Selective homo- and heterodehydrocouplings of phosphines catalyzed by rhodium phosphido complexes

Article abstract of DOI:10.1021/ja065346+

Reactions of the rhodium complex (dippe)Rh(η³-CH₂Ph) (1, dippe = ⁱPr₂PCH₂CH₂PⁱPr₂) with ArPH₂ (Ar = Ph, Mes) proceed via P-H oxidative additions to the phosphido complexes (dippe)Rh(μ-PHAr)₂Rh(dippe) (3a, Ar = Ph; 3b, Ar = Mes). The corresponding reaction of Ph₂PH occurs similarly, via the intermediate (dippe)Rh(PPh₂)PHPh₂ (4), to (dippe)Rh(μ-PPh₂)₂Rh(dippe) (3c). Complexes 3a-c and 4 are catalysts for the catalytic dehydrodimerizations of the corresponding phosphines to diphosphanes. Complex 1 is a more active dehydrocoupling catalyst, and substituent effects suggest that the active catalyst is mononuclear. Efficient dehydrocouplings of 2-EtC₆H₄PH₂, 2-ⁱPrC₆H₄PH₂, and 2,4,6-ⁱPr₃C₆H₂PH₂ were also observed. Complex 1 also catalyzes the heterocoupling of Ph₂PH with PhSH (to Ph₂P-SPh), and stoichiometric reactions in this system allowed isolation of (dippe)Rh(μ-SPh)₂Rh(dippe) (6) and (dippe)Rh(SPh)PHR₂ (7a, R₂PH = MesPH₂; 7b, R₂PH = Ph₂PH). Copyright

Full text of DOI:10.1021/ja065346+

Published on Web 09/29/2006
Selective Homo- and Heterodehydrocouplings of Phosphines Catalyzed by
Rhodium Phosphido Complexes
Li-Biao Han and T. Don Tilley*
Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460
Received July 25, 2006; E-mail: tdtilley@berkeley.edu
The increasing importance of phosphorus compounds in synthesis
and biology has stimulated extensive studies on new phosphorus-
element bond-forming reactions via transition metal catalysis.1
Given the success of dehydrocouplings of silanes and related
compounds,2 similar metal-catalyzed reactions of phosphines should
provide useful routes to phosphorus-element (P-E) bonded
compounds, especially since such reactions produce hydrogen as
the only byproduct (eq 1). However, to date, this strategy has met
with limited success, perhaps because phosphine and phosphido
ligands inherently form strong, relatively nonlabile interactions with
unsaturated metal species.2-4 Group 4 metallocene catalysts dehy-
drocouple primary phosphines to cyclic oligomers,3a-c and Cp*Rh-
(CH2dCHSiMe3)2 is a catalyst for the coupling of secondary
phosphines to the corresponding diphosphanes.3d Recently, our
therefore consistent with the sequence of reactions shown in eq 2.
Similar results were observed for the reaction of 1 with MesPH2,
which gave 3b as a red solid in 71% yield. The dimeric nature of
this product was confirmed by X-ray crystallography.9
Interestingly, the reaction of Ph2PH with 1 takes a somewhat
different course, to initially form 4, which contains a diphenylphos-
phine ligand. When 1 and Ph2PH were combined in a 1:1 ratio,
half of 1 was left unreacted (by NMR spectroscopy) after 50 min.
Thereafter, 4 reacted slowly with the remaining amount of 1 to
give 3c after 2 days. Both 3c and 4 are red solids that were isolated
in 91% and 87% yields, respectively, from reactions using 1 and 2
equiv of Ph2PH.10,11
attention was attracted to the highly reactive (dippe)Rh (dippe )
iPr2PCH2CH2PiPr2) fragment,5 since we previously observed that a
catalyst derived therefrom is active in promoting an intramolecular
C-H/P-H dehydrocoupling.6 This result suggested that related,
intermolecular dehydrocouplings might be possible via the general
mechanism of Scheme 1. Herein we communicate our preliminary
Scheme 1
The phosphido complexes 3a-c and 4 exhibit catalytic activity
(5 mol % in benzene) in the selective dehydrocoupling of
phosphines to the corresponding diphosphanes. With 3a as a
catalyst, 13% of PhPH2 was converted to PhHP-PHPh after 20 h
at 20 °C, and 3b provided a 51% conversion of MesPH2 to the
corresponding diphosphane after 2 h at 110 °C. Whereas 3c did
not produce diphosphane from reaction with Ph2PH over 10 h at
70 °C, 4 gave a 92% yield of Ph2P-PPh2 under the same conditions.
