = 2.9 Hz, p of o-tol), 133.9 (d, 3 J PC = 11.0 Hz, m′ of o-tol), 127.6 (d, 3 J PC = 12.7 Hz, m of o-tol), 125.1 (br, i-C6F5), 117.5 (d, 1 J PC = 80.6 Hz, i of o-tol), 99.2 [br q (1:1:1:1), 1 J BC = ca. 51 Hz, =CH], 80.7 (br m, =C), 31.3 (d, 1 J PC = 47.8 Hz, CH2), 23.1 (d, 3 J PC = 3.8 Hz, CH3 of o-tol), 20.4 (d, 3 J PC = 4.6 Hz, CH3). 31P{1H} NMR (202 MHz, 298 K, CD2Cl2): δ = 25.4 (ν1/2 = ca. 10 Hz). 11B{1H} NMR (160 MHz, 298 K, CD2Cl2): δ = –15.7 (ν1/2 = ca. 20 Hz). 19F NMR (470 MHz, 298 K, CD2Cl2): δ = –132.2 (m, 2 F o-C6F5), –163.6 (t, 3 J FF = 20.3 Hz, 1 F, p-C6F5), –167.4 (m, 2 F, m-C6F5), [Δδ19F m,p  = 3.7].