Valence Isomerization of Phosphepines - American Chemical Society

Dec 1, 2010 - Helen Jansen, J. Chris Slootweg, Andreas W. Ehlers, and Koop Lammertsma*. Department of Chemistry and Pharmaceutical Sciences, Vrije ...
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Organometallics 2010, 29, 6653–6659 DOI: 10.1021/om1004379

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Valence Isomerization of Phosphepines† Helen Jansen, J. Chris Slootweg, Andreas W. Ehlers, and Koop Lammertsma* Department of Chemistry and Pharmaceutical Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV, Amsterdam, The Netherlands Received May 6, 2010

Valence isomerization of phosphepines into phosphanorcaradienes has been studied computationally to facilitate the development of novel, metal-free phosphinidene precursors that controllably release singlet phosphinidenes upon heating. This target becomes viable by benzannulation of the seven-membered phosphepine ring and the introduction of P-amino substituents.

Introduction Valence isomerization of phosphepines, the fully unsaturated seven-membered phosphacycles A, into their bicyclic isomers, phosphanorcaradienes B, has received little attention when compared to their heteroelement analogues (O,1 S,2 NH3). Part of the reason may be the difficulty in synthesizing these phosphacycles. Only the sterically encumbered 1a was reported by M€ arkl et al.,4 but the parent phosphepine 2a and the phenyl-substituted 3a remain elusive despite evidence for the existence of the aromatic phosphatropylium ion 4a.5 Also the isomeric phosphanorcaradienes B have not been isolated to date, presumably because they undergo a [1þ2]-retroaddition to release a phosphinidene [R-P] for which the formation of an aromatic system is the thermodynamic driving force.6 Hampering the A a B valence isomerization seems to enhance the stability of the phosphepine. For instance, benzophosphepines 5a and 6a have half-life times at 80 C

of 1207 and 90 min,8a respectively, and double benzannulation (7a and 8a) stabilizes the phosphepine ring even further,8b while oxidized phosphepine 9a9 generates benzene only at temperatures of g150 C and benzannulated phosphine oxide 10a is stable at 140 C.7a

† In honor of Dietmar Seyferth. *To whom correspondence should be addressed. Fax: (þ31)20-5987488. E-mail: [email protected]. (1) (a) Jerina, D. M.; Daly, J. W. Science 1974, 185, 573–582. (b) Boyd, D. R.; Sharma, N. D. Chem. Soc. Rev. 1996, 25, 289–296. (c) Katritzky, A. R.; Rees, C. W.; Scriven, E. F., In Comprehensive Heterocyclic Chemistry II; Seven Membered and Larger Rings with Fused Derivatives, Vol. 9; Newkome, G. R., Ed.: Elsevier Science Ltd.: Oxford, 1996; pp 45-66. (d) Hashmi, A. S. K.; Rudolph, M.; Weyrauch, J. P.; W€olfle, M.; Frey, W.; Bats, J. W. Angew. Chem., Int. Ed. 2005, 44, 2798–2801. (2) Schwan, A. L. In Science of Synthesis; Weinreb, S. M., Ed.; Thieme: Stuttgart, 2004; Vol. 17, pp 717-748. (3) Meigh, J.-P. K. In Science of Synthesis; Weinreb, S. M., Ed.; Thieme: Stuttgart, 2004; Vol. 17, pp 825-927. (4) M€ arkl, G.; Burger, W. Angew. Chem., Int. Ed. Engl. 1984, 23 894–895. (5) Muedas, C. A.; Schr€ oder, D.; S€ ulzle, D.; Schwarz, H. J. Am. Chem. Soc. 1992, 114, 7582–7584. (6) (a) Wang, B.; Lake, C. H.; Lammertsma, K. Organometallics 1997, 16, 4145–4149. (b) Kassaee, M. Z.; Cheshmehkani, A.; Musavi, S. M.; Majdi, M.; Motamedi, E. J. Mol. Struct. (THEOCHEM) 2008, 865, 73–78. (7) (a) M€ arkl, G.; Burger, W. Tetrahedron Lett. 1983, 24, 2545–2548. (b) Yasuike, S.; Kiharada, T.; Kurita, J.; Tsuchiya, T. Chem. Commun. 1996, 2183–2184. (c) Yasuike, S.; Kiharada, T.; Tsuchiya, T.; Kurita, J. Chem. Pharm. Bull. 2003, 51, 1283–1288. (8) (a) Kurita, J.; Shiratori, S.; Yasuike, S.; Tsuchiya, T. J. Chem. Soc., Chem. Commun. 1991, 1227–1228. (b) Yasuike, S.; Ohta, H.; Shiratori, S.; Kurita, J.; Tsuchiya, T. J. Chem. Soc., Chem. Commun. 1993, 1817–1819.

