Dinuclear addition of the Pd-Pd moieties to 1,3-dienes

Article abstract of DOI:10.1246/cl.2006.754

The reaction of a substitutionally labile dipalladium(I) complex [Pd ₂(CH₃CN)₆][BF₄]₂ with isoprene or 1,3-butadiene in acetonitrile afforded [Pd₂(μ- η³:η¹-C₅H₈)(CH ₃CN)₅][BF₄]₂ or [Pd ₂(μ-η³:η¹-C₄H ₆)(CH₃CN)₅][BF₄]₂. The structure of the isoprene complex was determined by X-ray crystallographic analyses. The interconversion between the [Pd₂(μ- η³:η¹-C₅H₈)-(CH ₃CN)₅][BF₄]₂ and [Pd ₂(μ-η²:η²-C₅H ₈)-(μ-Cl)Cl₂][PPh₄] occurred facilely. Copyright

Full text of DOI:10.1246/cl.2006.754

754
Chemistry Letters Vol.35, No.7 (2006)
Dinuclear Addition of the Pd–Pd Moieties to 1,3-Dienes
Tetsuro Murahashi,ꢀ Tomoki Nagai, Hiromitsu Nakashima, Sadayuki Tomiyasu, and Hideo Kurosawaꢀ
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, and
PRESTO, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871
(Received April 26, 2006; CL-060501; E-mail: tetsu@chem.eng.osaka-u.ac.jp)
The reaction of a substitutionally labile dipalladium(I)
shown in Figure 1.10 The Pd–Pd bond in 1 was cleaved during
the formation of 2. Two palladium atoms are located at the op-
posite faces of the pseudo-plane composed of the diene carbons
(C1, C2, C3, and C4). The isoprene ligand coordinated to the Pd2
moiety in a ꢀ-ꢁ3:ꢁ1-mode. As expected, the C3–C4 bond length
complex [Pd2(CH3CN)6][BF4]2 with isoprene or 1,3-butadiene
in acetonitrile afforded [Pd2(ꢀ-ꢁ3:ꢁ1-C5H8)(CH3CN)5][BF4]2
or [Pd2(ꢀ-ꢁ3:ꢁ1-C4H6)(CH3CN)5][BF4]2. The structure of the
isoprene complex was determined by X-ray crystallographic
analyses. The interconversion between the [Pd2(ꢀ-ꢁ3:ꢁ1-C5H8)-
(CH3CN)5][BF4]2 and [Pd2(ꢀ-ꢁ2:ꢁ2-C5H8)(ꢀ-Cl)Cl2][PPh4]
occurred facilely.
˚
(1.488(8) A) is longer than the C1–C2 and C2–C3 bond lengths
˚
(1.411(8) and 1.407(7) A) due to its C–C single-bond character.
L
2+
L
Pd
L
2+
-
L
Pd
L
L
L
Pd L
(1)
Coordination of 1,3-dienes to a mononuclear palladium cen-
ter has been intensively studied in conjunction with the palladi-
um-catalyzed transformation of those substrates.1,2 However, the
coordination of 1,3-dienes to a Pd–Pd bonded center has not
been fully understood. Several dipalladium complexes contain-
ing ꢀ-ꢁ2:ꢁ2-1,3-diene ligands have been isolated, and some of
those were structurally characterized by X-ray crystallographic
studies.3,4 On the other hand, the dinuclear addition of Pd–Pd
bonded moieties to 1,3-dienes has not been reported.5 We recent-
ly reported that the dinuclear addition of [Pd2Ln]2þ (L = aceto-
nitrile) to 1,3,5-trienes proceeds smoothly to afford the bi-ꢁ3-al-
lyl type dipalladium complexes [Pd2{R(CH=CH)3R})L4][PF6]2
(R ¼ Ph and t-Bu).6 Through a mechanistic study including
demonstration of highly stereospecific nature of the reverse
reactions; i.e. dinuclear elimination reactions, we proposed that
the dimetalla-[4ꢂ þ 2ꢃ] Diels–Alder type process is possibly
involved in the dinuclear addition reactions of 1,3,5-trienes
(Scheme 1).6b While the initial product of such [4ꢂ þ 2ꢃ] addi-
tion may readily be converted to the more stable bi-ꢁ3-allyl
form, it seems of considerable interest to examine what type
of structure is taken by an analogous [4ꢂ þ 2ꢃ] adduct involv-
ing 1,3-diene, particularly because the ꢁ1-allylpalladium is
usually a less stable species.7 Herein, we report that the reaction
of [Pd2(CH3CN)6][BF4]2 (1)8 with 1,3-dienes in acetonitrile
afforded the ꢀ-ꢁ3:ꢁ1-1,3-diene dipalladium complexes.
