Lewis acid controlled regioselective 1,2 and 1,4 reaction of α,β-unsaturated carbonyl compounds with TiIV enolates derived from α-diazo β-keto carbonyl compounds

Article abstract of DOI:10.1002/1521-3773(20020802)41:15<2773::AID-ANIE2773>3.0.CO;2-I

The selective generation of 1,2- and 1,4-addition products (3 and 4, respectively) in the addition of Ti^IV enolates 1 to α,β-unsaturated carbonyl compounds 2 can be almost completely controlled by the choice of Lewis acid. The high selectivity

Full text of DOI:10.1002/1521-3773(20020802)41:15<2773::AID-ANIE2773>3.0.CO;2-I

COMMUNICATIONS
[6] a) D. W. Armstrong, L. He, Y.-S. Liu, Anal. Chem. 1999, 71, 3873
3876; b) A. Heintz, D. V. Kulikov, S. P. Verevkin, J. Chem. Eng. Data
2001, 46, 1526 1529; c) R. D. Rogers, A. E. Visser, H. James, C.
Koval, D. L. DuBoix, P. Scovazzo, R. D. Noble, Abstract of Papers,
223rd ACS National Meeting, Orlando, USA, April 7 11, 2002.
[7] a) A. G. Fadeev, M. M. Meagher, Chem. Commun. 2001, 295 296;
b) T. Sch‰fer, C. M. Rodrigues, C. A. M. Afonso, J. G. Crespo, Chem.
Commun. 2001, 1622 1623.
[8] a) J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser,
R. D. Rogers, Chem. Commun. 1998, 1765 1766; b) S. G. Cull, J. D.
Holbrey, V. Vargas-Mora, K. R. Seddon, G. J. Lye, Biotechnol. and
Bioeng. 2000, 69, 227 233; c) A. E. Visser, R. P. Swatloski, W. M.
Reichert, S. T. Griffin, R. D. Rogers, Ind. Eng. Chem. Res. 2000, 39,
3596 3604; d) A. E. Visser, R. P. Swatloski, W. M. Reichert, R.
Mayton, S. Sheff, A. Wierzbicki, J. H. Davies, R. D. Rogers, Chem.
Commun. 2001, 135 136.
seems particularly appealing for large-scale operations and
for different substrates; with the emergence of a considerable
number of new RTILs,[3, 11] it should be possible to design
RTILs with high selectivity for specific substrates. The
possibility of using nonpolar solvents with high boiling points
or water instead of diethyl ether, and supported RTIL in
hollow-fiber membranes will allow this technology to rein-
force its environmentally benign character and become
attractive for industrial application.
Experimental Section
For the batch studies, the cell indicated in Figure 1 was used without pumps.
The ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate
([bmim][PF6]) was immobilized in the porous structure of a polyvinylidene
fluoride (PVDF) hydrophilic membrane (Gelman Sciences, FP Vericel,
pore size 0.45 mm) by filtration in vacuo and placed in a metallic net (i.d.
1.65 cm) located between side A (Vˆ 30 mL) and side B (Vˆ 30 mL) of
the cell. The amines (1:1:1 molar mixture) hexylamine (470 mL), DIIPA,
(500 mL) and TEA (500 mL), and n-decane (400 mL; internal standard) in
diethyl ether (30 mL) were added to side A of the cell. n-Decane (400 mL;
internal standard) was added to diethyl ether (30 mL) in side B of the cell.
The transport of amines to side B at room temperature was monitored by
GLC by taking samples from side A and B of the cell at defined time
intervals (15, 30, 60, 120, 240, 360 min). The recovery of each amine was
determined by comparison of the areas of the peaks of each amine with
those of n-decane and relative to the areas initially observed in side A.
[9] a) R. A. Brown, P. Pollet, E. McKoon, C. A. Eckert, C. L. Liotta, P. G.
Jessop, J. Am. Chem. Soc. 2001, 123, 1254 1255; b) M. F. Sellin, P. B.