The latter results, and the influence of ancillary ligands and
substrates suggest that the active catalyst is mononuclear (vide
infra).
Higher catalytic activities are exhibited by complex 1, used as
an isolated compound or generated in situ from the reaction of (1,4-
cod)Rh(η3-CH2Ph)5a with dippe (Table 1).12 When 5 mol % of 1
was added to PhPH2 in benzene at room temperature, gas evolution
was immediately observed. After 3 h, PhHP-PHPh had selectively
formed in 20% yield as a 52/48 meso/rac mixture (by NMR
spectroscopy).13 The yield of PhHP-PHPh slightly increased to
36% after 1 day; however, no further conversion was observed
indicating deactivation of the catalyst. A similar result was obtained
with 1, generated in situ (run 2). It should be noted that the chelating
diphosphine ligand is crucial for the success of this coupling
reaction. Thus, (1,4-cod)Rh(η3-CH2Ph) alone did not catalyze the
dehydrocoupling, even at 70 °C. Although the corresponding (Cy2-
PCH2CH2PCy2)Rh(η3-CH2Ph) complex behaves similarly, giving
a 32% yield of PhHP-PHPh under the reaction conditions of run
results on efficient dehydrodimerizations of primary and secondary
phosphines and the first heterodehydrocoupling of a phosphine with
a thiol.
When PhPH2 (0.29 mmol) was added to a solution of the benzyl
rhodium complex (dippe)Rh(η3-CH2Ph)5a (1, 0.29 mol) in benzene-
d6 (2 mL) at room temperature, the color of the solution immediately
turned from orange to red. A 1H NMR spectrum after 0.5 h revealed
the formation of toluene and a rhodium hydride species 2 (br s, δ
-8.27) bearing a benzyl group (br s, δ 3.35, CH2Ph). This complex,
presumed to be the oxidative addition product (dippe)Rh(H)(PHPh)-
(CH2Ph),7 was observed to be part of a product mixture that also
included diastereomers of the phosphido derivative 3a (3a′ and 3a′′),
observed as broad signals shifted to high field in the 31P NMR
spectrum, at -119.5 (3a′) and -125.2 (3a′′) (eq 2; 3a′/3a′′/2 ratio
1
) 3/3/1). As indicated by H and 31P NMR spectroscopies, the
hydride 2 gradually transformed to 3a at room temperature.
Concentration and cooling of the reaction mixture gave a brown
solid of the binuclear phosphido-bridged rhodium 3a as a 1/1
mixture of diastereoisomers of 3a′ and 3a′′.8 The above results are
9
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J. AM. CHEM. SOC. 2006, 128, 13698-13699
10.1021/ja065346+ CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Dehydrocouplings of Phosphines Catalyzed by 1
in Scheme 1. Ongoing studies focus on the optimization and scope
of these reactions and on further characterization of the mechanism.
run
phosphine
conditions
% yieldb
1
2
3
4
5
6
7
8
PhPH2
20 °C, 20 h (3 h)
20 °C, 20 ha
5 mol % dippe, 20 °C, 20 h
20 °C, 20 h (3 h)
20 °C, 20 h (3 h)
36 (20)
31
51
74 (61)
80 (64)
Acknowledgment. The authors thank the National Science
Foundation for support of this work. L.-B.H. was on leave from
the National Institute of Advanced Industrial Science and Technol-
ogy, and thanks a JSPS Overseas Research Fellowship for the
support of his visit.
