2,4,6-tri-tert-butylphenyl and 2,4-di-tert-butyl-6-methylphenyl groups: Look similar, react differently

Article abstract of DOI:10.1002/hc.10083

The crystal structures of molecules with two phosphaalkene groups have been determined. Differences in the stabilization of the P=C π-bond by the 2,4, 6-tri-tert-butylphenyl and 2, 4-di-tert-butyl-6-methylphenyl groups were observed. It has been found that lithium supermesityl(trimethylsilyl)phosphide could be a very efficient base to remove a protort from acetonitrile.

Full text of DOI:10.1002/hc.10083

Heteroatom Chemistry
Volume 13, Number 7, 2002
2,4,6-Tri-tert-butylphenyl and
2,4-Di-tert-butyl-6-methylphenyl Groups:
Look Similar, React Differently
Alex S. Ionkin and William J. Marshall
DuPont Central Research & Development, Experimental Station, Wilmington, DE 19898-0328
Received 22 March 2002; revised 24 April 2002
ABSTRACT: The crystal structures of molecules with
two phosphaalkene groups have been determined. Dif-
ferences in the stabilization of the P C π-bond by
the 2,4,6-tri-tert-butylphenyl and 2,4-di-tert-butyl-6-
methylphenyl groups were observed. It has been found
that lithium supermesityl(trimethylsilyl)phosphide
could be a very efficient base to remove a pro-
ꢀ
C
ton from acetonitrile.
2002 Wiley Periodicals, Inc.
Heteroatom Chem 13:662–666, 2002; Published online
in Wiley InterScience (www.interscience.wiley.com). DOI
10.1002/hc.10083
INTRODUCTION
Phosphorus is homologous to nitrogen and could
increase the electron density on the metal center.
Therefore, it can make the complex more stable and
extend the lifetime of the catalyst in the polymeriza-
tion. In order to replace nitrogen atoms in the tri-
dentate [N,N,N] ligand in A by phosphorus atoms,
we had to deal with low-coordinated phosphorus.
Some synthetic methods to stabilize such molecules
bearing 2,4,6-tri-tert-butylphenyl groups at the phos-
phorus atom have been published [4–8]. Our task
was to make the low-coordinated phosphorus triden-
tate [P,N,P] ligands with additional steric hindrance
near the imino functions of the pyridine ring, or the
above kind of ligands with less steric hindrance at the
phosphorus atom. In some cases, building additional
bulk could facilitate the dissociation of the formed
catalyst-product, and thus, lead to an enhancement
of the reaction rates [9].
The past decade has witnessed tremendous advances
in the design and application of organometallic
complexes as alpha-olefin polymerization catalysts;
many are now reaching the early stages of com-
mercialization [1]. Some of the most recent addi-
tions to the small but growing number of highly ac-
tive nonmetallocene polymerization catalysts are the
2,6-bis(ꢀ-iminoalkyl)pyridine iron(II) complexes A
[2,3].
Dedicated to Professor Louis D. Quin on the occasion of his 74th
birthday.
Correspondence to: Alex S. Ionkin; e-mail: alex.s.ionkin@usa.
dupont.com.
Contract grant sponsor: DuPont.
Contract grant number: 08297.
2002 Wiley Periodicals, Inc.
c
ꢀ
662

