2,6-Diisopropylphenylphosphane: A new, bulky primary phosphane and its mono- and disilylated Si(CH3)3 and Si(CH3)2-t-Bu derivatives - A synthetic, crystallographic, and dynamic NMR investigation

Article abstract of DOI:10.1139/v02-150

The bulky primary phosphane 2,6-diisopropylphenylphosphane, DipPH₂, has been prepared from 1-bromo-2,6-diisopropylbenzene via the reaction of the Grignard reagent with PCl₃. The resulting mixed phosphonous dihalides DipP(Cl,Br)₂

Full text of DOI:10.1139/v02-150

1
607
2,6-Diisopropylphenylphosphane: A new, bulky
primary phosphane and its mono- and disilylated
Si(CH ) and Si(CH ) -t-Bu derivatives — A
3
3
3 2
synthetic, crystallographic, and dynamic NMR
1
investigation
René T. Boeré and Jason D. Masuda
Abstract: The bulky primary phosphane 2,6-diisopropylphenylphosphane, DipPH , has been prepared from 1-bromo-
2
2
,6-diisopropylbenzene via the reaction of the Grignard reagent with PCl . The resulting mixed phosphonous dihalides
3
DipP(Cl,Br) are reduced with LiAlH to yield the title compound in reasonable yield and on a synthetically useful
2
4
scale. DipPH is also used to prepare the monosilylated derivatives DipPHSi(CH ) and DipPH{Si(CH ) -t-Bu} as well
2
3 3
3 2
as the disilylated compounds DipP{Si(CH ) } and DipP{Si(CH ) -t-Bu} . All products have been fully characterized
3
3
2
3 2
2
by IR, mass, and NMR spectroscopy. The crystal structure of DipPH{Si(CH ) -t-Bu} was determined from single-
3
2
crystal diffraction data: C H PSi, P2 /c, Z = 4, a = 8.5768(10), b = 28.104(3), c = 8.1102(4) Å, ꢀ = 93.341(3)°
1
8
33
1
(
R = 0.0518). Changes in the NMR spectrum of DipPH{Si(CH ) -t-Bu} were observed over the temperature range
3 2
1
ꢁ
78–380 K. Barrier heights were determined from the peak separation at low temperature and the coalescence points:
G W 40 kJ mol for C(aryl)—P bond rotation and W 72 kJ mol for pyramidal inversion at phosphorus.
‡
–1
–1
Key words: crystal structure, 2,6-diisopropylphenyl, bulky substituents, steric congestion, dynamic NMR, primary
phosphane, silyl phosphane.
Résumé : On a préparé le phosphane primaire encombré 2,6-diisopropylphénylphosphane, DipPH , à partir du 1-
2
bromo-2,6-diisopropylbenzène, par le biais d’une réaction de Grignard avec le PCl . On a réduit les dihalogénures
3
phosphoneux mixtes qui en résultent, DipP(Cl,Br) , à l’aide de LiAlH pour obtenir le composé mentionné dans le titre
2
4
avec un rendement raisonnable et à une échelle utile d’un point de vue synthétique. On a aussi utilisé le DipPH pour
2
préparer les dérivés monosilylés DipPHSi(CH ) et DipPH{Si(CH ) -t-Bu} ainsi que les produits disilylés
3
3
3 2
DipP{Si(CH ) } et DipP{Si(CH ) -t-Bu} . On a caractérisé tous les produits par spectroscopie IR, de masse et RMN.
3
3
2
3 2
2
On a déterminé la structure cristalline du DipPH{Si(CH ) -t-Bu} par diffraction des rayons X par un cristal unique; les
3
2
données pour C H PSi, P2 /c, Z = 4, a = 8,5768(10), b = 28,104(3) et c = 8,1102(4) Å, ꢀ = 93,341(3)° (R = 0,0518).
1
8
33
1
En faisant varier la température de 178 à 310 K, on a observer des changements dans le spectre RMN du
DipPH{Si(CH ) -t-Bu}. Les hauteurs de barrières ont été déterminées sur la base de la séparation des pics à basse
3
2
‡
–1
température et des points de coalescence: ꢁG W 40 kJ mol pour la rotation autour de la liaison C(aryle)—P et
W 72 kJ mol–1 pour l’inversion pyramidale au niveau du phosphore.
Mots clés : structure cristalline, 2,6-diisopropylphényle, substituants volumineux, congestion stérique, RMN dynamique,
phosphane primaire, phosphane silylé.
[
Traduit par la Rédaction] Boeré and Masuda 1617
Introduction
chemistry (1–8) and coordination chemistry (9–18) is under
active investigation. Aryl phosphanes bearing bulky sub-
stituents in the ortho positions have been particularly popu-
lar, as they have considerably greater stability and more
The chemistry of primary phosphanes has received much
attention in recent years, and their utility in both main group
Received 14 February 2002. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 29 November 2002.
This work is dedicated to Professor Tristram Chivers in grateful acknowledgment for his constant support and sound collegial
advice. Tris has served his Department, University, and Canadian chemistry impeccably and enthusiastically. His efforts have
helped put main group chemistry in the forefront of Canadian science.
2
R.T. Boeré and J.D. Masuda. Department of Chemistry and Biochemistry, University of Lethbridge, Lethbridge, AB T1K 3M4,
Canada.
1
Presented in part at the 84th Canadian Chemical Conference and Exhibition, Montreal, QC, 26–30 May 2001.
Corresponding author (e-mail: boere@uleth.ca).
2
Can. J. Chem. 80: 1607–1617 (2002)
DOI: 10.1139/V02-150
© 2002 NRC Canada

1
608
Can. J. Chem. Vol. 80, 2002
Scheme 1.
Scheme 2.
controlled reactions (3, 4, 17). The risk associated with the
high toxicity of primary phosphanes such as PhPH is also
2
mitigated by the bulky substituents, if by nothing more than
lowering the volatility of the compounds. The most com-
monly used bulky primary phosphane (Scheme 1) has un-
doubtedly been 2,4,6-tri-tert-butylphosphane, Mes*PH (1)
2
(
19). The closely related 2,4,6-triisopropylphenylphosphane,
TripPH (2) is known, and its chemistry has received some
attention (20). A laborious, multi-step, small-scale synthesis
of 2,4-di-tert-butyl-6-isopropylphenylphosphane, DtbiPH (3)
Many applications of primary phosphanes either require,
or are enhanced by, replacement of one or both hydrides
with silyl groups (6, 25–27). We have therefore prepared the
2
2
has recently been reported (3). The terphenyl compound 2,6-
silyl compounds DipPHSi(CH3)3 (6), DipPH{Si(CH ) -t-
3 2
(
2,4,6-triisopropylphenyl)-phenylphosphane, TterphPH2 (4)
Bu} (7), DipP{Si(CH3)3}2 (8), and DipP{Si(CH ) -t-Bu}2
3 2
has been prepared and its crystal structure determined (4).
