Article pubs.acs.org/Organometallics
Water Addition to Alkynes Promoted by a Dicationic Platinum(II) Complex† Spring Carlisle, Angel Matta, Homer Valles, Jason B. Bracken, Mayra Miranda, Jinah Yoo, and Christine Hahn* Department of Physical Sciences, University of Texas of the Permian Basin, 4901 East University Boulevard, Odessa, Texas 79762, United States S Supporting Information *
ABSTRACT: Dicationic platinum alkyne complexes were generated in situ by substitution of ethylene in [Pt(PNP)(C 2H4)](BF4)2 (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine) with alkynes at low temperature. The dicationic acetylene complex readily adds water to form the platina-acetaldehyde complex [Pt(PNP)(CH 2CHO)]BF4, which was analyzed by X-ray diffraction. 1 H and 31P NMR studies were performed to elucidate the mechanism of formation of [Pt(PNP)(CH 2CHO)]BF4. A reversible acid−base equilibrium between the platina-acetaldehyde and the corresponding η 2-vinyl alcohol complex [Pt(PNP)(CH2 CHOH)]2+ was observed. The complexes with terminal alkynes (propyne and 1-hexyne) gave with water a mixture of Markovnikov and anti-Markovnikov addition products [Pt(PNP){CH 2C(O)R1}]BF4 and [Pt(PNP){C(O)CH2R1}]BF4 (R1 = Me, n-Bu) in a ratio of 1:4. However, with tert -butyl- and phenylacetylene C−H bond activation occurred, yielding the σ-alkynyl complexes [Pt(PNP)(CCR2)]BF4 (R2 = t-Bu, Ph). Complexes with internal alkynes R3CCR4 (R3 = Me; R4 = Me, n-Pr) react with water and form the corresponding β-ketonyl complexes [Pt(PNP){CHR3C(O)R4}]BF4. Moderate regioselectivity was observed for 2-hexyne.
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alkynes in order to replace the traditional acid and the highly toxic mercury catalysts. 1 The first platinum(II)-catalyzed hydration of alkynes was reported by Chatt and Duncanson using Na2PtCl4·H2O as catalyst. 2 Later, Jennings and coworkers found that Zeise’s dimer and Pt II halides efficiently catalyze the hydration of unactivated alkynes. 3 Terminal alkynes were hydrated with Markovnikov selectivity and unsymmetrical internal alkynes with moderate regioselectivity. Atwood and co-workers have synthesized water-soluble platinum(II) complexes containing sulfonated mono- and bidentate phosphine ligands. 4 Furthermore, platinum(IV) chloride was shown to catalyze the hydration of various alkynes at 1.38 MPa CO pressure. 5 Also, for the study of hydrative cyclizations of allenynes and trialkynes PtCl 2 has been frequently used as catalyst. 6 While various new gold complexes were reported more recently as very efficient catalysts,7 there seem to be currently
INTRODUCTION
The hydration of alkynes is an important reaction to synthesize aldehydes or ketones, because it is atom-economical and inexpensive starting materials are used. With terminal alkynes either Markovnikov or anti-Markovnikov products can be formed, while with unsymmetrical alkynes two different regioisomers can be expected (Scheme 1). Scheme 1. Hydration of Terminal and Internal Alkynes
In the last few decades increasing efforts have been made to develop new transition-metal catalysts for the hydration of © 2011 American Chemical Society
Received: August 23, 2011 Published: November 14, 2011 6446
dx.doi.org/10.1021/om2007908 | Organometallics 2011, 30, 6446−6457
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no further efforts to design structurally new complexes of platinum(II) and to study their catalytic activity, mechanistic details, or reaction intermediates. Many questions concerning transition-metal-catalyzed hydration of alkynes remain unanswered. For example, the regioselectivity of the hydration of unfunctionalized internal alkynes is still a challenge. The catalytic anti-Markovnikov addition of water to terminal alkynes to form aldehydes succeeded only with ruthenium catalysts.8 In analogy to our previous studies of electrophilic activation of alkenes by coordination in dicationic platinum(II) complexes of the type [Pt(PNP)(CHRCHR)](BF4)2 (PNP = 2,6-bis(diphenylphosphinomethyl)pyridine) and their reactions with water and other weakly basic nucleophiles,9 we explored also the activation of alkynes at the Pt(PNP)2+ complex fragment. Since water addition to the coordinated C−C double bond gave the stable β-hydroxyalkyl complexes [Pt(PNP)(CH2CHROH)]BF4 in a stoichiometric reaction, it was interesting to study similarly the products resulting from water addition to the coordinated C−C triple bond. In this paper, we report the generation of dicationic platinum(II) alkyne complexes of the type [Pt(PNP)(RC CR′)](BF4)2 with acetylene and terminal and internal alkynes. These complexes were reacted with water and gave platinaacetaldehyde, ketonyl, acyl, or alkynyl σ complexes, which were isolated and characterized by NMR, IR, and X-ray structure analysis and studied in solution. These results provide not only support for the relevant intermediates and reaction steps suggested by Jennings and co-workers3 for the platinum(II)catalyzed alkyne hydration but also demonstrate the possibility of the anti-Markovnikov addition of water to terminal alkynes using a platinum(II) complex.