2,4,6-Tri-tert-butylphenyl and 2,4-Di-tert-butyl-6-methylphenyl Groups: Look Similar, React Differently 663
RESULTS AND DISCUSSION
Since 1 equiv of unreacted basic lithium tri-
methylsilyl(2,4,6-tri-tert-butylphenyl)phosphide was
left in the reaction mixture, and taking into account
work [12] about the addition of lithium aryl phos-
phides to the triple bond between the carbon and
nitrogen in acetonitrile to form phosphaalkenes, we
added a controlled amount of acetonitrile to the re-
action mixture. To our surprise, we did not detect
any amino-substituted phosphaalkenes in the reac-
tion mixture. Instead, lithium trimethylsilyl(2,4,6-
tri-tert-butylphenyl)phosphide behaved as a highly
efficient base toward the weak acidic protons
of the acetonitrile and lithiated it. The in-
dication that lithium trimethylsilyl(2,4,6-tri-tert-
butylphenyl)phosphide can be an efficient base has
been discussed in reports of the synthesis of phos-
phasilenes [13,14]. The in situ formed lithium methy-
lene nitrile 5 added to the carbonyl group of 4.
The formed adduct was purified by column chro-
matography, with the isolation of the correspond-
ing alcohol 6 containing also the phosphaalkene
moiety.
The preparation for the nitrogen backbone of the lig-
and was the first step. From the commercially avail-
able 2,6-pyridinedicarbonyl dichloride (1) and tert-
butylmagnesium chloride, the corresponding dike-
tone 2 could be made but with substantial amounts
of the tris-adduct 3.
Compounds 2 and 3 could be easily identified by
GC/MS with m/z 247 for 2 and m/z 305 for 3. The
ratio between 2 and 3 was 4:1.
The formation of tris-adduct 3 was a well-known
complication for this kind of reaction. To overcome
this complication cuprate chemistry was employed
in the manner of polysilylcuprate [10]. By using tert-
butyl cuprate, the diketone 2 was isolated with a 70%
yield after chromatography.
The synthesis of the phosphorus part of the ligand
was the second step. Initially, lithium trimethylsi-
lylphosphide bearing the 2,4,6-tri-tert-butylphenyl
group was prepared [11]. Coupling the diketone 2
with it at room temperature led to the monosubsti-
tuted phosphaalkene 4.
A crystal of 6 suitable for X-ray analysis was ob-
tained from pentane and its ORTEP drawing is
shown in Fig. 1. 1H and 31P NMR spectral data are in
The phosphaalkene 4 is stable enough to pass
through a GC column and give the molecular ion at
507. Compound 4 has a typical downfield chemical
shift δ 31P = 256.0. There are two downfield signals
in the 13C NMR spectrum proving a crucial bond-
ing sequence in the molecule: δ = 205.5 (carbonyl
FIGURE 1 ORTEP drawing of 2-[6-(1ꢁ-hydroxo-1ꢁ-cyano-
methyl-2ꢁ,2ꢁ -dimethylpropyl)pyridyl]tert-butylmethylene(2,4,
6-tri-tert-butylphenyl)phosphine (6).
1
sp2-carbon) and 191.7 (doublet, JPC 53.7 Hz, phos-
phaalkene sp2-carbon).

664 Ionkin and Marshall
agreement with the assigned structure of compound
6.
phetane (11).
Because we were unable to obtain the bis-
phosphaalkene through the use of the 2,4,6-tri-
tert-butylphenyl group at phosphorus, we decided
to apply the 2,4-di-tert-butyl-6-methylphenyl group
for the protection of a two coordination status
for the phosphorus atom. The 2,4-di-tert-butyl-6-
methylphenyl group has been shown to allow the sta-
bilization of diphosphenes [15,16], phosphaalkenes
[17], dithiophosphoranes [18], and arsaalkenes
[19].
In contrast to the lithium trimethylsilyl (2,4,6-
tri-tert-butylphenyl)phosphide, lithium trimethylsi-
lyl(2,4-di-tert-butyl-6-methylphenyl)phosphide con-
densed with diketone 2 at room temperature with
formation of the bisphosphaalkene 7.
The ORTEP drawing of 11 is presented in Fig. 3.
The chemical shift δ 31P of 11 is −45.5, practically
identical to that published earlier [16]. The com-
pound 11 has been prepared by the dehalo-
genation reaction of a 2,4-di-tert-butyl-6-methyl-
phenyldihalophosphine with magnesium [16].
Presently, we do not know the role of the pyridine
moiety in the reaction. The mechanism of the
formation of 11 has not been investigated yet.
A crystal of compound 7 suitable for X-ray anal-
ysis was obtained from methylene chloride. The
ORTEP drawing of 7 is shown in Fig. 2. 1H, 13C, and
31P NMR spectra and elemental analysis data are in
agreement with the assigned structure 7. The 2,4,6-
tri-tert-butylphenyl group has been reported [5] to
allow the stabilization of bisphosphaalkene 8 with-
out the presence of tert-butyl groups at the carbon
atoms of the P C π-bond.
Our attempt to synthesize an analogous
bis-phosphalkene with the presence of the 2,4-di-
tert-butyl-6-methylphenyl moiety led to isolation of
tetrakis[2,4-di-tert-butyl-6-methylphenyl]tetraphos-
FIGURE 2 ORTEP drawing of 2,6-bis[1-(2ꢁ,4ꢁ-di-tert-butyl-
6ꢁ -methylphenyl)phosphinidene-2ꢁꢁ,2ꢁꢁ-dimethylpropyl]-pyri-
dine (7).