We have been exploiting for some time the 2,6-
diisopropylphenyl (Dip) group as a bulky substituent on ni-
trogen, in particular for the preparation of N,N>-disubstituted
amidines (21, 22) and N,N>,N>>-trisubstituted guanidines
(9). Although the Si(CH ) compounds could be prepared
3
3
quite easily in sufficient purity to be of synthetic utility, it
was difficult to obtain analytically pure samples of these
low-melting compounds. The Si(CH ) -t-Bu derivatives, on
3
2
the other hand, are easily purified, crystalline solids and
have been fully characterized. The strong asymmetry in-
duced by the bulky substituent in 7 induces a dynamic pro-
cess observable by NMR spectroscopy, and the results of
investigations in C D CD are reported over the temperature
(
23). We are currently in the process of preparing directly
analogous phosphorus-containing compounds via primary
phosphanes (24) and we were therefore surprised to find that
DipPH (5) has not been reported heretofore in the literature
2
6
5
3
and indeed there are no reports of the Dip group attached to
phosphorus in any compound. At the cost of being slightly
less sterically shielding, the use of ortho diisopropyl groups
imparts to bulky substituents the advantage over tert-butyl of
providing a wealth of stereochemical information from the
range 178–380 K. These solution state changes are corre-
lated to the structure found in the solid state by a single-
crystal X-ray diffraction study.
Results and discussion
1
H NMR signals of the diastereotopic isopropyl groups. The
2
,4,6-triisopropylphenyl (Trip) substituent has such groups,
Synthesis and characterization
but has the disadvantage that the sterically unnecessary para
isopropyl substituent tends to obscure the often complex and
always interesting H NMR signals of the sterically essential
ortho isopropyl substituents. For example, in 2 the chemical
shifts of the ortho and para i-Pr CH groups are 1.23 and
1
drogen atoms on the aromatic backbone of Dip groups tend
to be well resolved and can be useful spectroscopic probes
in a region of the spectrum removed from the crowded
aliphatic regions. There is considerable merit in the use of
the Dip substituent, and in this paper we report a practical
large-scale synthesis of 5.
Our synthetic route (Scheme 2) starts from DipBr (10) for
which a large-scale synthesis has been worked out starting
from the commercially available aniline (28). The Grignard
reagent prepared from 10 is added to a cold solution of PCl3
in THF. This reaction leads consistently to mixtures of all
three possible halides, which co-distil as identified by their
1
3
.24 ppm (20). In our experience, the meta and the para hy-
3
1
P chemical shifts (Table 1), i.e., DipPCl (11a), DipPClBr
2
(11b), and DipPBr (11c). Typical product distributions of
2
the three are 1.6:2.4:1.0, which by mass balance means that
virtually all of the bromide contained in the Grignard re-
agent ends up on phosphorus, remarkable testimony to the
higher nucleophilicity of bromide versus chloride. Mixed
©
2002 NRC Canada

Boeré and Masuda
1609
Table 1. 31P NMR data for phosphonous dihalides, phosphanes, and silylated phosphanesa.
3
1
1JPH, Hz
No.
Compound
ꢂ ( P)
Conditions
Reference
1
1
1
1
1
1
1
1
1
1a
1b
1c
2
DipPCl2
+164.5
+159.2
+151.9
+164.5
+153.8
+191b
—
CDCl , RT
This work
This work
This work
(3)
3
DipPClBr
DipPBr2
DtbiPCl2
Mes*PCl2
—
CDCl , RT
3
—
CDCl , RT
3
—
CDCl , RT
3
3
—
CDCl , RT
(3)
3
4
CH PCl2
—
Not specified
Not specified
Not specified
Not specified
(29)
3
5
CH PClBr
+190b
—
(29)
3
6
CH PBr2
+184b
–122b
—
(29)
3
7
PhPH2
207
211
208
207
204
208
203
212
210
211
211
212
—
(29)
1
Mes*PH2
–128.9
–139.8
–143.9
–153.0
–156.4
–158.2
–130.1
–136.2
–163.8
–169
CDCl , RT
(30)
3
4
TterphPH2
DtbiPH2
CDCl , RT
(4)
3
3
CDCl , RT
(3)
3
1
8
MesPH2
Not specified
(17)
5
DipPH2
CDCl , RT
This work
(20)
3
2
TripPH2
CDCl , RT
3
1
2
9
Mes*PHSi(CH3)3
Mes*PH{Si(CH ) -t-Bu}
THF
(30)
0
CDCl , RT
(30)
3
2
3
6
DipPHSi(CH3)3
TripPHSiPh3
CDCl , RT
This work
(27)
3
2
1
C D , RT
6
6
7
DipPH{Si(CH ) -t-Bu}
–175.6
–132.8
–142.7
–167.1
–169.3
–193.3
CDCl , RT
This work
(31)
3
2
3
2
2
2
PhP{Si(CH ) }
3 2
Not specified
C D , RT
3
3
Mes*P{Si(CH ) }
—
(26)
3
3
2
6
6
8
DipP{Si(CH ) }
—
CDCl , RT
This work
(20)
3
3
2
3
2
4
TripP{Si(CH ) }
—
CDCl , RT
3
3
2
3
9
DipP{Si(CH ) -t-Bu}
—
CDCl , RT
This work
3
2
2
3
a
Referenced to external 85% H PO ; positive chemical shifts to low field (IUPAC recommendation).
Sign of chemical shift adjusted to the IUPAC convention.