Webb, D. J. Cole-Hamilton, Chem. Commun. 2001, 781 782; c) L. A.
Blanchard, J. F. Brennecke, Ind. Eng. Chem. Res. 2001, 40, 287 292;
d) F. Liu, M. B. Abrams, R. T. Baker, W. Tumas, Chem. Commun.
¡
2001, 433 434; e) A. Bˆsmann, G. Francio, E. Janssen, M. Solinas, W.
Leitner, P. Wasserscheid, Angew. Chem. 2001, 113, 2769 2771;
Angew. Chem. Int. Ed. 2001, 40, 2697 2699.
√
[10] P. Bonhote, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M.
Gr‰tzel, Inorg. Chem. 1996, 35, 1168 1178.
[11] a) J. D. Holbrey, K. R. Seddon, J. Chem. Soc. Dalton Trans. 1999, 13,
2133 2139; b) A. S. Larsen, J. D. Holdbrey, F. S. Tham, C. A. Reed, J.
Am. Chem. Soc. 2000, 122, 7264 7272; c) J. G. Huddleston, A. E.
Visser, W. M. Reichert, H. D. Willauer, G. A. Broker, R. D. Rogers,
Green Chem. 2001, 3, 156 164; d) J. Pernak, A. Czepukowicz, R.
Poz×niak, Ind. Eng. Chem. Res. 2001, 40, 2379 2383.
For continuous operation conditions, the cell indicated in the Figure 1 was
used, with each side of the cell connected to a piston pump (FMI lab pump,
model QSY) to promote the circulation in each side. The RTIL
[bmim][PF6] was immobilized as indicated above. The amines (1:1 molar
mixture) DIIPA (100 mL) and TEA (100 mL) in diethyl ether (5 L) were
circulated with a flow rate of 1 mL/min in side A of the cell. Diethyl ether
(5L) was circulated with a flow rate of 1 mLminÀ1 in side B of the cell. Both
solutions were renewed every 2days. The transport of each amine to side B
was monitored by sequentially collecting samples from the outlet tube of
side B every 12h. During the 14 days of continuous operation, 23 samples
were collected with a total volume of 20410 mL. The DIIPA/TEA ratio was
determined for each sample by GLC. Eighteen samples (15460 mL) were
fractionally distilled to afford a mixture of DIIPA/TEA (250 mL; 89.2:10.8,
determined by GLC); 1H and 13C NMR spectral data were identical to
those of authentic samples. The distilled diethyl ether fraction and the
remaining six samples contained a mixture of DIIPA/TEA (142.2 mL
(81.6:18.4) and 105.7 mL (80.9:19.1), respectively, ratio determined by
GLC analysis with n-decane as internal standard).
Lewis Acid Controlled Regioselective 1,2 and
1,4 Reaction of a,b-Unsaturated Carbonyl
Compounds withTi IV Enolates Derived from a-
Diazo b-Keto Carbonyl Compounds**
Guisheng Deng, Xue Tian, Zhaohui Qu, and
Jianbo Wang*
The addition of nucleophiles to a,b-unsaturated carbonyl
compounds is a fundamental transformation in organic syn-
thesis. Since there are two reaction sites in the a,b-unsatu-
rated carbonyl functional group, this addition reaction can
only be of practical synthetic utility in organic synthesis if one
Received: January 25, 2002 [Z18583]
[1] J. G. Crespo, I. M. Coelhoso, R. M. C. Viegas in Encyclopedia of
Separation Processes, Academic Press, San Diego, 2000, pp. 3303
3311.
[2] J. J. Pellegrino, R. D. Noble, Trends Biotechnol. 1990, 8, 216 225.
[3] a) K. R. Seddon, Kinet. Catal. 1996, 37, 693 697; b) T. Welton, Chem.
Rev. 1999, 99, 2071 2084; c) M. Freemantle, Chem. Eng. News 2001,
January 1, 21 25; d) J. Dupont, C. S. Consorti, J. Spencer, J. Braz.
