Phosphepines: Convenient access to phosphinidene complexes

Article abstract of DOI:10.1021/ja050817y

Reaction of o-diethynylbenzene with transition metal-complexed primary phosphines gives in a single base-induced step stable phosphepine complexes as confirmed by X-ray data. At 75-80 °C these phosphepines undergo clean cheletropic elimination of naphthalene to give transient carbene-like phosphinidene complexes that can be trapped in high yield by alkenes, alkynes, and alcohols. Copyright

Full text of DOI:10.1021/ja050817y

Published on Web 03/31/2005
Phosphepines: Convenient Access to Phosphinidene Complexes
Mark L. G. Borst, Rosa E. Bulo, Christiaan W. Winkel, Danie`le J. Gibney, Andreas W. Ehlers,
Marius Schakel, Martin Lutz, Anthony L. Spek, and Koop Lammertsma*
Department of Organic Chemistry, Faculty of Sciences, Vrije UniVersiteit, De Boelelaan 1083, NL-1081 HV
Amsterdam, The Netherlands, and BijVoet Center for Biomolecular Research, Crystal and Structural Chemistry,
Utrecht UniVersity, Padualaan 8, 3584 CH Utrecht, The Netherlands
Received February 8, 2005; E-mail: k.lammertsma@few.vu.nl
Scheme 1 . Valence Isomers of Cyclohexatriene
The cycloheptatriene-norcaradiene (CHT-NCD) equilibrium
exemplifies a 6-el electrocyclic reaction with a modest barrier
separating the valence tautomers 1C and 2C.1 Less is known about
the heterocyclic analogues (X ) O, S, NR, PR) (Scheme 1). For
those with an oxygen atom, benzene oxide 2O is the more stable
form, but entropy factors shift the equilibrium toward oxepin 1O
at room temperature.2 Only the monocyclic form is observed for
thiepins3 1S and 1H-azepines4 1N, and both require bulky groups
Table 1. B86-P88/TZP Relative Energies (in kcal/mol) for 1P, 2P,
3P, and Their W(CO)5 and Benzannelated Derivativesa
and/or extended conjugation for stability. Of the phosphorus
analogue, phosphepine 1P, only its oxide5 and 2,7-dialkyl-
substituted6 and annelated7,8 derivatives are known, but without
structural details. Here we describe a computational analysis of the
1P-2P equilibrium and present new stable phosphepine derivatives
and a novel application.
R(Y)
1P
2P
3P
H
0.0
0.0
0.0
0.0
-4.2
1.5
11.0
16.8
11.3
7.8
4.2
1.3
H(W(CO)5)
H + benzob
H(W(CO)5) + benzo-b
a The lone pair and W(CO)5 group occupy the axial (1) or syn (2, 3)
Phosphepines equilibrate with phosphanorcaradienes (benzene
phosphines) such as CHT with NCD. DFT calculations for the
parent system give an energetic preference for NCD 2P over CHT
1P (∆E2-1 -4.2 kcal/mol, Table 1) with a modest 1P f 2P barrier
(10.6 kcal/mol), reflecting the same behavior as the thia analogues
(∆E2-1 -7.8 (S) kcal/mol). A 1,5-sigmatropic shift relates NCD
2P with the 15.5 kcal/mol less stable 7-phosphanorbornadiene 3P,
neither of which is known experimentally, except for a strained
derivative of 3P,10 presumably due to release of the phosphorus
group to give pentamers, (PR)5.
position.9 b Benzannelation as indicated in Scheme 1.
Scheme 2 . HOMO-LUMO Interactions between 2P and W(CO)5
Scheme 3 . Synthesis of Benzophosphine Complex 7a
Transition metal coordination at phosphorus stabilizes 1P over
2P because the distal C-C bond is weakened by σ,π-interactions
(Scheme 2), reversing the order for W(CO)5 (∆E2-1 1.5 kcal/mol).
Due to extended conjugation, benzannelation has an even stronger
influence, reversing the CHT-NCD equilibrium in favor of 1P
by 11.0 kcal/mol. These cumulative electronic effects give a
16.8 kcal/mol energetic preference of benzophosphepine W(CO)5
complex over its valence isomer 2P with a 24.7 kcal/mol barrier
for electrocyclization.11
a
Reaction conditions: i) CBr4, PPh3; ii) LDA; iii) KOH, THF, at
25 °C.
(Figure 1) that has the phosphorus atom shifted out of the
hydrocarbon plane by 67.20(13)°. The orthogonal phenyl group is
on the mirror plane and bisects the C1-P-C1i angle. The P-C1-
(C1i) bonds are very short (1.814(3) Å).
On the basis of these calculations benzophosphepine complexes
7 are expected to be stable at room temperature. Indeed, 7 could
be synthesized from the complexed phosphine and 1,2-diethynyl-
benzene (5) by a modified Ma¨rkl’s procedure7 (Scheme 3). Dialkyne
5 was obtained in 91% yield from commercially available
o-phthaldialdehyde (4) by a one-carbon homologation-oxidation
sequence. The base-promoted double hydrophosphination of 5 with
PH2Ph-W(CO)5 (6a) gave 3H-3-benzophosphepine-W(CO)5 7a