reaction conditions, but this time 15 equiv of water was added to the solution of the ethylene complex prior to the addition of acetylene. Water addition to the coordinated ethylene9 is very slow at low temperature and is not a competing side reaction. In contrast, the far more reactive acetylene complex [Pt(PNP)(C2H2)](BF4)2 (I) (eq 1) immediately reacts upon its
formation with the excess water provided in advance. 12 This prevents practically all unwanted side reactions. Under these optimized conditions (eq 1) the platina-acetaldehyde complex [Pt(PNP)(CH2CHO)]BF4 (1) was isolated in analytically pure form. The platina-acetaldehyde complex [Pt(PNP)(CH2CHO)]BF4 (1) was characterized by NMR and IR spectroscopy as well as by X-ray crystal structure analysis. The 1H NMR spectrum shows a characteristic doublet of triplets for the PtCH 2 group at δ 2.94 with platinum satellites. The triplet at δ 8.95 is diagnostic for an aldehyde proton. In the 13C NMR spectrum a signal at δ 201.7 is observed for the carbonyl carbon atom. The IR bands at ν̃ 1673 and 2722 cm−1 are assigned to the CO and C−H stretching frequencies of the aldehyde functional group. Similar IR bands were observed for the CO group in other platinaacetaldehyde complexes.13,14 X-ray Structure Analysis of [Pt(PNP)(CH 2CHO)]BF4 (1). Suitable crystals for X-ray structure analysis were obtained by overlayering a solution of complex 1 in CH2Cl2 with diethyl ether. An ORTEP drawing of the complex cation is shown in Figure 1. The complex cation 1 has a square-planar coordination
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RESULTS AND DISCUSSION Reaction of [Pt(PNP)(C2H4)](BF4)2 with Acetylene and Water. A common method to generate platinum(II) alkyne complexes is the displacement of an alkene by the alkyne, since the alkyne is a stronger donor ligand.10 When the ethylene complex [Pt(PNP)(C2H4)](BF4)29 was reacted with an excess of acetylene in CD2Cl2 at room temperature, the solution turned immediately dark brown, indicating fast acetylene oligoor polymerization. The 1H NMR spectra of the reaction solution showed a number of signals between 0 and 5 ppm, which can be assigned to oligomerization products. This is not unusual for platinum(II) complexes.10 The formation of uncontrolled oligomerization byproduct could be strongly reduced by performing the reaction at lower temperature with only a slight excess of acetylene. When 3 equiv of acetylene was added to the ethylene complex in CH2Cl2 at −78 °C, the reaction solution turned only light brown. After 15 min the reaction mixture was warmed to room temperature and a light brown solid was precipitated by addition of diethyl ether. However, the 1H and 31P NMR spectra of the isolated product show the formation of a mixture of compounds. A dicationic acetylene complex could not be identified among them. This is not surprising, since it is expected to be extremely reactive and can easily undergo numerous side reactions. There are also no examples known for isolated platinum acetylene complexes.11 However, in the 1H NMR spectrum of the compound mixture a characteristic set of signals could be identified. It suggests the formation of the platina-acetaldehyde complex [Pt(PNP)(CH2CHO)]BF4 (1) as a result of reaction of the coordinated acetylene with water which was adventitiously present in solution. The reaction was repeated under the same
Figure 1. ORTEP drawing of the complex cation 1. Selected bond lengths (Å) and angles (deg): Pt−P1 = 2.2693(9), Pt−P2 = 2.3033(10), Pt−N = 2.112(3), Pt−C1 = 2.132(3), C1−C2 = 1.410(6), C2−O = 1.218(5); P1−Pt−P2 = 165.90(3), N−Pt−P1 = 82.82(9), N−Pt−P2 = 83.15(9), N−Pt−C1 = 178.10(12), C1−Pt− P1 = 95.34(10), C1−Pt−P2 = 98.70(10), Pt−C1−C2 = 102.2(2), C1−C2−O = 126.7(4).