(9) M€arkl, G.; Schubert, H. Tetrahedron Lett. 1970, 15, 1273–1276. (10) (a) Slootweg, J. C.; Lammertsma, K. In Science of Synthesis; Trost, B. M.; Mathey, F., Eds.; Georg Thieme Verlag: Stuttgart, 2009; Vol. 42, pp 15-36. (b) Mathey, F. Dalton Trans. 2007, 1861–1868. (11) (a) Carbene Chemistry; Bertrand, G., Ed.; Marcel Dekker: New York, 2002. (b) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39–92. (c) Arduengo, A. J., III. Acc. Chem. Res. 1999, 32, 913–921. (d) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (12) (a) Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science 2000, 288, 834–836. (b) Lavallo, V.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Science 2006, 312, 722–724. (c) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran, P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556–559.

r 2010 American Chemical Society

Published on Web 12/01/2010

Our interest concerns phosphepines of moderate stability that controllably release singly substituted phosphinidenes [R-P].10 Such transients, isoelectronic with the metal-free carbenes [R2C],11,12 are detectable under forcing conditions

pubs.acs.org/Organometallics

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only, such as for the parent [H-P] by MCD and UV spectroscopy,13 [H3Si-P] by its IR spectrum,14 and [Mes-P] (Mes = 2,4,6-trimethylphenyl) by ESR, UV, and IR spectroscopy.15 Theoretical calculations suggest the singlet ground state to be preferred upon amino substitution, e.g., [Me2N-P],16 but there are as of yet no suitable precursors available to verify this hypothesis. Oxidation17 and transition metal complexation18 of the phosphorus atom also stabilize the low-valent phosphinidenes and change the electronic ground state and hence their reactivity. For example, [Ph-PdO] behaves as a singlet phosphinidene,17 whereas [Ph-P] is a diradical bearing a triplet ground state.19 Of all transient electrophilic phosphinidenes, particularly the metal-complexed [Ph-PdW(CO)5] has been applied to afford a wealth of structurally different organophosphorus compounds by way of carbene-like [1þ2]-cycloadditions and insertion reactions.10,20 Experimental confirmation of the transient’s existence was established only recently.21 To date, the parent metal-complexed phosphepines, such as phenyl-substituted 11a, remain unknown, but the 3H-3benzophosphepine complexes 12a are available and do serve as precursor to generate singlet phosphinidenes by cheletropic elimination from their valence isomers 12b (MLn = W(CO)5, Mo(CO)5, Cr(CO)5, Mn(CO)2Cp).22,23 Metal-complexed (13a) and metal-free phosphepines (15a, 16a) containing a dimethylamino substituent are also promising but yet unknown precursors, except for 14a.22 It is therefore of interest to obtain a better understanding of the stability of the phosphepine moiety for designing new singlet phosphinidene precursors. This requires an investigation of the factors that influence the formation of phosphanorcaradiene B from which the phosphinidene is obtained.

In the present study, we examine the influence of benzannulation (5a-7a) on the phosphepine (A) a phosphanorcaradiene (B) valence isomerization in conjunction with oxidation (13) Harrison, J. J.; Williamson, B. E. J. Phys. Chem. A 2005, 109, 1343–1347. (14) Glatthaar, J.; Maier, G. Angew. Chem., Int. Ed. 2004, 43, 1294– 1296. (15) (a) Li, X.; Weismann, S. I.; Lin, T. -S.; Gasper, P. P.; Cowley, A. H.; Smirnov, A. I. J. Am. Chem. Soc. 1994, 116, 7899–7900. (b) Bucher, G.; Borst, M. L. G.; Ehlers, A. W.; Lammertsma, K.; Ceola, S.; Huber, M.; Grote, D.; Sander, W. Angew. Chem., Int. Ed. 2005, 44, 3289–3293. (16) At the QCISD(T)/6-311G(d,p) level of theory, see: Nguyen, M. T.; Van Keer, A.; Vanquickenborne, L. G. J. Org. Chem. 1996, 61, 7077–7084. (17) Gaspar, P. P.; Qian, H.; Beatty, A. M.; d’Avignon, D. A.; Kao, J. L.-F.; Watta, J. C.; Rath, N. P. Tetrahedron 2000, 56, 105–119, and references therein. (18) Ehlers, A. W.; Baerends, E. J.; Lammertsma, K. J. Am. Chem. Soc. 2002, 124, 2831–2838. (19) (a) Lammertsma, K.; Ehlers, A. W.; McKee, M. L. J. Am. Chem. Soc. 2003, 125, 14750–14759. (b) Galbraith, J. M.; Gaspar, P. P.; Borden, W. T. J. Am. Chem. Soc. 2002, 124, 11669–11674. (20) (a) Mathey, F.; Tran Huy, N. H.; Marinetti, A. Helv. Chim. Acta 2001, 84, 2938–2957. (b) Lammertsma, K.; Vlaar, M. J. M. Eur. J. Org. Chem. 2002, 1127–1138. (21) Jansen, H.; Samuels, M. C.; Couzijn, E. P. A.; Slootweg, J. C.; Ehlers, A. W.; Chen, P.; Lammertsma, K. Chem.;Eur. J. 2010, 16, 1454–1458.

