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C-N Bond Insertion of a Complexed Phosphinidene Into a Bis(Imine)

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C-N Bond Insertion of a Complexed Phosphinidene Into a Bis(Imine) FULL PAPER C؊N Bond Insertion of a Complexed Phosphinidene into a Bis(imine) ؊ A Novel 2,3-Sigmatropic Shift Mark J. M. Vlaar,[a] Pieter Valkier,[a] Marius Schakel,[a] Andreas W. Ehlers,[a] Martin Lutz,[b] Anthony L. Spek,[b] and Koop Lammertsma*[a] Keywords: Complexed phosphinidene / Cycloadditions / Density functional calculations / Heterocycles / P,N-ylides The electrophilic phosphinidene complex PhPW(CO)5 reacts energetically much more demanding. Ring-closure to an aza- with bis(imine) 5, possessing a single carbon spacer, to give phosphirane is also less favorable than the C−N insertion, the C−N insertion product 6. B3LYP/6-31G* calculations in- which is in agreement with the absence of such a product in dicate that this process occurs by way of a 2,3-sigmatropic the experiment. shift from the initially formed phosphane ylide. The alternat- ive, 1,3-dipolar cycloaddition, followed by ring-opening of ( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, the bicyclic structure, can be ruled out since both steps are 2002) Introduction Phosphane ylides are emerging as viable low-valent or- ganophosphorus synthons. These highly reactive species are already commonly employed in the phospha Wittig reac- tion,[1] while their transition metal complexes are useful re- agents for the synthesis of three- to eight-membered hetero- cycles.[2] These ylides are readily available. For example, the frequently used complexed nitrilium phosphane ylides I can be generated thermally[3] and photochemically[4] from 2H- azaphosphirene complexes. Reminiscently of regular yl- ides,[5] however, most transient P,N-ylides (I,[3,4,6] II,[7] and III[8]), P,O-ylides (IV[9]), P,S-ylides (V[10]), and P,P-ylides (VI[11]) are formed from direct interaction (or exchange) be- [12] tween complexed phosphinidenes RPW(CO)5, generated extends recent studies on the formation of azaphosphiridi- in situ, and the lone pair of the heteroatom-containing sub- nes[7,9a] and the 1,3-dipolar cycloadditions of P,N-ylides.[6,7] strate. Of the many types of reactions that these phosphane yl- ides can undergo, the 2,3-sigmatropic shift[13,5] has not yet been observed except for the somewhat related conversions of P,N-ylide IIIa into the formal CϪH insertion product 1 and of P,O-ylide IVa into the bicyclic compound 2. The initially formed ylides were obtained by treatment of [8] RPW(CO)5 with azobenzene (PhϪNϭNϪPh) and benzo- phenone [PhϪC(O)ϪPh],[9a] respectively (Scheme 1). In this study, we provide evidence for the first 2,3-sigmatropic shift of a phosphane ylide formed from a complexed phosphinid- ene and a bis(imine) with a single carbon spacer. This work [a] Faculty of Sciences, Department of Chemistry, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands [b] Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The Netherlands Scheme 1 Eur. J. Org. Chem. 2002, 1797Ϫ1802 WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ193X/02/0611Ϫ1797 $ 20.00ϩ.50/0 1797 FULL PAPER K. Lammertsma et al. Results and Discussion While exploring reactions between the phosphinidene complex PhPW(CO)5 (4) Ϫ generated in situ from complex 3 at 110 °C[12,14] Ϫ and a broad range of imines, we were surprised to see this carbene-like synthon insert into a CϪN single bond of bis(imine) 5[15] to give 6 in 58% isolated yield. No 1,2-addition product could be detected in the re- action mixture. Figure 1. Displacement ellipsoid plot (50% probability level) of 6 in the crystal; only the major isomer was crystallized; the first of two crystallographically independent molecules is shown, with hy- drogen atoms omitted for clarity; selected bond lengths [A˚ ], angles and torsion angles [°]; the values for the second molecule are in square brackets: C1ϪN1 1.268(3) [1.265(3)], N1ϪP1 1.7148(19) [1.712(2)], P1ϪC2 1.889(2) [1.892(2)], C2ϪN2 1.460(3) [1.449(3)], N2ϪC3 1.268(3) [1.266(3)], P1ϪW1 2.5104(6) [2.5070(6)], P1ϪC10 1.819(2) [1.826(2)], C1ϪC4 1.471(3) [1.470(3)], C2ϪC16 1.507(3) Compound 6 was obtained after column chromatography [1.510(3)], C3ϪC22 1.475(3) [1.468(3)]; N1ϪC1ϪC4 122.9(2) [122.3(2)], C1ϪN1ϪP1 121.30(17) [122.89(17)], N1ϪP1ϪC2 as a 4:1 diastereomeric mixture as determined by integra- Ϫ Ϫ 31 [16] 99.81(9) [97.19(10)], P1 C2 N2 104.63(14) [107.56(15)], tion of its P NMR resonances at δ ϭ 81.8 and 81.2. C2ϪN2ϪC3 117.43(19) [117.1(2)], N2ϪC3ϪC22 122.2(2) The