Phosphole-N-phenylmaleimide [4+2] cycloadducts as synthetic equivalents of nucleophilic phosphinidenes

Article abstract of DOI:10.1039/b901154d

Using 1-R-3,4-dimethylphosphole-N-phenylmaleimide cycloadducts as synthetic equivalents of phosphinidenes [R-P], the following sequence has been developed: [R-P] + R¹X → RR¹PX → RR¹P-OR ². The Royal Society of C

Full text of DOI:10.1039/b901154d

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Phosphole-N-phenylmaleimide [4+2] cycloadducts as synthetic
equivalents of nucleophilic phosphinidenes
Rongqiang Tian,a Zheng Duan,*a Xiaoxu Zhou,a Dingjin Geng,a Yangjie Wua
and Franc¸
ois Mathey*ab
Received (in Cambridge, UK) 19th January 2009, Accepted 13th March 2009
First published as an Advance Article on the web 26th March 2009
DOI: 10.1039/b901154d
Using 1-R-3,4-dimethylphosphole-N-phenylmaleimide cycloadducts
as synthetic equivalents of phosphinidenes [R–P], the following
sequence has been developed: [R–P] + R1X - RR1PX -
RR1P–OR2.
in situ by iodomethane or benzyl bromide. Next, the resulting
phosphonium salts, 3, were subjected to alcoholysis in the
presence of triethylamine at room temperature. Our reasoning
was that the intermediate pentacoordinate species would
collapse easily. In fact, we observed the release of the
phosphorus bridge at room temperature (Scheme 1). The
formation of the expected phosphinites, 4, was monitored by
31P NMR, and they were fully characterised as their P-sulfides,
5.w The overall yields of pure 5 from 1 were in the range
27.5–55.0%. What we have is a synthetic sequence that is
equivalent to the unprecedented oxidative addition of RX to a
nucleophilic phenyl-phosphinidene.
Although there are several methods to generate them,1
transient free phosphinidenes [R–P] have seen only limited
use as synthetic tools in organophosphorus chemistry until
now. Among the reasons for this state of affairs, one is
certainly that most phosphinidenes have a triplet ground
state,2 favouring oligomerisation and other erratic reactions.
In the few cases where the phosphinidene ground state is a
singlet, e.g. [R2P–P], an interesting cycloaddition chemistry
It remains to be seen whether this sequence can be
transposed to other phosphinidenes. We were not able to
duplicate the synthetic sequence with 1,3,4-trimethylphosphole,
but we were successful with 1-n-pentyl-3,4-dimethylphosphole,
prepared in situ from 1 (Li in THF, then 1-bromopentane),
subjecting it without purification to the same series of
reactions as described in Scheme 1. Phosphinite 6 and its
sulfide 7 (Scheme 2) were characterized in the usual way.
The overall yield of pure 7 from 1 was an acceptable 22.6%.
Similarly, we were able to convert the 1-bithienyl-3,4-di-
methylphosphole, prepared as described in the literature,9 into
the corresponding phosphinite, 8, in a 21.7% overall yield. The
synthetic equivalency between phosphinidenes and phosphole-
N-phenylmaleimide cycloadducts is thus quite general.
According to 31P NMR solution monitoring, the phosphorus
bridge of phosphonium salts 3 collapses as soon as the
3
has been developed; however, of course, its scope is very
limited. This is why the carbene-like reactivity of electrophilic
singlet terminal phosphinidene complexes has been so
extensively studied recently.4 The drawback to this approach
is that the products resulting from this chemistry are formed in
the coordination sphere of a metal, whose elimination is often
quite difficult. One way to circumvent this problem is to
replace the phosphinidene by a synthetic equivalent, RPX2,
displaying normal P(III) behaviour and susceptible to losing X2
by reductive elimination during the reaction pathway. With
this idea in mind, our attention was drawn to the easy
reductive elimination that takes place under moderate heating
in pentacoordinate phospholenes with the loss of dienes.5 On
this basis, we decided to investigate the possibility of using the
[4+2] cycloadducts between 1-R-3,4-dimethylphospholes and
N-phenylmaleimide6 as synthetic equivalents of [R–P]. Several
reasons guided our choice: the strain of the bicyclic structure
must favour the collapse of the phosphorus bridge; such
structures have already been used to generate [R–PQS] species
under UV irradiation.7 A wide range of R substituents are also
easily accessible for phospholes.8
Our initial experiments were carried out with the already
described cycloadduct of 1-phenyl-3,4-dimethylphosphole, 1,
with the maleimide.6 Since we tried to devise a practical
reaction sequence, neither the adduct nor the subsequent
intermediate products were isolated; we simply monitored
the reaction medium for completion of the various
transformations by 31P NMR. Adduct 2 was quaternised
a Chemistry Department, International Phosphorus Laboratory,
Zhengzhou University, Zhengzhou 450052, P.R. China.
