Selective and efficient cycloisomerization of alkynols catalyzed by a new ruthenium complex with a tetradentate nitrogen-phosphorus mixed ligand

Article abstract of DOI:10.1002/chem.200903441

The new ruthenium complex [Ru(N₃P)(OAc)][BPh₄] (4), in which N₃P is the N,P mixed tetradentate ligand N,N-bis[(pyridin-2-yl)methyl]-[2(diphenylphosphino)phenyl]methanamine was synthesized. The complex was found to be catalytically active for the endo cycloisomerization of alkynols. The catalytic reactions can be used to synthesize five-, six-, and seven-membered endo-cyclic enol ethers in good to excellent yields. A catalytic cycle involving a vinylidene intermediate was proposed for the catalytic reactions. Treatment of complex 4 with PhC≡CH and H₂O gave the alkyl complex [Ru(CH₂PhJ(CO)(N ₃P)][BPh₄] (30), which supports the assumption that the catalytic reactions involve addition of a hydroxyl group to the C=C bond of vinylidene ligands.

Full text of DOI:10.1002/chem.200903441

FULL PAPER
DOI: 10.1002/chem.200903441
Selective and Efficient Cycloisomerization of Alkynols Catalyzed by a
New Ruthenium Complex with a Tetradentate Nitrogen–Phosphorus
Mixed Ligand
Pei Nian Liu,[a, b] Fu Hai Su,[a] Ting Bin Wen,[a] Herman H.-Y. Sung,[a]
Ian D. Williams,[a] and Guochen Jia*[a]
Abstract: The new ruthenium complex
[Ru(N3P)(OAc)][BPh4] (4), in which
used to synthesize five-, six-, and
seven-membered endo-cyclic enol
ethers in good to excellent yields. A
catalytic cycle involving a vinylidene
intermediate was proposed for the cat-
alytic reactions. Treatment of complex
ꢀ
G
E
R
4 with PhC CH and H2O gave the
N3P is the N,P mixed tetradentate
ligand N,N-bis[(pyridin-2-yl)methyl]-[2-
(diphenylphosphino)phenyl]methana-
mine was synthesized. The complex
was found to be catalytically active for
the endo cycloisomerization of alky-
nols. The catalytic reactions can be
alkyl complex [Ru
A
G
AHCTUNGERTG[NNUN BPh4] (30), which supports the as-
sumption that the catalytic reactions in-
volve addition of a hydroxyl group to
the C=C bond of vinylidene ligands.
Keywords: alkynes · isomerization ·
N,P ligands · ruthenium · vinylidene
Introduction
provides a straightforward and efficient approach to numer-
ous oxygen-containing heterocycles.[4]
Oxygen-containing heterocycles are important structural
components presented in a diverse range of naturally occur-
ring and biologically active molecules.[1] The widespread oc-
currence of oxygen heterocycles and the limitations associat-
ed with the traditional synthetic methodologies for heterocy-
cles[1a,b] have stimulated considerable interest in developing
efficient homogeneous catalytic methods for the synthesis of
such heterocyclic compounds.[2] Cycloisomerization of alky-
nols represents a direct means for the synthesis of cyclic
enol ethers with the advantage of 100% atom efficiency,
which fulfills the requirements for green chemistry[3] and
Cycloisomerization of alkynols can lead to either exo- or
endo-cyclic enol ethers.[5] There have been growing efforts
to develop efficient and selective catalysts for this challeng-
ing transformation. Cycloisomerization of alkynols to give
exo-cyclic enol ethers was firstly described for reactions cat-
alyzed by HgO and BF3·Et2O.[6] Other complexes, such as
Pd
plexes [Ln{N
demonstrated to be effective to mediate the exo cycloisome-
rization of alkynols. [RuCl2A(p-cymene)
(PPh3)][11] and palladi-
um catalysts, such as PdCl2,[12] Pd(OAc)2,[13] and K2PdI4,[14]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
C
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
were found to catalyze the exo cycloisomerization/isomeriza-
tion tandem reactions of enynols to form endo-cyclic enol
ethers or furans.
[a] Dr. P. N. Liu, Dr. F. H. Su, Dr. T. B. Wen, Dr. H. H.-Y. Sung,
Prof. Dr. I. D. Williams, Prof. Dr. G. Jia
Department of Chemistry, Hong Kong University of Science
& Technology Institution, Clear Water Bay
Kowloon, Hong Kong (P. R. China)
The endo cycloisomerization of alkynols could afford
endo-cyclic enol ethers, which are very useful synthetic inter-
mediates in the construction of diverse oxygen-containing
heterocycles.[15–19] As the pioneering work, McDonald and
co-workers have developed the endo cycloisomerization of
alkynols by employing molybdenum or tungsten carbonyls
as the catalysts.[20] The molybdenum catalyst is effective for
the synthesis of five-membered cyclic enol ethers from 4-hy-
droxy-1-alkynes,[5,17,18,21] whereas the tungsten catalyst can
also effect the formation of six-[15,16,20–22] and seven-mem-
bered[23] cyclic enol ethers from 5-hydroxy-1-alkynes and 6-
Fax : (+852)23581594
E-mail: chjiag@ust.hk
[b] Dr. P. N. Liu
Key Laboratory for Advanced Materials and
Institute of Fine Chemicals, East China University of
Science and Technology, 130 Meilong Road
Shanghai, 200237 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903441.