major isomer was obtained after fractional crystalliza- [123.3(2)], N1ϪP1ϪC10 99.18(10) [99.77(10)], N1ϪP1ϪW1 ϭ 117.93(7) [119.46(7)]; P1ϪN1ϪC1ϪC4 179.15(16) [177.41(17)], tion. The presence of its two N CH groups was evident C1ϪN1ϪP1ϪC2 Ϫ98.19(19) [Ϫ100.4(2)], N1ϪP1ϪC2ϪN2 from the 13C NMR resonances at δ ϭ 163.4 and 174.7 and Ϫ167.85(14) [Ϫ166.94(15)], P1ϪC2ϪN2ϪC3 129.65(18) [102.2(2)], 1 ϭ C2ϪN2ϪC3ϪC22 Ϫ175.4(2) [179.4(2)], N1ϪC1ϪC4ϪC5 16.2(3) the lowfield H NMR resonances at δ 7.99 and 8.72, the Ϫ Ϫ Ϫ Ϫ Ϫ 3 [ 3.2(4)], N2 C3 C22 C23 14.4(4) [ 11.3(4)] latter of which showed a typical JP,H coupling of 28.1 Hz. Its structure was established unequivocally by a single-crys- tal X-ray structure determination. The major isomer of 6 What are the alternatives? Ylides can be subjected to 1,3- crystallized as two crystallographically independent molec- dipolar cycloadditions. For example, we showed recently ules, of which one is depicted in Figure 1.[16b] This molecule that the transient P,N-ylide complexes 8, in which the two has a P1ϪN1 bond 1.7148(19) A˚ long and two CϭN bonds imine groups are separated by two to four methylene units, of 1.268(3) A˚ for both C1ϪN1 and N2ϪC3, which are con- undergo such a transformation to give 9 diastereoselectiv- [7a] jugated with the phenyl rings as is evident from the rela- ely. In the case of 10 this would result in a rather strained tively short C1ϪC4 and C3ϪC22 bonds of 1.471(3) A˚ and compound (11), which might ring-open to give product 6 ϩ 1.475(3) A˚ , respectively. The phenyl rings are not coplanar [Equation (1)]. In fact, such a retro [2 2] cycloaddition of with the CϭN bonds, with torsion angles of 16.2(3), and a 1,3-diazacyclobutane to yield two imines has been re- ° [23] 14.4(4)°. ported to proceed at temperatures below 35 C. Another The formation of 6 is exceptional. So far, insertion of possible route to 6 is a 2,3-sigmatropic shift of P,N-ylide 10 [Equation (2)], but these ylides are better known to give RPW(CO)5 into a CϪN bond has been observed only for the reaction with a highly strained aziridine.[17] Insertions azaphosphiridines instead, which in this case would result into CϪH,[18] CϪC,[19] and CϪP[20] bonds are equally rare, in 12, or to give extended structures by incorporation of another imine.[7,9] whereas they are well established for acidic NϪHorOϪH bonds.[21] However, ab initio MO theory showed that singlet phosphinidene 1PH should give an addition complex with [22] NH3 prior to NϪH bond insertion. Hence, formation of 6 by direct insertion of PhPW(CO)5 into a CϪN bond seemed unlikely. (1) (2) 1798 Eur. J. Org. Chem. 2002, 1797Ϫ1802 CϪN Bond Insertion of a Complexed Phosphinidene into a Bis(imine) FULL PAPER The theoretical calculations indeed support the view that the first step in the reaction giving rise to the CϪN inser- tion product is the presumed formation of P,N-ylide com- plex 10. Namely, formation of 13 by the addition of singlet 1PH to the nitrogen lone pair of one of the CϭN groups of Ϫ A priori, no differentiation between the two reaction the parent bis(imine) is exothermic by 66.3 kcal·mol 1. This pathways can be made. We therefore resorted to B3LYP/6- ylide prefers the gauche conformation (N1ϪC2ϪN2ϪC3 ϭ 31G* calculations to explore the potential energy surface of 5.2°)overtheanti form (N1ϪC2ϪN2ϪC3 ϭ 132.2°), albeit Ϫ1 a simplified parent system in which the W(CO)5 group was by only 0.2 kcal·mol . Its ylide character is evident from absent and all the phenyl groups were replaced by hydrogen the 1.315 A˚ CϭN iminium bond, which is much longer atoms.[24] Four minima were found in anti and gauche con- than that of the imine group (1.267 A˚ ). formations: P,N-ylide 13, bicyclic 14 (1 isomer), the formal Starting from this P,N-ylide 13, the transition structures insertion product 15, and azaphosphiridine 16. Their abso- 17, 18, and 19 correspond to a 1,3-dipolar cycloaddition lute and relative energies are listed in Table 1. The optim- to bicyclic 14, a 2,3-sigmatropic shift to CϪN insertion ized geometries of the energetically preferred gauche con- product 15, and ring-closure to azaphosphiridine 16,re- formers are shown in Figure 2. spectively. A fourth transition structure (20) concerns the ring-opening of the four-membered ring of 14 to give prod- uct 15. The nature of the transition structures was con- Table 1. B3LYP/6-31G* absolute (in Ϫa.u.) and relative (in (kcal·molϪ1) energies for 13؊19 firmed by intrinsic reaction coordinate calculations (IRC that connected them with the associated minima.
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