Three different fates for phosphinidenes generated by photocleavage of phospha-wittig reagents ArP=PMe3

Full text of DOI:10.1021/ja015767l

J. Am. Chem. Soc. 2001, 123, 6925-6926
6925
Communications to the Editor
PMe3 may lead to photodissociation of PMe3 to produce free
phosphinidenes.
Three Different Fates for Phosphinidenes Generated
by Photocleavage of Phospha-Wittig Reagents
ArPdPMe3
Pale yellow solutions of 1 in benzene-d6 subjected to 355 nm
laser irradiation quickly (15 min) fade in color.9 Analysis of such
solutions by 31P and 1H NMR spectroscopy reveal that PMe3 and
4 (>95%), are produced in high yields (Scheme 1). Compound 4
is formally derived by insertion of the phosphinidene {Mes*P}
into a vicinal CH bond of a methyl group.
Shashin Shah, M. Cather Simpson,* Rhett C. Smith, and
John D. Protasiewicz*
Department of Chemistry
Case Western ReserVe UniVersity
CleVeland, Ohio 44106-7708
Scheme 1. Photolyses of ArPdPMe3
ReceiVed March 6, 2001
A longstanding challenge in the chemistry of free phosphin-
idenes (RP) has been the obtainment of definitive evidence for
their generation.1 The chemistry of free phosphinidenes remains
sparse, in contrast to free carbenes and other reactive intermedi-
ates, due in large part to the limited set of available precursors to
these electron deficient species. Although alkali metal reduction
of RPCl2 readily leads to cyclopolyphosphines (RP)n and, in select
cases, diphosphenes (RPdPR), mechanisms involving metalloid
or radical species are equally or more viable compared to
mechanisms involving simple phosphinidene intermediates. Ad-
ditionally, Mathey has stressed that identification of products
formally derived from phosphinidenes does not constitute absolute
proof, since alternative mechanisms can often be presented that
do not postulate phosphinidene intermediates.2 Photochemical
methods for the generation of phosphinidenes have produced some
of the best evidence for generation of these intermediates.3,4 In
this report we describe evidence for the photochemical generation
of free phosphinidenes from three closely related phospha-
nylidene-σ4-phosphoranes (ArPdPMe3). Quite amazingly, each
precursor spawns phosphinidenes that undergo different reaction
paths that lead to products of intramolecular CH bond insertion,
net dimerization to diphosphenes, or novel CC bond insertion.
We recently reported the synthesis and characterization of the
stable phosphanylidene-σ4-phosphoranes Mes*PdPMe3 (1) and
2,6-Mes2C6H3PdPMe3 (2).5 These materials act as phospha-Wittig
reagents upon reaction with aldehydes.6 Early studies of unstable
CF3PdPMe3 led to a recognition of the potential of RPdPR3 as
precursors to free phosphinidenes.7 Theoretical work suggests that
the LUMO in HPdPH3 is σ* in character,8 and thus we
investigated the possibility that photochemical irradiation of ArPd
The same phosphaindan is formed by a number of other
experiments that have been proposed to generate {Mes*P}, such
4
as photolysis of either Mes*P(N3)2 or the phosphirane Mes*P-
[CH2]2.3a In addition, the diphosphene Mes*PdPMes*,10 the
possible product from dimerization of two {Mes*P} units, is itself
photochemically active and will also produce 4 over time, quite
possibly by initial cleavage to two {Mes*P}.11 This process is
much slower than conversion of 1 to 4, and no Mes*PdPMes*
is detected by 31P NMR spectroscopy during the photolysis of 1.
The behavior of compound 2 bearing a m-terphenyl-protecting
group is strikingly different. The products of photolysis are the
diphosphene 2,6-Mes2C6H3PdPC6H3-2,6-Mes212 (5, 90-95%) and
PMe3 (Scheme 1). Diphosphene 5 is very stable to photolysis
under the reaction conditions, even in the presence of added PMe3.
The closely related diphosphene 2,6-Mes2-4-MeC6H3PdPC6H3-
4-Me-2,6-Mes2 is also inert to photolysis.13
Recent work from Power’s group suggested yet a new, third
possible reaction channel for free phosphinidenes.14 During the
reduction of 2,6-Trip2C6H3PCl2 with magnesium metal, a phos-
phafluorene (6, 68%), arising from formal intramolecular insertion
of a putative phosphinidene into an Ar-iPr carbon-carbon bond,
is obtained. The analogous reduction using potassium metal
provides the diphosphene 2,6-Trip2C6H3-PdP-C6H3-2,6-Trip as
the main product, however. Although these reductions of ArPCl2
(1) (a) Mathey, F. Angew. Chem., Int. Ed. Engl. 1987, 26, 275-286. (b)
Mathey, F. In Multiple Bonds and Low Coordination in Phosphorus Chemistry;
Regitz, M., Schere, O. J., Eds.; Thieme Verlag: Stuttgart, 1990; pp33-57.