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(9a, 10a) and transition metal complexation of the phosphorus center (11a, 12a). The results are then used to explore the generation of singlet, metal-free amino-phosphinidenes from the amino-substituted phosphepines (13a-16a).24

Computational Method Density functional theory calculations were carried out with Gaussian 0325 at the B3PW91 level of theory, which has already been used successfully in mechanistic studies of phosphorus compounds.26 In this functional, the exchange energy is described with contributions from local and nonlocal (B3 parameter Becke27 and Hartree-Fock (HF)) exchange terms, and the correlation energy is given by the Perdew and Wang 9128 nonlocal functional within the generalized gradient approximation (GGA).29 The 6-311þG(d,p) basis set for atoms C, H, N, O, and P has been used in combination with a pseudorelativistic ecp approach30 (LANL2DZ) for W. All possible conformations were calculated, but only the structures with the lowest energies are displayed. The nature of each stationary point was confirmed by a frequency calculation. The nature of the transition-state structures for valence isomerization was also examined by IRC calculations.31 Gibbs free energies (298.15 K, 1.0 atm) are given throughout the paper for better comparison with experimental work. The nucleusindependent chemical shifts (NICS(1)) were obtained 1 A˚ above the center of the phosphepine rings.32 Geometrical details are given in the Supporting Information.

Results and Discussion We start with the parent, metal-free phenyl-substituted phosphepine 3a, for which the potential energy hypersurface for isomerization and fragmentation is displayed in Figure 1, (22) (a) Borst, M. L. G.; Bulo, R. E.; Gibney, D. J.; Ehlers, A. W.; Schakel, M.; Lutz, M.; Spek, A. L.; Lammertsma, K. J. Am. Chem. Soc. 2005, 127, 5800–5801. (b) Borst, M. L. G.; Bulo, R. E.; Gibney, D. J.; Alem, Y.; de Kanter, F. J. J.; Ehlers, A. W.; Schakel, M.; Lutz, M.; Spek, A. L.; Lammertsma, K. J. Am. Chem. Soc. 2005, 127, 16985–16999. (23) (a) Couzijn, E. P. A.; Ehlers, A. W.; Slootweg, J. C.; Schakel, M.; Krill, S.; Lutz, M.; Spek, A. L.; Lammertsma, K. Chem.;Eur. J. 2008, 14, 1499–1507. (b) Jansen, H.; Rosenthal, A. J.; Slootweg, J. C.; Ehlers, A. W.; Lutz, M.; Spek, A. L.; Lammertsma, K. Organometallics 2008, 27, 2868–2872. (24) Alternative routes for amino-functionalized phosphinidene generation have been proposed theoretically: (a) Benk~ o, Z.; Gudat, D.; Nyulaszi, L. Chem.;Eur. J. 2008, 14, 902–908. (b) Clendenning, A.; Hitchcock, P. B.; Lappert, M. F.; Merle, P. G.; Nixon, J. F.; Nyulaszi, L. Chem.;Eur. J. 2007, 13, 7121–7128. (25) Frisch, M. J.; et al. et al. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford CT, 2004. For the full citation see the Supporting Information. (26) Lappert, M. F.; Nixon, J. F.; Nyulaszi, L. Chem.;Eur. J. 2007, 13, 7121–7128. (27) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (28) (a) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671– 6687. (b) Perdew, J. P.; Wang, Y. Phys. Rev. B. 1992, 45, 13244–13249. (c) Perdew, J. P. Unified Theory of Exchange and Correlation Beyond the Local Density Approximation. In Electronic Structure of Solids, Vol. 11; Ziesche, P.; Eschrig, H., Eds.; Akademie Verlag: Berlin, 1991. (d) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1993, 48, 4978. (e) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533–16539. (f) Burke, K.; Perdew, J. P.; Wang, Y. Derivation of a Generalized Gradient Approximation: The PW91 Density Functional. In Electronic Density Functional Theory and New Directions, Vol. 81; Dobson, J. F.; Vignale, G.; Das, M. P., Eds.; Plenum: New York, 1998. (29) Jensen, F. Introduction to Computational Chemistry, 3rd ed.; Wiley: Chichester, 1999; p 429. (30) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–31098. (31) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154– 2161. (b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523–5527.

