η3-diphosphavinylcarbene: A P2 lthe doetz Intermediate

Full text of DOI:10.1002/anie.200805037

Communications
DOI: 10.1002/anie.200805037
Carbene Ligands
h3-Diphosphavinylcarbene: A P2 Analogue of the Dꢀtz Intermediate**
Halil Aktas, J. Chris Slootweg, Andreas W. Ehlers, Martin Lutz, Anthony L. Spek, and
Koop Lammertsma*
Olefin metathesis[1] constitutes a powerful tool for the
construction of a plethora of unsaturated building blocks,
pharmaceuticals, and advanced materials. The principle steps
involve a transition-metal carbene complex (A) that under-
herein report on a diphosphorus analogue of h3-vinylcarbene
complex C (P2-C).[14] Our approach involves the reaction of in
=
situ generated phosphinidene [M PMes*] (Mes* = 2,4,6-
tBu3C6H2;[15] M = (h6-pCy)Ru (pCy = para-cymene), (h5-
Cp*)Ir (Cp* = C5Me5)), the phosphorus analogues of car-
ꢀ
benes, with phosphaalkynes (RC P; R = Mes*, tBu).
Treatment of primary phosphine complexes [Cl2M-
(PH2Mes*)] (3) with two equivalents 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) in the presence of one equivalent
3
ꢀ
Mes*C P afforded h -diphosphavinylcarbenes 5a (P2-C) as
the sole products in 89% (Ru) and 76% (Ir) yield after
crystallization (Scheme 1). The synthesis of 5a is remarkably
goes a [2+2] cycloaddition/cycloreversion protocol with
alkenes according to the Chauvin mechanism.[2] For the
widely used Grubbs catalysts, however, support for the key
intermediate of this process, the four-membered metallacy-
clobutane, relies on theoretical studies[3] and low-temperature
NMR spectroscopy.[4,5] The unsaturated analogues provide
more insight. For example, reaction of alkynes with the
second-generation Grubbs catalyst gives stable h3-vinylcar-
bene complexes (1, Mes = 2,4,6-Me3C6H2) that form after
rearrangement of the initial ruthenium cyclobutenes (B).[6,7]
These puckered metal h3-vinylcarbene complexes[8] (C) are
highly potent and represent the key intermediate in enyne
metathesis,[9] alkyne polymerization (via D),[10] and the
versatile Fischer carbene mediated Dꢀtz (benzannulation)
reaction (which involves complexes such as 2).[11,12]
Scheme 1. Synthesis and reactivity of h3-diphosphavinylcarbene 5.
selective, as evidenced by the single AB spin system in the
1
31P NMR spectrum (Ru-5a: d = 52.1 and ꢁ4.8 ppm, JP, P
=
431.4 Hz; Ir-5a: d = 55.7 and ꢁ21.1 ppm, 1JP, P = 394.0 Hz);
ꢁ
the large P,P coupling indicates the presence of a P P bond.
In our quest for novel reactivity patterns and in light of the
The highly deshielded resonances in the 13C NMR spectrum
at d = 306.9 (Ru-5a) and 260.3 ppm (Ir-5a) are diagnostic for
metal alkylidenes.[6,16] They unambiguously support the
carbenoid nature of the ring carbon atom and exclude the
diphosphametallacyclobutene (P2-B) conformation.
diagonal relationship between carbon and phosphorus,[13] we
[*] H. Aktas, Dr. J. C. Slootweg, Dr. A. W. Ehlers,
Prof. Dr. K. Lammertsma
X-ray crystal structure determination[17] of dark green Ru-
5a (Figure 1) and deep red crystals of Ir-5a revealed
unequivocally the puckered structure of the unique h3-
diphosphavinylcarbene ligand. Complex Ru-5a displays
Department of Chemistry and Pharmaceutical Sciences
VU University Amsterdam
De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands)
Fax: (+31)20-598-7488
E-mail: k.lammertsma@few.vu.nl
ꢁ
short metal–alkylidene and P2 C19 bonds (1.928(2) and
Dr. M. Lutz, Prof. Dr. A. L. Spek
Bijvoet Center for Biomolecular Research, Crystal and Structural
Chemistry
ꢁ
1.759(2) ꢁ, respectively) and typical Ru P (2.4339(5) and
ꢁ
2.4478(6) ꢁ) and P P bonds (2.1771(8) ꢁ).