CH3CN (CD3CN)
L
Pd
2 BF4
-
2 BF4
L
L
1
2
The reaction of 1 with 1,3-butadiene in CH3CN afforded
[Pd2(ꢀ-ꢁ3:ꢁ1-C4H6)(CH3CN)5][BF4]2 (3) as a mixture of two
isomers (49% isolated yield after recrystallization from
CH3CN/Et2O, syn/anti = 93/7 at ꢁ40 ꢂC). The syn- or anti-al-
lyl structure in each isomer was assumed by the J-coupling con-
stants in the 1H NMR spectra; JHc{Hd ¼ 11:9 Hz for syn isomer,
JHc{Hd ¼ 7:9 Hz for anti isomer (see Scheme 2 for the labels of
the butadiene protons).11 It should be noted that the isoprene
complex 2 contained only syn isomer in CD3CN solution, as
was confirmed by difference NOE measurements. Relatively
large geminal coupling constant at one of the butadiene termini
was observed (J ¼ 5:6 Hz). Raising the temperature from
The reaction of 1 with excess amount of isoprene in CD3CN
at room temperature afforded [Pd2(ꢀ-ꢁ3:ꢁ1-C5H8)(CH3CN)5]-
[BF4]2 (2) almost quantitatively (eq 1).9 No other isomer was
formed. A considerable sp3-hybridized character at the CHCH2
terminal carbon is suggested by the geminal coupling constant
(J ¼ 6:6 Hz), as well as the high-field shifted 13C{1H} resonance
(ꢄ ¼ 18:3 ppm). The complex 2 was isolated in 55% yield after
recrystallization from CH3CN/Et2O, and a single crystal suita-
ble for X-ray structure analysis was obtained by recrystallization
from CH3CN/CH2Cl2/benzene. The molecular structure of 2 is
Figure 1. ORTEP drawing of [Pd2(ꢀ-ꢁ3:ꢁ1-C5H8)(CH3CN)5]-
[BF4]2 (2) (50% probability ellipsoids, BF4 anions were omitted
˚
for clarity). Selected bond lengths (A): Pd1–C1 2.094(6), Pd1–
[Pd]+
C2 2.132(5), Pd1–C3 2.162(5), Pd2–C4 2.041(5), Pd1–N1
2.091(5), Pd1–N2 2.103(5), Pd2–N3 2.001(5), Pd2–N4
2.041(5), Pd1–N5 1.999(5), C1–C2 1.411(8), C2–C3 1.407(7),
C3–C4, 1.488(8), C2–C5 1.472(7).
2+
+
Pd
Pd
[Pd]+
[Pd]+
[Pd]+
Scheme 1.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
755
L
Pd
12, 4503. b) P. Leoni, M. Pasquali, M. Sommovigo, A.
Albinati, P. S. Pregosin, H. Ruegger, Organometallics
1996, 15, 2047. c) T. Murahashi, N. Kanehisa, Y. Kai, T.
Otani, H. Kurosawa, Chem. Commun. 1996, 825.
T. Murahashi, T. Otani, E. Mochizuki, Y. Kai, H. Kurosawa,
S. Sakaki, J. Am. Chem. Soc. 1998, 120, 4536.
The dinuclear addition reactions of other metal–metal bonds
such as Mn–Mn or Re–Re to 1,3-diene are known to take
place under the irradiation condition: a) C. G. Kreiter, W.
Lipps, Angew. Chem., Int. Ed. Engl. 1981, 20, 201. b) C. G.
Kreiter, W. Lipps, Chem. Ber. 1982, 115, 973. c) C. G.
