Nonmetal-mediated fragmentation of P4: Isolation of P 1 and P2 bis(carbene) adducts

Full text of DOI:10.1002/anie.200902344

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DOI: 10.1002/anie.200902344
À
P P Activation
Nonmetal-Mediated Fragmentation of P4: Isolation of P1 and P2
Bis(carbene) Adducts**
Olivier Back, Glenn Kuchenbeiser, Bruno Donnadieu, and Guy Bertrand*
Transition metals are well-known for activating small mole-
cules and for stabilizing highly reactive species. A recent trend
in the accomplishment of either of these goals is the use of
nonmetals,[1] especially stable singlet carbenes. Robinson and
co-workers have reported that N-heterocyclic carbenes
(NHCs) give rise to stable adducts with HBBH[2] and Si2 ,[3]
which are otherwise not isolable.[4] We have shown that cyclic
(alkyl)(amino)carbenes (CAACs) can activate CO,[5] H2 ,[6]
and even NH3 ,[7] which is difficult when attempted with
transition-metal centers.[7,8]
White phosphorus (P4) is a small molecule that is of
industrial interest as it is the classical starting material for the
large-scale preparation of organophosphorus derivatives.[9]
The reactivity of P4 with transition metals has been widely
studied,[10–12] and is therefore an excellent model to further
test if carbenes can undergo reactions in the same manner as
transition metals. Our research group has already shown that
the bulky rigid CAAC 1 opens P4 and simultaneously
stabilizes the resulting acyclic P4 species (A,[13a] Scheme 1),
fragmentation of P4 , which can be accomplished by using
transition metals.[10] Cummins et al. have even shown that the
resulting P1– and P2–niobium complexes can be used as
phosphorus transfer agents.[14]
Herein we report that carbenes can induce the fragmen-
tation of white phosphorus in the same manner as transition
metals. Depending on the nature of the activator, carbene-
stabilized P2 and P1 species can be isolated. Preliminary
mechanistic studies of the reaction that leads to A and B, in
particular trapping experiments, showed that both carbenes 1
and 2 first formed an unstable monocarbene adduct of type C.
A second carbene molecule subsequently induces a ring-
opening reaction and the formation of bis(carbene) adducts of
type A.[13] The different outcomes of the reactions shown in
Scheme 1 can be rationalized by the different electronic
properties of CAACs and NHCs. CAACs are more electro-
philic (p acceptor) and strengthen the PC bonds of A,[5,6]
while NHCs are less basic and are therefore better leaving
groups, and favor the formation of clusters such as B. This
analysis indicates that strongly basic but electrophilic car-
benes should be the best candidates to induce the fragmenta-
tion of white phosphorus, provided that they are small enough
to further attack the P4 fragments of adducts of type A or C.
According to our previous investigations, the acyclic
(alkyl)(amino)carbene 3[15] (Scheme 2) is one of the most
Scheme 1. Activation and aggregation of white phosphorus by singlet
carbenes. Dipp=2,6-diisopropylphenyl.
Scheme 2. Reaction of white phosphorus with acyclic (alkyl)-
(amino)carbene 3.
which is otherwise highly reactive. Moreover, when a bulky
NHC 2 was used, the NHC-stabilized P12 cluster B was
isolated in high yield.[13b] Therefore, singlet carbenes can
activate and induce the aggregation of white phosphorus and
stabilize the resulting species, in a similar manner to transition
metals. However, the most synthetically useful organophos-
phorus derivatives contain only one or two phosphorus atoms,
and therefore it is of primary importance to induce the
electrophilic stable carbenes known. An excess of carbene 3
(3.5 equivalents) was added to a suspension of white phos-
phorus in ether. After two hours at room temperature and
subsequent workup, the bis(carbene) P4 adduct D was
isolated as light-yellow crystals in 66% yield (based on P4),
and its bis(carbene) P4 adduct structure was unambiguously
assigned by a single crystal X-ray diffraction study[16]
(Figure 1). This bicyclic compound clearly results from the
cycloaddition of carbene 3 onto the adduct of type C.
This result indicates that the acyclic (alkyl)-
(amino)carbene 3 is sufficiently nucleophilic to open the P4
tetrahedron, but so electrophilic that it undergoes a cyclo-
propanation reaction, rather than inducing the ring-opening
of the initally formed adduct C. Therefore, we chose the
cyclohexyl CAAC 4,[17] in the hope that such a small CAAC
would be able to induce the formation of an adduct of type A,
but then react again and induce the fragmentation of the P4
chain. Upon addition of three equivalents of CAAC 4 to an
[*] O. Back, G. Kuchenbeiser, B. Donnadieu, Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957),
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/
guybertrandwebpage/
[**] Financial support from the NSF (CHE 0808825) and Rhodia Inc. are
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902344.
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mation of tris(carbene) P4 adduct E implies that two CAACs 4
react on the initially formed adduct of type C.