3
4
b
phosphorus halides have been observed previously, for ex-
the PH signal is observed. In addition, almost all the 13C sig-
nals are doublets owing to coupling to the single phosphorus
nucleus.
ample, in the Grignard preparation of Mes P(Cl,Br), where
2
Mes = 2,4,6-trimethylphenyl (32). Reduction of mixtures of
1
1a–c with LiAlH in THF yielded the phosphane 5, as a
After repeated handling of a container of 5, a new signal
4
3
1
clear, colourless liquid. It has very little odour and is quite
easy to handle if stored below room temperature and under
nitrogen.
was detected in the P NMR spectrum at ꢂ –14.6, consisting
1
of a 1:2:1 triplet with J = 477 Hz. We attribute this signal
PH
to the phosphane oxide, DipP(O)H , formed by aerobic oxi-
2
Compound 5 has been fully characterized by H, 31P, and
1
dation of the phosphane, and base our assignment on the
close correspondence of its NMR parameters to those of
1
3
C NMR spectroscopy, as well as by mass spectrometry and
IR spectroscopy. The P–H stretch (recorded as a liquid
1
Mes*P(O)H2: triplet, JPH = 490.7 Hz at ꢂ –10.0 (33). Satu-
–
1
smear) has a frequency of 2306 cm , comparable to
ration of the NMR sample with dry O2 caused the intensity
of this signal to increase, consistent with our hypothesis. We
have not attempted to isolate bulk phosphane oxide.
The general procedure for the preparation of the silylated
compounds (Scheme 2) was to cleave P—H bonds with n-
BuLi, then quench the resulting phosphide anions with
–
1
2
302 cm in CH Cl2 solution reported for MesPH2 (18)
2
3
1
(
17). The P NMR spectrum displays the expected 1:2:1
1
31
triplet with J = 208 Hz at ꢂ –156.4. As the P NMR
PH
spectral data for primary phosphanes with bulky substituents
and their silyl derivatives are extremely scattered in the liter-
ature, we have collected values for as many derivatives as
we could locate and compiled them in Table 1. A survey of
these data shows that bulky primary phosphanes differ quite
widely in P chemical shift but that the J values are re-
markably consistent and hence diagnostic. The H NMR
R R>SiCl. However, either these reactions do not go to com-
2
pletion, or the base also cleaves P—Si bonds, so that the
cleaving and quenching sequences had to be repeated more
than once per step. The success of this procedure was
3
1
1
PH
1
31
greatly assisted by the use of P NMR spectroscopy.
spectrum measured in CDCl (Table 2) contains the typical
Aliquots taken directly from the reaction mixture were used
to assess the progress of each reaction, and in this way, we
were able to prepare phosphanes 6–9 in acceptable yields.
3
aromatic and aliphatic signals of the Dip group, with the ad-
ditional aspect that the isopropyl methine septet appears to
be split further into doublets with 3.0 Hz coupling to phos-
phorus. Whether this is four-bond dipolar or through space
coupling is yet to be determined. The expected doublet for
Use of the larger t-BuMe Si group yielded analytically pure
2
samples of 7 and 9, whereas 6 and 8 always contained some
minor impurities. The monosilylated phosphane 7 was
©
2002 NRC Canada

1
610
Can. J. Chem. Vol. 80, 2002
Table 2. Solution NMR data for 5–7 and related compoundsa.
Nuclei
Feature
5
6
7b
8
9c
27d
21e
CH3(A,D)
ꢂ ( H)
1
1.25
6.7
= (A,D)
1.21
1.21
1.18
6.9
= (A,D)
1.21
6.7
= (A,D)
1.2
6.8
= (A,D)
0.98
1.09
3.52
3J
f
f
(Hz)
6.7
1.26
6.9
1.26
HH
1
CH3(B,C)
CH(1,2)
ꢂ ( H)
3J
(Hz)
6.7
3.41
6.7
3.8
3.46
6.7
3.43
6.8
4.0
3.51
HH
1
ꢂ ( H)
3.36
6.8
3.0
3.84
207.2
4.06
6.6
4.29
6.7
3.8
6.8
4.8
5.6
216.6
3
f
f
JHH (Hz)
4J
(Hz)
PH
1
g
PH
ꢂ ( H)
4.44
211
1
g
J
(Hz)
210.7
0.18
211.0
0.06
PH
1
SiCH3(E)
ꢂ ( H)
0.22
0.18
3J (Hz)
4.3
= (E)
5.3
0.02
6.1
= (E)
4.3
= (E)
PH
SiCH3(F)
ꢂ ( H)
1
3J (Hz)
0.9
1.01
2.4
7.26–7.17
PH
1
Si-t-Bu
ꢂ ( H)
0.91
0.8
7.28–7.21
3
J
(Hz)
PH
1
H–C4
ꢂ ( H)
7.35–7.25
7.35–7.19
7.35–7.23
1
H–C3
ꢂ ( H)
7.22–7.10
23.84
7.17–7.08
23.97
7.12–7.08
24.00
7.20–7.07
25.32
7.12–7.08
25.67
CH3(A,D)
ꢂ ( C)
13
24.9
f
f
CH3(B,C)
ꢂ ( C)
13
24.18
24.16
CH(1,2)
13
f
f
ꢂ ( C)
33.15
11.7
33.50
13.7
0.67
33.53
13.7
–4.20
33.98
16.1
2.55
33.96
16.1
–1.54
32.7
14.4
3
JPC (Hz)
SiCH3(E)
ꢂ ( C)
13
2JCH (Hz)
10.3
15.6
–2.93
17.0
7.8
= (C)
SiCH3(F)
Cipso
ꢂ ( C)
13
1
3
f
ꢂ ( C)
126.04
129.21
129.04
128.29
130.44
130.4
1J (Hz)
f
f
13.7
152.08
20.5
152.03
21.5
152.47
14.2
156.44
9.3
156.57
15.7
153.0
PC
1
3
Cortho
Cmeta
Cpara
ꢂ ( C)
2J (Hz)
f
f
9.8
122.91
9.3
122.94
9.3
122.97
10.7
123.43
11.2
128.92
11.1
121.6
PC
1
3
ꢂ ( C)
3J (Hz)
f
f
2.4
128.53
3.4
127.44
3.4
127.52
5.4
128.55
5.4
128.59
3.4
149.7
PC
1
3
ꢂ ( C)
4J (Hz)
f
2.0
–167.1
1.95
–192.9
PC
3
1
P
ꢂ ( P)
–156.4
207.5
–163.8
210.6
–175.6
212.1
–113.2
214.1
–169
211
1
JHP (Hz)
a
Measured in CDCl at room temperature unless otherwise noted; label scheme:
3
b
c
1
4
13
4
2
t-Bu signals: H NMR: 1.01 (d, J = 2.4 Hz, 9H). C NMR: 27.02 (d, J = 2.9 Hz, C(CH ) ), 18.70 (d, J = 9.8 Hz, C(CH ) ).