(2) More recently, ESR observation of a triplet phosphinidene produced
by the photolysis of trans-2,3-dimethyl-1-mesitylphosphirane has been
achieved. See: Li, X.; Weissman, S. I.; Lin, T.-S.; Gaspar, P. P.; Cowley, A.
H.; Smirnov, A. I. J. Am. Chem. Soc. 1994, 116, 7899-7900.
(3) (a) Li, X.; Lei, D.; Chiang, M. Y.; Gaspar, P. P. J. Am. Chem. Soc.
1992, 114, 8526-8531. (b) Tsuji, K.; Sasaki, S.; Yoshifuji, M. Heteroatom
Chem. 1998, 9, 607-613.
(4) Cowley, A. H.; Gabba¨ı, F.; Schluter, R.; Atwood, D. J. Am. Chem.
Soc. 1992, 114, 3142-3144.
(9) Samples (ca. 0.04 M) were photolyzed in quartz NMR tubes (Wilmad)
in C6D6 using a nanosecond Nd:YAG laser (Continuum, Inc) at a wavelength
of 355 nm and power of 200-220 mW and monitored by 1H (yields are versus
internal standard) and 31P NMR spectroscopy.
(5) (a) Shah, S.; Protasiewicz, J. D. J. Chem. Soc., Chem. Commun. 1998,
1585-1586. (b) Shah, S.; Yap, G. P.; Protasiewicz, J. D. J. Organomet. Chem.
2000, 608, 12-20.
(10) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am.
Chem. Soc. 1981, 103, 4587-4589.
(6) Shah, S.; Protasiewicz, J. D. Coord. Chem. ReV. 2000, 210/1, 181-
201.
(11) (a) Yoshifuji, M.; Sato, T.; Inamoto, N. Chem. Lett. 1988, 1735-
1738. (b) Caminade, A. M.; Verrier, M.; Ades, C.; Paillous, N.; Koenig, M.
J. Chem. Soc., Chem. Commun. 1984, 875-877.
(7) (a) Burg, A. B.; Mahler, W. J. Am. Chem. Soc. 1961, 83, 2388-2389.
(b) Cowley, A. H.; Cushner, M. C. Inorg. Chem. 1980, 19, 515-518. (c)
Schmidpeter, A. In Multiple Bonds and Low Coordination in Phosphorus
Chemistry; Regitz, M., Schere, O. J., Eds.; Thieme Verlag: Stuttgart, 1990;
pp 338-351. (d) Fritz, G.; Scheer, P. Chem. ReV. 2000, 100, 3341-3402.
(8) (a) Sudhakar, P. V.; Lammertsma, K. J. Am. Chem. Soc. 1991, 113,
1899-1906. (b) Rak, J.; Skurski, P.; Liwo, A.; Blazejowski, J. J. Am. Chem.
Soc. 1995, 117, 2368-2648.
(12) Urnezius, E.; Protasiewicz, J. D. Main Group Chem. 1996, 1, 369-
372.
(13) Tsuji, K.; Fujii, Y.; Sasaki, S.; Yoshifuji, M. Chem. Lett. 1997, 855-
856.
(14) Twamley, B.; Sofield, C. D.; M. M. Olmstead, M. M.; Power, P. P.
J. Am. Chem. Soc. 1999, 121, 3357-3367.
10.1021/ja015767l CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/20/2001

6926 J. Am. Chem. Soc., Vol. 123, No. 28, 2001
Communications to the Editor
Scheme 2. Reactions of Free Phosphinidenes
may involve free phosphinidenes, pathways involving radicals
or metalloid species such as RP(Cl)MgCl cannot be ruled out.
Similar questions are unresolved in related insertions into C-C
bonds of m-terphenyl ligands by reactive boron and silicon
centers.15
To ascertain if a phosphinidene would insert into a carbon-
carbon bond under a metal-free environment we prepared the
phosphanylidene-σ4-phosphorane 2,6-Trip2C6H3PdPMe3 (3). Com-
pound 3 was prepared in 70% yield in the same manner as 1 and
2, that is, by the reduction of 2,6-Trip2C6H3PCl2 by zinc dust in
the presence of PMe3.16 Yellow crystalline 3 displays a pair of
diagnostic doublets in the 31P NMR spectrum at -1.6 and -113.4
ppm (JPP ) 564 Hz).
As anticipated, photolysis of 3 does indeed gives 6 in 90%
yield (equation 1) in addition to minor amounts of the corre-
sponding diphosphene. Our data thus support the proposition that
been noted.20 The phosphinidene derived from 3 could face a
greater steric barrier to phosphinidene abstraction from 3, thus
allowing enough time for the CC bond-insertion reaction to occur.