Chem. Soc. 2000, 11, 337 344; e) P. Wasserscheid, K. Wilhelm,
Angew. Chem. 2000, 112, 3926 3945; Angew. Chem. Int. Ed. 2000, 39,
3772 3789; f) R. Sheldon, Chem. Commun. 2001, 2399 2407;
g) C. M. Gordon, Appl. Catal., A: 2001, 222, 101 107; h) R. T. Carlin,
J. Fuller, Chem. Commun. 1997, 1345 1346.
[4] L. A. Blanchard, D. Hancu, E. J.Beckman, J. F. Brennecke, Nature
1999, 399, 28 29.
[5] a) R. M. Lau, F. van Rantwijk, K. R. Seddon, R. A. Sheldon, Org.
Lett. 2000, 2, 4189 4191; b) S. H. Schˆfer, N. Kaftzik, P. Wasserscheid,
U. Kragl, Chem. Commun. 2001, 425 426; c) K.-W. Kim, B. Song,
M.-Y. Choi, M.-J. Kim, Org. Lett. 2001, 3, 1507 1509.
[*] Prof. J. Wang, G. Deng, X. Tian, Z. Qu
Key Laboratory of Bioorganic Chemistry
and Molecular Engineering of the Ministry of Education
Department of Chemical Biology, College of Chemistry
Peking University, Beijing 100871 (P. R. China)
Fax : (‡86)10-6275-1708
E-mail: wangjb@pku.edu.cn
[**] The project is generously supported by the Natural Science Founda-
tion of China (Grant Nos. 29972002, 20172002) and Trans-Century
Training Program Foundation for the Talents by the Ministry of
Education of China.
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2002, 41, No. 15
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4115-2773 $ 20.00+.50/0
2773

COMMUNICATIONS
can control the selectivity for the two possible regioisomers.[1]
There are several factors that control the regioselectivity (1,2
vs 1,4 addition). These include the attacking nucleophiles,[2]
solvent,[3] temperature,[4] steric bulk,[5] and transition-metal
additives.[6, 7] In general, the softness or hardness of the
nucleophiles is of primary importance. Hard nucleophiles,
such as alkyl lithium compounds, give predominantly 1,2
addition, whereas soft nucleophiles, such as the anion of the
activated methylene compounds, give primarily 1,4 addition
products.[8]
On the other hand, the complexation of Lewis acids with
the carbonyl oxygen atom can dramatically affect the proper-
ties of the a,b-unsaturated carbonyl compounds.[9] For exam-
ple, both the reactivity and the selectivity of the Diels Alder
reaction of a,b-unsaturated carbonyl compounds with dienes
could be greatly enhanced by Lewis acids.[10] Lewis acids play
an indispensable role in organic chemistry, especially in
catalytic asymmetric synthesis. Herein we report the Lewis
acid promoted nucleophilic addition of the TiCl4-derived
enolate of b-keto a-diazo carbonyl compounds to a,b-
unsaturated carbonyl compounds. We found that by choosing
appropriate Lewis acids, it is possible to control the selectivity
for either 1,2or 1,4 addition.
The Ti enolate 1a or 1b was generated by treating the b-
keto a-diazo carbonyl compounds with TiCl4/Et3N in anhy-
drous CH2Cl2 at À788C (Scheme 1).[11] When the enolate 1a
reacts with enone 2a at À788C, a mixture of 1,2- and 1,4-
addition products (60:40) was isolated in 70% yield (Table 1,
entry 1). If the enone 2a was stirred with another equivalent
of TiCl4 in CH2Cl2 before adding to the enolate 1a, the 1,4-
addition product was obtained as the major product (1,2/1,4
17:83) (Table 1, entry 4). If the enone 2a was activated with
SnCl4 (1 equiv) instead of TiCl4, the selectivity for 1,4 addition
was further enhanced (1,2/1,4 5:95; Table 1, entry 5). On the
other hand, when the enone 2a was activated with BF3 ¥ OEt2,
the 1,2-addition product became predominant (1,2/1,4 83:17;
Table 1, entry 2). The activation of enone with Ti(OiPr)4
further enhanced the selectivity for 1,2addition (1,/21,4
96:4; Table 1, entry 3). For Ti enolate 1b, a similar enhance-
ment of regioselectivity was observed (Table 1, entries 6 10).