Kreiter, M. Leyendecker, J. Organomet. Chem. 1985, 292,
C18. d) E. Guggolz, F. Oberdorfer, M. L. Ziegler, Z. Natur-
forsch., B: Chem. Sci. 1981, 36, 1060.
a) T. Murahashi, T. Nagai, Y. Mino, E. Mochizuki, Y.
Kai, H. Kurosawa, J. Am. Chem. Soc. 2001, 123, 6927. b)
T. Murahashi, H. Nakashima, T. Nagai, Y. Mino, T. Okuno,
M. A. Jalil, H. Kurosawa, J. Am. Chem. Soc. 2006, 128,
4377.
L
Pd
2+
2+
2+
L
L
L
L
L
¨
L
Pd
L
Hc
Hf
−L
L
Hb
Ha
He
Hd
L
Pd
Pd
L
L
Pd
L
L
-
-
-
L
L
2 BF
2 BF
2 BF
4
4
4
5
4
Scheme 2. Interconversion between ꢀ-ꢁ3:ꢁ1- and ꢀ-ꢁ1:ꢁ3-
complexes (3-syn).
ꢁ40 ꢂC resulted in the coalescence of the four proton resonances
(Ha, Hb, He, and Hf) and two proton resonances (Hc and Hd) at
around 40 ꢂC, and two broad resonances (ꢄ 3.2 and 5.2 ppm) with
relative intensities of 2:1 appeared at 70 ꢂC. This temperature-
dependent NMR behavior can be explained by the occurrence
of rapid exchange between ꢀ-ꢁ3:ꢁ1-mode and ꢀ-ꢁ1:ꢁ3-mode,
through a di-ꢃ-bonded intermediate, as depicted in Scheme 2.
Then, we examined the transformation of the ꢀ-ꢁ3:ꢁ1-com-
plexes synthesized here to the ꢀ-ꢁ2:ꢁ2-complexes. When the
isoprene complex 2 was treated with [PPh4]Cl (3 equiv.), the
known ꢀ-ꢁ2:ꢁ2-isoprene trichloride complex [Pd2(ꢀ-ꢁ2:ꢁ2-
C5H8)(ꢀ-Cl)Cl2][PPh4] (4)3c containing a Pd–Pd bond was
formed almost quantitatively. Addition of AgBF4 (3 equiv.) to
the complex 4 in CD3CN gave the complex 2 in a quantitative
manner (eq 2). During the interconversion of eq 2, the dimetal-
la-[4ꢂ þ 2ꢃ] process is probably involved. Similarly to the Pd–
Pd bond formation in the forward reaction of eq 2, the dinuclear
elimination of a [Pd–Pd]2þ moiety was observed to take place
from the bi-ꢁ3-allyl dipalladium(II) complexes (i.e. the reverse
reaction of Scheme 1), although no Pd–Pd complex having ꢀ-
ꢁ2:ꢁ2-bound triene ligand was detected.6b
6
7
8
9
H. Kurosawa, S. Ogoshi, Bull. Chem. Soc. Jpn. 1998, 71,
973.
T. Murahashi, T. Nagai, T. Okuno, T. Matsutani, H.
Kurosawa, Chem. Commun. 2000, 1689.
Synthesis of 2: To a solution of 1 (94.4 mg, 0.149 mmol) in
CH3CN was added isoprene (60.0 mL, 0.600 mmol) and the
mixture stirred for 30 min at room temperature. The yellow
reaction mixture was filtered. Recrystallization from
CH3CN/Et2O gave yellow microcrystals of 2 (53.3 mg,
1
55% yield). H NMR data (CD3CN) for 2 : ꢄ 4.26 (dd, J ¼
11:4 Hz, J ¼ 5:7 Hz, 1H), 3.93 (s, 1H), 2.86 (s, 1H), 2.78
(dd, J ¼ 11:4 Hz, J ¼ 6:6 Hz, 1H), 2.64 (dd, J ¼ 5:7 Hz,
J ¼ 6:6 Hz, 1H), 2.07 (s, 3H). 13C1H NMR (CD3CN): ꢄ
130.0 (C2), 118.3 (CH3CN), 89.4 (C3), 61.9 (C1), 19.2
(CH3), 18.3 (C4). Anal. Calcd for C15H23N5B2F8Pd2: C,
27.30; H, 3.51; N, 10.61%. Found: C, 27.24; H, 3.57; N,
10.66%.
