Using monovalent phosphorus compounds to form P-C bonds

Article abstract of DOI:10.1002/anie.201306643

User friendly P-C: Electrophilic terminal phosphinidene complexes [RP-W(CO)₅] react with Ar-B(OH)₂ to give the corresponding secondary phosphine complexes [{R(Ar)PH}W(CO)₅]. This method enables the formation of P-C bonds through the combination of a wide array of heterocyclic and aryl boronic acids with various electrophilic phosphinidene complexes. The final products incorporate a P-H bond that can be used for further transformations. Copyright

Full text of DOI:10.1002/anie.201306643

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Angewandte
Communications
DOI: 10.1002/anie.201306643
Phosphinidene Complexes
Using Monovalent Phosphorus Compounds to Form P–C Bonds**
Yong Xiang Ng and Francois Mathey*
Table 1: Reaction of 7-phosphanorbornadienes 1 with arylboronic acids
2. Th=thienyl, Fu=furyl.
Since its inception in 1855 with the work of Hofmann on
trialkyphosphines,[1] organophosphorus chemistry has relied
À
on the tools available for forming P C bonds. Not surpris-
1 R
2 Ar
Reaction time [h]
3 Yield [%]
ingly, a lot of work has been done on this topic.[2] The most
1a Ph
1a Ph
1a Ph
1a Ph
1b Me
1b Me
1b Me
1b Me
1c Bn
1c Bn
1d CH2CH2Cl
1e OMe
2a Ph
2b 2-Th
2c 2-Fu
2d 4-MeO-C6H4
2a Ph
2b 2-Th
2c 2-Fu
2d 4-MeO-C6H4
2a Ph
2b 2-Th
2b 2-Th
2a Ph
17
17
15
17
24
24
14
16
18
18
24
3
3a 48
3b 76
3c 37
3d 52
3e 46
3f 69
3g 20
3h 45
3i 52
3j 65
3k 35
3l 10
recent advances concern the use of catalysts.[3] Herein, we
describe a new route to P C bonds. The flourishing develop-
À
ment of the chemistry of electrophilic terminal phosphinidene
complexes[4] that derive from monovalent phosphorus offers
À
the possibility of creating a new approach to P C bonds, one
that relies on the conversion of PI into PIII just as the classical
Arbuzov reaction relies on a PIII-to-PV transformation.
Unfortunately, to date, no reasonably general synthesis of
À
P C bonds from phosphinidenes is available, except for
a range of versatile cycloaddition reactions with unsaturated
hydrocarbons. The literature contains a few reports of
[5]
À
phosphinidene insertions into special C H bonds and two
examples of reactions with soft carbon nucleophiles, that is,
malonate anions[6] and stabilized phosphorus ylids.[7] Based on
these last results, it appeared that the key to success was the
choice of an appropriate mild carbon nucleophile. This led us
to consider the possible use of the organoborates widely used
in the Suzuki cross-coupling reaction.[8]
Our preliminary experiments were carried out with the 7-
phenyl-7-phosphanorbornadiene precursor 1a (R = Ph),[9a]
phenylboronic acid 2a (Ar= Ph), and a variety of bases.
The best conditions are given in Equation (1):
verified that the reaction is quite general as shown in Table 1.
The X-ray crystal structure of 3b is shown in Figure 1.
The mechanism of this reaction is necessarily complex
since the role of the base is to allow the transfer of the aryl
group from the electrophilic boronic acid to the electrophilic
phosphinidene complex. What is known is that a simple base
such as an amine is totally inefficient because it gives a loose
adduct with the phosphinidene, which becomes nucleo-
philic.[11] A possible explanation would be that a single
anionic phosphate complexes both the phosphinidene and the
boronic acid and thus allows the transfer of the aryl
anion from boron to phosphorus to give a phosphido
complex. To find evidence for this phosphido com-
plex, we repeated the reaction with 7-b-chloroethyl-
7-phosphanorbornadiene (1d)[9b] [Eq. (2)].
According to the 31P monitoring of the reaction mixture,
the reaction is extremely clean. The reaction must be run
under strictly anhydrous conditions since terminal phosphi-
À
nidene complexes readily insert into the O H bond of
water.[10] Potassium phosphate appears to be the best base;
potassium carbonate and triethylamine are inefficient. Cata-
lysts such as CuCl or [Pd2(dba)3] (dba = dibenzylidenacetone)
lower the yields and induce the formation of by-products. We
The formation of the phosphiranes 4 and 5 supports the
intermediacy of a nucleophilic phosphido intermediate. Note
that the nucleophilicity of this intermediate strongly depends
on the nature of the aryl group since 3k does not fully cyclize
under the reaction conditions used.
The next step of our investigation concerned the possible
use of alkylboronic acids. Surprisingly, we obtained the
reduction product 6 [Eq. (3)].
In the Suzuki cross-coupling, the intermediate alkylpalla-
dium species can undergo b or a-H elimination with
formation of a palladium hydride, an observation that
explains why the coupling reaction fails. This type of b or a-
[*] Y. X. Ng, Prof. F. Mathey
Division of Chemistry & Biological Chemistry
Nanyang Technological University
21 Nanyang Link, Singapore 637371 (Singapore)
E-mail: fmathey@ntu.edu.sg
[**] We thank Dr. Yongxin Li for the X-ray crystal structure analysis of 3b
and the Nanyang Technological University in Singapore for the
financial support of this work.
Supporting information (including experimental procedures) for
this article is available on the WWW under http://dx.doi.org/10.
1002/anie.201306643.
14140
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Changing the conditions and using the milder sodium
tetraphenylborate, more satisfactory but still insufficient
results were obtained [Eq. (6)].
À
This new method for forming P C bonds has great
potential because it can use the wide array of aryl and
heterocyclic boronic acids that has been developed for the
popular Suzuki cross-coupling reaction and combine them
with the numerous electrophilic phosphinidene complexes
that have been reported. Furthermore, the final products
Figure 1. X-ray crystal structure of 3b. Main bond lengths [ꢀ] and
angles [deg]: P–W 2.4940(10), P–C6 1.816(4), P–C12 1.799(15); C6-P-
C12 100.0(4), C6-P-W 119.10(14), C12-P-W 120.0(6).[13]
À
incorporate a P H bond that can be used for further
transformations.
Received: July 30, 2013
Revised: September 25, 2013
Published online: November 19, 2013
Keywords: boronic acids · phosphinidenes · phosphorus–
.
carbon bonds · secondary phosphines · tungsten
H elimination cannot take place at phosphorus, so we have no
simple explanation for why the coupling failed in this case.
The final experiments concerned the possible use of
alkoxy- and aminophosphinidenes in this kind of reaction.
The reaction of methoxyphosphinidene, produced from the
appropriate 7-phosphanorbornadiene precursor 1e,[9c] proved
to be less satisfactory than the use of P–Me or P–Ph
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Similarly, using the 1-phenylaminophosphirane complex 8
as the phosphinidene precursor,[12] we noticed the high
À
reactivity of the P N bond, an effect that can supersede the
cycloreversion in the case of the reaction with 2-thienylbor-
onic acid. This leads to a complex mixture of products
[Eq. (5)].
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data for this paper. These data can be obtained free of charge
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ccdc.cam.ac.uk/data_request/cif.
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