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Figure 1. Relative B3PW91/6-311þG(d,p) Gibbs free energies (kcal 3 mol-1) for phenyl-phosphinidene release from syn and anti 1-phenyl-1H-phosphepine (3a) via valence isomer syn/anti-3b. The NICS(1) values are given within the rings in italics.

to evaluate the influence of this synthetically much used P-substituent before evaluating the various modifications. 1-Phenyl-1H-phosphepine (3a). Two phosphepine conformations are feasible, with the phenyl group in either the syn (syn-3a) or the energetically slightly preferred anti orientation (anti-3a -2.4 kcal 3 mol-1; Figure 1). The barrier for their interconversion via a ring-flip is small (TSinv 5.2 kcal 3 mol-1) and proceeds via an almost planar seven-membered ring. Each of the phosphepine conformers has a modest barrier (TSanti 11.4; TSsyn 10.3 kcal 3 mol-1) for electrocyclic ring closure to the corresponding phosphanorcaradienes anti- and syn-3b, which is for both thermodynamically favored products (anti3b -4.8; syn-3b -6.3 kcal 3 mol-1). The sizable NICS(1)33 value of the transition-state structure (e.g., TSanti -13.4 ppm; Figure 1) reflects the pericyclic pathway; the homoaromatic character present in the monocyclic systems gives rise to a less negative NICS(1) value (e.g., anti-3a -4.8 ppm).34 The energy profile displayed in Figure 1 is similar to that for the unsubstituted phosphepine 2 (R = H)6b,22b and indicates that the phenyl substituent exercises little influence on the valence isomerization. The phosphanorcaradiene dissociates exergonically (anti-3b -3.5; syn-3b -4.4 kcal 3 mol-1) into triplet [Ph-P] and benzene. This process could be concerted or stepwise, as suggested for the expulsion of ethylene from phosphiranes,35 and proceeds usually without a barrier. Only for the fragmentation into transition metal stabilized phosphinidenes36 are (32) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. Rev. 2005, 105, 3842–3888. (33) As NICS is an easily accessible index and its interpretation is straightforward, it has been applied numerous times since its introduction. However, its applicability is still discussed in the scientific literature, i.e., (a) Lazzeretti, P. Phys. Chem. Chem. Phys. 2004, 6, 217–223. (b) Islas, R.; Martinez-Guajardo, G.; Jimenez-Halla, J. O. C.; Sola, M.; Merino, G. J. Chem. Theory Comput. 2010, 6, 1131–1135. (34) For comparison, benzene has at the same level of theory a NICS(1) value of -10.36 ppm. (35) Lam, W. H.; Gaspar, P. P.; Hrovat, D. A.; Trieber, D. A., II; Davidson, E. R.; Borden, W. T. J. Am. Chem. Soc. 2005, 127, 9886–9894. (36) Bulo, R. E.; Ehlers, A. W.; de Kanter, F. J. J.; Schakel, M.; Lutz, M.; Spek, A. L.; Lammertsma, K.; Wang, B. Chem.;Eur. J. 2004, 10, 2732–2738. (37) Lammertsma, K.; Ehlers, A. W.; McKee, M. L. J. Am. Chem. Soc. 2003, 125 (48), 14750–14759.

small barriers reported (i.e., 8.6 kcal/mol for [H-PdW(CO)5] and benzene).37 Therefore, we focus here on the analysis of the valence isomerization, noting that simple substituents at phosphorus and at the carbon 3- and 4-positions6b have little influence. To help understand the influence of other substituents and complexing groups, it is instructive to inspect the geometrical features of the valence isomers A and B including their shapes. 1-Phenyl-1H-phosphepine (3a), with its three alternating double bonds (dav = 1.354 A˚) and long transannular ˚ ), has a boat-shape conformation, distance C2-C7 (2.745 A with slightly more pronounced bow (R = 180 - — P1-C7C2-C3) and stern (β = 180 - — C4-C3-C6-C7) angles for isomer anti-3a (R = 48.3, β = 27.8) than for syn-3a (R = 40.4, β = 24.9). The corresponding phosphanorcaradienes anti/syn-3b, with two olefinic bonds (dav = 1.350 A˚ ) and a transannular single bond (dav = 1.512 A˚ ), have stronger bows but flattened sterns (syn-3a R = 68.2, β = 4.4; syn-3b R = 64.2, β = 4.7), resulting in similar curvatures (γ = R þ β) for the monocyclic (anti-3a γ = 76.1; syn-3a 65.3) and bicyclic (anti-3b γ = 72.6; syn-3b 68.9) structures. Whereas the transition structures for electrocyclic ring closure have bond lengths and angles intermediate to those of the valence isomers, they are more curved (TSanti γ = 85.3; TSsyn γ = 77.0), with bows similar to the bicyclic isomer and sterns like the monocyclic isomer. The nearly flat transition structure for interconversion (TSinv γ = 9.8) has alternating carbon-carbon double and single bonds (1.346, 1.449, 1.351 A˚), and consequently the seven-membered ring displays antiaromatic character (NICS(1) = þ6.4 ppm). Benzannulation. Adding a fused benzene ring substantially influences the geometries, relative energies, and reaction barriers of the valence isomers (Table 1).22b The molecular shapes show that the effect of benzannulating the sides of 1-phenyl-1H-phosphepine (6 and twice in 7) is larger than that on its bow (5; Figure 2). For example, the anti-conformer of 3-phenyl-3H-3-benzophosphepine (anti-5a γ = 67.9) is less curved than the parent anti-3a (γ = 76.1), whereas both anti-6a (γ = 84.8) and anti-7a (γ = 84.2) are more curved. The effect is also pronounced for the benzophosphanorcaradiene isomers, with large curvatures of 90.2 and 88.5 for anti-6b and anti-7b,

