1 Introduction
It has been known for some time that ethoxyacetylene and its derivatives decompose to ketenes and ethylene upon heating [1–3]. Some time ago, we studied the reaction of ethoxyacetylene with electrophilic terminal phosphinidene complexes 2 as generated from the appropriate 7-phosphanorbornadiene complexes 1 [4]. To our surprise, we did not get the expected 3-membered phosphirene complexes 3, but instead the C-unsubstituted phosphirane complexes 4 resulting from the trapping of ethylene as produced by the thermal decomposition of ethoxyacetylene (Eq. (1)).
It was clear that parent ketene was a by-product of this reaction, but could not be trapped by the electrophilic terminal phosphinidene complexes.
It is also known for some time that electrophilic terminal phosphinidene complexes react with electron-rich nitriles to give 1,3-dipolar species 5, in which the phosphorus centre is nucleophilic (Eq. (2)) [5].
Thus it was tempting to add an electron-rich nitrile to the reaction depicted in Eq. (1). We expected that 5 would not react any more with ethylene but, instead, would selectively trap parent ketene. The experiments were carried out with N-cyano-P,P,P-triphenylphosphazene 6 [6]. The results are shown in Eq. (3) and confirm our expectations.
The formula of 7a was established by X-ray crystal structure analysis (Fig. 1). The compound incorporates a 1,3,4-oxazaphosphole ring that, to our knowledge, has never been described in the literature until now [7]. The structural parameters deserve no special comments. The 13C NMR spectrum suggests that the very contracted exocyclic C=C double bond (C=C 1.303(4) Å) is highly polarized δ (=CH2) 161.5, δ (C=H2) 96.5.
ORTEP drawing of one molecule of 7a. Selected bond lengths (6 Å) and angles (deg): W(2)–P(3) 2.4890(8), P(3)–C(4) 1.833(3), C(4)–C(5) 1.303(4), C(4)–O(2) 1.394(4), O(2)–C(6) 1.391(4), C(6)–W(4) 1.340(4), W(4)–P(4) 1.603(3), C(6)–W(3) 1.298(4), W(3)–P(3) 1.683(3) ; W(3)–P(3)–C(4) 91.9(1), P(3)–C(4)–O(2) 106.8(2), C(4)–O(2)–C(6) 111.1(2), O(2)–C(6)–W(3) 119.1(3), C(6)–W(3)–P(3) 111.0(2).
2 Experimental section
NMR spectra were recorded on a multinuclear Bruker AVANCE 300 MHz spectrometer operating at 300.13 MHz for 1H, 75.47 for 13C and 121.50 MHz for 31P. Chemical shifts are expressed in parts per million (ppm) downfield from internal tetramethylsilane (1H and 13C) and external 85% aqueous H3PO4 (31P).
2.1 General procedure for the synthesis of 1,3,4-oxazaphospholes 7a,b
To a solution of 7-phosphanorbornadiene complexes (1.2 mmol each), dissolved in 1,2-dichlorobenzene (4 ml), N-cyano-P,P,P-triphenyl-phospha-λ5-azene (2.4 mmol) and ethoxyacetylene (40% in n-hexane, 4 ml) was added and the solutions heated at 120 °C for 45 min with slow stirring. All volatile components were removed in vacuo (ca. 0.01 mbar) and the products separated by low-temperature column chromatography (SiO2, –10 °C, 10 × 2 cm, n-pentane/diethyl ether 2:1). Evaporation of the third fractions yielded complexes 7a,b as brown oils.