Chem. Eur. J. 2010, 16, 7889 – 7897
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7889

hydroxy-1-alkynes. Although a rather large amount of cata-
lyst loading (25–40 mol%) was usually required in these cat-
alytic reactions, the catalytic reaction has been successfully
applied for the synthesis of various natural products,[15] such
as glycals,[16] nucleosides,[17] and clinically valuable drugs.[18]
More recently, Trost, Saa, and Zacuto et al. reported the cy-
cloisomerization of 4-hydroxy-1-alkynes and 5-hydroxy-1-al-
kynes (homo and bis(homopropargylic) alcohols) catalyzed
by ruthenium complexes, such as [Ru(Cp)(L)n]+/phosphine
Complex 4 has been characterized by 1H, 13C{1H}, and
31P{1H} NMR spectroscopy and mass spectroscopic methods.
Consistent with the structure shown in Scheme 1, the
1H NMR spectrum (in CD2Cl2) displays the methyl signal of
the acetate ligand at d=2.00 ppm and the proton signal of
the methylene linked to the aryl group at d=3.37 ppm. The
four protons of the methylenes linked to the pyridyls exhibit
two doublets at d=3.83 and 4.37 ppm. The 13C{1H} NMR
spectrum (in CD2Cl2) shows the signals of the acetate ligand
at d=24.3 and 189.7 ppm and the signals of the three meth-
ylenes at d=67.5, 67.6, and 68.7 ppm. The 31P{1H} NMR
spectrum displays a singlet at d=63.4 ppm.
The structure of 4 has also been confirmed by X-ray dif-
fraction. Single crystals of complex 4 suitable for the X-ray
crystallographic study were readily obtained by slow diffu-
sion of Et2O into a CH2Cl2 solution of complex 4. The crys-
tal data and refinement details are given in Table 1 and se-
lected bond lengths and angles are listed in Table 2. Figure 1
shows the X-ray structure of the complex cation of 4. The
coordination geometry of ruthenium in 4 can be described
as a distorted octahedron with a tetradentate N3P ligand
and a bidentate acetate ligand. The two pyridine rings of the
N3P ligand are trans to each other. One of the oxygen atoms
of the acetate ligand is trans to the P atom and the other
ligands (20–40 mol%) and [RuCl(Cp)
the rhodium complex [RhCl
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
55 mol%).[25] It was reported very recently that AgNO3,
PdII/CuI, or AuI complexes can catalyze the cycloisomeriza-
tion of cis-4-hydroxy-5-alkynylpyrrolidinones and cis-5-hy-
droxy-6-alkynylpiperidinones.[26] In a related work, Qing
et al. have studied the [PdCl2ACHTNUGRTENUNG(MeCN)2]-mediated endo cy-
cloisomerization of 2-alkynyl-3-trifluoromethyl allylic alco-
hols to give tetrahydrofuran derivatives.[27] The development
of more efficient and selective catalysts for the endo cycloi-
somerization of alkynols still remains a challenging objec-
tive.
Reported herein is our work in the design and synthesis
of a new ruthenium complex with a P/N tetradentate ligand
and its application for the cycloisomerization of alkynols.
The catalytic reactions exhibit very high regioselectivity to
give exclusively endo products in good to excellent yields.
Table 1. Crystal data and structure refinement for complexes 4 and 30.
4
30
Results and Discussion
empirical formula
Mw
T [K]
C57H51BN3O2PRu·CH2Cl2 C63H55BN3OPRu
1037.78
100(2)
0.71073
triclinic
P1
11.5070(15)
13.4495(17)
16.409(2)
82.474(2)
82.791(2)
77.904(2)
2449.0(5)
2
1012.95
173(2)
1.54178
monoclinic
P21/c
16.6548(2)
11.33680(10)
26.4042(2)
90
102.0530(10)
90
4875.53(8)
4
1.380
0.30ꢂ0.28ꢂ0.15
5.79 to 71.65
À19ꢁhꢁ20
À13ꢁkꢁ11
À30ꢁlꢁ32
15205
Synthesis of the catalytic precursor [Ru
The synthetic route to the catalytic precursor [Ru
(OAc)][BPh4] (4) N3P=N,N-bis[(pyridin-2-yl)methyl][2-(di-
ACHTUNGTRENNU(GN N3P)ACHTUNGTREG(NNUN OAc)]ACUTHNGTRENNUNG
AHCTUNGTRENNUNG
l [ꢁ]
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
G
ACHTUNGTRENNUNG
¯
phenylphosphino)phenyl]methanamine) is outlined in
Scheme 1. Reductive amination of compound 1 with di(2-pi-
colyl)amine (2) in the presence of sodium triacetoxyborohy-
dride produced the tetradentate ligand 3 (abbreviated as
N3P), which could be isolated in 85% yield. Treatment of
a [8]
b [8]
g [8]
ligand 3 with [Ru
ACHTUNGTRENNUNG(OAc)2ACHTUNGTR(EUNNNG PPh3)2] and NaBPh4 produced the
V [ꢁ3]
Z
new ruthenium complex [Ru
N
N
ACHTUNGTRENNUNG
1 [gcmÀ3
]
1.407
was isolated as a yellow solid in 63% yield.
crystal size [mm3]
q range [8]
0.38ꢂ0.25ꢂ0.20
1.82 to 26.00
À14ꢁhꢁ14
À16ꢁkꢁ16
À20ꢁlꢁ20
20681
index ranges
no. of reflns collected
no. of independent reflns
9454
8913
(R
G
(RACHTNGUTERN(UNG int)=0.0284)
completeness to q=25.008 98.5
[%]
96.3
max. and min. transmission 1.00 and 0.94
1.00 and 0.83
8913/0/631
1.023
data/restraints/parameters
9454/0/633
GOF on F2
1.044
final R indices [I>2s(I)]
R1=0.0338,
wR2=0.0767
R1=0.0402,
wR2=0.0790
0.582 and À0.497
R1=0.0310,
wR2=0.0850
R1=0.0334,
wR2=0.0864
0.389 and
R indices (all data)
largest diff peak and hole
[eꢁÀ3
]
À0.451
Scheme 1. Synthesis of complex 4.