When enone 2b was employed as the substrate, the direct
reaction with the Ti enolate 1a gave equal amounts of 1,2- and
1,4-addition products (Table 1, entry 11). TiCl4 activation of
the enone significantly enhanced 1,4 addition (1,2/1,4 1:99;
Table 1. Regioselective nucleophilic addition of Ti enolate 1 with a,b-
unsaturated carbonyl compounds 2.
Entry
1
2
Lewis acid[a]
t [h]
3
4
3/4[b]
Yield [%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
a
a
a
a
a
b
b
b
b
b
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
a
b
b
b
b
c
none
6
8
8
6
10
8
9
8
9
8
9
8.5
8
8
7
a
a
a
a
a
b
b
b
b
b
c
c
c
c
d
d
e
e
e
f
a
a
a
a
a
b
b
b
b
b
c
c
c
c
d
d
e
e
e
f
60:40
83:17
96:4
17:83
5:95
50:50
76:24
94:6
22 :78
0:100
50:50
49:51
76:24
1:99
100:0
100:0
40:60
76:24
0:100
71:29
100:0
0:100
70
67
78
63
50
42
48
60
75
58
71
83
90
73
50
71
73
82
76
78
81
54
BF3 ¥ OEt2
Ti(OiPr)4
TiCl4
SnCl4
none
BF3 ¥ OEt2
Ti(OiPr)4
TiCl4
SnCl4
none
BF3 ¥ OEt2
Ti(OiPr)4
TiCl4
none
TiCl4
none
Ti(OiPr)4
SnCl4
none
Ti(OiPr)4
SnCl4
c
5
d
d
d
e
e
e
8.5
8.5
8.5
8
8
8
f
f
f
f
[a] Lewis acid (1 equiv) was used to activate the substrate. [b] The product
ratio was determined by 1H NMR spectroscopic analysis (400 MHz) and
was confirmed by separation by column chromatography. [c] Yield of
isolated products.
Table 1, entry 14), whereas BF3 ¥ OEt2 did not affect the
selectivity for 1,2 addition (1,2/1,4 49:51; Table 1, entry 12).
Evidently, the steric bulk of the phenyl group in enone 2b
overrides the activation of BF3 ¥ OEt2 for the carbonyl
group.[5a] However, activation by Ti(OiPr)4 can still enhance
the selectivity for the sterically less favored 1,2addition (1,2/
1,4 76:24; Table 1, entry 13).
For enal 2c, only the 1,2-addition product was isolated, even
in the TiCl4-activated reaction (Table 1, entries 15, 16). The
high reactivity of the aldehyde carbonyl group is the
determining factor in controlling the regioselectivity in this
case. On the other hand, for the enones 2d and 2e, similar
control of diastereoselectivity as that for 2a and 2b was
observed (Table 1, entries 17 22).
Regiocontrol by Lewis acids has also been observed for
cyclic enones. Without the activation of the Lewis acid, the
reaction of Ti enolate 1a with cyclohexenone 5b (Scheme 2)
gave a mixture of 1,2- and 1,4-addi-
tion products in low selectivity (1,2/1,4
67:33; Table 2, entry 4). TiCl4 activation
gave almost only 1,4-addition product
(1,2/1,4 1:99; Table 2, entry 7), whereas
O
Cl
Ti
Cl
O
Cl
O
N2
OH
R2
O
O
O
R
Lewis acid
CH2Cl2
+
+
R1
R
R1
R2
O
O
R
–78 o
C
N2
BF3 ¥ OEt2 activation slightly increases
the amount of 1,2-addition product ob-
tained (1,2/1,4 86:14; Table 2, entry 5).