PH
P-C
3
3
PC
3 3
1
13
4
2
t-Bu signals: H NMR: 25.67 (s, C(CH ) ). C NMR: 27.79 (d, J = 3.9 Hz, C(CH ) ), 20.18 (d, J = 20.5 Hz, C(CH ) ).
3
3
PC
3
3
PC
3 3
d
Reference (35), measured in C D solution at room temperature.
6
6
e
Reference (27), measured in C D solution at room temperature.
6
6
f
No data provided for this parameter.
g
Data taken from the closely related TripPHPh, 25.
sufficiently stable to allow the investigation of its dynamic
behaviour in solution by VT NMR, and this compound
yielded crystals suitable for an X-ray diffraction study.
The P–H stretch of 7 (recorded as a KBr pellet) has a fre-
1JPH values, which are found between 203 and 212 Hz for all
the trivalent P–H compounds listed in Table 1.
Structure of 7 in the solid state
–
1
quency of 2330 cm , thus considerably higher in energy
The crystal structure of DipPH{Si(CH ) -t-Bu} (7) was
3
2
–
1
1
13
31
than the 2301 cm value for 5. H, C, and P data are
compiled in Tables 1 and 2. We again note the large varia-
tion in phosphorus chemical shift and the consistency of the
determined by X-ray diffraction at –80°C. Crystal data and
structure solutions for 7 are given in Table 3, and its selected
inter-atomic distances and angles in Table 4. The compound
©
2002 NRC Canada

Boeré and Masuda
Table 3. Crystal data for 7.
1611
Formula
C H PSi
18 33
Molecular weight
Colour, habit
Dimensions (mm)
Crystal system
Space Group
a (Å)
308.50
Colourless, block
0.14 × 0.19 × 0.20
Monoclinic
P2 /c
1
8.5768(10)
28.104(3)
8.1102(4)
93.341(3)
1951.6(4)
4
b (Å)
c (Å)
ꢀ
(°)
V (Å )
3
Z
–
3
Dcalcd. (g cm )
1.050
T (K)
193(2)
0.194
–
1
Absorption coefficient (ꢃ) (mm )
Radiation
Crystal growth method
Data collection
ꢈ range for data collection (°)
Collection ranges
Reflections collect
Mo Kꢄ (ꢅ=0.71073)
Sublimation
Bruker CCD area detector; ꢆ and ꢇ scans
2.38–26.41
–10 ? h ?10; –29 ? k ? 35; –8 ? l ? 10
11 570
Independent reflections
4009 (Rint = 0.0618)
Absorption correction
Refinement
None
2
Refinement on F (all data); H atoms constrained, except H(1)
Number of parameters refined
Final R indices [F >2ꢉ(F )]
194
2
2
R = 0.0518, wR = 0.1114
1
2
R indices (all data)
R = 0.0918, wR = 0.1308
1
2
–
3
Largest diff. peak and hole (e Å )
0.338 and –0.196
Fig. 1. An ORTEP diagram (25% probability) of the structure of
as found in the solid state by single-crystal X-ray crystallography.
The atom numbering scheme is indicated. Hydrogen atoms are
indicated by spheres of minimum radii for the sake of clarity.
Table 4. Selected bond lengths (Å) and angles (°) for 7.
Bond lengths (Å)
7
P(1)—C(1)
P(1)—H(1)
Si(1)—C(14)
C(1)—C(2)
C(2)—C(6)
C(7)—C(8)
C(2)—C(3)
C(3)—C(4)
C(10)—C(11)
1.854(2)
1.31(4)
P(1)—Si(1)
Si(1)—C(13)
Si(1)—C(15)
C(1)—C(9)
C(6)—C(7)
C(8)-—C(9)
C(9)—C(10)
C(3)—C(5)
2.2681(9)
1.861(3)
1.894(3)
1.410(3)
1.375(4)
1.396(3)
1.521(3)
1.524(3)
1.527(4)
1.864(3)
1.415(3)
1.396(3)
1.374(3)
1.521(3)
1.523(4)
1.5331(4) C(10)—C(12)
Bond angles (°)
C(1)-P(1)-Si(1)
Si(1)-P(1)-H(1)
C(1)-C(2)-C(3)
C(2)-C(3)-C(5)
C(9)-C(10)-C(11)
P(1)-Si(1)-C(13)
P(1)-Si(1)-C(15)
104.03(7) C(1)-P(1)-H(1)
105(2)
93(2)
C(2)-C(1)-P(1)
C(9)-C(1)-P(1)
C(2)-C(3)-C(4)
C(1)-C(9)-C(10)
118.49(17)
121.30(18)
111.54(2)
123.1(2)
123.4(2)
112.6(2)
113.3(2)
111.97(9) C(9)-C(10)-C(12) 110.98(2)
106.42(8) P(1)-Si(1)-C(14) 111.20(9)
crystallizes as discrete molecules. The shortest inter-
molecular contacts are between H12C and H16A (2.357 Å)
and H4B and H16C (2.385 Å), fractionally less than the sum
of the van der Waals’ radii of two H atoms. A drawing of
the molecular structure and the atom-numbering scheme is
presented in Fig. 1. The essential co-planarity of C(7), C(1),
P(1), Si(1), C(15), and C(16) is noticeable. The RMS devia-
tion of these atoms from a least-squares plane is 0.05 Å,
with Si(1) deviating most. This molecular plane is almost
exactly orthogonal to that of the aryl ring (86.38(7)° dihed-
ral angle), with the result that the Si(CH ) -t-Bu group is
3
2
©
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1
612
Can. J. Chem. Vol. 80, 2002
Fig. 2. Calculated geometries with full optimization at the B3LYP/6–31G(d) level of theory for: (a) 5; (b) 6; (c) 7; (d) 8; (e) 9. In
each representation, the atom labelled C(2) is the aryl ring carbon located above the carbon C(1) ipso to phosphorus, while C(9) is on the
lower side. See footnote (a) of Table 5.
