Remarkable, sterically induced rate enhancement in the insertion of allenes into palladium-methyl bonds

Article abstract of DOI:10.1021/om991018p

The rate of insertion of methyl-substituted allenes into the Pd-Me bond in chelate pyridine-thioether complexes [PdCl(Me)(R′N-SR)] (R′N-SR = 6-R′-C₅H₃N-2-CH₂SR) to give 2-methylallyl derivatives is remarkably enhanced by t

Full text of DOI:10.1021/om991018p

Organometallics 2000, 19, 1461-1463
1461
Rem a r k a ble, Ster ica lly In d u ced Ra te En h a n cem en t in
th e In ser tion of Allen es in to P a lla d iu m -Meth yl Bon d s
Luciano Canovese,*,† Fabiano Visentin,† Gavino Chessa,† Paolo Uguagliati,† and
Giuliano Bandoli‡
Dipartimento di Chimica, Universita` di Venezia, Venice, Italy, and Dipartimento di Scienze
Farmaceutiche, Universita` di Padova, Padua, Italy
Received December 21, 1999
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lexes 1a a n d 1b
Summary: The rate of insertion of methyl-substituted
allenes into the Pd-Me bond in chelate pyridine-
thioether complexes [PdCl(Me)(R′N-SR)] (R′N-SR ) 6-R′-
C5H3N-2-CH2SR) to give 2-methylallyl derivatives is
remarkably enhanced by the presence of a methyl group
in position 6 of the pyridine ring, which induces distor-
tion on the main coordination plane, resulting in a metal
substrate more prone to allene insertion. The flexibility
of the sulfur-donor chelate ligand appears to be a
paramount requisite.
1a
1b
1a
1b
Pd-Cl
Pd-S
2.347(1) 2.336(2) Pd-N
2.259(1) 2.268(2) P-C(11)
2.162(5) 2.229(4)
2.035(7) 2.048(5)
Cl-Pd-S
Cl-Pd-N
178.8(1) 168.4(1) S-Pd-N
84.8(1) 82.8(1)
95.3(1) 99.4(1) S-Pd-C(11) 90.7(2) 89.9(2)
Cl-Pd-C(11) 89.3(2) 87.6(2) N-Pd-C(11) 174.9(2) 172.7(2)
In the course of our systematic studies on the syn-
thesis and reactivity of palladium(II) chelate complexes
with hemilabile, mixed donor polydentate ligands,5 we
conceived that the rate of insertion of allenes into the
palladium-methyl bond in pyridine-thioether com-
plexes [PdCl(Me)(R′N-SR)] (R′N-SR ) 6-R′-C5H3N-2-
CH2SR) could be enhanced by the presence of the sub-
stituent in position 6 of the pyridine ring. This was ex-
pected to induce distortion of the main coordination
plane, resulting in a metal center more prone to prior
coordination of allene, the latter being considered a req-
uisite preliminary step in these insertion reactions.1b,3d,e
To this purpose we have prepared the complexes 1a ,b
and 2a ,b.6-8 The crystal structures of substrates 1a and
1b are shown in Figure 1,9 and selected bond lengths
and angles are given in Table 1. A superimposition of
The insertion reactions of unsaturated molecules into
the metal-carbon bond of transition metal-alkyl and
-aryl chelate complexes is a subject of great current
interest.1 Particularly important appear to be the
interactions of organopalladium species with allenic
compounds under catalytic conditions involving the
polymolecular assembly of allenes, carbon monoxide,
and a wide range of nucleophiles. The ensuing reaction
products may be allylamines, methacrylate derivatives,
or etherocycles and carbocycles.1c-e Despite the wealth
of preparative and mechanistic studies, the factors
affecting the course and rates of these reactions have
yet to be rationalized. In particular, steric, structural,
and electronic properties of the chelating ligand may
lead to different results. Thus, insertion is made easier
by flexible diphosphine ligands with a large bite angle,2
while rigid diimine ligands with a small bite angle will
enhance the rate of insertion.3 Also, hemilability of the
polydentate ligand plays an important role.4
(5) (a) Canovese, L.; Visentin, F.; Uguagliati, P.; Chessa, G.; Luc-
chini, V.; Bandoli, G. Inorg. Chim. Acta 1998, 275, 385. (b) Canovese,
L.; Visentin, F.; Uguagliati, P.; Chessa, G.; Pesce, A. J . Organomet.