The minor product F was determined to be the desired
bis(carbene) P2 adduct, and therefore the reaction that leads
to F represents the first example of fragmentation of P4 with a
neutral organic species. Interestingly, compound F is reminis-
cent of compounds F’[18] and F’’,[19] which were prepared as
shown in Scheme 4, and characterized as 2,3-diphosphabuta-
diene and NHC-stabilized bis(phosphinidene), respectively.
Figure 1. Thermal ellipsoids plot (40% probability surface) for D
(hydrogen atoms omitted for clarity). Selected bond distances [ꢀ] and
angles [8]: P(1)–P(2) 2.2320(19), P(2)–P(3) 2.220(2), P(2)–P(4)
2.2342(17), P(3)–P(4) 2.1700(19), C(1)–P(1) 1.747(2), C(18)–P(3)
1.912(2), C(18)–P(4) 1.925(2), N(1)–C(1) 1.3893(19), N(2)–C(18)
1.4472(18); P(1)-P(2)-P(3) 93.29(3), P(1)-P(2)-P(4) 91.99(6), P(3)-P(2)-
P(4) 58.31(7), P(2)-P(4)-P(3) 60.51(5), P(3)-P(4)-P(2) 60.51(5).
suspension of P4 in ether, two new products E and F were
isolated in moderate yields (67% and 12%, respectively,
based on P4 ; Scheme 3).
Scheme 4. Reported syntheses[18,19] and electronic structure of com-
pounds related to F.
The 31P NMR chemical shift for F (d = + 59.4 ppm) is com-
parable to that of F’ (d = 54.2 ppm), and is at a lower field
À
than F’’ (d = À52.4 ppm); similarly, the P C bond length
(Figure 3) in F (1.719(7) ꢀ) is shorter than in F’’ (1.750(2) ꢀ).
Both of these data indicate that F has a 2,3-diphosphabuta-
diene structure, which is expected because of the higher
electrophilicity of CAACs than NHCs.
Scheme 3. Reaction of white phosphorus with the nonhindered CAAC
4.
The 31P NMR spectrum of the major product E shows a
doublet signal at d = + 68.1 ppm and a quartet signal at d =
À66.2 ppm, with a large P–P coupling constant (227 Hz).
These data indicated the presence of three magnetically
equivalent P nuclei that were directly connected to another
P center. This hypothesis was confirmed by a single crystal X-
ray diffraction study (Figure 2). Very surprisingly, the for-
Figure 3. Thermal ellipsoids plot (40% probability surface) for F
(hydrogen atoms omitted for clarity). Selected bond distances [ꢀ] and
angles [8]: Selected bond distances [ꢀ] and angles [8]: P(1a)–P(2a)
2.184(3), C(1a)–P(1a) 1.719(7), N(1a)–C(1a) 1.387(9); C(1a)-P(1a)-
P(2a) 105.1(2); C(1a)-P(1a)-P(2a)-C(24a) 149.22.
Compound F most probably results from the attack of the
carbene to the b-phosphorus centers of a bis(carbene) adduct
of type A. In order to obtain a P1 adduct by attack at a
phosphorus center in a position that is a to the carbene(s) of
adducts A or C, we have used the least sterically demanding
stable carbene known to date, namely bis(diisopropylami-
no)cyclopropenylidene (5).[20] The 31P NMR spectrum of the
reaction mixture of P4 and three equivalents of carbene 5 after
12 hours at room temperature revealed an ABX system (dA =
Figure 2. Thermal ellipsoids plot (40% probability surface) for E
(hydrogen atoms omitted for clarity). Selected bond distances [ꢀ] and
angles [8]: P(1a)–P(2a) 2.2168(6), P(1a)–P(3a) 2.2270(6), P(1a)–P(4a)
2.2192(7), C(1a)–P(2a) 1.7328(19), C(24a)–P(3a) 1.7343(18), C(47a)–
P(4a) 1.7324(18), N(1a)–C(1a) 1.374(2), N(2a)–C(24a) 1.370(2),
N(3a)–C(47a) 1.368(2); P(2a)-P(1a)-P(4a) 90.15(2), P(3a)-P(1a)-P(4a)
90.15(2), P(2a)-P(1a)-P(4a) 90.15(2), C(1a)-P(2a)-P(1a) 108.22(6), C-
(24a)-P(3a)-P(1a) 106.62(6), C(47a)-P(4a)-P(1a) 108.45(7).