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Chem. Eur. J. 2010, 16, 7889 – 7897

Selective and Efficient Cycloisomerization of Alkynols
FULL PAPER
Table 2. Selected bond lengths and angles in complex 4.
the conversion of the substrate was then analyzed by
1H NMR spectroscopy. The results are listed in Table 3. As
shown in Table 3, the reactions proceeded well in solvents
À
À
Ru1 N1
2.0797(18)
2.0523(18)
2.1406(14)
1.270(3)
1.509(3)
1.499(3)
Ru1 N10
2.0478(18)
2.1866(14)
2.2446(6)
1.266(3)
1.505(2)
1.502(3)
À
À
Ru1 N20
Ru1 O51
À
À
Ru1 O52
Ru1 P1
À
À
C50 O51
C50 O52
À
À
C1 C2
N1 C1
À
À
N1 C20
N1 C10
Table 3. Cycloisomerization of 5-pentyne-1-ol 5 under various condi-
tions.[a]
À
À
C10 C11
1.504(3)
C20 C21
1.510(3)
O52-Ru1-O51
N1-Ru1-O52
O51-Ru1-P1
N10-Ru1-N20
N10-Ru1-P1
N20-Ru1-O52
N20-Ru1-O51
N10-Ru1-N1
C50-O52-Ru1
C3-P1-Ru1
60.61(6)
O52-Ru1-P1
N1-Ru1-O51
N20-Ru1-N1
N1-Ru1-P1
N20-Ru1-P1
N10-Ru1-O52
N10-Ru1-O51
C50-O51-Ru1
O52-C50-O51
C1-N1-Ru1
C20-N1-Ru1
C10-N1-Ru1
C41-P1-Ru1
C21-N20-Ru1
C11-N10-Ru1
107.91(4)
99.60(6)
83.37(7)
91.88(5)
91.59(5)
160.20(6)
168.00(4)
165.29(7)
94.86(5)
95.26(6)
86.34(6)
Entry
Solvent
Loading of cat. 4 [mol%]
t [h]
Conv. [%][b]
1
2
3
4
5
6
7
8
acetone
CHCl3
benzene
CH3CN
CH2Cl2
THF
1
1
1
1
1
1
0.5
2
2
2
2
2
48
1
84
85
100
9
95.32(6)
89.97(6)
83.22(7)
89.14(12)
118.91(19)
119.43(13)
104.41(12)
102.74(12)
120.07(7)
112.34(14)
111.83(14)
91.33(13)
107.14(7)
112.73(16)
111.72(18)
110.06(17)
117.95(7)
129.20(16)
129.08(15)
22[c]
100
70
N1-C1-C2
THF
THF
7
7
N1-C20-C21
N1-C10-C11
C31-P1-Ru1
C25-N20-Ru1
C15-N10-Ru1
3[d]
[a] The reactions were carried out by using 0.50 mmol of 5 and 0.5 mL of
solvent under N2 at 808C, unless otherwise noted. [b] Determined by
1H NMR spectroscopy. [c] The reaction was carried out at 508C. [d] The
reaction was carried out at room temperature.
such as acetone, chloroform, and benzene to give exclusively
six-membered endo-enol ether 17 (Table 3, entries 1–3).
However, the reaction in CH3CN gave a poor conversion
probably due to the strong coordination ability of CH3CN
with ruthenium (entry 4). The conversion of the reaction is
also poor (only 22% conversion) if the reaction was carried
out in boiling CH2Cl2 (ca. 508C; entry 5). THF was found to
be the best solvent for the reaction and complete cycloiso-
merization of 5-pentyn-1-ol could be accomplished in reflux-
ing THF in 1 h with a catalyst loading of only 1 mol%
(entry 6). In contrast, 25 mol% or more of tungsten carbon-
yl,[20,21] 5–10 mol% of [Ru(Cp)(L)n]+/phosphine ligands (20–
40 mol%),[24] or 2.5–7.5 mol% of rhodium complex [RhCl-
AHCTUNGTRENNUNG
(PR3)3]/phosphine ligands (30–55 mol%)[25] were required
for a similar transformation. Our catalytic system is still ef-
fective even when the catalyst loading is decreased to
0.5 mol%, although a longer reaction time is required. For
example, approximately 70% conversion of 5 was achieved
after 7 h of reaction (entry 7). However, we noted that the
catalytic reaction proceeded very slowly at room tempera-
ture (entry 8).
Figure 1. ORTEP diagram for the cation of complex 4.
À
one trans to the tertiary amine N atom. The Ru O (acetyl)
bond lengths (2.1866(14) and 2.1406(14) ꢁ) are similar to
those of other RuII–h2-acetate complexes.[28] The solid-state
structure is in agreement with the solution NMR spectro-
scopic data.