Chem. 1998, 566, 61.
(6) Complexes 1 and 2 were obtained by reacting the appropriate
pyridine-thioether ligand with a stoichiometric amount of [PdCl(Me)-
(COD)]7 (COD ) 1,5-cycloctadiene) in anhydrous, freshly distilled
toluene under an inert atmosphere (N2). The reaction mixture was
stirred at room temperature for several hours. The resulting yellowish
solution was dried under vacuum, dissolved in anhydrous CH2Cl2, and
treated with charcoal. Filtration on a Celite filter, reduction to small
volume, and addition of diethyl ether yields the required complex. In
any case only one isomer was detected, which was identified with the
species containing the thioetheric sulfur trans to the chlorine atom on
the basis of structural data and of the mutual trans influence.3b,8 1a :
* To whom correspondence should be addressed. E-mail: cano@
unive.it. Fax: ++39412578517.
† Universita` di Venezia.
‡ Universita` di Padova.
(1) (a) van Leeuwen, P. W. N. M.; van Koten, G. In Catalysis: An
Integrated Approach to Homogeneous, Heterogeneous and Industrial
Catalysis; Moulijn, J . A. et al., Eds.; Elsevier: Amsterdam, 1993, and
references therein. (b) Yamamoto, A. J . Chem. Soc. Dalton Trans. 1999,
1027. (c) Besson, L.; Gore´, J .; Cazes, B. Tetrahedron Lett. 1995, 36,
3853, 3857. (d) Grigg, R.; Monteith, M.; Sridharan, V.; Terrier, C.
Tetrahedron 1998, 54, 3885. (e) Zenner, J . M.; Larock, R. C. J . Org.
Chem. 1999, 64, 7312.
yield 89%. Anal. Found: C, 39.28; H, 5.61; N, 4.21. Calcd for C11H18
-
ClNPdS: C, 39.07; H, 5.36; N, 4.14%. 1H NMR (298 K, CDCl3, ppm):
δ 0.92 (3H, s, Pd-CH3), 1.34 (9H, s, S-C (CH3)3), 4.27 (2H, bs, CH2-
S), 7.34 (1H, dd, J ) 7.8, 5.1 Hz, H5pyr), 7.45 (1H, d, J ) 7.8 Hz, H3pyr),
7.69 (1H, td, J ) 7.8, 1.7 Hz, H4pyr), 9.21 (1H, d, J ) 5.1 Hz, H6pyr). 1b:
(2) Dierkes, P.; van Leeuwen, P. W. N. M. J . Chem. Soc., Dalton
Trans. 1999, 1519.
yield 80%. Anal. Found: C, 41.01; H, 5.81; N, 4.02. Calcd for C12H20
-
(3) (a) Dekker, P. G. C. M.; Elsevier, C. J .; Vrieze, K.; van Leeuwen,
P. W. N. M. Organometallics 1992, 11, 1598. (b) van Asselt, R.; Gielens,
E. E. C. G.; Ru¨lke, R. E.; Vrieze, K.; Elsevier, C. J . J . Am. Chem. Soc.
1994, 116, 977. (c) Ankersmit, H. A.; Veldman, N.; Spek, A. L.; Eriksen,
K.; Goubitz, K.; Vrieze, K.; van Koten, G. Inorg. Chim. Acta 1996, 252,
203. (d) Ru¨lke, R. E.; Delis, J . G. P.; Groot, A. M.; Elsevier, C. J .; van
Leeuwen, P. W. N. M.; Vrieze, K.; Goubitz, K.; Schenk, H. J .
Organomet. Chem. 1996, 508, 109. (e) Delis, J . G. P.; Groen, J . H.;
Vrieze, K.; van Leeuwen, P. W. N. M.; Veldman, N.; Spek, A. L.
Organometallics 1997, 16, 551.