2,4,6-Tri-tert-butylphenyl and 2,4-Di-tert-butyl-6-methylphenyl Groups: Look Similar, React Differently 665
Preparation of
2-[6-(2ꢁ,2ꢁ-Dimethylpropionyl)pyridyl]tert-
butylmethylene(2,4,6-tri-tert-butylphenyl)
Phosphine (4) and 2-[6-(1ꢁ-Hydroxo-1ꢁ-
cyanomethyl-2ꢁ,2ꢁ-dimethylpropyl)pyridyl]tert-
butylmethylene(2,4,6-tri-tert-butylphenyl)
Phosphine (6)
A
1.6 M solution of butyllithium in hexane
(14 ml) was added to a solution of 2,4,6-tri-tert-
butylphenylphosphine (5.59 g, 0.02 mol) in 250 ml
of THF at 0◦C. After that, 2.39 g of chlorotrimethyl-
silane was added dropwise and warmed to room
temperature. Butyllithium (14 ml) was added again
at 0◦C into the solution that had been cooled
to −78◦C and 2.48 g (0.01 mol) of 2,6-bis(2ꢁ,2ꢁ-
dimethylpropionyl)pyridine 2 in 20 ml of THF was
added dropwise. After the addition, the solution was
warmed to room temperature and 40 ml of dry
acetonitrile was added. The solvents were evaporated
under reduced pressure. The residue was purified by
silica-gel chromatography with hexane/methylene
chloride (1:1) as the eluent to give 0.710 g (7%
yield) of phosphaalkene 4, as a white solid (from
FIGURE 3 ORTEP drawing of tetrakis[2,4-di-tert-butyl-6-
methylphenyl]tetraphosphetane (11).
1
acetonitrile) with mp 212.95◦C. H NMR: (CD2Cl2)
δ = 1.15 (s, 9H, p-t-Bu), 1.31 (s, 9H, t-Bu CO), 1.42
(s, 9H, t-Bu C P), 1.43 (s, 9H, o-t-Bu), 6.4–7.5 (mul-
tiplets, 5H, Py and Ph-protons); 31P NMR: (CD2Cl2)
δ = 256.0. Chemical ionization m/z calculated for
C33H51NOP: 508.3708; found 508.3719. Anal calcd for
C33H50NOP: C, 78.11%; H, 9.86%; P, 6.11%. Found:
C, 78.06%, H, 10.02%, P, 6.56%. Phosphaalkene 6
was isolated with 16% yield (1.75 g), as a white
solid with with mp 154.02◦C. 1H NMR: (CD2Cl2)
δ = 0.80 (s, 9H, p-t-Bu), 1.10 (s, 9H, t-Bu COH),
1.31 (s, 9H, t-Bu C P), 1.39 (s, 9H, o-t-Bu), 1.53
(s, 9H, o-t-Bu), 2.98 (m, 2H, CH2), 6.0–9.5 (multi-
plets, 5H, Py and Ph-protons); 31P NMR: (CD2Cl2)
δ = 251.0. Chemical ionization m/z calculated for
C34H54N2OP: 549.3974; found 549.3995. Anal calcd
for C35H53N2OP: C, 76.64%; H, 9.67%; N, 5.11%.
Found: C, 76.40%, H, 9.71%, N, 4.99%.
By carefully tuning up the steric hindrance
near the phosphorus and carbon atoms of the P C
π-bond, tridentate [P,N,P] ligands with prepar-
ative yields can be obtained for the catalysis
applications.
EXPERIMENTAL
Preparation of
2,6-Bis(2ꢁ,2ꢁ-dimethylpropionyl)pyridine (2)
A 1.7 M solution of tert-butyllithium in pentane
(60 ml) was added to a suspension of 18.66 g
(0.098 mol) of CuI in 50 ml of THF at once at
−78◦C with vigorous stirring. The reaction mixture
was kept for 1 h at this temperature and 10.0 g
(0.049 mol) of 2,6-pyridinedicarbonyl dichloride in
40 ml of THF was added. The reaction mixture
was allowed to warm to room temperature, treated
with 10% NaOH in water, filtered, and extracted
with pentane. Extracts were concentrated and the
residue was subjected to chromatography on sil-
ica gel with hexane/ethyl acetate (10:1) as the elu-
ent. The yield of 2 was 8.47 g (70%) as oil with
Preparation of 2,6-Bis[1-(2ꢁ,4ꢁ-di-tert-butyl-
6ꢁ-methylphenyl)phosphinidene-2ꢁꢁ,2ꢁꢁ-
dimethylpropyl]-pyridine (7)
A 1.6 M solution of butyllithium in hexane (36 ml)
was added to a solution of 2,4,6-di-tert-butyl-6-
methylphenylphosphine (12.42 g, 0.0526 mol) in
100 ml of THF at 0◦C. After that, 6.3 g of chloro-
trimethylsilane was added dropwise and the mix-
ture was warmed to room temperature. Butyllithium
(36 ml) was added again at 0◦C to the solution that
had been cooled to −78◦C and 6.5 g (0.0263 mol)
1
m/z = 247, H NMR: (CD2Cl2) δ = 7.82 (broad, 3H,
Py), 1.30 (s, 18 H, t-Bu); 13C NMR: (CD2Cl2) δ =
206.92 (s, C O), 154.39 (s, C N), 138.60 (s, C C),
126.57 (s, C C), 44.45 (s, CMe3), 28.05 (s, Me).
The above data are close to the literature values
[20].