located entirely to one side of the aryl ring. Along the t-Bu–
Si–P–aryl vector, the structure has adopted a staggered con-
formation about each bond. The two ortho isopropyl groups
are oriented in the “Vee-back” conformation that we have
observed in all solid-state structures determined for Dip
groups (21–23, 34). However, these groups are significantly
tilted such that C(5) and C(11) avoid contact with the
Si(CH ) -t-Bu group. In the N{Si(CH ) } substituted Dip
3
2
3 3 2
group found in the structure of Cl SeC H -i-
3
6
2
Pr N{Si(CH ) } , where the symmetrical disilylamino group
2
3 3 2
is co-planar with the aryl ring, such a tilt is not observed
(34). Another noticeable feature of the structure is the large
©
2002 NRC Canada

Boeré and Masuda
1613
Table 5. Calculated geometry at B3LYP/6–31G(d) level with full
optimization of 5–9.a
substituent at each side of the aryl ring. This is unlike the re-
sults from crystallography for Mes*PX derivatives, such as
2
Mes*P{OCH2}2 or 23 (25), where both substituents are
found on the same side of a (distorted) aryl ring. The pyram-
idal geometry at phosphorus causes a slight distortion of the
bond angles, such that the angle C –C –P (the upper side
5
6
7
8b
9
Distance (Å)
C(1)—P
1.871 1.874
1.424 1.424
1.425 2.298
1.874
1.416
2.307
1.881
2.276
2.276
1.881
2.276
2.296
(
2)
(1)
H(1),Si(1)––P
H(2),Si(2)—P
for each picture in Fig. 2) is in each case somewhat larger
than the angle C –C –P. The unsymmetrically substituted
(9)
(1)
6
and 7 have significantly twisted isopropyl substituents to
Angle (°)
C(1)-P-H(1),Si(1)
minimize steric interactions, whereas the symmetrically sub-
stituted compounds have the isopropyl groups symmetrically
disposed to the aryl rings and in the “Vee-back” conforma-
tion.
98.9
99.0 105.1
93.3 95.3
99.9
99.8
103.9
96.3
111.2
111.3
113.7
117.3
111.2
117.6
C(1)-P-H(2),Si(2)
H(1),Si(1)-P-H(2),Si(2)
ꢊ(<P)
C(2)-C(1)-P
291.7 300.3
122.8 124.1
300.3
124.1
336.2
125.3
346.1
125.5
Finally, we note the great similarity in the sum of angles
at phosphorus calculated for 5–7, and the agreement of cal-
culated and measured values for 7 (within experimental er-
ror). In contrast, when two silyl groups are attached as in 8
and 9, the sum of angles is greatly increased (by 36–46°).
We note that the calculated values of 336.2 and 346.1° are
significantly greater than the average measured for Mes3P
C(9)-C(1)-P
a
117.4 116.3
116.3
115.9
115.9
Full geometry optimization and stationary point verification, unless
otherwise indicated. Label scheme:
(
329.0°) from two independent molecules in the unit cell of
a single crystal X-ray structure (36). However, these values
b
Frequency calculation to verify stationary point fails.
are comparable to that obtained in a crystal structure for 23,
3
43.2° (25)
deviations of the phosphorus (0.318(4) Å) and the ipso car-
bon C(1) (0.038(3) Å) atoms from the mean aryl plane
defined by the remaining five carbon atoms. Such a devia-
tion for the phosphorus atom has been observed previously
in other “secondary” and “tertiary” phosphane structures
Solution structures by NMR
The pertinent NMR data for all five phosphanes as deter-
mined in CDCl3 solution have been compiled in Table 2.
Data from two previously reported compounds, TripPHCH3
(27) (35) and TripPHSiPh3 (21) (27) have also been included
to substantiate the assignment and interpretation of our re-
sults. The former is of course a secondary phosphane and
with bulky ortho groups: in 23, 0.85
Å (25), in
Mes*P{OCH } , 0.82 Å (25), and in TripPHPh (25), 0.48 Å
2
2
(
35). However, both the structures with ortho isopropyl
3
1
groups have the P atom displaced on the side opposite to
hence has a P chemical shift that is not comparable to that
of ours. However, there is remarkable agreement between
the NMR data for 27 and the more symmetrical of our
phosphanes DipPR2 (R = H, Si{CH3}3, and Si{CH3}2-t-Bu).
that of the “R” substituent, while the Mes*PR structures
2
have the phosphorus pushed out on the same side of the ring
as the “axial” substituent.
The sum of angles around phosphorus is only 301.5(26)°,
indicative of a highly pyramidal structure at phosphorus.
This is commonly observed for secondary phosphanes with
bulky Trip or Mes* substituents (25, 304° (35);
1
13
This applies to the H and C parameters of the ortho
1
3
isopropyl groups and the C parameters of the aromatic
rings, for which all the carbon nuclei except the para atom
C4 are doublets from coupling to phosphorus. We have also
found that the silyl methyl and even the methyl hydrogen at-
oms of the t-Bu groups in 6–9 show evidence of coupling to
the phosphorus nucleus.
Mes*PHSiPh (26), 303.9° (27)), values that are statistically
3
indistinguishable from our results.
The unsymmetrically substituted phosphanes 6 and 7 have
room temperature (RT) H NMR spectra in which there are
Calculated (gas phase) structures of 5–9
1
The structures of 5–9 have been calculated at the
B3LYP/6–31G(d) level. Illustrations of the calculated struc-
tures are provided in Fig. 2, and some of the core geometric
parameters are presented in Table 5. We started our analysis
by comparing the calculated structure of 7 with that deter-
mined from the crystal structure. There is a remarkable
agreement between the two, right down to the conformations
adopted by the aryl isopropyl and the triorganosilyl groups.
These results serve to confirm that the structure obtained in
the solid state is relatively unaffected by intermolecular
forces. The calculated bond lengths are 1–2% longer than
the measured values, typical for the DFT methodology that
we employed. However, the calculated sum of angles at
phosphorus was within experimental error of the measured
value.
two isopropyl methyl environments, but only a single CH
environment. Similar effects are reported in the literature for
the ortho isopropyl groups of 21 (27). Additionally, 7 also
displays two distinct silyl methyl resonances, which have
3
1
different degrees of coupling to P, i.e., 5.3 and -1 Hz.