ClNPdS: C, 40.92; H, 5.72; N. 3.98. 1H NMR (298 K, CDCl3, ppm): δ
1.12 (3H, s, Pd-CH3), 1.25 (9H, s, S-C(CH3)3), 3.08 (3H, s, C5H3N-6
-CH3, 4.05, 4.51 (2H, AB sys, J ) 17.18 Hz, CH2-S), 7.14 (1H, d, J )
7.7 Hz, H5pyr), 7.19 (1H, d, J ) 7.7 Hz, H3pyr), 7.61 (1H, t, J ) 7.7 Hz,
H4pyr). 2a : yield 93%. Anal. Found: C, 43.81; H, 3.66; N, 3.82. Calcd
for C13H14ClNPdS: C, 43.59; H, 3.94; N, 3.91. 1H NMR (298 K, CDCl3,
ppm): δ 0.91 (3H, s, Pd-CH3), 4.52 (2H, bs, CH2-S); H5pyr), 7.58 (8H,
m, H3pyr, H5pyr, H4pyr, Hphen), 9.29 (1H, d, J ) 5.0 Hz, H6pyr). 2b: yield
94%. Anal. Found: C, 45.11; H, 4.51; N, 3.74. Calcd for C14H16ClNPdS:
C, 45.17; H, 4.33; N, 3.76. 1H NMR (298 K, CDCl3, ppm): δ 1.14 (3H,
s, Pd-CH3), 3.10 (3H, s, C5H3N-6-CH3), 4.61 (2H, bABsys, CH2-S), 7.02
(1H, d, J ) 7.7 Hz, H5pyr), 7.12 (1H, d, J ) 7.7 Hz, H3pyr), 7.52 (1H, t,
J ) 7.7 Hz, H4pyr), 7.33 (3H, m, Hphen), 7.63 (2H, m, Hphen).
(4) Ru¨lke, R. E.; Kaasjager, V. E.; Kliphuis, D.; Elsevier, C. J .; van
Leeuwen, P. W. N. M.; Vrieze, K.; Goubitz, K. Organometallics 1996,
15, 668.
10.1021/om991018p CCC: $19.00 © 2000 American Chemical Society
Publication on Web 03/21/2000

1462 Organometallics, Vol. 19, No. 8, 2000
Communications
F igu r e 2. Superimposition of complexes 1a (bold line) and
1b (dotted line).
makes a dihedral angle of 10.5° in 1a and 34.4° in 1b
with the pyridine ring plane, whereas the dihedral angle
between the planes N-Pd-S and Cl-Pd-C(11) is 2.6°
in 1a and 11.4° in 1b. The five-membered ring Pd-S-
C(6)-C(5)-N adopts an envelope conformation (Cs) with
Pd, N, C(5), and C(6) coplanar, whereas S lies off by
0.49 Å in 1a and by as much as 0.93 Å in 1b. Torsion
angles range from -25.5 to +21.5° in 1a and from
-44.2° to +33.5° in 1b. The five-membered ring forms
dihedral angles with the pyridine ring and with the
plane Cl-Pd-C(11) of 9.9 and 9.0°, respectively, in 1a
and of 17.6 and 28.4°, respectively, in 1b. The Pd-N
distances and the Cl-Pd-S and Pd-N-C(5) bond
angles are 2.162(5) Å and 178(1) and 118.2(4)°, respec-
tively, in 1a , whereas they are 2.229(4) Å and 168.4(1)
and 112.3(3)°, respectively, in 1b. The bite angles and
other relevant structural features of the two compounds
are comparable.
F igu r e 1. Perspective views of the complexes 1a (top) and
1b (bottom), showing the numbering scheme and the
thermal ellipsoids at the 40% probability level.