geometry with the η 1-acetaldehyde ligand in the fourth position. The Pt−C1 (2.132(3) Å) and C1−C2 (1.410(6) Å) bond lengths are similar to those found in [Pt(PNP){CH2CH2C6H2(OCH3)3}]SbF615 (2.157(9) and 1.421(16) Å; cf. Table 1), indicating a Pt−C σ bond and a C−C single bond type. The C2−O bond length of 1.218(5) Å is characteristic for a C−O double bond, and the C1−C2−O angle of 126.7° suggests the sp2 character of the C2 atom. These structural features agree with the spectroscopic data. 6447
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Table 1. Selected Bond Lengths (Å) and Angles (deg) of [Pt(PNP)(CH 2CHO)]BF4 (1) and Those of Structurally Related Complexesa [Pt(PNP)(CH2CHO)]BF4 (1) [PtBr(CH3)(dmphen)(CH2CHOH)]b [PtCl(acac)(CH2CHOH)]c [PtIII2(NH3)4{(CH3)3CCONH}2(CH2CHO)](NO3)3·H2Od [Pt(PNP)(CH2CH2)](BF4)2e [Pt(PNP){CH2CH2C6H2(OCH3)3}]SbF6f a b
Pt−C1
Pt−C2
C1−C2
C2−O
Pt−C1−C2
C1−C2−O
2.132(3) 2.081(9) 2.098(10) 2.121(9) 2.181(8) 2.157(9)
2.7937(3) 2.062(9) 2.222(9) 2.93 2.180(6) 3.06(1)
1.410(6) 1.45(1) 1.387(5) 1.35(2) 1.359(10) 1.421(16)
1.218(5) 1.41(1) 1.297(12) 1.30(2)
102.2(2) 70(1) 76.3 113.5 71.8(4) 116.3(7)
126.7(4) 121(1) 121.9(9) n.a.
For the sake of uniformity the atom labeling was adopted for all complexes to that of complex 1 and may not agree with the original labeling. dmphen = 2,9-dimethyl-1,10-phenanthroline.15 cReference 17. dReference 14. eReference 9. fReference 15.
It is interesting to compare the structure and bonding in complex 1 with those of the following platinum complexes: [PtBr(CH3)(dmphen)(CH2CHOH)],16a [PtCl(acac)(CH2 CHOH)],17 and [PtIII2(NH3)4{(CH3)3CCONH}2(CH2CHO)](NO3)3 ·H2O14 (cf. Table 1), which display structural features intermediate between vinyl alcohol π complex A and platinaacetaldehyde σ complex C (Scheme 2). Structures A and C are
the coordinated vinyl alcohol in [PtBr(CH3)(dmphen)(CH2 CHOH)] is lengthened due to the relatively strong π backdonation, which is expected for a relatively electron-rich fivecoordinate platinum(II) complex. In contrast, the solid-state structure of [PtCl(acac)(CH 2 CHOH)] reported by Cotton et al.17 displays a vinyl alcohol π complex with a strong geometric bias to the intermediate structure B (Scheme 2). In this structure the Pt−C2 bond (2.222(9) Å) is significantly longer than Pt−C1 (2.098(10) Å). The C2−O distance (1.29(12) Å) seems to be intermediate between C−O single- and a double-bond lengths.18,19 The C−C bond length of 1.387(5) Å of the vinyl alcohol in [PtCl(acac)(CH2CHOH)] lies between the lengths observed for the coordinated C−C double bonds in [PtBr(CH 3)(dmphen)(CH2CHOH)]16a and [Pt(PNP)(C2H4)](BF4)29 (cf. Table 1). On the other hand, the platina-acetaldehyde σ complex [PtIII2(NH3)4{(CH3)3CCONH}2(CH2CHO)](NO3)3·H2O reported by Matsumoto14 represents essentially structure C but with a bias to structure B. The Pt−C1 and Pt−C2 distances (2.121(9) and 2.93 Å) are similar to those observed for complex 1. The Pt−C1−C2 bond angle of 113.5° lies between the values found for η 1 complexes 1 and [Pt(PNP){CH2CH2C6H2(OCH3)3}]SbF615 (cf. Table 1) and indicates a σ-alkyl bond of the CH2 group. However, the C1−C2 bond length in the PtIII complex at 1.35(2) Å is shorter than a C−C single bond, while the C2−O distance (1.30(2) Å) appears somewhat longer than a C−O double-bond length.