E-mail: duanzheng@zzu.edu.cn
b Nanyang Technological University, Division of Chemistry and
Biological Chemistry, SPMS-04-01, 21 Nanyang Link,
Singapore 637371, Singapore. E-mail: fmathey@ntu.edu.sg
Scheme 1 Reagents and conditions: (i) N-phenylmaleimide, 100 1C,
3 h, xylene; (ii) RX, from r.t. to 110–120 1C, 4–5 h; (iii) Et3N, then,
after 20 min, R1OH, 30 min., r.t.; (iv) sulfur, r.t.
ꢀc
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Exact mass calc. for C17H23NOPS: 320.1238 [M + H]+. Found:
320.1238.
4d: 31P NMR: d 120.9.
5d: yield 40% from 1; 31P NMR (CDCl3): d 92.9; 1H NMR
2
(CDCl3): d 2.23 (d, 3H, JHP = 14.1 Hz, PMe), 2.91 (s, 6H, NMe),
3.47–3.49 (m, 2H, NCH2), 3.86–3.96 (m, 1H, OCH2), 4.40–4.49
(m, 1H, OCH2), 7.50–8.08 (m, 5H, Ph); 13C NMR (CDCl3): d 22.03
(d, JCP = 81.9 Hz, PMe), 41.49 (s, NMe), 54.84 (d, JCP = 8.5 Hz,
1
CH2), 55.29 (d, JCP = 4.9 Hz, CH2), 126.72 (d, JCP = 13.0 Hz,
Ph-CH), 129.21 (d, JCP = 11.9 Hz, Ph-CH), 129.92 (d, 1JCP = 95.7 Hz,
Ph-Cipso), 130.73 (d, JCP = 3 Hz, Ph-CHpara). Exact mass calc. for
C11H19NOPS: 244.0925 [M + H]+. Found: 244.0925.
Scheme 2 Synthetic equivalence with phosphinidenes [R2P].
4e: 31P NMR: d 117.4.
triethylamine is added to the reaction mixture. In the case of 3e,
we observed
5e: yield 27.5% from 1; 31P NMR (CDCl3): d 90; 1H NMR (CDCl3):
d 3.72 (unresolved ABX, 2H, CH2), 6.93–7.89 (m, 15H, Ph); 13C NMR
(CDCl3): d 45.11 (d, 1JCP = 73.8 Hz, PCH2), 121.43 (d, JCP = 5.1 Hz,
PhO-Cortho), 124.70 (d, JCP = 1.3 Hz, PhO-Cpara), 127.22 (d, JCP = 4.1
Hz, CH2Ph-Cpara), 128.22 (d, JCP = 3.5 Hz, CH2Ph-CH), 128.38 (d,
a complex series of signals in the range
d À35.4 to +74.7, including an AB system at d À30.5 and
À34.5 (JAB = 223 Hz), which suggests the formation of a
dinuclear P(V) intermediate with a P–P bond. Upon adding
phenol, all of these signals almost completely disappeared. The
mechanism probably involves a nucleophilic attack of the
triethylamine onto the positive phosphonium centre, similar to
that observed in the reaction of N-methylimidazole with
7-phosphanorbornadiene complexes.10 Finally, it must be
stressed that another P–C bond can be easily created from 4
through an Arbuzov reaction. We therefore have at our disposal
a new tool that allows P-chiral phosphines to be built at will.
The authors thank the National Natural Science Foundation
of China (no. 20702050) and Zhengzhou University for
financial support.