Whereas no intermediates could be detected by variable-
temperature NMR spectroscopy during the formation of Ru-
5a, selective monodehydrohalogenation of iridium precursor
3 occurred at ꢁ808C to give syn/anti-Ir-4 (31P NMR d = 119.5
(1JP, H = 398.9 Hz) and 128.3 ppm (1JP, H = 369.6 Hz)) in a 5:1
Utrecht University (The Netherlands)
[**] This work was partially supported by the Council for Chemical
Sciences of the Netherlands Organization for Scientific Research
(NWO/CW). We thank Dr. M. Smoluch and J. W. H. Peeters for
measuring high-resolution mass spectra.
ꢀ
ratio. At this temperature, Ir-4 is unreactive towards Mes*C
P, suggesting that the second dehydrohalogenation to afford
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805037.
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ment.[19] As a result, the P2C2 unit displays p delocalization, as
ꢁ
evidenced by the similar P C bond lengths (1.7787(16)–
1.7926(15) ꢁ) and an alternating bond pattern for the
dearomatized Mes* ring.
Monitoring the reaction of ruthenium precursor 3 with
ꢀ
tBuC P by variable-temperature NMR spectroscopy showed
full conversion at ꢁ808C to dark green Ru-5b (31P NMR: d =
1
ꢁ2.0 and ꢁ35.7 ppm, JP, P = 400.3 Hz) and above ꢁ408C its
subsequent rearrangement to yellow Ru-6b. Monitoring the
corresponding reaction of the iridium congener of 3 at ꢁ808C
showed, besides the monodehydrohalogenation to syn/anti-Ir-
4 (ca. 3:1 ratio), the formation of two new P2 species (10:1
ratio) with sizable P,P and P,H couplings (major isomer,
Figure 1. Displacement ellipsoid plot of Ru-5a with ellipsoids drawn at
the 50% probability level. Hydrogen atoms and toluene solvent are
omitted for clarity. Selected bond lengths [ꢀ], angles [8], and torsion
angle [8]: Ru1–P1 2.4339(5), Ru1–P2 2.4478(6), Ru1–C19 1.928(2),
Ru1–pCy(cg) 1.7473(7), P1–C1 1.879(2), P1–P2 2.1771(8), P2–C19
1.759(2), C19–C20 1.480(3); P2-P1-Ru1 63.84(2), P1-P2-C19 88.26(7),
Ru1-C19-P2 83.06(8); P1-P2-Ru1-C19 111.06(9).
1
1
31P NMR d = 358.7 and ꢁ138.9 ppm, JP, P = 164.0 Hz, JP, H
=
387.8 Hz). We ascribe these products to the syn and anti
adducts of Ir-7b (Figure 3), which suggests that in this case the
=
the transient [M PMes*] species takes place prior to phos-
phaalkyne addition. At ꢁ608C, the gradual formation of Ir-5a
was observed.
Surprisingly, reaction of 3 with the less sterically con-
ꢀ
gested phosphaalkyne tBuC P using two equivalents DBU
resulted, at room temperature, in yellow crystalline 6b (80%
(Ru), 87% (Ir); Scheme 1) instead of the expected carbene
5b, as indicated by a different set of signals in the 31P NMR
spectrum. The smaller P,P couplings are characteristic (Ru-
6b: d = ꢁ1.5 and ꢁ92.1 ppm, 2JP, P = 20.7 Hz; Ir-6b: d = ꢁ32.7
and ꢁ113.0 ppm, 2JP, P = 24.5 Hz), as is the absence of a
carbenoid resonance in the 13C NMR spectrum. The molec-
ular structure of Ru-6b, established by single-crystal X-ray
structure determination,[17] exhibits a number of fascinating
features. First and foremost, 6b contains an unprecedented
1,3-diphospha-3H-indene moiety[18] bearing a dearomatized
Mes* substituent (Figure 2). The striking resemblance of this
stable P2 entity with the benzannulation (Wheland) inter-
mediate of the Dꢀtz reaction is evident.[12c] Secondly, h4
coordination of the P2C2 moiety to the ruthenium center is
favored over coordination of the all-carbon butadiene frag-
Figure 3. Intermediate Ir-7b and model structure Ir-7’, calculated at
the B3PW91/6-31G(d,p) (LANL2DZ for Ir) level of theory. Selected
bond lengths [ꢀ] and a torsion angle [8] of Ir-7’: Ir–P1 2.304, Ir–C1
1.992, P1–P2 2.214, P2–C1 1.707; P1-P2-C1-Ir 4.86.