When the enone was activated with
Ti(OiPr)4, the 1,2-addition product is
greatly increased (1,2/1,4 99:1; Table 2,
entry 6). For cyclopentenone 5a, the re-
action without Lewis acid activation gave
a mixture of unidentified products (Ta-
ble 1, entry 1). TiCl4 activation gave the
N2
R1
R2
1
2
4
3
a: R1 = Ph, R2 = Me
b: R1 = Ph, R2 = Ph
c: R1 = Ph, R2 = H
d: R1 = H, R2 = Ph
a: R = OEt, R1 = Ph, R2 = Me
b: R = Ph, R1 = Ph, R2 = Me
c: R = OEt, R1 = Ph, R2 = Ph
d: R = OEt, R1 = Ph, R2 = H
e: R = OEt, R1 = H, R2 = Ph
a: R = OEt
b: R = Ph
e: R1 =
, R2 = Me
O
f: R = OEt, R1 =
, R2 = Me
O
Scheme 1. Lewis acid promoted nucleophilic addition of Ti enolate 1 with a,b-unsaturated carbonyl
compounds 2.
2774
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4115-2774 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 15

COMMUNICATIONS
activator, 1,2addition was again greatly enhanced (Table 1,
entries 3, 8 13, 18, 21; Table 2, entry 6).
N2
O
O
R
Cl
Ti
Cl
O
O
( )n
5
Cl
O
O
HO
Since the nucleophilic addition products bear diazo func-
tionality, both 1,2- and 1,4-addition products can be subjected
to further synthetically useful transformations.[16] For exam-
ple, when 3a was treated with [Rh2(OAc)4] (1 mol%) in
benzene, highly efficient chemoselective intramolecular in-
O
O
Lewis acid
+
+
( )n
CH2Cl2
–78 oC
R
R
( )n
N2
N2
6
1
7
a: R = OEt
b: R = Ph
a: n = 1
b: n = 2
a: R = OEt, n = 1
b: R = OEt, n = 2
c: R = Ph, n = 2
À
sertion into the O H bond occurs to give tetrahydrofuran
derivative 9 as a mixture of two diastereomeric isomers in
Scheme 2. Lewis acid promoted nucleophilic addition of Ti enolate 1 with
cyclic enones 5.
excellent yield (Scheme 3).
O
OH O
Me
O
Table 2. Regioselective nucleophilic addition of Ti enolate 1 with cyclic
enones 5.
[Rh2(OAc)4] (1% mol)
Me
OEt
Ph
OEt
Ph
O
benzene
reflux, 93 %
N2
O
Entry
1
5
Lewis acid[a]
t [h]
6
7
6/7[b]
Yield [%][c]
9
3a
[d]
[d]
1
2
3
4
5
6
7
8
9
a
a
a
a
a
a
a
b
b
b
b
a
a
a
b
b
b
b
b
b
b
b
none
BF3 ¥ OEt2
TiCl4
8
6.5
7
8
8
8.5
7
8
8.5
8.5
8
a
a
a
b
b
b
b
c
a
a
a
b
b
b
b
c
À
Scheme 3. [Rh2(OAc)4]-mediated intramolecular O H insertion.
0:100
45
63
40
68
52
51
none
67:33
86:14
99:1
In summary, we have demonstrated that both 1,2and 1,4
selectivity of the nucleophilic addition could be controlled by
Lewis acids. Similar control of selectivity may be possible for
other types of nucleophiles. Investigations along this line are
underway in our laboratory.
BF3 ¥ OEt2
Ti(OiPr)4
TiCl4
none
BF3 ¥ OEt2
TiCl4
1:99
70:30
100:0
19:81
0:100
c
c
c
c
c
c
31[e]
47
34[e]
10
11
SnCl4
[a] Lewis acid (1 equiv) was used to activate the substrate. [b] The product
ratio was determined by 1H NMR spectroscopic analysis (400 MHz) and
was confirmed by separation by column chromatography. [c] Yields of
isolated products. [d] The reaction gave a complex mixture, 1,4-addition
product could be isolated in low yield. [e] Considerable amounts of starting
materials were recovered.