the two structures is shown in Figure 2. It appears that
complex 1b is significantly distorted as compared to 1a
We have carried out a kinetic study of the reaction of
insertion of 1,1′-dimethylpropadiene (DMA) and 1,1′,3,3′-
tetramethylpropadiene (TMA) across the Pd-Me bond
in these complexes in CH2Cl2 at room temperature, to
(9) Crystal data are as follows. 1a : C11H18ClNPdS, fw ) 338.2,
triclinic, space group I1h, a ) 8.626(5) Å, b ) 10.610(6) Å, c ) 15.051(6)
Å, V ) 1374(1) Å3, Z ) 4, F ) 1.635 g/cm3, µ ) 1.67 mm-1. 1b: C12H20
-
ClNPdS, fw ) 352.2, monoclinic, space group P21/n, a ) 11.792(5) Å,
b ) 10.013(5) Å, c ) 13.301(6) Å, V ) 1513(1) Å3, Z ) 4, F ) 1.546
g/cm3, µ ) 1.517 mm-1. Suitable crystals (obtained by slow diffusion
of hexane in CH2Cl2 solutions) of complexes 1a and 1b were mounted
on a glass fiber at room temperature (20 °C). Data collection for both
crystals was performed on a Nicolet Siemens R3m/V diffractometer
(oriented graphite monochromator; Mo KR radiation, λ ) 0.710 73 Å
at 293(2) K). No sign of crystal deterioration was revealed by monitor-
ing 3 standard reflections after every 200 measurements for 1a (2
standards every 150 reflections for 1b). The stuctures were solved by
heavy-atom methods and completed by a combination of least-squares
techniques (on F2) and Fourier syntheses with the SHELXTL program
library. Totals of 2890 and 2151 unique reflections with I > 2σ(I) were
observed for 1a and 1b, respectively. All hydrogen atoms, apart from
those at C(11), were included in the idealized positions with C-H
distances and isotropic temperature factors fixed at 1.2 times the Ueq
value of the preceding carbon atom. All non-hydrogen atoms were
refined anisotropically. Selected bond distances and angles are sum-
as far as the C(7)-S-C(6)-C(5)-N-C(1) backbone is
concerned. In fact the trans donors S and Cl lie below
the average coordination plane by 0.03 Å in 1a and by
0.10 Å in 1b, with the other two donors N and C(11)
lying above the plane by 0.03 Å in 1a and 0.10 Å in 1b.
The central metal lies in the plane in 1a , whereas it is
above it by 0.13 Å in 1b. The average coordination plane
marized in Table 1. Final agreement factors were R ) 0.045, Rw
)
(7) Ru¨lke, R. E.; Ernsting, J . M.; Spek, A. L.; Elsevier, C. J .; van
Leeuwen, P. W. N. M.; Vrieze, K. Inorg. Chem. 1993, 32, 5769.
(8) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev.
1973, 10, 335.
0.105, and GOF ) 1.187 for 1a and R ) 0.037, Rw ) 0.105, and GOF
) 1.110 for 1b. Additional crystallographic data (atomic coordinates,
full listings of bond lengths and angles, anisotropic thermal param-
eters) are provided as Supporting Information.

Communications
Organometallics, Vol. 19, No. 8, 2000 1463
Ta ble 2. Secon d -Or d er Ra te Con sta n ts, k2 (m ol-1
d m 3 s-1), for th e Rea ction s of DMA a n d TMA w ith
1 a n d 2 a t 25 °C
A 6-methyl-substituted pyridyl group in a pyridyl-
phosphine ligand bound to palladium(II) has been
shown earlier to play a marked role in enhancing the
rate and selectivity of formation of methyl methacrylate
by methoxycarbonylation of propyne catalyzed by such
palladium(II) complexes.13
[PdCl(Me)(R′N-SR)]
DMA
TMA
1a : R′ ) H, R ) t-Bu 2.3 × 10-4 a
b
1b: R′ ) Me, R ) t-Bu (3.3 ( 0.1) × 10-1 c (1.07 ( 0.05) × 10-2 c
2a : R′ ) H, R ) Ph
(4.0 ( 0.3) × 10-2 c
b
2b: R′ ) Me, R ) Ph 23 ( 1c
(4.6 ( 0.1) × 10-1 c
Su ppor tin g In for m ation Available: Tables giving atomic
coordinates, bond distances and angles, and anisotropic dis-
placement parameters for 1a and 1b. This material is available
free of charge via the Internet at http://pubs.acs.org.