2.1.1 Complex 7a
Yield: 800 mg, (85.9%); 1H NMR (CDCl3) δ 4.68 (dd, 2JHH = 2.4 Hz, 3JPH = 6.1 Hz, 1H, =CH2), 5.02 (dd, 2JHH = 2.7 Hz, 3JPH = 22.8 Hz, 1H, =CH2), 7.00 (mc, 5H, Ph), 7.25 (mc, 10H, Ph), 7.58 (mc, 5H, Ph); 13C{1H} NMR (CDCl3) δ 96.5 (d, 2JPC = 18.5 Hz, =CH2), 126.3 (d, 1JPC = 102.6 Hz, ipso-Ph3P), 127.8 (d, 4JPC = 3.2 Hz, para-Ph), 128.1 (d, 3JPC = 12.1 Hz, meta-Ph), 128.2 (d, 2JPC = 9.8 Hz, ortho-Ph), 128.8 (d, 1JPC = 61.4 Hz, ipso-Ph), 128.7 (d, 3JPC = 12.6 Hz, meta-Ph3P), 132.7 (d, 4JPC = 2.7 Hz, para-Ph3P), 133.2 (d, 2JPC = 10.1 Hz, ortho-Ph3P), 161.5 (dd, 1JPC = 20.1 Hz, 4JPC = 3.0 Hz, PCO), 164.6 (dd, 2JPC = 12.4 Hz, 2JPC = 6.0 Hz, PNC), 196.2 (d, 2JPC = 8.0 Hz, 1JWC = 126.0 Hz, cis-CO), 200.0 (d, 2JPC = 23.7 Hz, trans-CO); 31P{1H} NMR (CDCl3) δ 24.6 (s, PPh3), 69.0 (s, 1JWP = 266.6 Hz); UV/vis (CH3CN) λ (log ε) = 228 (4.85), 250 (4.51), 262 (4.33), 270 (4.18), 282 (3.88); IR (KBr) 2047 (s, CO), 2011 (s, CO), 1947 (vs, CO), 1921 (vs, CO) cm–1; MS (pos.-DCI(NH3) ; 35Cl, 184W): m/z (%) = 776 (60) [M]+ ; MS (neg.-DCI(NH3), 35Cl, 184W); m/z (%) = 776 (18) [M]–•, 748 (32) [M–CO]–•, 324 (100) [W(CO)5]–•.
2.1.2 Complex 7b
240 mg, (50.0%); 1H NMR (CDCl3) δ 1.16 (t, 3JHH = 7.1 Hz, 3H, CO2CH2CH3), 1.74 (mc, 2H, CH2CH2CO2CH2CH3), 2.14 (mc, 2H, CH2CH2CO2CH2CH3), 3.99 (q, 3JHH = 7.1 Hz, 2H, CO2CH2CH3), 4.63 (dd, 2JHH = 2.7 Hz, 3JPH = 6.4 Hz, 1H, =CH2), 5.22 (dd, 2JHH = 2.7 Hz, 3JPH = 22.3 Hz, 1H, =CH2), 7.40 (mc, 5H, Ph), 7.51 (mc, 3H, Ph), 7.73 (mc, 7H, Ph); 13C{1H} NMR (CDCl3) δ 15.2 (s, CH2CH3), 29.1 (d, 2JPC = 3.7 Hz, CH2CH2CO2CH2CH3), 37.2 (d, 1JPC = 18.1 Hz, –CH2CH2CO2CH2CH3), 61.8 (s, OCH2CH3), 97.5 (d, 2JPC = 17.1 Hz, =CH2), 127.3 (d, 1JPC = 102.7 Hz, ipso-Ph), 129.7 (d, 3JPC = 12.7 Hz, meta-Ph), 133.8 (d, 4JPC = 2.8 Hz, para-Ph), 134.2 (d, 2JPC = 10.1 Hz, ortho-Ph), 160.4 (dd, 1JPC = 19.2 Hz, 4JPC = 2.9 Hz, PCO), 165.1 (dd, 2JPC = 12.4 Hz, 2JPC = 5.8 Hz, PNC), 173.1 (d, 3JPC = 13.7 Hz, CO2CH2CH3), 197.1 (d, 2JPC = 8.0 Hz, 1JWC = 125.6 Hz, cis-CO), 200.9 (d, 2JPC = 23.5 Hz, trans-CO); 31P{1H} NMR (CDCl3) δ 23.6 (s, PPh3), 72.0 (s, 1JWP = 264.0 Hz, 2JPH = 20.9 Hz); MS (EI, 70 eV, 184W); m/z (%) = 774 (5) [M–CO]+•, 746 (17) [M–2CO]+•, 261 (100) [PPh3–H]+.
3 Supplementary Material
The supplementary material has been sent to the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK/Fachinformationzentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany, as supplementary material CCDC 212951 and can be obtained by contacting the CCDC/FIZ (quoting the article details and the corresponding SUP number).