Experimental Section
Typical procedure: TiCl4 (209 mg, 1.1 mmol) and Et3N (111 mg, 1.1 mmol)
were added dropwise to a solution of 1a (156 mg, 1 mmol) in anhydrous
CH2Cl2 (10 mL) at À788C. The dark-red mixture was stirred at À788C for
1 h. Ti(OiPr)4 (284 mg, 1 mmol) and 2a (146 mg, 1 mmol) in anhydrous
CH2Cl2 (2mL) were added to this mixture. The reaction mixture was
stirred for another 8 h and then quenched with saturated aqueous NH4Cl
(5 mL). The organic layer was separated; upon workup, a crude product
was obtained which was purified by flash chromatography to yield major
product 3a as an oil (224 mg) and minor product 4a as a white solid (10 mg,
m.p. 68 698C) in 78% total yield. Simultaneously, 1a (13%) and 2a
(19%) were recovered.
1,4-addition compound as the sole product (Table 1, entry 3).
The 1,2-addition product could not be isolated when the
reaction was activated with BF3 ¥ OEt2. This is believed to be a
result of the low stability of the 1,2-addition product 6a.
The Lewis acid controlled selectivity described above could
be rationalized as follows. As the anion of the activated
methylene compound, the Ti enolate 1 is considered to be a
soft nucleophile.[8b] From ab initio calculations it is known that
the complexation of the Lewis acid with the oxygen atom of
an a,b-unsaturated carbonyl compound increases its carbonyl
coefficient of LUMO relative to that of the remote b-carbon
atom,[12] and hence the Lewis acid coordination should
promote 1,2addition. Therefore,
Received: February 4, 2002 [Z18651]
[1] For a review, see: P. Perlmutter, Conjugate Addition Reactions in
Organic Synthesis, Pergamon Press, Oxford, 1992.
[2] a) D. A. Hunt, Org. Prep. Proced. Int. 1989, 21, 707 749; b) W. C.
Still, A. Mitra, Tetrahedron Lett. 1978, 30, 2659 2662.
[3] a) T. Cohen, W. D. Abraham, M. Myers, J. Am. Chem. Soc. 1987, 109,
7923 7924; b) H. J. Reich, W. H. Sikorski, J. Org. Chem. 1999, 64,
14 15; c) W. H. Sikorski, H. J. Reich, J. Am. Chem. Soc. 2001, 123,
6527 6535.
[4] a) A. G. Schultz, Y. K. Lee, J. Org. Chem. 1976, 41, 4044 4045;
b) P. C. Ostrowski, V. V. Kane, Tetrahedron Lett. 1977, 3549 3552;
c) K. Ogura, M. Yamashita, G.-I. Tsuchihashi, Tetrahedron Lett. 1978,
1303 1306.
[5] a) D. Seebach, R. Locher, Angew. Chem. 1979, 91, 1024 1025; Angew.
Chem. Int. Ed. Engl. 1979, 18, 957 958; b) K. Maruoka, M. Ito, H.
Yamamoto, J. Am. Chem. Soc. 1995, 117, 9091 9092; c) J. Lucchetti,
A. Krief, J. Organomet. Chem. 1980, 194, C49-C52.
BF3 ¥ OEt2 enhances the 1,2selectivity.
Cl
Cl
Ti
Cl
O
Cl
O
Cl
Cl
However, when enones are activated
by TiCl4 or SnCl4,[13] there is another
factor that overrides the Lewis acid
activation for 1,2addition. It is known
that TiCl4 can form dimeric structures
that involve bridging chlorine
atoms.[14, 15] Therefore we speculate
M
O
Cl
R
N2
8 (M = Sn or Ti)
[6] For reviews, see: a) G. H. Posner, Org. React. 1977, 19, 1 113; b) B. H.
Lipshutz, S. Sengupta, Org. React. 1992, 41, 135 631.
that the complex 8 formed in the TiCl4- or SnCl4-activated
reactions in which the two chlorine atoms serve as bridges for
the two transition metals. Because of the steric proximity in
this structure, 1,4 addition occurs much easier. Strong evi-
dence to support this rationalization is that when Ti(OiPr)4,
which has no chlorine atoms for the bridging, is used as the
[7] J.-M. Lefour, A. Loupy, Tetrahedron 1978, 34, 2597 2605.