JCP = 13.1 Hz, Ph-CH), 129.34 (d, JCP = 0.8 Hz, PhO-Cmeta), 130.46
(d, JCP = 5.9 Hz, CH2Ph-CH), 130.95 (d, JCP = 7.8 Hz,
2
1
CH2Ph-Cipso), 132.01 (d, JCP = 10.9 Hz, Ph-CH), 132.36 (d, JCP
98.6 Hz, Ph-Cipso), 132.43 (d, JCP = 3.0 Hz, Ph-Cpara), 150.78 (d, 2JCP
=
=
.
9.8 Hz, PhO-Cipso). Exact mass calc. for C19H18OPS: 325.0816 [M + H]+
Found: 325.0815.
6: 31P NMR: d 136.9.
7: yield 22.6% from 1; 31P NMR (CDCl3): d 102.0; 1H NMR
(CDCl3): d 0.88 (t, 3H, Me), 1.30 (m, 4H, CH2), 1.60 (m, 2H, CH2),
1.82 (m, 2H, CH2), 3.36 (unresolved ABX, 2H, PhCH2), 3.60 (d, 3H,
3JHP = 13.2 Hz, OMe), 7.24–7.36 (m, 5H, Ph); 13C NMR (CDCl3): d
13.85 (s, Me), 22.05 (d, JCP = 3.8 Hz, CH2), 22.17 (s, CH2), 32.48 (d,
JCP = 6.9 Hz, CH2), 33.06 (d, 1JCP = 48.7 Hz, PCH2), 42.40 (d, 1JCP
= 63.4 Hz, PCH2Ph), 51.77 (d, 2JCP = 7.0 Hz, OMe), 127.16 (d, JCP
= 3.7 Hz, CH2Ph-Cpara), 128.55 (d, JCP = 3.0 Hz, CH2Ph-CH),
2
129.86 (d, JCP = 5.4 Hz, CH2Ph-CH), 131.78 (d, JCP = 8.3 Hz,
CH2Ph-Cipso). Exact mass calc. for C13H22OPS: 257.1129 [M + H]+
Found: 257.1129.
Notes and references
.
w 4a: 31P NMR: d 118.4.
8: 31P NMR: d 112.3.
5a: yield 49.2% from 1; 31P NMR (CDCl3): d 89.7; 1H NMR
(CDCl3): d 2.01 (d, 3H, 2JHP = 13.8 Hz, Me), 3.56 (d, 3H, 3JHP = 14.1
Hz, OMe), 7.29–7.96 (m, 5H, Ph); 13C NMR (CDCl3): d 23.65
9: yield 21.7% from the bithienylphosphole; 31P NMR (CDCl3): d
80.6; 1H NMR (CDCl3): d 3.59 (unresolved ABX, 2H, PhCH2), 3.64
(d, 3H, JHP = 14.1 Hz, OMe), 7.01–7.29 (m, 10H, Ph-CH and
3
1
2
(d, JCP = 83.1 Hz, PMe), 51.02 (d, JCP = 6.0 Hz, OMe), 128.53
Th-CH); 13C NMR (CDCl3): d 45.20 (d, 1JCP = 80.7 Hz, PCH2Ph), 51.70
(d, JCP = 12.8 Hz, Ph-CH), 131.00 (d, JCP1 = 11.3 Hz, Ph-CH), 132.18
(d, JCP = 2.9 Hz, Ph-Cpara), 133.14 (d, JCP = 99.4 Hz, Ph-Cipso).
2
(d, JCP = 7.0 Hz, OMe), 124.51 (d, JCP = 13.7 Hz, CH-Th), 125.19
4
4b: 31P NMR: d 123.3.
(s, Th-CH), 126.07 (s, Th-CH), 127.28 (d, JCP = 4.2 Hz, CH2PhCpara),
128.16 (s, Th-CH), 128.30 (d, JCP = 3.6 Hz, CH2PhCH), 130.32
(d, JCP = 6.1 Hz, CH2Ph-CH), 131.03 (d, 2JCP = 8.4 Hz, CH2Ph-Cipso),
131.72 (d, 1JCP = 105.7 Hz, C–P-Th), 135.95 (s, Th-C), 137.83 (d, JCP
= 9.7 Hz, Th-C), 146.46 (d, JCP = 5.4 Hz, Th-C). Exact mass calc. for
C16H16OPS3: 351.0101 [M + H]+. Found: 351.0138.
