'Phospha-Wittig' reactions using isolable phosphoranylidenephosphines ArP=PR3 (Ar = 2,6-Mes2C6H3 or 2,4,6-But3C6H2)

Article abstract of DOI:10.1039/a802722f

Phosphoranylidenephosphines DmpP=PMe₃ (1a, Dmp = 2,6-Mes₂C₆H₃) and Mes*P=PMe₃ (1b, Mes* = 2,4,6-Bu^t₃C₆H₂) act as 'Phospha-Wittig' reagents with aldehydes pr

Full text of DOI:10.1039/a802722f

View Article Online / Journal Homepage / Table of Contents for this issue
‘Phospha-Wittig’ reactions using isolable phosphoranylidenephosphines
ArPNPR3 (Ar = 2,6-Mes2C6H3 or 2,4,6-But3C6H2)
Shashin Shah and John D. Protasiewicz*†
Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106-7708, USA
Phosphoranylidenephosphines DmpPNPMe3 (1a, Dmp =
Mes*PH2. For example, compound 2b has been prepared in
2,6-Mes2C6H3) and Mes*PNPMe3 (1b, Mes*
=
80% yield after purification by chromatography [eqn. (2)].10
The primary phosphine Mes*PH2 is obtained by LiAlH4
reduction of Mes*PCl2 and isolated in 80% yield after
recrystallization.11 Our procedure thus represents not only a
saving in time but also of material due to phosphaalkene access
from the more readily available dichlorophosphine precursors.
A more dramatic advance in the utility of the current reaction
was realized by the discovery that compounds 1a and 1b can be
generated and used in situ. For example, reaction of DmpPCl2,
benzaldehyde, zinc dust and excess PMe3 gives an isolated yield
of 95% of DmpPNC(H)Ph. Likewise, Mes*PNC(H)Ph is
obtained in 87% yield from Mes*PCl2 under the same
conditions.
2,4,6-But3C6H2) act as ‘Phospha-Wittig’ reagents with
aldehydes providing phosphaalkenes [ArPNC(H)R] in high
yields.
The successful synthesis of
a ‘true phosphobenzene’,
Mes*PNPMes* (Mes* = 2,4,6-But3C6H2),1 signaled a new era
in the study of phosphorus–phosphorus double bonds.2 We have
recently uncovered reactions of [Cp2ZrNPDmp(PR3)] (Dmp =
2,6-Mes2C6H3) which produce phosphoranylidenephosphines
DmpPNPR3 (R = Me or Bu).3 Phosphoranylidenephosphines
are formally the products of phosphinidene transfer to phos-
phines.4 These novel materials contain PP multiple bonding of
a very differing nature, as exemplified by the following
BunLi Me2ButSiCl
Ph(H)C=O
BunLi
resonance forms:
(2)
2b
Mes*PH2
Mes*P(Li)SiMe2But
+
¯
ArPNPR3 Ô ArP–PR3
Similar resonance forms are commonly drawn for Wittig
reagents R2CNPR3, and the nature of the bonding between the P
and C atoms in these species has been reviewed.5 Bearing such
a close kinship to Wittig reagents, it was anticipated that
phosphoranylidenephosphines could act as potential ‘phospha-
Wittig’ reagents by reacting with aldehydes to generate
phosphaalkenes RPNC(H)R [eqn. (1)]. Several transition metal
containing systems have been reported that accomplish similar
transformations.6–8 Herein we present the reactivity of the
The scope of the phosphaalkene forming reactions using 1a
was also investigated. Pentafluorobenzaldehyde, ferrocene-
carboxaldehyde and pivaldehyde provided phosphaalkenes
7a–9a in good yields, demonstrating the remarkable tolerance
of the phosphoranylidenephosphines to varying functional
groups. Reactions of 1a with ketones proved more problematic,
Table 1 Reactions of aldehydes to give phosphaalkenes
phosphoranylidenephosphines
Mes*PNPMe3 1b with aldehydes to generate phosphaalkenes.