L
2+
L
Pd
L
-
[PPh4]Cl (3 equiv.)
CD3CN
(2)
Pd
Pd
Cl
Cl
PPh4
Pd
-
+
AgBF4 (3 equiv.)
CD3CN
Cl
2 BF4
L
L
2
4
10 Crystal data for 2: C15H23Pd2B2F8N5, Mr 659.79, space
˚
In summary, it has been proven that the dinuclear addition
of a [Pd2Ln]2þ moiety to isoprene or 1,3-butadiene takes place
to afford the ꢀ-ꢁ3:ꢁ1-1,3-diene dipalladium complexes.
group P21/n (No. 14), a ¼ 13:3229ð4Þ A, b ¼ 10:3235ð2Þ
ꢂ
3
˚
˚
˚
A, c ¼ 17:5397ð5Þ A, ꢅ ¼ 92:6824ð8Þ , V ¼ 2409:7ð1Þ A ,
Z ¼ 4, Fð000Þ ¼ 1288, Dcalcd ¼ 1:818 g cmꢁ3, ꢀðMo KꢆÞ
¼ 15:66 cmꢁ1, 289 variables refined with 3774 reflections
with I > 3ꢃðIÞ to R ¼ 0:034.
This work was supported by PRESTO, Japan Science and
Technology Agency (JST), and Grant-in-Aid for Scientific
Research, Ministry of Education, Culture, Sports, Science and
Technology of Japan. Thanks are also due to the Analytical
Center, Faculty of Engineering, Osaka University for the use
of NMR facilities.
11 Synthesis of 3: To a solution of 1 (113.4 mg, 0.179 mmol) in
CH3CN was bubbled 1,3-butadiene gas for 5 min and the
mixture stirred for 30 min at room temperature. The yellow
reaction mixture was filtered. Recrystallization from
CH3CN/Et2O gave yellow microcrystals of 3 (56.4 mg,
49% yield). H NMR (CD3CN, ꢁ40 ꢂC) for 3-syn: ꢄ 5.73
1
References and Notes
(ddd, J ¼ 12:0 Hz, J ¼ 6:8 Hz, J ¼ 11:9 Hz, 1H), 4.41
(ddd, J ¼ 11:9 Hz, J ¼ 5:3 Hz, J ¼ 12:1 Hz, 1H), 4.00 (d,
J ¼ 6:8 Hz, 1H), 3.00 (d, J ¼ 12:0 Hz, 1H), 2.72 (dd,
J ¼ 12:1 Hz, 5.6 Hz, 1H), 2.66 (dd, J ¼ 5:3 Hz, 5.6 Hz,
1H). 1H NMR (CD3CN, ꢁ40 ꢂC) for 3-anti: ꢄ 5.56 (dt,
J ¼ 12:1 Hz, J ¼ 7:9 Hz, 1H), 5.35 (ddd, 1H), 4.07 (d,
J ¼ 7:9 Hz, 1H), 3.41 (d, J ¼ 12:1 Hz, 1H), 2.3 (m). Anal.
Calcd for C14H21N5B2F8Pd2: C, 26.04; H, 3.28; N,
10.84%. Found: C, 25.26; H, 3.35; N, 10.58%.
1
P. M. Maitlis, P. Espinet, M. J. H. Russell, in Comprehensive
Organometallic Chemistry, ed. by G. Wilkinson, F. G. A.
Stone, E. W. Abel, Pergamon, New York, 1982, Vol. 6,
Chap. 38.7.
J. Tsuji, Palladium Reagents and Catalysts, John Wiley &
Sons, Chichester, 2004.
a) P. Leoni, M. Pasquali, M. Sommovigo, A. Albinati, F.
2
3
Lianza, P. S. Pregosin, H. Ruegger, Organometallics 1993,
¨
Published on the web (Advance View) June 3, 2006; doi:10.1246/cl.2006.754