in 74% yield as yellow crystals. W(CO)5 coordination causes
shielding of the 31P NMR resonance (δ -15 vs -33 ppm), shielding
of the C1,C5 13C NMR resonances (by 6 ppm), and reduces the
2JCP coupling constants (1.6 vs 21.1 Hz).7 Distinctive are the
Because the phosphorus group is introduced in the final step,
the synthesis allows for versatility in substituents and metal
complexes. Illustrative is the condensation of 5 with PhPH2-
Mn(CO)2Cp that gives manganese-complexed phosphepine 7b
(35%). The molecule in the crystal shows again Cs-symmetry, but
has a flatter phosphepine ring with a dihedral angle of 34.49(8)°
between the C1-P1-C1i and hydrocarbon planes (Figure 2). This
is best explained by a steric effect of the Cp, hanging over the
phosphepine ring, in which CdC bond alternation is again evident.
The orthogonal phenyl group bisects the C1-P-C1i angle, and
the Mn-P bond has a normal distance of 2.1954(5) Å.12 The more
shielded 31P NMR resonance at +64 ppm points to less electrophilic
character for the phosphorus group than in 7a; the 1H and 13C NMR
3
coupling constants for the olefinic protons with a sizable JHH
3
(12.4 Hz) and with a JHP (33.4 Hz) being much larger than the
2JHP (21.3 Hz).
2
3
The crystal structure (Cs-symmetry) shows alternating CdC
bonds (no homoaromaticity) for the boat-shaped phosphepine ring
parameters are similar, including the JHP (20.7 Hz) and JHP
(32.1 Hz) coupling constants.
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10.1021/ja050817y CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 4. Phosphinidene Reactions of 7 (and 10)
Figure 1. ORTEP diagram of 3-phenyl-benzophosphepine-W(CO)5.
phosphinidenes is illustrated by the reaction of manganese complex
7b with phenylacetylene that results in a Mn(CO)2Cp complexed
phosphirene (81%, 31P NMR δ -59.0), suggesting the intermediacy
of the novel phosphinidene [R-PdMn(CO)2Cp].
In conclusion, we presented a very short synthesis to diverse
stable 3H-3-benzophosphepine complexes that are excellent precur-
sors to electrophilic phosphinidene complexes, requiring very mild
reaction conditions and an extremely simple workup.
Acknowledgment. This work was supported by the Council
for Chemical Sciences of The Netherlands Organization for
Scientific Research (NWO/CW). We thank Dr. M. Smoluch for
exact mass determinations.
Supporting Information Available: Experimental section including
NMR spectra and computational data. X-ray crystallographic data for
7a, b in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 2. ORTEP diagram of 3-phenyl-benzophosphepine-Mn(CO)2Cp.
Chart 1 Stabilized Phosphepines and Phosphinidene Precursor
10.
References
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Phosphepines 8 and 9 are known to decompose readily to the
aromatic hydrocarbon and (RP)5, presumably by expelling [RP]
from the NCD intermediates7,8 (Chart 1). Interestingly, in the case
of complex 7 this would result in the expulsion of a transition metal-
stabilized phosphinidene [R-PdMLn]. Extremely few synthetic
routes enable access to such phosphinidene complexes,13 especially
those with electrophilic properties. In fact, cheletropic elimination
from complexed 7-phosphanorbornadienes 10 is the only general
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Benzophosphepine 7a reacts with the test set of molecules, first
used for the reaction with 10, to give at 75-80 °C the same
addition/insertion products,13a,16,17 but in better yields (Scheme 4)
and requiring only removal of solvent and naphthalene (by
sublimation). The 30 °C advantage in reaction temperature is evident
from the formation of vinylphosphirane 14 from 2,3-dimethyl-
butadiene, whereas phospholene 15 is the main product with 10a13
due to a 1,3-sigmatropic shift that occurs above 80 °C.16 Reaction
of molybdenum complex 7c, obtained from 5 and PhPH2-Mo-
(CO)5 (67%), with tolan gives phosphirene 11c in 66% yield versus
29% with 10c.17 Access to unprecedented far less electrophilic
(9) Equatorial (1) or anti (2, 3) orientation of the lone pair and the W(CO)5
group is energetically preferred by up to 1.6 kcal/mol for all structures.
Because the crystal structure of 7a shows syn complexation, we employ
the same orientation.
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the energy difference with the phosphepine complex to 1.3 kcal/mol.
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