Scheme 2. Structural Relationship between Vinyl Alcohol π Complex and Platina-Acetaldehyde σ Complex
related through an acid−base equilibrium which was studied in solution,13,14,16 while the intermediate structure B is considered to be important in the mechanism of the alkyne hydration (cf. Schemes 2 and 3; see discussion below). The five-coordinate complex [PtBr(CH3)(dmphen)(CH2 CHOH)] is one of the few structurally characterized η 2-vinyl alcohol complexes of platinum16 and provides an example for structure A (Scheme 2). The vinyl alcohol is coordinated with essentially equal Pt−C1 and Pt−C2 bond lengths, as similarly found for the π-coordinated ethylene in [Pt(PNP)(C2H4)](BF4)2 (cf. Table 1).9 The C−C double bond (1.45(1) Å) of
Scheme 3. Proposed Mechanism for the Formation of Complex 1, Where All Nonhighlighted Species Are Proposed Intermediates
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acidity of the corresponding vinyl alcohol PtIII complex is expected. Similarly, intermediate IV should also display a relatively high acidity due to the high positive complex charge, and the equilibrium lies far on the side of complex 1. This may explain the generally low stability of IV and the failure to isolate it. By addition of Na2CO3 to the mixture of complex 1 and HBF4·Et2O the equilibrium was immediately shifted back to the pure form of complex 1. The 1H NMR spectrum of this solution showed the restored signals for complex 1. The similar conversion of five-coordinate vinyl alcohol PtII π complexes to the corresponding square-planar platina-acetaldehyde σ complexes by addition of KOH was demonstrated by Panunzi16a and De Felice16b previously. While the addition of HCl to those square-planar platinaacetaldehyde σ complexes regenerated the corresponding fivecoordinate vinyl alcohol PtII π complexes,15 the addition of HCl to complex 1 resulted in the formation of [Pt(PNP)Cl]+ and acetaldehyde (eq 2). This reaction is fast and complete after
A comparison of these structures may provide some insight into the progress of the acid−base equilibrium of the vinyl alcohol π complex and the platina-acetaldehyde σ-complex, where complex 1 represents “pure” structure C. NMR Studies of Complex 1 in Solution. According to the mechanism for the alkyne hydration proposed by Jennings and co-workers,3 it is suggested that the formation of complex 1 involves the nucleophilic addition of water to the in situ generated acetylene complex I (cf. Scheme 3). The β-hydroxonium vinyl species II is most likely the primary addition product from which the proton dissociates, and the η 1-enol species III is formed, which tautomerizes to the more stable platinaacetaldehyde complex 1. It was interesting to see if the proposed equilibrium reaction can be shifted back by addition of HBF4 and if in this way the dicationic acetylene π complex I can be generated back and spectroscopically characterized. It should be noted that the methanol addition to [Pt(PNP)(CH 2CH2)](BF 4) 2 could be completely reversed by addition of HBF 4 to [Pt(PNP)(CH2CH 2OCH3)]BF 4.9 To a solution of complex 1 in CD2Cl2 were added 3 drops of HBF4·Et2O. In the 1H NMR spectrum the signal at δ 8.95 is no longer observed and the characteristic doublet of triplets for the PtCH2 group has shifted downfield from δ 2.94 to δ 3.78 and the Pt−H coupling constant 2JPt−H has decreased from 108 to 81 Hz (see Figure S2 in the Supporting Information). This indicates the formation of the η 2-enol complex IV rather than the acetylene complex I, because very similar 1H NMR data were observed for K[PtCl(acac)(CH2CHO)] and the corresponding η 2-enol complex (cf. Table 2).13b All attempts to isolate the η 2-vinyl alcohol intermediate IV, however, failed.
mixing (