We then explored the substrate scope of the cycloisomeri-
zation reactions catalyzed by complex 4 in THF at 808C and
the results are summarized in Table 4. With only 1 mol% of
the catalyst, both aliphatic and aromatic alkynols 5–8 were
converted exclusively to the corresponding six-membered
endo-enol ethers 17–20 in high yields within 1 h (Table 4, en-
tries 1–4), which indicated that the reactivity of primary and
secondary hydroxyl groups is similar. With 5 mol% of the
catalyst, cycloisomerization of propargyl alcohol 9 also oc-
curred to give the endo-cyclic enol ether 21 in 90% yield in
1 h (entry 5), and no byproduct was detected in the reaction.
Catalytic reactions: The catalytic properties of complex 4
for the cycloisomerization of alkynols was initially tested
with 5-pentyn-1-ol (5) as the substrate in various solvents. In
a typical reaction, a mixture of 5 (0.50 mmol) and catalyst
(0.005 mmol) in a solvent was heated at 808C for 2 h and
Chem. Eur. J. 2010, 16, 7889 – 7897
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7891

G. Jia et al.
Table 4. Cycloisomerization of various alkynols by using catalyst 4.[a]
Entry Substrate
Product
Loading of 4
[mol%]
t
Yield
Entry Substrate
Product
Loading of 4
[mol%]
t
Yield
[h] [%][b]
[h] [%][b]
1
1
1
1
1
91[c]
8
5
5
44 83
2
3
98[d]
9
17 98
1
1
97
10
5
5
15 89
4
5
1
5
1
1
94
11
20 93
90[d]
12
5
1
20 91[d]
6
7
5
5
20 78[d]
17 80[d]
13[e]
2
90[c]
[a] The reactions were catalyzed by complex 4 by using 0.5 mmol substrate and THF (0.5 mL) under N2 at 808C, unless otherwise noted. [b] Isolated
1
1
yields. [c] Yields were determined by H NMR spectroscopic integration with CH3NO2 as the internal standard. [d] Yields were determined by H NMR
spectroscopic integration with tBuOH as the internal standard. [e] The reaction was carried out by using 5 (10 mmol) and THF (10 mL) at 808C.
Our catalytic system also effects the cycloisomerization of
4-hydroxy-1-alkynols to give five-membered cyclic enol
ethers. With 5 mol% of the catalyst, cycloisomerization of
4-hydroxy-1-alkynols generally gave the five-membered
cyclic enol ethers in high yields, although a longer reaction
time is required relative to the formation of six-membered
enol ethers. For example, cycloisomerization of aliphatic al-
kynols 10 and 11 gave the corresponding endo-cyclic enol
ethers 22 and 23 in 78 and 80% yields, respectively,
(Table 4, entries 6 and 7). When 4,5-dihydroxy-1,7-octadiyne
(12) was used, the reaction produced bicyclic compound 24
in 83% yield (entry 8). Catalytic cycloisomerization of alky-
nols 13 and 14, which bear sterically hindered tertiary hy-
droxyl groups, also proceeded smoothly to afford the five-
membered products 25–26 in high yields (entries 9 and 10).
Under similar conditions, 4-hydroxy-6-phenyl-1,5-enyne (15)
was also transformed into the five-membered endo-cyclic
enol ether 27 in 93% yield, which suggests that the presence
of the alkenyl group did not exhibit any obvious adverse
effect on the catalytic reaction (entry 11). We were delight-
ed to note that the reaction of compound 16 proceeded
smoothly to give the seven-membered product 28 in excel-
lent yield (entry 12). To evaluate the practical applicability
of this catalytic reaction, we have carried out the catalytic
reaction by using 10 mmol (0.84 g) of 5-pentyn-1-o1 (5). In
the presence of 1 mol% of complex 4, the reaction gave the
cyclized product 17 in 90% yield in 2 h (entry 13).
We have tried the catalytic reaction of 10-undecyn-1-ol,
with the hope of obtaining a 12-membered enol ether. How-
ever, the catalytic reaction did not proceed. In addition, no
reaction was observed for 3-hexyn-1-ol, which is expected to
give a four-membered enol ether.
Mechanism: The endo cycloisomerization reactions of termi-
nal alkynols catalyzed by Mo and W systems were generally
believed to involve the initial formation of vinylidene inter-
mediates that undergo intramolecular nucleophilic addition
of an OH group to the vinylidene ligand to afford an endo-
cyclic enol ether linked to the transition metals, followed by
protonation of the metal–carbon bond.[29] The mechanism is
supported by DFT studies,[30] although experimental evi-
dence for the mechanism is still rare.[31] Since it is well-
7892
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Chem. Eur. J. 2010, 16, 7889 – 7897

Selective and Efficient Cycloisomerization of Alkynols
FULL PAPER
ꢀ
known that ruthenium(II) complexes can react with terminal
alkynes to give vinylidene complexes[32] and ruthenium vi-
nylidene complexes can be attacked by weak nucleophiles,
such as alcohols,[24,33] water,[34] and carboxylic acids,[35] it is
likely that the reactions catalyzed by complex 4 proceed
through a vinylidene intermediate.
A plausible mechanism for the endo cycloisomerization of
alkynols catalyzed by complex 4 is depicted in Scheme 2 by
using substrate 5 as an example. The alkynol reacts with
acids catalyzed by [Ru
G
(PMeiPr2)(Tp)]
(Tp=hydrotris(pyrazolyl)borate).[35e]
Consistent with the catalytic cycle, no reaction was ob-
served when using internal alkynols, such as 5-decyn-1-ol
and 3-hexyn-1-ol, as the catalytic reaction substrates. Un-
fortunately, we have failed to isolate and identify experi-
mentally the active species involved in the catalytic reac-
tions. In an effort to gain indirect evidence for the involve-
ment of a vinylidene intermediate in the catalytic reactions,
we have carried out the reaction of complex 4 with phenyla-
cetylene (29) in the presence of H2O. The reaction was
found to give the benzyl carbonyl complex 30 (Scheme 3).