2-EtC6H4PH2
2-i-PrC6H4PH2
MesPH2
70 °C, 18 h (3 h); 110 °C 3 h 68 (59); 81
2,4,6-i-Pr3C6H2PH2 70 °C, 18 h (3 h); 110 °C 3 h 71 (66); 93
Ph2PH 70 °C, 8(3) h 91 (76)
Supporting Information Available: Full characterization data for
3a-c, 4, 5, 6, 7b, and NMR spectra of related Rh intermediates; detailed
reaction conditions and identification of the coupling products. This
material is available free of charge via the Internet at http://pubs.acs.org.
a Catalyst generated in situ from a Rh(cod)Bz/dippe ) 1/1 ratio mixture.
b Estimated from 31P NMR. All (ArHP)2 are ca. 1/1 meso/rac mixtures (see
Supporting Information for details).
References
2, other phosphines, both monodentate R3P (R ) Et, t-Bu, Ph) and
bidentate R2PCH2CH2PR2 (R ) Me, Ph), did not produce the
coupling product at all (20 °C, overnight). The nature of the rhodium
precursor complex also seems critical, as the [Rh(cod)Cl]2/dippe
combination exhibited no catalytic activity. This presumably results
from the inability of this system to form a phosphido complex upon
addition of the phosphine substrate. Interestingly, in the presence
of an additional dippe ligand (run 3), the yield of PhHP-PHPh
was improved from 36% to 51%.13 The dehydrocoupling reaction
is highly sensitive to the nature of ortho substituents on the aryl
ring of the phosphine substrate. Thus, the introduction of 2-Et (run
4) and 2-iPr (run 5) groups to the aryl ring of a primary phosphine
greatly increased the reactivity, resulting in high yields of the
coupling products even at room temperature. With more hindered
aryl substituents (runs 6 and 7), however, the dehydrocoupling did
not proceed at room temperature and heating was required. The
dehydrocoupling of the secondary phosphine Ph2PH proceeds
sluggishly at room temperature. Upon heating at 70 °C, however,
a high yield of the product was obtained (run 8).
(1) (a) Hirao, T.; Masaunaga, T.; Yamada, N.; Oshiro, Y.; Agawa, T. Bull.
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L.-B.; Tanaka, M. Chem. Commun. 1999, 395. (d) Hirai, T.; Han, L.-B.
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Jr.; Clark, R.; Guzei, I. Organometallics 2002, 21, 3278. (f) Depre`le, S.;
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(h) Shulyupin, M. O.; Kazakova, M. A.; Beletskaya, I. P. Org. Lett. 2002,
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(2) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem. 1998, 42,
363.
(3) Oligomerizations of RPH2 with Ti and Zr: (a) Fermin, M. C.; Stephan,
D. W. J. Am. Chem. Soc. 1995, 117, 12645. (b) Stephan, D. W. Angew.
Chem., Int. Ed. 2000, 39, 314. (c) Xin, S.; Woo, H. G.; Harrod, J. F.;
Samuel, E.; Lebuis A.-M. J. Am. Chem. Soc. 1997, 119, 5307. For Cp*Rh-
(olefin)2-catalyzed dehydrocouplings of phosphines at high temperatures,
see (d) Bohm, V. P. W.; Brookhart, M. Angew. Chem., Int. Ed. 2001, 40,
4694 (for Ph2PH, 60% yield after 27 h at 70 °C with CH2dCHtBu; for
PhPH2, 3% yield at 145 °C after 18 h).
(4) Hetereodehydrocouplings. (a) Ar2PH/RSiH3: Shu, R.; Hao, L.; Harrod,
J. F.; Woo, H.-G.; Samuel, E. J. Am. Chem. Soc. 1998, 120, 12988. (b)
Copolymerization of PhPH2 and BH3: Dorn, H.; Singh, R. A.; Massey,
J. A.; Nelson, J. M.; Jaska, C. A.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2000, 122, 6669. (c) Dehydrocouplings of phosphine-borane ad-
ducts: McWilliams, A. R.; Dorn, H.; Manners, I. Top. Curr. Chem. 2002,
120, 141.
Significantly, the application of this new rhodium catalyst system
is not restricted to homodehydrocouplings. Investigations on the
possibility of phosphorus-sulfur coupling began with examination
of reactions of phosphido rhodium complexes with PhSH, which
proceed readily with elimination of the phosphine (eq 4). Thus,
when an excess of PhSH (3 equiv) was added to a suspension of
3b in benzene at 25 °C, the formation of MesPH2 (-156.8 ppm)
and 6 (88.3 ppm)16 occurred as shown by 31P NMR spectroscopy.