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respectively, and smaller ones for both anti-5b (73.7) and anti-3b (72.6). “Bow”-benzannulation majorly impacts the potential energy surface of the valence isomerization (Figure 3). Illustrative is the endergonic ring closure of benzophosphepine 5a to phosphanorcaradiene 5b (anti-5b 11.8; syn-5b 8.8 kcal 3 mol-1), while that for the nonbenzannulated 3a occurs exergonically (Table 1). The origin of this difference lies in disruption of the aromaticity of the annulated ring, as exemplified by both the increase of the NICS(1) value (from -9.2 for 5a to -3.0 ppm for 5b (Figure 2)) and the C-C/CdC bond alternation in the annulated ring (e.g., ˚ C10-C11/C9-C10: 1.441/1.354 (anti-5b) vs 1.393/1.385 A (anti-5a)). The barrier for ring closure is correspondingly higher for the benzannulated system (22.0 for TSanti-5b vs Scheme 1. Definition of the Bow (r) and Stern Angles (β)

11.4 kcal 3 mol-1 for TSanti-3b) due to the reduced electron delocalization, which is reflected in a less negative NICS(1) value for the seven-membered ring, cf., -9.9 for TSanti-5b and -13.4 ppm for TSanti-3b. Still, fragmentation of 5b into phenyl-phosphinidene and naphthalene is feasible (anti-5b -13.7; syn-5b -14.1 kcal 3 mol-1), as, in fact, has been observed for 5a by heating at 80 C in decaline.7 Benzannulating a “side” (C2-C3) of the phosphepine ring (6a) increases the endergonicity for electrocyclic ring closure even more (anti-6b 16.7; syn-6b 15.6 kcal 3 mol-1; Table 1). This is no surprise, as the C7-carbon becomes sp3 hybridized (Figure 2), thereby diminishing the π-conjugative stabilization, which is reflected in the reduction of the NICS(1) value of the annulated ring from -10.0 to -1.2 ppm on converting 6a into 6b. Still, also 6a is thermolabile and gradually decomposes in toluene at 80 C, as shown by Tsuchiya et al., who isolated naphthalene in 95% yield.8 Akin to benzophosphepine 5a, the anti orientation for the phenyl group of 1-phenyl-1H-1-benzophosphepine 6a is slightly favored (1.3 kcal 3 mol-1) over the syn form. Instead, dibenzannulated 7a favors by 2.0 kcal 3 mol-1 the syn isomer, which is less curved

Table 1. Relative B3PW91/6-311þG(d,p) Gibbs Free Energies (kcal 3 mol-1) with ΔH Values in Parentheses for the Valence Isomerization of Phosphepine 3a, Benzophosphepines 5a and 6a, and Dibenzophosphepine 7a

3 5 6 7

anti-b

TSanti

anti-a

TSinv

syn-a

TSsyn

syn-b

[Ph-P]

-4.8 (-5.7) 11.8 (11.1) 16.7 (16.4) 37.8 (37.9)

11.4 (10.2) 22.0 (20.8) 23.5 (22.6) 38.7 (37.8)

0.0 0.0 0.0 0.0

5.2 (4.7) 4.7 (3.6) 6.0 (5.1) 6.3 (5.7)

2.4 (2.1) 3.4 (3.3) 1.3 (1.2) -2.0 (-1.5)

12.7 (11.4) 22.9 (21.6) 25.0 (23.8) 39.4 (38.5)

-3.9 (-4.8) 12.2 (11.3) 16.9 (16.3) 37.7 (37.8)

-8.3 (3.1) -1.9 (9.6) -1.6 (10.2) 2.3 (13.2)

Figure 2. B3PW91/6-311þG(d,p)-optimized structures of benzophosphepines anti-5a-7a and phosphanorcaradienes anti-5b-7b. The NICS(1) values are given within the rings in italics. For 7a, the syn isomer is slightly lower in energy; see text.