666 Ionkin and Marshall
of 2,6-bis(2ꢁ,2ꢁ-dimethylpropionyl)pyridine 2 in 20
ml of THF was then added dropwise. After the ad-
dition, the solution was warmed to room tempera-
ture. The solvents were evaporated under reduced
pressure. The residue was extracted and recrystal-
lized from pentane. The yield of 7 was 14.56 g (81%)
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1
as a white solid with 215.63◦C. H NMR: (CD2Cl2)
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A 1.6 M solution of butyllithium in hexane (11 ml)
was added to a solution of 2,4,6-di-tert-butyl-6-
methylphenylphosphine (3.67 g, 0.0155 mol) in
100 ml of THF at 0◦C. After that, 1.86 g of
chlorotrimethylsilane was added dropwise, and the
mixture was warmed to room temperature. Butyl-
lithium (11 ml) was added again at 0◦C to the solu-
tion that had been cooled to −78◦C, and 1.0 g (0.0074
mol) of 2,6-pyridinedicarboxaldehyde 9 in 20 ml of
THF was added dropwise. After the addition, the so-
lution was warmed to room temperature. The sol-
vents were evaporated under reduced pressure. The
residue was extracted and recrystallized from ben-
zene. The yield of 11 was 2.47 g (68%), orange crys-
tals 31P NMR: (CD2Cl2) δ = −45.5.0 [16]. The X-ray
structure is shown in Fig. 3.
ACKNOWLEDGMENT
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GC/MS support.
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