ꢀ
Thus C C bond cleavage occurred in the reaction. Several
ꢀ
related metal-assisted C C bond-cleavage reactions of 1-al-
kynes with water leading to the formation of complexes con-
taining a carbonyl and an h1-alkyl with one less carbon atom
have been reported, especially for metal complexes of ruthe-
nium,[40] osmium,[41] and iridium.[42] The related reactions of
metal acetylides or vinylidenes with water are also
known.[43]
The structure of 30 has been determined by an X-ray dif-
fraction study. The crystal data and refinement details of
complex 30 are given in Table 1 and selected bond lengths
and angles are listed in Table 5. A view of the molecular ge-
ometry for the complex cation of 30 is shown in Figure 2.
The geometry around ruthenium in 30 can be viewed as a
distorted octahedron. The tertiary amine N atom is trans to
the CO ligand. The two pyridyl units in 30 are cis to each
other, which is different from the trans arrangement in com-
Scheme 2. The proposed working mechanism of the catalytic reaction.
complex 4 to give initially an
h2-alkyne complex
A which
could then rearrange to the vi-
nylidene intermediate B. Intra-
molecular attack by the hydrox-
yl group on the a-carbon atom
of the vinylidene ligand in B
followed
by
deprotonation
would afford the vinyl complex
C. Protolysis of the metal–
carbon bond would then give
the endo-cyclic enol ether 17
Scheme 3. Reaction of complex 4 with phenylacetylene and H2O to afford complex 30.
and regenerate complex 4. Nu-
cleophilic attack on vinylidene
intermediates has been proposed for several ruthenium-cat-
alyzed reactions of terminal alkynes, for example, coupling
of allyl alcohols with alkynes,[24,33d–m] addition of carboxylic
acids to terminal alkynes to give enol esters,[35] hydration of
terminal alkynes to give aldehydes,[34] hydrophosphination
of alkynes,[36] and reactions of hydrazines with terminal al-
kynes to give nitriles.[37] It should be noted that the mecha-
nism involving nucleophilic attack on simple h2-alkyne com-
plexes rather than vinylidene intermediates has also been
suggested for some of the transformations, for example, in
the addition reactions of amides to alkynes,[38] and in the ad-
dition of carboxylic acids to terminal alkynes.[39] The catalyt-
ic reactions in this study are closely related to the formation
of unsaturated lactones by endo cyclization of a,w-alkynoic
Table 5. Selected bond lengths and angles in complex 30.
À
À
Ru1 C1
1.825(2)
Ru1 N20
2.1253(15)
2.1692(17)
2.2890(4)
1.495(2)
À
À
Ru1 N10
2.1627(15)
2.1934(15)
1.162(3)
Ru1 C2
À
À
Ru1 N1
Ru1 P1
À
À
O1 C1
C2 C3
C1-Ru1-C2
C1-Ru1-N10
C1-Ru1-N20
N10-Ru1-C2
C2-Ru1-P1
N1-Ru1-P1
N10-Ru1-P1
N20-Ru1-P1
C3-C2-Ru1
C4-C3-C2
91.99(8)
96.16(7)
97.22(7)
167.87(7)
91.40(5)
93.02(4)
97.56(4)
172.64(5)
114.27(12)
121.52(17)
C1-Ru1-N1
C1-Ru1-P1
C2-Ru1-N1
N20-Ru1-C2
N10-Ru1-N1
N20-Ru1-N1
N20-Ru1-N10
O1-C1-Ru1
C8-C3-C2
174.55(7)
89.94(6)
92.51(6)
86.68(6)
78.92(6)
79.98(6)
83.40(6)
176.20(18)
121.33(17)
Chem. Eur. J. 2010, 16, 7889 – 7897
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7893

G. Jia et al.
previously for similar transformations, for example, in the
reaction of [RuCl
N
ACHTUNGERTN(NUNG PNP)] with water to give
[RuCl(CH2Ph)(CO)
A
ACHTUNGTRENNUNG
ꢀ
the reaction of [Os
G
ACHTUGNTRENNUN(G PiPr3)2] with H2O to give
ꢀ
[Os
[IrCl
T
(CH2Ph)(CO)
G
(Cp*)]2 (Cp*=pentamethylcyclopentadienyl) with ter-
ꢀ
minal alkynes RC CH and water to give [IrCl
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG(CH2R)-
have also tried the reaction of complex 4 with phenylacety-
lene in the presence of methanol with a hope to obtain a
carbene complex related to 33. Unfortunately, the expected
ruthenium carbene complex could not be isolated.
In support of the mechanism, we found that reaction of
complex
4
with phenylacetylene and approximately
50 equivalents of D2O (99.9 atom% D) gave the partially
1
deuterated complex 35 (Scheme 5). An analysis of the H
Figure 2. ORTEP diagram for the complex cation of 30.
plex 4. The benzyl group is trans to one of the two pyridyl
nitrogen atoms.