Figure 3. Relative B3PW91/6-311þG(d,p) Gibbs free energies (kcal 3 mol-1) depicted for the extrusion of the triplet phenyl-phosphinidene from syn and anti 3-phenyl-3H-3-benzophosphepine (5a). NICS(1) values are given within the rings in italics.

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Figure 4. Relative B3PW91/6-311þG(d,p) Gibbs free energies (kcal 3 mol-1) for phenyl-phosphinidene oxide (singlet) release from 1-phenyl-1H-phosphepine oxide (syn/anti-9a). NICS(1) values are given within the rings in italics. Table 2. Relative B3PW91/6-311þG(d,p) Gibbs Free Energies (kcal 3 mol-1) with ΔH Values in Parentheses for the Valence Isomers of Phosphepine 3, Phosphepine Oxides 9 and 10, and Metal-Complexed Phosphepines 11 and 12

3 9 10 11 12 a

anti-b

TSanti

anti-a

TSinv

syn-a

TSsyn

syn-b

[Ph-P(X)]b

-4.8 (-5.7) 13.0 (12.4) 29.4 (28.9) -0.9 (-0.6) 19.1 (17.0)

11.4 (10.2) 19.4 (18.3) 32.0 (30.9) 14.3 (13.1) 29.0 (26.0)

0.0 0.0 0.0 0.0 0.0

5.2 (4.7) 3.1 (2.1) -a -a -a

2.4 (2.1) 1.5 (2.6) 1.9 (2.4) 0.6 (-1.4) 1.0 (0.1)

12.7 (11.4) 20.6 (19.9) 33.5 (32.1) 12.2 (11.8) 24.8 (23.5)

-3.9 (-4.8) 14.6 (14.5) 31.2 (31.1) -1.6 (-0.9) 15.5 (16.0)

-8.3 (3.1) -22.6 (10.6) -17.5 (-5.8) -9.2 (3.2) -1.8 (9.5)

Could not be found, presumably due to the flat potential energy surface in the region of the transition state. b X = O or W(CO)5.

and therefore sterically less strained (γ = 68.0) than anti-7a (γ = 84.2). With two benzannulated rings in the C2-C3 and C6-C7 positions the destabilizing effect for forming the P-norcaradiene 7b more than doubles. This is reflected in the 39.7 kcal 3 mol-1 endergonicity for the syn form (barrier 41.4 kcal 3 mol-1) as well as in the low NICS(1) value of -7.7 ppm for TSsyn-7. Obviously, syn-7a is a robust phosphepine. Our experimental results showed indeed that no fragmentation of 7 was observed on prolonged heating at 150 C,38 which is consistent with the calculated endergonic dissociation into the Ph-P and phenanthrene (4.3 kcal 3 mol-1). Oxidation. Oxidizing the phosphorus atom has a significant influence on the valence isomerization.39 In contrast to the parent 3a, 1-phenyl-1H-phosphepine oxide 9a is rather a stable compound with a reported mp of 91-92 C.9 The oxygen substituent renders a less curved geometry (anti-9a γ = 55.3, syn-9a 31.3) than 3a (anti-3a γ = 76.1, syn-3a 65.3), but the anti form is likewise favored (1.5 kcal 3 mol-1). The stability of 9a is underscored by the 13.0 (anti) and 13.1 (syn) kcal 3 mol-1 endergonic ring closure to phosphanorcaradiene 9b with respective barriers of 19.4 and 19.1 kcal 3 mol-1 (Figure 4, Table 2) and is also reflected in reduced NICS(1) values (e.g., TSanti -6.6 ppm). The origin of this behavior is (38) Freshly prepared 7 (15 mg, 0.05 mmol) was dissolved in xylene (0.5 mL) and heated for 24 h at 150 C, which resulted in no fragmentation of the dibenzophosphepine core. Over time, only traces of the corresponding phosphine oxide (δ31P 16.1 ppm) were observed. (39) Gilheany, D. G. Chem. Rev. 1994, 94, 1339–1374.

Figure 5. Walsh diagrams of phosphanorcaradiene oxide 9b.

the strong σ-donor bonding with electron transfer from phosphorus to oxygen that depopulates the symmetric C2-C7 bonding Walsh orbital, thereby favoring a ring-opened structure (Figure 5a). In addition, there is weaker π-back-bonding due to electron donation of the oxygen into the unoccupied orbitals of bicyclic 9b. Population of the antisymmetric C2-C7 antibonding Walsh orbital destabilizes this bond even further (Figure 5b).40,41 These effects are reflected in a C2-C7 bond lengthening from 1.511 (anti-3b) to 1.591 A˚ (anti-9b). Generating singlet phenyl-phosphinidene oxide17 and benzene from phosphanorcaradiene 9b is calculated to be exergonic (anti-9b 35.6; syn-9b -37.2 kcal 3 mol-1) and indeed has been observed experimentally by heating 9 above 150 C.9 Expectedly, benzannulating phosphepine oxide 9a increases the endergonicity for electrocyclic ring closure of 10a to 10b to 29.4 (syn) and 29.6 kcal 3 mol-1 (anti), respectively, with corresponding barriers of 32.0 (TSanti) and 31.6 kcal 3 mol-1 (40) Goumans, T. P. M.; Ehlers, A. W.; van Hemert, M. C.; Rosa, A.; Baerends, E.-J.; Lammertsma, K. J. Am. Chem. Soc. 2003, 125, 3558–3567. (41) G€ unther, H. Tetrahedron Lett. 1970, 11, 5173–5176.