DmpPNPMe3
1a
and
Aldehyde
R(H)C=O
Phosphaalkene Yield (%) 31P{1H} (C6D6)§
R(H)C=PAr
d
O
H
P
H
Ar
O
H
+ ArP=PMe3
+
O=PMe3
(1)
R
C
R C
X
C
X = H
Cl
2a
2b
3a
3b
4a
4b
5a
5b
94
93
90
90
92
92
97
87
78
96
97
240.9
258.510
Compounds 1a and 1b are conveniently prepared by
reduction of either DmpPCl2 or Mes*PCl2 with Zn dust in the
presence of excess PMe3 in 88–95% yields [1a: 31P NMR
(C6D6), d 22.8, 2114.7 (JPP 582 Hz); 1b: 31P NMR (C6D6), d
4.7, 2134.0 (JPP 581 Hz)].3‡ In the absence of air and water,
compounds 1a, b are stable yellow crystalline solids. Both 1a
and 1b slowly decompose in solution to lose PMe3 and form
DmpPNPDmp and Mes*PNPMes*, respectively (days to
weeks).9
Reactions of 1a and 1b with CNO containing molecules were
thus examined. A series of para-substituted benzaldehydes
reacted with 1a, b in THF to produce the desired phosphaalk-
enes in excellent isolated yields (Table 1). Work-up involves
removal of THF and extraction of the phosphaalkene into
hexanes to remove the relatively insoluble ONPMe3. Reaction
times, as well as product yields, varied with the nature of the
substituent; the most electron releasing substituents required the
longest reactions times (2–24 h) and provided the lowest yields.
Each reaction produced a single isomer of the phosphaalkene,
and the 2JPH coupling constants (24–25 Hz) are consistent with
an assignment of E-isomers for the products.10§
243.8
261.4
NO2
265.6
284.6
OMe
225.4
245.115
NMe2
6a
7a
7b
210.4
O
F5
286.8 (t, JPF 98Hz)
304.3 (t, JPF 93Hz)
C
H
H
O
8a
61
222.1
Fe
Me
O
H
9a
91
221.2
C
Me
Me
Our new protocol can be contrasted to multistep procedures
utilizing sterically hindered primary phosphines such as
Chem. Commun., 1998
1585

View Article Online
H), 1.35 (s, 9 H). 3a: mp 113–115 °C; 1H NMR(C6D6), d 8.80 (d, 2JHP 24.9
Hz, 1 H), 7.20 (t, JHH 7.6 Hz, 1 H), 6.98 (d, JHH 7.4 Hz, 2 H), 6.83 (m, 2 H),
6.80 (s, 4 H), 6.66 (d, JHH 8.5 Hz, 2 H), 2.18 (s, 12 H), 2.08 (s, 6 H); HRMS
however. Acetophenone, benzophenone and cyclohexanone
showed no evidence of phosphaalkene formation and yielded
extensive amounts of DmpPNPDmp over time.
(EI) m/z calc. for
C31H30PCl 468.1776; found 468.1788. 3b: mp
Effords to extend the reactivity of phosphoranylidenephos-
phines to systems having less steric hindrance than Dmp or
Mes* have been partially successful. Attempts to isolate
TripPNPMe3 (Trip = 2,4,6-Pri3C6H2) by reduction of TripPCl2
with Zn dust in the presence of PMe3 resulted in rapid formation
of (TripP)3.12 Addition of benzaldehyde, however, to such
reactions results in mixtures of (TripP)3 and TripPNC(H)Ph
{31P{1H} (C6D6), d 254.7; 1H NMR, d 8.99 [TripPNC(H)Ph, d,
JHP 25.6 Hz]}, suggesting the presence of a transient
TripPNPMe3 capable of effecting phosphaalkene formation.
Reactions of phosphoranylidenephosphines with aldehydes
would be of greater synthetic value if the more readily handled
(and cheaper) PPh3 could replace PMe3 in these reactions.
Unfortunately, efforts to prepare DmpPNPPh3 by reduction of
DmpPCl2 with Zn in the presence of PPh3 resulted in isolation
of DmpPNPDmp. Attempts to generate DmpPNPPh3 in situ for
reaction with benzaldehyde also failed. Exchange of the PMe3
unit in 1a with added PPh3 also proved futile. The PMe3 groups
in 1a and 1b do undergo exchange with certain non-hindered
trialkylphosphines in solution. For example, 1a and 1b react
quickly with PBu3 to produce mixtures of 1a, PMe3 and
DmpPNPBu3 [1c, 31P NMR(C6D6), d 24.1, 2151.3 (JPP 589
Hz)] and mixtures of 1b, PMe3 and Mes*PNPBu3 [1d, 31P
NMR(C6D6), d 19.9, 2153.7 (JPP 612 Hz)], respectively.13,14
Compound 1c can also be generated in situ (as above) from
PBu3 and DmpPCl2, which in the presence of benzaldehyde
yields the phosphaalkene DmpPNC(H)Ph and ONPBu3 in good
yields. Work-up, however, requires more effort than the PMe3
system due to the decreased volatility of PBu3.