The solid-state structure of complex 30 is supported by
the 1H, 13C{1H}, 31P{1H} NMR, and HRMS spectroscopic
1
data. In the H NMR spectrum of 30 (in CD2Cl2), the meth-
ylene protons of the benzyl group gave rise to two signals at
d=1.38 and 2.50 ppm. The six protons of the three methyl-
enes of the tetradentate ligand exhibit six doublets at d=
3.41, 3.65, 3.86, 4.37, 4.43, and 4.84 ppm. In the
13C{1H} NMR spectrum (in CD2Cl2), the signal of RuCH2
appears as a doublet at d=21.3 ppm, and the signal of the
CO ligand was observed at d=204.6 ppm. The 31P{1H} NMR
spectrum shows a singlet at d=55.9 ppm. The mass spec-
trum displays the parent peak of the cation [Ru-
Scheme 5. Reaction of complex 4 with phenylacetylene and D2O to
afford complex 35.
and 2D NMR spectra suggests that the methylene carbon
atom of the benzyl group has approximately 86% deuteri-
um. The formation of 35 is consistent with the mechanism
profile outlined in Scheme 4. Incorporation of 86% rather
than 50% deuterium at the methylene carbon atom is likely
due to reversible formation of 33 from 32. It is also possible
that the terminal alkyne is enriched with deuterium before
forming 32 due to H/D exchange between the terminal
alkyne and D2O in the presence of the ruthenium complex.
Formation of 30–35 provides indirect evidence that com-
plex 4 can react with terminal alkynes to give a vinylidene
intermediate that can be attacked by weak nucleophiles,
such as water and alcohols.
A
ACHTUNGTRENNUNG
(N3P)]+ (30+) at m/z: 694.2952 (calcd:
Scheme 4 shows a plausible mechanism for the formation
of complex 30. Complex 4 could react with phenylacetylene
to give an alkyne complex 31 that rearranges to the vinyli-
dene intermediate 32, which could react with H2O to afford
intermediate 33. Losing HOAC from 33 would afford inter-
mediate 34, which could undergo a deinsertion reaction to
afford complex 30. A similar mechanism has been proposed
Conclusion
We have synthesized a new ruthenium complex 4 that is an
effective catalyst for the catalytic endo cycloisomerization of
a range of alkynols. The cycloisomerization is highly selec-
tive and effective to give exclusively endo-cyclic enol ethers
of five-, six-, and seven-membered rings in high yields under
nonbasic conditions. The ruthenium catalytic precursor is
air-stable and can be easily prepared. The catalytic reaction
can be carried out with a low loading of catalyst without ad-
ditional co-catalyst. The isolation of complex 30 from the
Scheme 4. The mechanism of the reaction of complex 4 with phenylacety-
lene and H2O to afford complex 30.
7894
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 7889 – 7897

Selective and Efficient Cycloisomerization of Alkynols
FULL PAPER
ꢀ
Complex 30: A mixture of complex 4 (0.15 g, 0.16 mmol), phenylacety-
lene (0.16 mL, 1.5 mmol), and H2O (0.20 mL, 11 mmol) in THF (6 mL)
was stirred at 808C for 10 h. The reaction mixture was cooled to room
temperature and Na2SO4 was added. Two hours later, the solid was re-
moved by filtration and the filtrate was concentrated to 1–2 mL and
Et2O was added to give a white precipitate. The solid was collected by fil-
tration, washed with a mixed solvent of CH2Cl2 and Et2O (1:5), and dried
under vacuum to afford 30 as a white solid (69%, 0.11 g). 31P{1H} NMR
(162.0 MHz, CD2Cl2, 258C): d=55.9 ppm (s); 1H NMR (CD2Cl2,
400.1 MHz, 258C): d=8.12 (d, J=5.6 Hz, 1H); 7.82 (t, J=2.8 Hz, 1H);
7.32–7.63 (m, 19H); 7.15–7.21 (m, 3H); 6.88–7.06 (m, 16H); 6.62–6.72
(m, 5H); 6.47 (d, J=7.2 Hz, 2H); 4.84 (d, J=13.6 Hz, 1H; RuCH2Ar);
4.43 (d, J=16.0 Hz, 1H; RuCH2Py); 4.37 (d, J=17.6 Hz, 1H; RuCH2Py);
3.86 (dd, J=13.6, 2.0 Hz, 1H; RuCH2Ar); 3.65 (d, J=16.0 Hz, 1H;
RuCH2Py); 3.41 (d, J=17.2 Hz, 1H; RuCH2Py); 2.50 (dd, J=9.6, 8.4 Hz,
1H; RuCH2Ph); 1.38 ppm (d, J=10.0 Hz, 1H; RuCH2Ph); 13C{1H} NMR
(100.6 MHz, CD2Cl2, 258C): d=204.7 (RuCO), 204.5, 165.2, 164.7, 164.2,
163.7, 157.8, 155.1, 153.1, 151.5, 151.2, 141.1, 141.0, 138.0, 136.7, 136.5,
135.0, 134.5, 134.3, 134.2, 133.6, 133.5, 133.1, 132.5, 132.4, 130.9, 130.5,
130.2, 130.1, 130.0, 129.3, 129.2, 128.8, 128.7, 127.9, 127.7, 127.5, 126.6,
126.2, 125.1, 125.0, 122.4, 121.8, 70.6 (RuCH2Py), 66.5 (RuCH2Py), 63.9,
63.8 (RuCH2Ar), 21.3, 21.2 ppm (RuCH2Ph); elemental analysis calcd
(%) for C63H55BN3OPRu: C 74.70, H 5.47, N 4.15; found: C 74.35, H
reaction of complex 4 with PhC CH and H2O supports that
the catalytic cycle involves a vinylidene intermediate and
hydroxyl-group addition. Further investigations are under-
way to expand the applications of complex 4 in other cata-
lytic reactions.