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Figure 6. Relative B3PW91/6-311þG(d,p) Gibbs free energies (kcal 3 mol-1) for W(CO)5-complexed phenyl-phosphinidene (singlet) release from syn/anti 1-phenyl-1H-phosphepine pentacarbonyltungsten(0) (11a). NICS(1) values are given within the rings in italics.

(TSsyn; Table 2). These results are in accord with the reported observation that 10a does not isomerize at 140 C.7 Complexation. The commonly employed stabilizing W(CO)5 group influences only modestly the energy profile of the A a B valence isomerization. The most obvious change is the virtual elimination of the exergonicity for formation of the phosphanorcaradiene 11b (anti -0.9; syn -2.2 kcal 3 mol-1) from the complexed phosphepine 11a (Figure 6).22b The transition metal group has little influence on the phosphepine geometry. Noticeable is the lesser curvature of 11a (anti γ = 64.1; syn 66.7) compared to metal-free 3a (anti γ = 76.1; syn 65.3), which results in an increased barrier of 14.3 kcal 3 mol-1 for electrocyclic ring closure to anti-11b; the barrier for the syn-conformer (11.6 kcal 3 mol-1) is hardly affected, resulting in the preferred formation of syn-11b. Isolation of W(CO)5-complexed phosphepine 11a will therefore be challenging, also because of the exergonic dissociation of 11b into singlet phosphinidene [Ph-PdW(CO)5] and benzene (anti-11b -8.3; syn-11b -7.6 kcal 3 mol-1), which most likely proceeds via a facile, concerted [1þ2]-retroaddition, as determined computationally for [H-PdW(CO)5].19 Benzannulating the bow of 11a has a similar effect to that on the parent 3a. In fact, anti 3-phenyl-3H-3-benzophosphepine 12a (MLn = W(CO)5) is a known phosphinidene precursor (mp 115 C)22,23 that generates [Ph-PdW(CO)5] with a halflife time of 203 min at 75 C in the presence of a trapping agent.22b The effect of benzannulation raises the barrier by over 12 kcal 3 mol-1 (anti 29.0; syn 23.8) for the endergonic ring closure to the corresponding P-norcaradiene 12b (anti 19.1; syn 14.5 kcal 3 mol-1, Table 2). P-Amino Substituents. The preceding computational analysis of the A a B valence isomerization agrees well with the limited experimental data. In this analysis, focused on evaluating precursors to generate singlet phosphinidenes, the phenyl substituent on phosphorus played a prominent role. The disadvantage of this group is that an additional oxo group or transition metal complex is required to prevent forming a triplet phosphinidene from precursor B. However, these additional groups may not be needed on replacing the phenyl with a dimethylamino group to generate a singlet species.16

We start with the W(CO)5-complexed phosphepine. The structure is hardly effected on changing the substituent from Ph (11a) to NMe2 (13a), with the difference in curvature as the most prominent one (anti-11a γ = 64.1; syn-11a 66.7; anti-13a 73.0; syn-13a 62.7). As a consequence, the amino derivative favors the flatter syn isomer (3.3 kcal 3 mol-1), which easily undergoes ring closure (barrier 10.9 kcal 3 mol-1) to the 2.7 kcal 3 mol-1 favored phosphanorcaradiene syn-13b (Table 3). Fragmentation of this isomer into singlet phosphinidene [Me2N-PdW(CO)5] and benzene is more exergonic than its phenyl analogue (25.8 vs 7.6 kcal 3 mol-1), thereby underscoring the stabilizing effect of the amino substituent. Introducing benzannulation at the bow (14) causes the expected relative destabilization of the phosphanorcaradiene (>14 kcal 3 mol-1 compared to 13), rendering the formation of 14b endergonic (anti 12.9; syn 12.9 kcal 3 mol-1) with a modest barrier (anti 22.3; syn 21.3 kcal 3 mol-1; Table 3). These data are in accord with the experimental observation on the diethyl derivative, which has shown to be a suitable precursor for generation a W(CO)5-complexed aminophosphinidene.22 To explore the access to singlet aminophosphinidenes [R2N-P],42 we examine the metal-free amino-substituted phosphepine (15a). Expectedly, replacing the Ph group of 3a with an amino group has little influence on both the geometrical parameters and energy profile (Figure 7). Thus, the 1.9 kcal 3 mol-1 preferred anti isomer readily undergoes ring closure (barrier 12.0 kcal 3 mol-1) to form the 3.9 kcal 3 mol-1 favored P-norcaradiene 15b, which fragments exergonically (10.6 kcal 3 mol-1) into singlet [Me2N-P] and benzene. Consequently, 15a is like its phenyl analogue 3a, not a suitable phosphinidene precursor, but the stability of the phosphepine can be increased by benzannulating the bow, i.e., 3-dimethylamino-3H-3-benzophosphepine (16a). The effect of benzannulation is similar to that for the phenyl-substituted phosphepines (3a f 5a), but the preference for the anti isomer has increased to 7.1 kcal 3 mol-1; just like 13-15, no barrier for anti-syn isomerization could be determined due to the flat potential energy surface (Table 3). + Z.; Streubel, R.; Nyulaszi, L. Dalton Trans. 2006 (42) Benko, 4321–4327.