5b: yield 55.0% from 1; 31P NMR (CDCl3): d 90; 1H NMR
2
(CDCl3): d 3.51 (unresolved ABX, 2H, CH2), 3.59 (d, 3H, JHP
=
13.5 Hz, OMe), 7.00–7.73 (m, 10H, Ph); 13C NMR (CDCl3): d 44.59
(d, 1JCP = 73.9 Hz, PCH2), 51.53 (d, 2JCP = 6.6 Hz, OMe), 127.00 (d,
JCP = 4.0 Hz, CH2Ph-Cpara), 128.12 (d, JCP = 3.47 Hz, CH2Ph-CH),
128.28 (d, JCP = 12.7 Hz, Ph-CH), 130.25 (d, JCP = 5.8 Hz, CH2Ph-CH),
1 J. C. Slootweg and K. Lammertsma, Sci. Synth., 2008, 42, 15.
2 M. T. Nguyen, A. Van Keer and L. G. Vanquickenborne, J. Org.
Chem., 1996, 61, 7077.
3 G. Fritz, T. Vaahs, H. Fleischer and E. Matern, Angew. Chem., Int.
Ed. Engl., 1989, 28, 315.
4 Recent references: R. Waterman, Dalton Trans., 2009, 18; M. Rani,
Synlett, 2008, 2078; F. Mathey, Dalton Trans., 2007, 1861.
5 P. J. Hammond, J. R. Lloyd and C. D. Hall, Phosphorus Sulfur
Relat. Elem., 1981, 10, 47.
2
131.26 (d, JCP = 7.9 Hz, CH2Ph-Cipso), 131.77 (d, JCP = 10.6 Hz,
Ph-CH), 131.78 (d, JCP = 98.2 Hz, Ph-Cipso), 132.15 (d, JCP =
1
2.9 Hz, Ph-Cpara). Exact mass calc. for C14H16OPS: 263.0659
[M + H]+. Found: 263.0656.
4c: 31P NMR: d 119.6.
5c: yield 36.7% from 1; 31P NMR (CDCl3): d 90; 1H NMR (CDCl3):
d 2.29 (s, 6H, NMe), 2.61–2.66 (m, 2H, NCH2), 3.55 (unresolved ABX,
2H, PCH2), 3.80–3.91 (m, 1H, OCH2), 4.13–4.23 (m, 1H, OCH2),
1
7.02–7.79 (m, 10H, Ph); 13C NMR (CDCl3): d 44.55 (d, JCP
=
6 F. Mathey and F. Mercier, Tetrahedron Lett., 1981, 22, 319.
7 S. Holand and F. Mathey, J. Org. Chem., 1981, 46, 4386.
8 S. Holand and F. Mathey, Organometallics, 1988, 7, 1796.
9 M.-O. Bevierre, F. Mercier, L. Ricard and F. Mathey, Angew.
Chem., Int. Ed. Engl., 1990, 29, 655.
73.6 Hz, PCH2), 45.42 (s, NMe), 58.85 (d, JCP = 7.6 Hz, NCH2),
62.22 (d, JCP = 6.6 Hz, OCH2), 127.00 (d, JCP = 4.2 Hz, CH2Ph-
Cpara), 128.07 (d, JCP = 3.5 Hz, CH2Ph-CH), 128.27 (d, JCP = 12.8
Hz, Ph-CH), 130.30 (d, JCP = 5.7 Hz, CH2Ph-CH), 131.13 (d, 2JCP
=
7.8 Hz, CH2Ph-Cipso), 131.70 (d, JCP = 10.7 Hz, Ph-CH), 132.17
4
(d, JCP = 2.9 Hz, Ph-Cpara), 132.27 (d, JCP = 96.1 Hz, Ph-Cipso).
1
10 I. Kalinina and F. Mathey, Organometallics, 2006, 25, 5031.
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2590 | Chem. Commun., 2009, 2589–2590