Experimental Section
General: All manipulations were carried out under a nitrogen atmos-
phere by using standard Schlenk techniques, unless otherwise stated. Sol-
vents were distilled under nitrogen from sodium benzophenone (hexane,
diethyl ether, THF, benzene) or calcium hydride (dichloromethane). The
(PPh3)2][44] and substrates 7–9,[45] and 12--
starting material [RuACHTUNGTRENNUNG(OAc)2ACHTUNGTRENNUGN
15[46] were prepared according to literature methods. Other chemicals
were used as received from Aldrich. Elemental analyses were performed
by M-H-W Laboratories (Phoenix, AZ). Mass Spectra were collected on
a MALDI Micro MX Mass Spectrometer (MALDI), an API QSTAR XL
System (ESI), or a GCT Premier Mass Spectrometer (CI). 1H, 13C{1H},
and 31P{1H} NMR spectra were collected on a Bruker AV 400 MHz NMR
spectrometer. 2D NMR spectra were collected on a JEOL EX 400 MHz
5.35,
N 4.08; HRMS (MALDI, Matrix: CHCA): m/z: calcd for
NMR spectrometer. H and 13C NMR chemical shifts are relative to TMS
1
C39H35N3OPRu+: 694.1556; found: 694.2952 [MÀBPh4]+.
or the residue of deuterium solvents, and 13P NMR chemical shifts are
relative to 85% H3PO4.
[D2]complex 35: A mixture of complex 4 (0.10 g, 0.11 mmol), phenylace-
tylene (0.11 mL, 1.0 mmol), and D2O (0.10 mL, 5.0 mmol) in THF (4 mL)
was stirred at 808C for 10 h. The reaction mixture was cooled to room
temperature and Na2SO4 was added. After 2 h, the solid was removed by
filtration and the filtrate was concentrated to approximately 1 mL and
Et2O was added to give a white precipitate. The solid was collected by fil-
tration, washed with a mixed solvent of CH2Cl2 and Et2O (1:5), and dried
under vacuum to afford 35 as a white solid (61%, 62 mg). 31P{1H} NMR
(162.0 MHz, CD2Cl2, 258C): d=55.7 ppm (s); 1H NMR (400.1 MHz,
CD2Cl2, 25 8C): d=8.13 (d, J=4.4 Hz, 1H); 7.82 (d, J=6.4 Hz, 1H);
7.32–7.65 (m, 19H); 7.12–7.20 (m, 3H); 6.86–7.04 (m, 16H); 6.59–6.74
(m, 5H); 6.47 (d, J=6.8 Hz, 2H); 4.87 (d, J=13.6 Hz, 1H; RuCH2Ar);
4.49 (d, J=15.6 Hz, 1H; RuCH2Py); 4.41 (d, J=17.2 Hz, 1H; RuCH2Py);
4.00 (dd, J=13.6 Hz, 2.0 Hz, 1H; RuCH2Ar); 3.88 (d, J=16.0 Hz, 1H;
RuCH2Py); 3.60 ppm (d, J=17.2 Hz, 1H; RuCH2Py); 2D NMR
(61.3 MHz, (CH3)2CO (set as d=2.20 ppm), 258C): d=1.66 (s, 1H;
RuCD2Ph); 0.99 ppm (s, 1H; RuCD2Ph); HRMS (MALDI, Matrix:
CHCA): m/z: calcd (%) for C39H33D2N3OPRu+: 696.1681; found:
696.2880 [MÀBPh4]+.
Typical procedure for the catalytic cycloisomerization of alkynols: Cata-
lyst 4 (0.005 or 0.025 mmol, as indicated in Table 4) was added to a solu-
tion of alkynol (0.5 mmol) in THF (0.5 mL). The resulting solution was
stirred at 808C and monitored by TLC or 1H NMR spectroscopy. When
the maximum conversion was reached, the desired product was isolated
by flash column chromatography on silica gel. For products of low boiling
points, the yields were determined by 1H NMR spectroscopic integration
with CH3NO2 or tBuOH as the internal standards.
N,N-Bis[(pyridin-2-yl)methyl][2-(diphenylphosphino)phenyl]methan-
amine (3): A mixture of 2-(diphenylphosphino)benzaldehyde (1, 0.85 g,
2.93 mmol), NaBHACHTUNGTRENNUNG(OAc)3 (1.04 g, 4.09 mmol), and 2,2’-dipicolylamine (2,
0.44 mL, 2.44 mmol) in CH2Cl2 (30 mL) was stirred at room temperature
for 12 h to give a light-yellow solution with a white precipitate. The reac-
tion mixture was filtered through Celite to remove the white solid. The
light-yellow filtrate was washed with a saturated aqueous solution of
sodium hydrogen carbonate and brine. The organic extract was dried
with magnesium sulfate. After filtration, the solvent was pumped away
under vacuum and the residue was purified by chromatography with a
silica-gel column to afford the product as a tacky solid (85%, 0.98 g).
31P{1H} NMR (CDCl3, 162.0 MHz, 258C): d=À16.6 ppm (s); 1H NMR
(CDCl3, 400.1 MHz, 258C): d=8.48 (d, J=4.8 Hz, 2H; Py), 7.82–7.79 (m,
1H), 7.59–7.54 (m, 2H), 7.39–7.26 (m, 9H), 7.23–7.19 (m, 4H), 7.15–7.09
(m, 3H), 6.88–6.85 (m, 1H), 3.99 (s, 2H; CH2Ph), 3.80 ppm (s, 4H;
2CH2Py); 13C{1H} NMR (CDCl3, 100.6 MHz, 258C): d=159.0, 148.7,
136.9, 136.7, 136.6, 136.4, 133.9, 133.7, 129.3, 129.2, 128.9, 128.6, 128.5,
127.2, 123.0, 121.9, 59.6 (ArCH2N), 56.8 (PyCH2N), 56.6 ppm (PyCH2N);
HRMS (ESI+): m/z: calcd for C31H29N3P+: 474.2094; found: 474.1947
[M+H]+.