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Table 3. Relative B3PW91/6-311þG(d,p) Gibbs Free Energies (kcal 3 mol-1) with ΔH Values in Parentheses for the Valence Isomerization of Phosphepine 3, Metal-Complexed Aminophosphepines 13 and 14, and Dimethylaminophosphepines 15 and 16

13 14 15 16 a

anti-b

TSanti

anti-a

TSinv

syn-a

TSsyn

syn-b

[Me2N-P(X)]b

-2.0 (-3.6) 12.9 (13.0) -3.9 (-4.3) 12.3 (12.3)

10.5 (9.8) 22.3 (21.1) 12.0 (11.1) 24.1 (23.2)

0.0 0.0 0.0 0.0

-a -a -a -a

-3.3 (-5.2) -2.9 (-5.5) 1.9 (2.0) 7.1 (6.3)

7.6 (7.2) 18.4 (17.4) 13.6 (12.5) 22.8 (21.5)

-6.0 (-5.0) 10.0 (10.6) -3.8 (-4.4) 11.4 (10.8)

-31.8 (-19.4) -25.6 (-13.9) -10.6 (2.0) -10.6 (2.3)

Could not be found, presumably due to the flat potential energy surface in the region of the transition state. b X = lone pair or W(CO)5.

Figure 7. Relative B3PW91/6-311þG(d,p) Gibbs free energies (kcal 3 mol-1) for dimethylamino-phosphinidene (singlet) release from syn/anti 1-dimethylamino-1H-phosphepine (15a). NICS(1) values are given within the rings in italics.

The preference for the anti isomer of 16a has its origin in the electronic repulsion between the amino and benzo groups. Both the 24.1 kcal 3 mol-1 barrier and the endergonicity of 12.3 kcal 3 mol-1 for electrocyclic ring closure to P-benzonorcaradiene 16b compare with those of phenyl derivative 5a (respectively 22.0 and 11.8 kcal 3 mol-1), which dissociates at 80 C into phenyl-phosphinidene and naphthalene. Adding to this the more favorable dissociation into singlet [Me2N-P] and naphthalene (10.6 kcal mol-1), we believe that benzannulated aminophosphepines, like 16a, are very promising precursors for the synthesis of metal-free singlet phosphinidenes. Synthetic efforts to confirm this hypothesis are currently under investigation in our laboratories.

Conclusions The influence of benzannulation, complexation, oxidation, and substitution at phosphorus on the valence isomerization of phosphepines was studied computationally with the aim to determine whether amino-substituted derivatives might be suitable precursors for the in situ generation of singlet aminophosphinidenes [R2N-P]. The present study, calibrated against experimental data, not only makes clear why phosphepines with simple substitution patterns are hard

to synthesize but also points to solutions. The parent phenylphosphepine 3a is shown to readily undergo exergonic ring closure to phosphanorcaradiene 3b, which liberates triplet phenylphosphinidene [Ph-P]. Benzannulation reverses the relative stabilities of the valence isomers, due to disruption of the aromaticity of the annulated ring in the norcaradiene structure (e.g., 5b). Replacing the phenyl group at phosphorus for a dimethylamino group gives a very similar picture, pointing to the viability of 1-dimethylamino-3H-3-benzophosphine 16a as precursor for generating singlet [R2N-P].

Acknowledgment. This work was supported by the Council for Chemical Sciences of The Netherlands Organization for Scientific Research (NWO-CW) and benefitted from interactions within the European PhosSciNet (CM0802). The National Center for Computing Facilities (SARA) is acknowledged for computer time. Supporting Information Available: All Cartesian coordinates (A˚) and energies (au) of all stationary points, together with a more detailed description of the geometries and NICS(1) values. This material is available free of charge via the Internet at http://pubs.acs.org.