124–126 °C; 1H NMR(C6D6), d 7.97 (d, 2JHP 25.1 Hz, 1 H), 7.63 (s, 2 H),
7.13 (m, 2 H), 6.96 (d, JHH 8.6 Hz, 2 H), 1.57 (s, 18 H), 1.35 (s, 9 H); HRMS
(EI) m/z calc. for C25H34PCl 400.2089; found 400.2086. 4a: mp
131–132 °C; 1H NMR(C6D6), d 8.67 (d, 2JHP 24.9 Hz, 1 H), 7.40 (d, JHH 8.6
Hz, 2 H), 7.20 (t, JHH 7.7 Hz, 1 H), 6.96 (d, JHH 7.7 Hz, 2 H), 6.80 (s, 4 H),
6.70 (m, 2 H), 2.14 (s, 12 H), 2.08 (s, 6 H); HRMS (EI) m/z calc. for
C
31H30PNO2 479.2016; found 479.2028. 4b: mp 129–131 °C; 1H
NMR(C6D6), d 7.83 (d, 2JHP 24.8 Hz, 1 H), 7.73 (d, JHH 8.8 Hz, 2 H), 7.62
(s, 2 H), 7.01 (m, 2 H), 1.52 (s, 18 H), 1.35 (s, 9 H); HRMS (EI) m/z calc.
for C25H34PNO2 411.2329; found 411.2329. 5a: mp 121–122 °C; 1H
NMR(C6D6), d 9.00 (d, 2JHP 24.9 Hz, 1 H), 7.22 (t, JHH 7.6 Hz, 1 H), 7.11
(m, 2 H), 7.02 (d, JHH 7.6 Hz, 2 H), 6.81 (s, 4 H), 6.34 (d, JHH 8.6 Hz, 2 H),
3.04 (s, 3 H), 2.23 (s, 12 H), 2.09 (s, 6 H); HRMS (EI) m/z calc. for
C32H33PO 464.2271; found 464.2260. 5b: mp 164–166 °C; 1H (C6D6): d
8.20 (d, 2JHP 25.1 Hz, 1 H), 7.66 (d, 4JHP 1 Hz, 2 H), 7.41 (m, 2 H), 6.44 (d,
JHH 8.4 Hz, 2 H), 3.20 (s, 3 H), 1.64 (s, 18 H), 1.37 (s, 9 H); HRMS (EI) m/z
calc. for C26H37PO 396.2584; found 396.2584. 6a: mp 181–183 °C; 1H
NMR (C6D6): d 9.06 (d, 2JHP 24.4 Hz, 1 H), 7.22 (m, 3 H), 7.04 (d, JHH 7.6
Hz, 2 H), 6.82 (s, 4 H), 6.09 (d, JHH 8.8 Hz, 2 H), 2.27 (s, 12 H), 2.22 (s, 6
H), 2.10 (s, 6 H); HRMS (EI) m/z calc. for C33H36PN 477.2588; found
477.2596. 7a: mp 159–161 °C; 1H NMR (C6D6), d 8.73 (d, 2JHP 24.9 Hz, 1
H), 7.21 (t, JHH 7.7 Hz, 1 H), 6.97 (d, JHH 7.4 Hz, 2 H), 6.82 (s, 4 H), 2.19
(s, 12 H), 2.06 (s, 6 H); HRMS (EI) m/z calc. for C31H26PF5 524.1694;
found 524.1704. 7b: mp 130–133 °C; 1H NMR (C6D6), d 7.94 (d, 2JHP 24.8
Hz, 1 H), 7.63 (d, JHP 1.0 Hz, 2 H), 1.58 (s, 18 H), 1.32 (s, 9 H); HRMS (EI)
m/z calc. for C25H30PF5 456.2007; found 456.2010. 8a: mp 104–106 °C; 1H
(C6D6), d 8.77 (d, 2JHP 24.2 Hz, 1 H), 7.20 (t, JHH 7.7 Hz, 1 H), 6.96 (d, JHH
8.1 Hz, 2 H), 6.84 (s, 4 H), 4.15 (m, 2 H), 3.89 (m, 2 H), 3.73 (d, J 0.5 Hz,
5 H), 2.21 (s, 12 H), 2.14 (s, 6 H); HRMS (EI) m/z calc. for C35H35PFe
542.1817; found 542.1837. 9a: mp 127–129 °C; 1H NMR (C6D6), d 8.37 (d,
2JHP 25.1 Hz, 1 H), 7.18 (t, JHH 7.6 Hz, 1 H), 6.97 (d, JHH 8.1 Hz, 2 H), 6.83
(s, 4 H), 2.16 (s, 12 H), 2.15 (s, 6 H), 0.79 (d, 4JHH 1.9 Hz, 9 H); HRMS (EI)
m/z calc. for C29H35P 414.2479; found 414.2474.