VT NMR study
A variable temperature NMR study of 7 was conducted in
C D CD solution. Compound 7 is superior to 6 for this pur-
6
5
3
pose, not only in being a pure crystalline material but also in
possessing a pair of diastereotopic methyl groups on silicon
which are direct probes for the chirality induced by the
asymmetric phosphorus atom in the absence of fast pyrami-
dal inversion. The basic spectral features of the dynamic
NMR sample, which was made up in C D CD , are very
The structures of the symmetrically substituted
phosphanes 5, 8, and 9 are quite similar, having one
6
5
3
similar to the RT spectrum in deuterochloroform as reported
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Can. J. Chem. Vol. 80, 2002
Scheme 3.
Fig. 3. NMR spectra of 7 at various temperatures as recorded in
C D CD . Spectra were monitored at 10° intervals over the range
6
5
3
1
78 to 380 K, and in the intervening spectra to those displayed
here, smooth transitions between the peaks were observed. Only
the aliphatic region of each spectrum is displayed, including the
silyl, the isopropyl, and the PH signals. The residual C D CHD
2
6
5
peak (X) is cut short in the low-temperature spectra. Similarly,
the t-Bu peak is cut short for the middle traces.
in Table 2. In Scheme 3, the molecule is viewed end-on to
highlight the symmetry relationships relevant to solution dy-
namics (the t-Bu group is located in front of the Si atom,
and is omitted for clarity). Figure 3 presents several traces
selected from the series of spectra that illustrate the key
3
spectral changes; full VT data have been deposited.
At 380 K, the dynamic processes are in the high tempera-
(
1,2)
ture limit. Here there is only one kind of isopropyl (CH
(
A–D)
equivalent), one isopropyl methyl (CH
equivalent), and
equivalent). This is
3
one silyl methyl group (CH3(
E,F)
achieved by a combination of rapid rotation about single
bonds and rapid inversion at phosphorus. We note that the
CH3(
E,F)
signal is a phosphorus-coupled doublet with a cou-
pling constant of -3 Hz.
(
A,B,C,D)
On cooling, coalescence of the CH3
as well as the
CH3(
E,F)
signals occurs, and the coalesced signals become
sharp at lower temperature, e.g., as in the 263 K spectrum.
Here CH3( appears as a singlet (the splitting of ~1 Hz ob-
served in CDCl3 is not resolved in the more viscous
F)
(
E)
C D CD ), while CH
is a doublet with coupling to phos-
phorus of -5 Hz. The isopropyl methyl signals break into
6
5
3
3
(
A,D)
(B,C)
two distinct sets, labelled CH
and CH3
signals remain identical, as in the 380 K spectrum.
These changes are consistent with slowed pyramidal inver-
, while the
3
CH(
1,2)
(
E)
(F)
sion at the chiral phosphorus atom. Thus CH
and CH3
3
as well as the CH3(
A,D)
and CH3
(B,C)
pairs become diastereo-
topic with respect to the chiral phosphorus atom.
Rapid rotation about single bonds has the effect of map-
ping CH( on CH , CH3 on CH3 , and CH3 on CH3
1)
(2)
(A)
(D)
(B)
(C)
(
Scheme 3), averaging their chemical shifts in the intermedi-
ate temperature regime. Below 263 K, this rotational averag-
ing slows down, with coalescence of the methine signals
occurring at 213 K. Partial coalescence of the methyl signals
is also observed at this temperature. By 178 K we had not
reached the low temperature limit of this additional dynamic
process, and further cooling of our solution led only to gen-
eral line broadening. However, at 178 K the methine signals
split up into two distinct, though broadened, signals labelled
with values previously reported in a dynamic study of
–
1
–1
TripPHSiPh (21): 39 kJ mol for rotation and 71 kJ mol
3
for inversion (27). The assignments of the processes in 21
were made by comparison to literature data on phosphorus
inversion barriers. The presence of diastereotopic groups
CH3(
E,F)
on silicon in 7 provides internal corroboration that
this is the correct assignment.
CH( and CH (Fig. 3). We attribute this inequivalence to
slowed rotation about single bonds. Other workers refer to
this as slow rotation about the C(aryl)—P bond (27), though
several bond rotations are likely coupled in a molecule as
sterically congested as 7.
1)
(2)
Experimental
Starting materials and general procedures
All experimental procedures were performed under a ni-
trogen atmosphere using modified Schlenk techniques. 2,6-
Diisopropylaniline (Aldrich) was distilled from KOH and
stored under nitrogen prior to use. PCl , SiMe Cl, SiMe -t-
Using the classical expression of Gutowsky and Holm
(
37) with Eyring absolute rate theory (38), we can estimate
3
3
2
‡
–1
the barrier heights as ꢁG - 40 kJ mol for rotation about
BuCl (Aldrich), and HBr (Merck) were commercial products
and used as received. Tetrahydrofuran was dried and distilled
under a nitrogen atmosphere from sodium/benzophenone.
‡
–1
single bonds and ꢁG - 72 kJ mol for pyramidal inversion
at phosphorus. These numbers are in remarkable agreement
3
Table S1 presents the full VT NMR Data for 7 at ~10 K intervals from 178 to 380 K.
©
2002 NRC Canada

Boeré and Masuda
1615
Hexanes and pentane were dried and distilled under a nitro-
Preparation of DipPHSi(CH ) , 6
3 3
1
13
31
gen atmosphere from LiAlH . H, C, and P NMR spectra
A solution of 28.4 mL of 1.6 M n-BuLi was added slowly
by syringe to a vigorously stirred solution of 8.412 g
(43.3 mmol) 5 in 80 mL THF held at 0°C. The now red so-
lution was warmed to RT and stirred for 20 min, then cooled
again to 0°C, whereupon 5.77 mL (45.5 mmol, 5% excess)
4
were recorded on a Bruker AC250/Tecmag Macspect spec-
trometer operating at 250.13, 101.26, and 62.90 MHz, re-
1
13
spectively. The H and C chemical shifts of CDCl3
solutions were referenced to TMS and of C D CD solutions
6
5
3
3
1
to the residual CHD pentet at ꢂ 2.09, while P was refer-
ClSi(CH ) was added via syringe, followed by warming to
2
3 3
enced to external 85% H PO . Infrared spectra were re-
RT and heating to reflux for 2 h. Removal of the volatile
components, extraction with 100 mL hexanes, filtering,
evaporation, and distillation (110–115°C, 0.5 mbar) of the
residue produced 7.39 g of clear oil (29.6 mmol, 64%).