242.8, dB = 237.0, dX = 157.0 ppm, JAX = À484.7 Hz, JAB
=
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resulting suspension stirred for 2 h. The solvent was removed under
vacuum, and the solid residue washed with hexane (6 mL) at À358C
to afford D (480 mg, 66.4% based on P4). Single crystals of D were
+ 38.8 Hz, JBX = À481.1 Hz) and a singlet signal at d =
À93.2 ppm. All attempts to purify the mixture led to the
disappearance of the second-order system, but, in the
presence of chloroform or any salts, the high-field singlet
remained unchanged. When chloroform was used, single
crystals suitable for X-ray diffraction readily formed upon
standing at room temperature. The isolated product was the
bis(carbene) P1 cation G with ClÀ as counterion (obtained in
74% yield based on P4; Scheme 5, Figure 4). Analogous
grown by layering acetonitrile over a THF solution. m.p. 1508C;
1
31P{1H} NMR (202.5 MHz, [D8]THF, 258C): d = 238.8 (dt, JPP
=
=
2
220 Hz and 2JPP = 87 Hz), À105.8 (dt, 1JPP = 220 Hz and JPP
167 Hz), À168.2 ppm (dd, JPP = 167 Hz and JPP = 87 Hz); 13C NMR
(125.75 MHz, [D8]THF, 258C): d = 227.6 (d, 1JPC = 89 Hz, Ccarb),
101.5 ppm (br s, Ccyclo).
1
2
Synthesis of E and F: A solution of carbene 4 (3.25 g, 10.01 mmol)
in ether (20 mL) was added at room temperature to a suspension of P4
(0.41 g, 3.34 mmol) in ether (20 mL). The mixture was stirred at room
temperature for 2 h and then half of the solvent was removed under
vacuum. The remaining solution was cooled to À308C and the
resulting precipitate filtered to give F as a bright yellow powder
(580 mg, 12.1% yield based on P4). Evaporation of the filtrate gave a
dark red powder, which was washed three times with acetonitrile (3 ꢁ
30 mL), and dried under vacuum to give E as a bright-yellow powder
(2.5 g, 67.4% yield based on P4). Single crystals were grown from a
saturated solution of F in diethyl ether at room temperature, and
single crystals of E were obtained by slow evaporation of a solution of
Scheme 5. Reaction of P4 with cyclopropenylidene 5.
E
in hexane at room temperature. E: m.p. 1428C; 13C NMR
(125.75 MHz, [D6]benzene, 258C): d = 207.4 ppm (m, Ccarb). F: m.p.
2168C; 13C{1H} NMR (125.75 MHz, [D6]benzene, 258C): d =
202.2 ppm (dd, 1JPC = 32 Hz, 2JPC = 26 Hz, Ccarb).
Synthesis of GClÀ: White phosphorus (P4; 0.17 g, 1.40 mmol) was
added to a solution of carbene 5 (1.00 g, 4.24 mmol) in THF (5 mL).
After stirring for 12 h at room temperature, chloroform (5 mL) was
added to the crude reaction mixture. After stirring at room temper-
ature for 30 min, the volatile components were removed under
vacuum, and the resulting red solid washed with Et2O (3 ꢁ 20 mL) to
afford a dark-orange powder. Yield: 564 mg (74% based on P4).
Single crystals of GCl were readily obtained by slow evaporation of a
solution of GCl in chloroform at room temperature. m.p. 1748C
Figure 4. Thermal ellipsoids plot (40% probability surface) for GCl. H
atoms are omitted for clarity. Selected bond lengths [ꢀ] and angles
[8]:P(1)–C(1) 1.787(2), P(1)–C(16) 1.788(2), N(1)–C(2) 1.323(3), N(2)–
C(3) 1.328(3), C(1)–C(3) 1.397(3), C(1)–C(2) 1.404(3), C(2)–C(3)
1.388(3); C(1)-P(1)-C(16) 104.68(9).
1
(dec.); H NMR (300 MHz, CDCl3, 258C): d = 3.79 (sept, J = 6.5 Hz,
4H), 1.25 ppm (d, J = 6.5 Hz, 24H); 13C{1H} NMR (75 MHz, CDCl3,
258C): d = 136.5 (Cring), 123.7 (d, 1JCÀP = 104.6 Hz, Ccarb), 51.3,
21.5 ppm. For compounds D–G, only selected NMR data are given.
For full details, see the Supporting Information.
(NHC)2P+ systems that used (R3P)2P+, (RNPCl)2 , or PCl3 as
PI source have previously been reported by Schmidpeter
et al.[21] and MacDonald et al.[22] On the other hand, although
we have not yet been able to identify the P3 fragment, its
anionic nature is clear, and therefore it is likely to be an allylic
triphosphorus anion that is unsymmetrically substituted by
cyclopropenylidenes.
Received: May 2, 2009
Published online: June 19, 2009
Keywords: carbenes · cyclopropenylidenes · fragmentation ·
.
À
phosphorus · P P activation
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P4 , namely, activation, aggregation, and importantly, frag-
mentation, which have previously been carried out by using
transition metals, can also be achieved by using stable singlet
carbenes. The next challenge is to use the resulting adducts,
especially F and G, to prepare useful organophosphorus
derivatives. This procedure would avoid the use of Cl2 gas,
which is important to meet the growing demand for phos-
phorus derivatives that are produced by using environmen-
tally friendly processes.
Experimental Section
All manipulations were performed under an atmosphere of dry argon
by using standard Schlenk techniques.
Synthesis of D: Ether (40 mL) was added to a mixture of carbene
3 (4.07 mmol) and P4 (1.16 mmol) at room temperature and the
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