In conclusion, we have demonstrated that readily prepared
and isolable phosphoranylidenephosphines are apt phosphini-
dene carriers in phospha-Wittig reactions. Our procedure
represents a significant advance for the synthesis of phosphaalk-
enes as it utilizes dichlorophosphines directly, rather than
derived primary phosphines. High yields and functional group
tolerance are further highlights of this phospha-Wittig ap-
proach. Further studies of the phosphinidene and atom transfer
reactions of these conveniently prepared phosphinidene-carriers
are underway.
1 M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and T. Higuchi, J. Am.
Chem. Soc., 1981, 103, 4587.
2 L. Weber, Chem. Rev., 1992, 92, 1839.
3 E. Urnezius, S. Shah, G. P. Yap and J. D. Protasiewicz, manuscript in
preparation.
4 In, Multiple Bonds and Low Coordination in Phosphorus Chemistry, ed.
M. Regitz and O. J. Schere, Thieme Verlag, Stuttgart, 1990.
5 D. G. Gilheany, Chem. Rev., 1994, 94, 1339; P. V. Sudhakar and K.
Lammertsma, J. Am. Chem. Soc., 1991, 113, 1899.
6 T. L. Breen and D. W. Stephan, J. Am. Chem. Soc., 1995, 117, 11 914;
Organometallics, 1996, 15, 4223; 1997, 16, 365.
7 C. C. Cummins, R. R. Schrock and W. M. Davis, Angew. Chem., Int. Ed.
Engl., 1993, 32, 756.
8 P. Le Floch, A. Marinetti, L. Ricard and F. Mathey, J. Am. Chem. Soc.,
1990, 112, 2407; P. Le Floch and F. Mathey, Synlett., 1990, 171; P.
Floch and F. Mathey, Synlett., 1991, 743.
9 E. Urnezius and J. D. Protasiewicz, Main Group Chem., 1996, 1, 369.
10 M. Yoshifuji, K. Toyota and N. Inamoto, Tetrahedron Lett., 1985, 26,
1727.
11 A. H. Cowley, N. C. Norman and M. Pakulski, Inorg. Synth., 1990, 27,
235.
12 C. N. Smit, T. A. van der Knaap and F. Bickelhaupt, Tetrahedron Lett.,
1983, 24, 2031.
13 A. B. Burg, and W. Mahler, J. Am. Chem. Soc., 1961, 83, 2388.
14 A. H. Cowley and M. C. Cushner, Inorg. Chem., 1980, 19, 515.
15 K. Issleib, H. Schmidt and E. Leibring, Z. Chem., 1986, 26, 406.
We thank the National Science Foundation (CHE-9733412)
and the Department of Chemistry (CWRU) for support of this
research.
Notes and References
† E-mail: jdp5@po.cwru.edu
‡ Compound 1a: 1H NMR(C6D6), d 7.08 (t, 1 H, JHH 8 Hz), 6.96 (d, 2 H,
J
HH 8 Hz), 6.90 (s, 4 H), 2.37 (s, 12 H), 2.22 (s, 6 H), 0.58 (dd, 9 H, 2JHP
3
12 Hz, JHPP 3 Hz). HRMS (EI) m/z calc. for C27H34P2 420.2138; found
420.2127. Compound 1a has also been structurally characterized.3 Com-
pound 1b: 1H NMR(C6D6), d 7.42 (s, 2 H), 1.90 (s, 18 H), 1.36 (s, 9 H), 0.69
(d, 9 H, 2JHP 11.5 Hz). HRMS (EI) m/z calc. for C21H38P2 352.2451; found
352.2446.
1
§ Other data for phosphaalkenes: 2a: mp 162–164 °C; H NMR(C6D6), d
9.00 (d, 2JHP 25.0 Hz, 1 H), 7.21 (t, JHH 8.0 Hz, 1 H), 7.16 (m, 2 H), 7.00
(d, JHH 7.6 Hz, 2 H), 6.78 (s, 4 H), 6.73 (m, 1 H), 2.20 (s, 12 H), 2.07 (s, 6
H); HRMS (EI) m/z calc. for C31H31P 434.2165; found 434.2141. 2b: mp
149–152 °C (lit. 152–153 °C10); 1H NMR (C6D6), d 8.19 (d, 2JHP 25.4 Hz,
1 H), 7.64 (d, 4JHP 1.2 Hz, 2 H), 7.46 (m, 2 H), 7.00 (m, 3 H), 1.60 (s, 18
Received in Bloomington, IN, USA; 14th April 1998; 8/02722F
1586
Chem. Commun., 1998