Computed from t1/2 values in CD2Cl2 by 1H NMR. Too slow
a
b
to measure. c In CH2Cl2 by UV/vis.
give the allyl complexes 3 and 4 upon addition of
NaClO4.10 The pyridine-thioether ligands chosen im-
part sufficient stability to both the reacting metal
substrates and the η3-allyl derivatives to warrant a
smooth reaction course which can be easily monitored
OM991018P
(10) Complexes 3 and 4 were obtained from the CD2Cl2 (or CDCl3)
1H NMR reaction mixtures. The solutions were dried under vacuum,
dissolved in a small volume of CH2Cl2, and treated with a methanolic
solution of NaClO4. Alternatively, complexes 3 and 4 were obtained
by reacting the appropriate allene with [PdCl(Me)(COD)] in freshly
distilled CH2Cl2. The resulting dimeric [PdCl(η3-allyl)]2 complexes were
converted into the corresponding complexes 3 and 4 by addition of the
required R′N-SR ligand and of stoichiometric NaClO4 dissolved in
MeOH (CH2Cl2/MeOH 3/1 v/v). Reduction to small volume and addition
of diethyl ether yield the required complex. The analysis and 1H NMR
spectra of the complexes obtained by the two alternative routes turn
out to be coincident. The isomers arising from the stereogenic character
of the sulfur atom and the pseudo-cis or pseudo-trans species (dimethyl-
substituted allylic carbon cis to sulfur and vice versa) were not detected
owing to the general fluxionality affecting these systems at room
temperature: i.e., sulfur inversion and allyl apparent rotation. How-
ever, a detailed analysis of this behavior for analogous complexes by
variable-temperature 1H NMR spectrometry has been carried out and
described in a previous paper.5b 3a : yield 80%. Anal. Found: C, 40.98;
H, 5.61; N, 2.93. Calcd for C16H26O4ClNPdS: C, 40.86; H, 5.57; N, 2.98.
1H NMR (323 K, CDCl3, ppm): δ 1.40 (9H, s, C(CH3)3), 1.53 (3H, s,
1
by H NMR or UV/vis techniques.
1
The course of the reactions was followed by either H
NMR or UV/vis techniques,11 depending on reaction
1
rates. Slower reactions were followed only by H NMR
to prevent partial decomposition occurring under UV/
vis concentration conditions (disappearance of Pd-CH3
protons at ∼1 ppm was monitored). Faster reactions
were studied by UV/vis techniques and confirmed, when
Meanti), 1.81 (3H, s, Mesyn), 2.18 (3H, s, Mecentral), 4.11 (2H, bs, Hanti
,
,
H
syn), 4.55 (2H, bs, CH2-S), 7.51 (1H, bs, H5pyr), 7.95 (2H, m, H3
pyr
H4pyr), 8.65 (1H, bs, H6pyr). 3b: yield 92%. Anal. Found: C, 41.98; H,
5.67; N, 2.83. Calcd for C17H28O4ClNPdS: C, 42.16; H, 5.82; N, 2.89.
1H NMR (298 K, CDCl3, ppm): δ 1.26 (9H, s, C(CH3)3), 1.50 (3H, s,
Meanti), 1.75 (3H, s, Mesyn), 2.16 (3H, s, Mecentral), 2.75 (3H, s, C5H3N-
6-CH3), 3.76 (1H, s, Hanti), 4.45 (1H, s, Hsyn), 4.51 (2H, bs, CH2-S),
7.36 (1H, d, J ) 7.7 Hz, H5pyr), 7.65 (1H, d, J ) 7.7 Hz, H3pyr), 7.84
(1H, t, J ) 7.7 Hz, H4pyr). 3c: yield 93%. Anal. Found: C, 44.15; H,
4.67; N, 2.81. Calcd for C18H22O4ClNPdS: C, 44.09; H, 4.52; N, 2.86.
1H NMR (298 K, CDCl3, ppm): δ 1.33 (3H, bs, Meanti), 1.61 (3H, bs,
Mesyn), 2.18 (3H, s, Mecentral), 4.02 (1H, bs, Hanti), 4.35 (1H, bs, Hsyn),
4.76 (2H, bs, CH2-S), 7.47 (5H, m, H5pyr, Hphen), 7.66 (1H, d, J ) 7.7
Hz, H3pyr), 7.92 (1H, td, J ) 7.7, 1.6 Hz, H4pyr), 8.88 (1H, bs, H6pyr). 3d :
yield 97%. Anal. Found: C, 45.31; H, 4.87; N, 2.80. Calcd for C19H24O4-
ClNPdS: C, 45.25; H, 4.80; N, 2.78. 1H NMR (298 K, CDCl3, ppm): δ
1.42 (3H, s, Meanti), 1.65 (3H, s, Mesyn), 2.21 (3H, s, Mecentral), 2.81 (3H,
s, C5H3N-6-CH3), 3.94 (1H, s, Hanti), 4.57 (1H, s, Hsyn), 4.82 (2H, bs,
CH2-S), 7.42 (7H, m, H3pyr, H5pyr, Hphen), 7.73 (1H, t, J ) 7.7 Hz, H4pyr).