3
4
corded on Bomem MB-100 or Nicolet Avatar 360
spectrometers as KBr pressed pellets (solids) or as neat liq-
uid smears. Mass spectra were undertaken in the Department
of Chemistry, University of Alberta, and elemental analyses
were done by MHW Laboratories, Phoenix, Ariz. 1-Bromo-
+
NMR, see Table 2. MS (70eV) m/z (%): 266.16084 ([M] ,
+
+
64), 193 ([DipPH] , 57), 149 ([C H P], 14), 73 ([SiMe ] ,
9
10
3
2
,6-diisopropylbenzene
was
prepared
from
2,6-
100). Analytically pure material was not obtained, and all
samples tended to be contaminated with very small amounts
of either 5 or 8.
diisopropylaniline by the literature method (28).
Preparation of 2,6-diisporopylphenylphosphonus mixed
halide, 11
Preparation of DipPH{Si(CH ) -t-Bu}, 7
3
2
A solution of Grignard reagent was prepared from 60.0 g
A solution of 4.00 g (20.6 mmol) 5 in 40 mL THF cooled
to 0°C was treated with 13.5 mL (21.6 mmol, 5% excess) of
1.6 M n-BuLi. After warming the now yellow solution to RT
for 30 min and cooling back to 0°C, 3.10 g (20.6 mmol)
ClSi(CH ) -t-Bu was added by syringe. After warming to RT
(
247 mmol) of 10 and 6.0 g of activated magnesium turnings
in 1150 mL of THF, and transferred via cannula to a solution
of 67.0 g (488 mmol) PCl in 200 mL THF held at –15°C.
3
The mixture was allowed to warm to room temperature with
stirring (12 h) and then heated to reflux for 2 h. Evaporation
of solvent, extraction with 500 mL ether, filtering, and distil-
lation (106–111°C, 0.5 mbar) produced 14.2 g of viscous,
colourless oil. Careful integration of the 31P NMR spectrum
3
2
3
1
and then refluxing for 1 h, the solution was shown by
P
NMR to have 70% conversion to 7. A further 4.5 mL of n-
BuLi followed by 1.05 g (3.6 mmol) was used in a repeat of
the above procedure. NMR indicated complete conversion at
this stage. The solids remaining after evaporation of the sol-
vent were extracted with 20 mL hexanes, filtered, evapo-
rated, and the residue sublimed under static vacuum onto a
water-cooled cold finger, yielding 3.60 g of colourless crys-
talline 7 (11.7 mmol, 57% yield, mp 45–51°C). X-ray qual-
ity crystals were grown by sublimation under dynamic
(
CDCl ) showed this to be the mixed phosphonous dihalides
3
DipPCl (11a) (ꢂ 164.5, 4.5 g), DipPClBr (11b) (ꢂ 159.2,
2
6
3
4
.9 g), and DipPBr (11c) (ꢂ 151.9, 2.8 g); total molar yield
2
1
8.1%. H NMR (CDCl ) ꢂ: 7.44–7.21 (m, 3H), 4.14–
3
.11(three sept, each with JHH = 6.8 Hz, 2H), 1.34–1.29
1
3
(
three d, each with JHH = 6.8 Hz, 6H). 3 C NMR ꢂ: 154 (d,
2
–1
JPC = 23 Hz), 133.49–133.41 (three d, J -1 Hz), 124.97,
vacuum in a three-zone tube furnace. IR (KBr pellet) (cm ):
PC
3
3
1
2
24.91 (two s), 31.25, (d, J = 27.8 Hz), 31.08, (d, J
7.8 Hz), 30.89, (d, J = 27.3 Hz), 24.91–24.81 (three
=
3050(w), 2958(vs), 2925(vs), 2856(s), 2330(m), 1567(w),
1463(s), 1417(w), 1381(w), 1362(s), 1246(m), 1176(w),
1050(w), 1006(w), 938(w), 839(s), 821(s), 804(vs), 773(w),
737(vs), 680(w), 601(w), 578(w), 444(w), 430(w). NMR,
PC
PC
3
PC
8
1
doublets, not resolved). MS (70eV) (calculated on Br and
35
+
+
Cl) m/z (%): 354 ([DipPBr ] , 5.5), 308 ([DipPBrCl] , 26.5),
2
+
+
+
+
273 ([DipPBr] , 9.8), 262 ([DipPCl ] , 44.9), 227 ([DipPCl] , 100).
see Table 2. MS (70 eV) m/z (%): 308.20866 ([M] , 53), 251
2
+
+
(
[DipP(Si(CH ) ] , 11), 193 ([DipPH] , 15), 115
3 2
+
+
Preparation of 2,6-diisopropylphenylphosphane, 5
A solution of 10.0 g (32.7 mmol) of 11 in 150 mL anhy-
drous ether was transferred via cannula to a vigorously
([(CH3)3CSi(CH3)2] , 13.7), 73 ([Si(CH3)3] , 100). Anal. calcd.
for C18H33PSi (%): C 70.08, H 10.78; found: C 69.84, H 10.62.
stirred suspension of 2.78 g (73.2 mmol) LiAlH in 150 mL
Preparation of DipP{Si(CH ) } , 8
4
3 3 2
anhydrous ether held at –5°C. After warming to room tem-
perature and heating to reflux for 22 h, the mixture was
cooled to 0°C and treated with 53 mL of saturated aqueous
A stirred solution prepared from 2.00 g (10.3 mmol) 5 and
20 mL THF held at 0°C was treated with 6.76 mL
(10.8 mmol, 5% excess) of 1.6 M n-BuLi. After 20 min, the
stirred orange solution was treated with 1.37 mL
(10.8 mmol) ClSi(CH ) , yielding a colourless solution with
NH Cl. The organic layer was decanted and the solids
4
washed with 3 × 100 mL degassed ether. The combined
ethereal solutions were dried, filtered, and evaporated to a
milky oil. Distillation (73–85°C, 0.5 mbar) produced 5 as a
3
3
3
1
a white precipitate. P NMR indicated that conversion to 7
was incomplete; a further 5.07 mL (8.11 mmol) of n-BuLi
and 1.03 mL (8.11 mmol) ClSi(CH ) were added following
–
1
clear oil (4.76 g, 24.6 mmol, 75.3% yield). IR (neat) (cm ):
056 (w), 2961 (vs), 2868 (m), 2627 (s), 2306 (m), 1572
3
3
3
1
3
the same procedure as before. P NMR analysis indicated
75% 8 and 25% 7. Again 5.07 mL (8.11 mmol) of n-BuLi
and 1.03 mL (8.11 mmol) ClSi(CH ) were added following
(
(
(
w), 1461 (s), 1423 (w), 1383 (m), 1362 (m), 1247 (w), 1181
w), 1162 (w), 1106 (w), 1086 (m), 1054 (m), 1037 (w), 923
w), 831 (m), 792 (s), 720 (s), 499 (w). NMR, see Table 2.