Complex 4: A mixture of [RuACTHNUTRGENN(UG OAc)2ACHTUGNTERN(NUGN PPh3)2] (0.89 g, 1.2 mmol) and 3
(0.74 g, 1.56 mmol) in benzene (50 mL) was stirred at room temperature
for 12 h to give a yellow solution with an orange precipitate. The mixture
was concentrated to approximately 10 mL. The solid was collected by fil-
tration, washed with benzene, THF, and Et2O, and dried under vacuum.
The resulting yellow solid was redissolved in CH3OH (30 mL), and then
a solution of NaBPh4 (0.86 g, 2.5 mmol) in CH3OH (10 mL) was added
dropwise. After completion of the addition, the reaction mixture was
stirred at room temperature for 1 h. The mixture was concentrated to ap-
proximately 5 mL. The solid was collected by filtration, washed with
methanol and Et2O, and dried under vacuum to afford 4 as a yellow solid
(63%, 0.72 g). 31P{1H} NMR (162.0 MHz, CD2Cl2, 258C): d=63.5 ppm
(s); 1H NMR (CD2Cl2, 400.1 MHz, 258C): d=8.40 (d, J=4.4 Hz, 2H;
Py), 7.40–7.15 (m, 23H), 7.06–6.88 (m, 17H), 6.73–6.63 (m, 4H), 4.37 (d,
J=15.2 Hz, 2H; PyCH2), 3.83 (2H, J=15.6 Hz; PyCH2), 3.37 (s, 2H;
ArCH2), 2.00 ppm (s, 3H; COCH3); 13C{1H} NMR (CDCl3, 100.6 MHz,
258C): d=189.7, 165.1, 164.6, 164.1, 163.6, 163.2, 154.0, 136.8, 136.6,
136.4, 132.8, 132.7, 132.1, 131.6, 130.8, 130.7, 130.6, 130.3, 129.9, 129.3,
128.9, 128.8, 126.1, 124.0, 122.3, 121.2, 68.7 (ArCH2), 67.6 (PyCH2), 67.5
(PyCH2), 24.3 ppm (COCH3); elemental analysis calcd (%) for
C57H52BN3O2PRu·0.5H2O: C 71.17, H 5.45, N 4.37; found: C 71.26, H
5.51, N 4.48; ESI-MS (CH2Cl2): m/z: 575 [MÀBPh4ÀOAc]+.
2,3-Dihydro-2-styrylfuran (27): 1H NMR (400.1 MHz, CD2Cl2, 258C): d=
7.31–7.13 (m, 5H; Ph), 6.52 (d, J=16.0 Hz, 1H; PhCHCH), 6.27–6.18 (m,
2H; PhCHCH, OCHCHCH2), 5.05–5.01 (m, 1H; CHCHCHACHTUNGTRENNUNG(CH2)O),
4.84 (q, J1 =5.2, J1 =2.8 Hz, 1H; OCHCHCH2), 2.80–2.73 (m, 1H;
OCHCHCH2), 2.42–2.36 ppm (m, 1H; OCHCHCH2); 13C{1H} NMR
(100.6 MHz, CDCl3, 258C): d=145.0 (OCHCHCH2), 136.5, 131.0, 129.1,
128.5, 127.7, 126.5, 99.1 (OCHCHCH2), 81.7 (CHCHCHACHTUNGTRENNUNG(CH2)O]),
35.5 ppm (OCHCHCH2); HRMS (CI+): m/z: calcd for C12H13O+:
173.0961; found: 173.0984 [M+H]+.
Crystallographic structure analysis of complexes 4 and 30: The diffraction
intensity data of 4 was collected with a Bruker Smart APEX CCD dif-
fractometer with monochromatized MoKa radiation (l=0.71073 ꢁ) at
173 K. Lattice determination and data collection were carried out by
using SMART v.5.625 software. Data reduction and absorption correc-
tions were performed by using SAINT v 6.26 and SADABS v 2.03. The
diffraction intensity data of 30 was collected with an Oxford Diffraction
Chem. Eur. J. 2010, 16, 7889 – 7897
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
7895

G. Jia et al.
Gemini S Ultra with monochromatized CuKa radiation (l=1.54178 ꢁ) at
173 K. Lattice determination, data collection, and reduction were carried
out by using CrysAlisPro 171.32.5. Absorption corrections (semi-empiri-
cal from equivalents) were performed by using the SADABS built-in the
CrysAlisPro program suite. Structure solution and refinement for all
three compounds were performed by using the SHELXTL v.6.10 soft-
ware package. They were solved by the direct methods and refined by
full-matrix least squares on F2. All non-hydrogen atoms were refined ani-
sotropically. Hydrogen atoms were introduced at their geometric posi-
tions and refined as riding atoms.
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Acknowledgements
This work was supported by the Hong Kong Research Grant Council
(project no. HKUST 601709), the Natural Science Foundation of China
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Selective and Efficient Cycloisomerization of Alkynols
FULL PAPER
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Received: December 15, 2009
Published online: June 2, 2010
Chem. Eur. J. 2010, 16, 7889 – 7897
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7897