4b: yield 85%. Anal. Found: C, 44.61, H, 5.98, N, 2.81. Calcd for
C19H32O4ClNPdS: C, 44.54, H, 6.29; N, 2.73. 1H NMR (298 K, CDCl3,
ppm): δ 1.21 (9H, s, C(CH3)3), 1.75 (6H, s, Meanti), 1.82 (6H, s, Mesyn),
2.10 (3H, s, Mecentral), 2.83 (3H, s, C5H3N-6-CH3), 4.48 (2H, bs, CH2-
S), 7.43 (1H, d, J ) 7.7 Hz, H5pyr), 7.64 (1H, d, J ) 7.7 Hz, H3pyr), 7.83
(1H, t, J ) 7.7 Hz, H4pyr). 4d : yield 82%. Anal. Found: C, 47.41; H,
5.28; N, 2.71. Calcd for C21H28O4ClNPdS: C, 47.38; H, 5.30; N, 2.63.
1H NMR (298 K, CDCl3, ppm): δ 1.79 (6H, s, Meanti), 1.86 (6H, s, Mesyn),
2.17 (3H, s, Mecentral), 2.84 (3H, s, C5H3N-6-CH3), 4.78 (2H, bs, CH2-
S), 7.35 (7H, m, H3pyr, H5pyr, Hphen), 7.67 (1H, t, J ) 7.7 Hz, H4pyr).
(11) The kinetics were followed under pseudo-first-order conditions
([allene] g 10[Pd]0) by mixing known aliquots of prethermostated CH2-
Cl2 solutions of reactants in the cell compartment of a Perkin-Elmer
Lambda 40 spectrophotometer. The reactions followed a monoexpo-
nential rate law with the pseudo-first-order constant kobsd equal to k2-
[allene]. No statistically significant allene-independent k1 path was
observed, at variance with earlier related systems.3d,e For the reaction
of 1a with TMA in CD2Cl2 monitored under second-order conditions
by 1H NMR techniques, the rate constant k2 was computed from half-
life values ([allene] ≈ 5[Pd]0 ) 0.05 mol dm-3).
1
possible, by H NMR. The results are summarized in
Table 2. Strictly speaking, the reactivity of 1a could not
be directly compared with other data reported in Table
2 owing to the larger error affecting NMR rate mea-
surements. However, the reactivities are so markedly
dissimilar that the accelerating effect of the methyl
group in position 6 of pyridine ring is borne out clearly
when 1a is compared with 1b. Complexes 2a and 2b
display the same reactivity trend, 2b being more reac-
tive than 2a by almost 3 orders of magnitude. To the
best of our knowledge, complex 2b displays the highest
reactivity toward insertion of allene observed so far.
This fact, in our opinion, can be traced back to the
peculiar electronic characteristics of the phenyl sub-
stituent to sulfur, which render the metal more efficient
as an electrophilic center compared with the bulky and
electron donating tert-butyl group, compounded with the
severe distortion induced by the methyl moiety on the
pyridine ring. Moreover, the high flexibility of the
S-donor chelate ligand allows for distortion of the
coordinating environment while providing for sufficient
stability of the metal-polydentate ring.
The resulting enhanced reactivity and the subsequent
facile insertion of the hindered TMA yields the corre-
sponding pentamethyl-substituted allyl derivative 4d ,
which can be easily converted to various different allylic
species owing to the lability of the MeN-SPh ancillary
ligand toward substitution by nucleophiles.5b
(12) Delis, J . G. P.; Groen, J . H.; Vrieze, K.; van Leeuwen, P. W. N.
M.; Veldman, N.; Spek, A. L. Organometallics 1999, 16, 551.
(13) Drent, E.; Arnoldy, P.; Budzelaar, P. H. M. J . Organomet. Chem.
1993, 455, 247.