Alternatively, the various reactions may reflect the inherent
differences in the chemical reactivity of the meta substituents.
The current work is paralleled by similar studies aimed at
increasing the lifetimes of free carbenes by photochemical
extrusion of dinitrogen from sterically congested azoalkanes.
These carbenes can competitively dimerize to olefins or undergo
formal intramolecular CH bond insertions.21
In conclusion, we have demonstrated remarkable reaction
diversity for free phosphinidenes generated from phosphanylidene-
σ4-phosphoranes that differ only slightly in the nature of the
sterically demanding groups. It must also be noted that while
photolysis of ArPdPMe3 appears to be a general method, attempts
to generate m-terphenyl phosphinidenes using other standard
approaches are not viable. For example, extension of the
photochemistry of Mes*P(N3)2 to other ArP(N3)3 produces not
phosphinidenes but cyclic azido phosphazenes.22 Likewise, the
phosphirane 2,6-Mes2-4-MeC6H3P[CH2]2 has been shown to be
relatively photochemically inert and thus not effective for the
photoinduced retroaddition approach to phosphinidenes.3b Further
studies aimed at the ESR detection of these species, as well as
other trapping experiments, are underway.
free phosphinidenes may be involved in the magnesium reduction
of 2,6-Trip2C6H3PCl2.17
To assay the reversibility of the initial photoscission of 1-3,
photolyses were conducted in the presence of excess PMe3. For
reactions of 1 and 3, no significant impact on the rate of product
formation was observed, suggesting that both CH and CC bond
insertion processes of the phosphinidene are faster than trapping
with phosphine to reform 1 or 3. Interestingly, the presence of
added PMe3 during the photolysis of 2 considerably slows the
formation of 5 (Supporting Information), indicating that in this
case the phosphinidene can be trapped by added phosphine to
reform 2.
A deceptively simple explanation for formation of diphosphene
5 is the dimerization of two {ArP}. This hypothesis seems
unlikely, given the high reactivity of {ArP} derived from 1 and
3. A more likely scenario is that the electrophilic {ArP} derived
from 2 reacts with nucleophilic 2. An interesting consequence of
this proposal is that photolysis of 2 in the presence of increasing
amounts of 1 yields more asymmetric diphosphene Mes*Pd
18
PC6H3-2,6-Mes2 (7, Scheme 2).
One possible explanation for the highly diverse array of
products derived by photolysis of 1-3 is that the phosphinidenes
thus generated are increasingly stabilized by varying the substit-
uents on the phenyl ring.19 The phosphinidene {Mes*P}, being
the least stable, and having vicinal CH bonds near the phosphorus
center, undergoes rapid intramolecular CH bond insertion. The
phosphinidene derived from 2 may have a longer lifetime,
sufficient such that abstraction of phosphinidene unit from 2 can
occur to afford diphosphene 5. The stability of m-terphenyl
containing systems having reactive multiple bonds, relative to
systems bearing the somewhat comparably sized Mes* group, has
Acknowledgment. We thank the National Science Foundation
(CAREER CHE-9733412 to J.D.P.) and the National Institutes of Health
(GM-56816 to M.C.S.) for support of this work.
Supporting Information Available: Synthesis and characterization
of 3 and complete experimental details (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
(15) (a) Grigsby, W. J.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 7981-
7988. (b) Pietschnig, R.; West, R.; Powell, D. R. Organometallics 2000, 19,
2724-2729.
JA015767L
(20) Twamley, B.; Haubrich, S. T.; Power, P. P. AdV. Organomet. Chem.
1999, 1-65.
(16) See Supporting Information for full details.
(17) Interestingly, compound 6 is photoreactive and slowly produces two
new species. Efforts to identify these secondary photoproducts are underway.
(18) Compound 7 (31P{1H} NMR (C6D6): δ 526.0 (d, JPP ) 571 Hz), 455.3
(d, JPP ) 571 Hz)) has been independently prepared and structurally
characterized. Details to be published elsewhere.
(21) (a) Tomioka, H. Acc. Chem. Res. 1997, 30, 315-321. (b) Tomioka,
H.; Okada, H.; Watanabe, T.; Banno, K.; Komatsu, K.; Hirai, K. J. Am. Chem.
Soc. 1997, 119, 1582-1593. (c) Bourissou, D.; Guerret, O.; Gabbai, F. P.;
Bertrand, G. Chem. ReV. 2000, 100, 39-91.
(22) Wehmschulte, R. J.; Masood, A. K.; Hossain, S. I. Inorg. Chem. 2001,
40, 2756-2762.
(19) Compounds 1-3 are thermally stable under the reaction conditions.