[8] For examples, see: a) E. M. Kaiser, C. L. Mao, C. F. Hauser, J. Org.
Chem. 1970, 35, 410 414; b) T. V. Rajan Babu, J. Org. Chem. 1984, 49,
2083 2089.
[9] For a comprehensive review, see: M. Santelli, J.-M. Pons, Lewis Acids
and Selectivity in Organic Synthesis, CRC Press, Boca Raton, 1996.
[10] P. Yates, P. Eaton, J. Am. Chem. Soc. 1960, 82, 4436 4437.
Angew. Chem. Int. Ed. 2002, 41, No. 15
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4115-2775 $ 20.00+.50/0
2775

COMMUNICATIONS
[11] a) M. A. Calter, P. M. Sugathapala, C. Zhu, Tetrahedron Lett. 1997, 38,
3837 3840; b) C. Zhu, M. A. Calter, J. Org. Chem. 1999, 64, 1415
1419.
[12] a) K. N. Houk, R. W. Strozier, J. Am. Chem. Soc. 1973, 95, 4094 4096;
b) A. Imammura, T. Hirano, J. Am. Chem. Soc. 1975, 97, 4192 4198;
c) A. Dargelos, D. Liotard, M. Chaillet, Tetrahedron 1972, 28, 5595
5605; d) O. F. Guner, R. M. Ottenbrite, D. D. Shillady, P. V. Alston, J.
Org. Chem. 1987, 52, 391 394; e) R. J. Loncharich, T. R. Schwartz,
K. N. Houk, J. Am. Chem. Soc. 1987, 109, 14 23; f) P. Laszlo, M.
Teston, J. Am. Chem. Soc. 1990, 112, 8750 8754.
[13] For a spectroscopic study on the complexation of TiCl4 and SnCl4 with
enones, see: S. E. Denmark, N. G. Almstead, Tetrahedron, 1992, 48,
5565 5578.
[14] a) L. Brun, Acta Crystallogr. 1966, 20, 739 749; b) I. W. Bassi, M.
Calcaterra, R. Intrito, J. Organomet. Chem. 1977, 127, 305 313.
[15] S. W. Ng, C. L. Barnes, M. B. Hossain, D. van der Helm, J. J. Zuker-
man, V. G. Kumer Das, J. Am. Chem. Soc. 1982, 104, 5359 5364.
[16] For a comprehensive review on a-diazo carbonyl compounds, see:
M. P. Doyle, M. A. McKervey, T. Ye, Modern Catalytic Methods for
Organic Synthesis with Diazo Compounds, Wiley, New York, 1998.
metal. Indeed, Sellmann et al. found strong intermolecular
À
S H ¥¥¥ S bridges in the crystal structure of [Ru(SH2)(PPh3)
™S4∫].[6a] An influence of these bridges on the reactivity of the
metal complexes has not been demonstrated, although intra-
À
molecular M SH ¥¥¥ hydride interactions have been proposed
in the initial stage of the mechanism of hydride protonation.[7]
À
Here we show that a fast S H ¥¥¥ S proton exchange takes
place in bimetallic platinum complexes with bridging SHÀ and
S
2À ligands.