After 2 h at 70 °C, 3b was completely consumed to give a
transparent orange solution containing 6, 7a, (-86.4 ppm, Rh-
PHMes) and MesPH2 in a ratio of 3/60/37. Reactions of PhSH with
3c and 4 occurred similarly, to produce the complex analogous to
7a (7b). However in this case, unlike for 7a, the equilibrium greatly
favors formation of the monorhodium complex, and this facilitated
its isolation. Furthermore, the catalytic dehydrocoupling of Ph2PH
(5) (a) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J. J. Organomet. Chem.
1993, 445, 245. (b) Rosenberg, L.; Fryzuk, M. D.; Rettig, S. J.
Organometallics 1999, 18, 958. (c) Fryzuk, M. D.; Rosenberg, L.; Rettig,
S. J. Organometallics 1996, 15, 2871. (d) Fryzuk, M. D.; Clentsmith, G.
K. B.; Rettig, S. J.; Haegele, G. Organometallics 1996, 15, 2083. (e)
Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Inorg. Chim. Acta 1994, 222,
345. (f) Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J. Organometallics 1991,
10, 2537. (g) Fryzuk, M. D.; Piers, W. E.; Einstein, F. W. B.; Jones, T.
Can. J. Chem. 1989, 67, 883. (h) Fryzuk, M. D.; Jang, M. L.; Jones, T.;
Einstein, F. W. B. Can. J. Chem. 1986, 64, 174.
(6) Stradiotto, M.; Fujdala, K. L.; Tilley, T. D. HelV. Chim. Acta 2001, 84,
2958.
(7) An attempted isolation of 2 was not successful owing to its ready
decomposition to 3a.
(8) Related phosphido-bridged complexes have been prepared by other
methods. (a) Kreter, P.; Meek, D. W. Inorg. Chem. 1983, 22, 319. (b)
Fultz, W.; Rheingold, A. L.; Kreter, P. E.; Meek, D. W. Inorg. Chem.
1983, 22, 860. (c) Burkhardt, E. W.; Mercer, W. C.; Geoffroy, G. L. Inorg.
Chem. 1984, 23, 1779. (d) Jones, R. A.; Norman, N. C.; Seeberger, M.
H. Organometallics 1983, 2, 1629. (e) Arif, A. M.; Jones, R. A.; Heaton,
D. E.; Nunn, C. M.; Schwab, S. T. Inorg. Chem. 1988, 27, 254.
(9) We thank B. V. Mork for performing this X-ray analysis (unpublished).
(10) Complex 4 was not regenerated from a mixture of 3c and additional Ph2-
PH at room temperature.
(11) Complexes 3a-c are stable to 110 °C for 10 h. However, when heated to
70 °C for 3 h, 4 decomposed to free Ph2PH, PhHPPHPh and a hydrido
rhodium complex, presumably [(dippe)Rh]2(µ-H)(µ-PPh2) (5). Complex
3c was not detected as a product. Note that 5 does not catalyze the
dehydrocoupling of Ph2PH.
(12) The generated catalyst in situ showed higher catalytic activity than 3a-
c, but activity similar to that of 4.
1
(13) The product PhHPPHPh was identified by comparison of its H and 31P
with PhSH was observed to readily form Ph2PSPh (eq 5).15
NMR data with those reported (ref 3c). When the reaction was carried
out at higher temperatures, further reaction of PhHPPHPh took place to
give a complicated mixture (see Supporting Information for details).
(14) We have confirmed that PhPH2 can replace dippe from complex 3a. Once
dippe is replaced the catalyst loses its activity. The addition of excess
dippe presumably delays this deactivation process.
(15) For a related rhodium-catalyzed dehydrocoupling of silanes with thiols,
see Baruah, J. B.; Osakada, K.; Yamamoto, T. Organometallics 1996,
15, 456.
In summary, a simple and versatile catalyst system has been
found for dehydrocoupling reactions of hydrophosphines. These
catalytic reactions appear to involve intermediate, mononuclear Rh
phosphido species which operate by the general mechanism depicted
(16) Complex 6 was separately generated via reaction of 1 with PhSH (see
Supporting Information).
JA065346+
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