3
3
the same procedure and NMR analysis showed complete
conversion to 8. Evaporation of solvent, extraction with
100 mL hexanes, filtration, evaporation, and distillation of
the residue (120–130°C, 0.5 mbar) gave 2.20 g of 8, which
cooled to a pasty white solid (6.5 mmol, 63% yield). IR
+
+
MS (70eV) m/z (%): 194.12306 ([M] , 100), 193 ([DipPH] ,
+
+
4
4), 179 ([C H P] , 30), 161 ([C H ] , 54), 152
11 16 12 17
+
+
+
(
[C H P] , 44), 151 ([C H P] , 26), 137 ([C H P] , 25),
9 13 9 12 8 10
10 ([C H P] , 52), 91 ([C H ] , 26). Anal. calcd. for
6 7 7 7
+
+
1
–
1
C H P (%): C 74.20, H 9.86; found: C 74.43, H 9.74.
(neat) (cm ): 3050 (m), 2957 (vs), 2867 (s), 1935 (w), 1861
1
2
19
©
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1
616
Can. J. Chem. Vol. 80, 2002
(
1
1
w), 1695 (w), 1651 (w), 1633 (w), 1589 (w), 1569 (m),
557 (w), 1463 (s), 1381 (m), 1361 (m), 1304 (m), 1246 (s),
177 (m), 1048 (m), 992 (m), 835 (s), 801 (m), 748 (m), 684
computers under Windows 2000 or on four processor
Compaq Alpha ES40 or ES45 workstations. Full frequency
calculations were performed on all models, and stationary
states were confirmed by the absence of imaginary frequen-
cies. In the case of 8 the frequency calculation was unsuc-
cessful, but the strong agreement with the structures of other
members of the series provides confidence in the calculated
structure.
(
m), 625 (m), 509 (w), 402 (w). NMR, see Table 2. MS
+
+
(70eV) m/z (%): 338.20229 ([M] , 49), 266 ([DipPHSi(CH ) ] ,
3
2
+
+
+
1
1
5), 193 ([DipPH] , 16), 147 ([C H ] , 13.7), 73 ([Si(CH ) ] ,
11 15 3 3
00). An analytically pure sample was not obtained.
Preparation of DipP{Si(CH ) -t-Bu} , 9
3
2
2
A solution of 2.00 g (10.3 mmol) 5 in 20 mL THF held at
°C was treated with 6.43 mL (10.29 mmol) of 1.6 M n-
Conclusions
0
BuLi. After warming to RT, the yellow solution was cooled
A new bulky phosphane and two mono- as well as two
disilyl derivatives were prepared by routes that provide these
materials in synthetically useful quantities. The silyl com-
pounds as well as the phosphane itself are expected to be
useful synthons for both main group and transition metal
chemistry. We have identified a further example of dynamic
again and treated with 1.55 g (10.29 mmol) ClSi(CH ) -t-
3
2
Bu. The faint yellow solution was allowed to warm to room
temperature over 30 min. Cooling again in an ice bath and
treating with another 6.43 mL of 1.6 M n-BuLi gave an or-
ange solution. Again 1.55 g ClSi{CH ) -t-Bu} was added af-
3
2
ter warming the reaction mixture to room temperature and
cooling to 0°C. The faint yellow solution was then heated to
reflux for 12 h and 31P NMR analysis showed complete con-
version to 9. Evaporation of solvent, extraction into 30 mL
pentane, filtering, and evaporation yielded a white solid,
which was purified by sublimation under dynamic vacuum
onto a water-cooled cold finger to give 2.934 g of a white
crystalline 9 (7.7 mmol, 75% yield, mp 57–62°C). IR (KBr
effects in compounds of the general class ArPHSiR where
3
both the aryl group and silicon bear sterically congested sub-
stituents. Studies on the utility of 5–9 as synthons are ongo-
ing in our laboratory.
Acknowledgments
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the University of
Lethbridge for financial support (R.T.B.). We thank Dr. Bob
Macdonald, Department of Chemistry, University of Alberta,
for the collection of the intensity data for the X-ray diffrac-
tion study, and Meg O’Shea for performing the DFT calcula-
tions. Access to the Multimedia Advanced Computation
Infrastructure (MACI) Alpha cluster at the University of
Calgary is gratefully acknowledged. We thank Dr. Paul
Hazendonk for helpful discussions, and anonymous referees
for useful suggestions.
–
1
pellet) (cm ): 3047 (w), 2958 (vs), 2929 (vs), 2894 (s),
857 (s), 2569 (w), 1471 (m), 1460 (m), 1403 (w), 1388
2
(
(
m), 1380 (m), 1360 (m), 1251 (s), 1245 (s), 1177 (w), 1048
w), 1024 (w), 1005 (w), 834 (vs), 820 (s), 794 (vs), 760 (s),
42 (m), 681 (w), 670 (m), 660 (w), 575 (w), 470 (w), 408
+
+
3
3
Anal. calcd. for C H PSi (%): C 68.18, H 11.21; found:
2
4
47
2
C 67.99, H 10.94.
X-ray diffraction analysis of DipPH{Si(CH ) -t-Bu}, 7
3
2
The crystal structure of 7 was determined using a Bruker
instrument fitted with a SMART 1000 area detector. Experi-
mental details are summarized in Table 3. The structure was
solved by direct methods using SHELXS-97 (39) and re-
fined by full-matrix least-squares on F against all reflec-
tions using SHELXL-97 (40). Neutral atom scattering
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