ꢀ
addition of the less congested phosphaalkyne tBuC P to Ir-4
is plausible. The intermediacy of Ir-7b is supported by the
calculated NMR spectral parameters[20] of the unsubstituted
model structure Ir-7’ (31P NMR d = 352.6 and ꢁ110.8 ppm,
1JP, P = 256.9 Hz; Figure 3). At ꢁ408C, Ir-7b underwent the
second base-induced dehydrohalogenation to yield Ir-5b
1
(31P NMR d = ꢁ3.2 and ꢁ24.4 ppm, JP, P = 371.6 Hz), which
upon warming to room temperature rearranged into Wheland
product Ir-6b, as was evident by the color change from red to
yellow.
The remarkable conversion of 5b into 6b was examined
by B3PW91/6-31G(d,p) calculations on model structures
containing H instead of tBu substituents (labeled 5’, 6’, etc.;
Figure 4). No simple direct pathway was found, but instead
one that involves phosphinidene complex 8’ and h3-phos-
phaalkenyl phosphinidene 9’, the 1,3-isomer of 5. Interest-
ingly, the planar diphosphametallacyclobutene conformer
(P2-B) is not an energy minimum but corresponds to the
transition-state structure for inversion of puckered 5’ (DE° =
24.4 (Ru), 22.1 (Ir) kcalmolꢁ1). Isomerization of carbene 5’ to
ꢁ
the less stable phosphinidene 8’ proceeds by P P bond
cleavage (Ru: DE = 19.8, DE° = 20.6 kcalmolꢁ1). This step is
then followed by facile rotation of the h2-coordinated
Figure 2. Displacement ellipsoid plot of Ru-6b with ellipsoids drawn at
the 30% probability level. Hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢀ] and torsion angle [8]: Ru1–P1 2.3664(4),
Ru1–P2 2.3863(4), Ru1–C1 2.2624(14), Ru1–C19 2.2055(14), Ru1–
pCy(cg) 1.7473(7), P1–C1 1.7926(15), P1–C19 1.7802(15), P2–C6
1.8777(15), P2–C19 1.7787(16), C1–C2 1.486(2), C1–C6 1.547(2), C2–
C3 1.351(2), C3–C4 1.461(2), C4–C5 1.341(2), C5–C6 1.518(2); C1-P1-
C19-P2 2.11(9).
ꢁ
phosphaalkyne ligand and P C bond formation to give
regioisomer 9’ (Ru: DE = ꢁ22.5, DE° = 4.8 kcalmolꢁ1).
Finally, the subsequent electrophilic attack of P2 affords the
unique Wheland product 6’ (Ru: DE = ꢁ0.6, DE° = 17.7 kcal
molꢁ1), of which the experimental analogues, Ru-6b and Ir-
6b, can be isolated.
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www.angewandte.org
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Scheme 2. Reactivity of Ru-5a towards phosphines.
afforded instead the terminal phosphinidene complex [(h6-
[14a]
=
1
pCy)(Ph3P)Ru PMes*] (Ru-10)
(t= = 100 min at 08C;
2
quantitative yield). Both conversions follow an unprece-
dented pathway for h3-vinylcarbenes for which an associative
ligand exchange at the phosphinidene stage (8) is proposed, as
=
ꢀ
the dissociative pathway (Ru-8’![C6H6Ru PPh] + HC P)
corresponds to an uphill process (DE = 28.8 kcalmolꢁ1).
In conclusion, phosphorus substitution of metal-bound h3-
vinylcarbenes has a marked influence on their reactivity.
Depending on the substituent pattern, the readily obtained
h3-diphosphavinylcarbene complexes 5 show facile ligand
exchange reactions or undergo an unprecedented rearrange-
ment to the stable Wheland product 6.
Received: October 15, 2008
Published online: December 15, 2008
Keywords: carbenes · cycloaddition ·
.
density functional calculations · NMR spectroscopy ·
phosphorus heterocycles
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Chemie
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[17] CCDC-687878 (Ru-5a), 687879 (Ru-6b), and 687880 (Ir-5a)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. For the experimental details of the X-ray crystal
structure determinations, see the Supporting Information.
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[20] DFT calculations were carried out with Gaussian 03 (Revision
C.02) at the B3PW91/6-31G(d,p) (LANL2DZ for Ru, Ir) level;
see the Supporting Information. The 31P NMR spectral param-
eters were calculated at the B3PW91/6-311G + + (2d,p)//
B3PW91/6-31G(d,p) level using PMe3 as reference (scalcd
=
ꢁ = ꢁ
367.8, dexptl = ꢁ62.0). The 1,3-isomer of Ir-7’ (Ir P C P) shows
distinct resonances: d = 652.8 and ꢁ93.0 ppm, 2JP, P = 53.4 Hz.
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