Sulfide-bridged aggregates with the Pt2S2 core have a rich
chemistry.[8, 9] We proved that the reactivity of the Pt2S2 core is
highly dependent on the nature of the terminal ligands.[9, 10]
We have now synthesized of the monoprotonated complexes
[Pt2{Ph2P(CH2)nPPh2}2(m-S)(m-SH)]ClO4 (n ˆ 2, dppe (1);
n ˆ 3, dppp (2)) by adding HClO4 to a solution of the
corresponding [Pt2(m-S)2P \ P)2] (P \ P ˆ dppe or dppp) com-
plex in benzene. The most remarkable spectroscopic feature
of 1 and 2 is the equivalence of the four phosphorus nuclei at
room temperature according to the 31P NMR spectrum
(Figure 1). The only analogous monoprotonated compound
À
First Evidence of Fast S H ¥¥¥ S Proton Transfer
in a Transition Metal Complex**
¬
¡
Gabriel Aullon, Merce Capdevila, William Clegg,
¬
¬
Pilar Gonzalez-Duarte,* AgustÌ Lledos,* and
Ruben Mas-Balleste
¬
¬
À
In the quest to control noncovalent interactions, S H ¥¥¥ S
hydrogen bonds are attracting great interest. Despite the
prevalence of the thiol group in cysteine residues and the
À
potential importance of S H ¥¥¥ S bridging bonds in biology,
little is known about this interaction.[1] Intermolecular
À
S H ¥¥¥ S chains that play an organizing role in the solid state
were found in X-ray structures of several compounds
[2]
À
À
containing S H groups. The S H ¥¥¥ S hydrogen bonds are
typically very weak, but may become moderately strong in
particular compounds. Resonance[3] and charge[4] assistances
have been put forward as being responsible for strong
Figure 1. Variable-temperature 31P{1H} NMR spectra of 1.
À
intramolecular S H ¥¥¥ S bonds. The greater acidity of dithiols
previously reported, namely, [Pt2(m-S)(m-SH)(PPh3)4]PF6, has
two distinct environments about the P nuclei, as the SH group
is cis to two phosphorus atoms and trans to the other two.
Consequently, at room temperature, it shows two 31P NMR
signals with two distinct 1JPt,P coupling constants.[8b, 11] Surpris-
ingly, each of the monoprotonated complexes 1 and 2 shows
relative to their monothiol analogues has been attributed to
enhanced stabilization of the thiolate anion by an intra-
molecular RSÀ ¥¥¥ HSR hydrogen bond.[5] Evidence of
À
S H ¥¥¥ S interactions in transition metal compounds are
scarce,[6] although the acidity of the SH group should be
enhanced when the sulfur atom is coordinated to a transition
only one pseudotriplet with the following apparent spectro-
1
scopic parameters in [D6]acetone: dP ˆ 42.8 ppm and JPt,P
ˆ
1
¬
¬
¬
[*] Prof. Dr. P. Gonzalez-Duarte, Prof. Dr. A. Lledos, Dr. G. Aullon,
3108 Hz for 1, and dP ˆ À3.3 ppm and JPt,P ˆ 2960 Hz for 2.
¬
Dr. M. Capdevila, Dipl.-Chem. R. Mas-Balleste
Departament de QuÌmica
We optimized the geometry of the model compounds
‡
[Pt2{H2P(CH2)nPH2}(m-S)(m-SH)] (n ˆ 2, dhpe (1t); n ˆ 3,
¡
Universitat Autonoma de Barcelona
dhpp (2t)) by B3LYP calculations.[12] Two conformations with
a hinged Pt2S2 skeleton were found as minima in both
complexes; they differ in the endo (e) or exo (x) orientation
of the thiol proton (see Figure 2). As expected, two different
08193 Bellaterra, Barcelona (Spain)
Fax : (‡34)935-812-920
E-mail: Pilar.Gonzalez.Duarteßuab.es, agusti@klingon.uab.es
Prof. Dr. W. Clegg
Department of Chemistry
University of Newcastle
À
À
Pt P and two different Pt S distances were found in all cases
À
À
À
(e.g., in 1t(x) Pt P(trans-S) 2.338, Pt P(trans-SH) 2.279, Pt S
Newcastle upon Tyne NE17RU (UK)
À
2.389, Pt SH 2.465 ä), and this reflects the different trans
influences of the sulfide and thiol ligands. The exo form is
slightly more stable than the endo form, although the exo/
¬
[**] Financial support is acknowledged from the Spanish Direccion
ƒ
General de Ensenanza Superior (DGES) under projects PB98-0916-
CO2-01 and BQU2001-1976, and from EPSRC (UK).
2776
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
1433-7851/02/4115-2776 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 15