Au(I) π-bis(tert-butyldimethylsilyl)acetylene triphenylphosphine complex, an effective pre-catalyst for Au(I)-catalyzed reactions

Abstract

A linear η²-alkyne π complex of [Ph₃PAu]⁺, namely Au(I) π-bis(tert-butyldimethylsilyl)acetylene triphenylphosphine complex (1), was prepared and characterized. The new complex is found to be shelf-stable and effective for a range of the Au(I)-catalyzed alkyne π activation reactions, including a glycosylation reaction with glycosyl ortho-alkynylbenzoates as donors.

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Introduction

The success of homogeneous gold catalysis with Au(I) complexes, such as [(L)Au][SbF₆] (L = R₃P, NHC, etc.), has been testified by numerous alkyne π activation towards nucleophilic attack in assembling structurally complex molecules (Fig. 1A).¹ Among these, we have developed an effective glycosylation protocol employing glycosyl ortho-alkynylbenzoates as donors (Fig. 1C);^2a–d the generality and versatility of this method has been demonstrated in the synthesis of complex oligosaccharides and glycoconjugates.² Nevertheless, the loading of the cationic Au(I) complex in these Au(I)-catalyzed reactions is usually ≥10 mol%, due partly to loss of the active Au(I) species via disproportionation.³ In only a few gold-catalyzed reactions high turnover numbers have been achieved,⁴ but for the important class of glycosylation reactions such systems have so far not been developed. In this regard, new Au(I) complexes that own better a reactivity-stability balance profile are highly desirable for practical use in the Au(I)-catalyzed reactions.

Fig. 1 (A) Au(I)-catalyzed alkyne π activation transformations. (B) Representative [R₃PAu(η²-alkyne)][SbF₆] pattern complexes. (C) The gold(I)-catalyzed glycosidation of glycosyl ortho-alkynylbenzoates.

Among the reported Au(I) coordination complexes,¹ the [(R₃P)Au(η²-alkyne)][SbF₆] pattern (R = alkyl or aryl group) has been suggested as a compelling choice based on several notable merits.⁵ The formation of a Au(I) π-[η²-alkyne] coordination bond could give improved stability to the designed pre-catalysts, for example [(t-Bu₃P)Au(η²-alkyne)][SbF₆] (alkyne = MeC≡C-t-Bu or Me₃SiC≡C-t-Bu) (Fig. 1B).⁵ In addition, this reversible coordination mode could regulate the release of the active catalytic Au(I)⁺ species, making the promoting activity of the catalyst in different reactions adjustable. While [Ph₃PAu]⁺ is one of the most applied gold(I) species for alkyne π activation in Au(I)-catalyzed reactions,^1a–c,5,6 to the best of our knowledge, a linear [Ph₃PAu]⁺ catalyst in this coordination pattern has not yet been prepared and fully characterized with catalytic activity. Herein, we report a shelf-stable linear η²-alkyne π complex [Ph₃PAu]⁺ species, namely Au(I) π-bis(tert-butyldimethylsilyl)acetylene triphenylphosphine, and its catalytic activity in the glycosylation reaction and a few other Au(I)-catalyzed reactions.

Results and discussion

Preparation of complex 1 and structural characterization

Given the alkyne ligand is activated once coordinated to Au(I) and could thus be readily attacked by nucleophiles, a sterically hindered alkyne is required in preparation of such a stable complex. We first tried to employ TMS-C≡C-TMS (2) as the ligand,⁷ however, the expected complex 3 made from PPh₃AuCl/AgSbF₆ (4) was found instable as indicated by the nonuniform ³¹P signals of the ³¹P NMR spectra at different temperatures. Thus, acetylene bearing bulky TBS groups, i.e., TBS-C≡C-TBS (5),⁸ was used as ligand. Upon addition of PPh₃AuCl/AgSbF₆ (4) into a solution of bis(tert-butyldimethylsilyl)acetylene (5) in dry CH₂Cl₂ at −70 °C and gradually warming the mixture up to rt, a sharp singlet at 39.0 ppm on ³¹P NMR showed up, which indicated clearly the formation of a single complex. After removal of the precipitated AgCl, concentration of the filtrate, and recrystallization of the resulting residue from CH₂Cl₂ and hexane (1 : 5) at 0 °C, the desired Au(I) π-bis(tert-butyldimethylsilyl) acetylene triphenyl phosphine complex (1) was obtained in 87% yield as colourless crystals, which was stable on a shelf for at least half a year (Scheme 1).

Scheme 1 Preparation of Au(I) π-[η²-alkyne] complexes 3 and 1.

The alkynyl carbon signal in complex 1 shifted downfield to 122.8 ppm and turned much wider than those of the free acetylene at 112.9 ppm on ¹³C NMR spectra. The overlapped signal of the two sp-hybridized carbon atoms demonstrated the existence of an symmetrical Au-[η²-acetylene] coordination, but not asymmetric bonding, i.e., η²→η¹ slippage along the alkyne. The ligand's Raman spectra also showed big differences after coordination with an [AuPPh₃]⁺ species, in which the ν_C≡C band signal shifted toward the red end at 1986 cm⁻¹ relative to the free ligand peak at 2103 cm⁻¹, which demonstrated the occurrence of the new π coordination.

The X-ray structure of 1 (CCDC 1043329) (Fig. 2) showed an ion-separated complex with perfect symmetrical coordination of the TBS-C≡C-TBS unit to the gold(I) centre intuitively. The C₁≡C₂ bond distance of 1.204(7) Å is identical to a typical C≡C bond length of 1.202(5) Å within the limit of error.^5b Because of the coordination effect of Au(I) as well as the steric hindrance from the PPh₃ group, the two TBS groups bend significantly to the back of the acetylene, leading to the Si₁–C₁–C₂ bond angle of 169.3(4)°, so that the C≡C bond is greatly sheltered by the t-butylsilyl groups from backside of the molecule. A search of the Cambridge Structural Database⁹ reveals that this titled complex is the first example of a linear [Ph₃PAu]⁺ η²-alkyne π complex structurally identified by X-ray diffraction.⁵

Fig. 2 The X-ray structure of complex 1 (CCDC 1043329). Thermal ellipsoids are at 30% probability; all hydrogen atoms and the solvent of crystallization have been omitted for clarity. Key bond lengths (Å) and angles (°): Au(1)–C(1) 2.226(4), Au(1)–P(1) 2.260(1), C(1)–C(2) 1.204(7); C(15)–P(1)–Au(1) 112.9(1), P(1)–Au(1)–C(1) 164.7(1), Au(1)–C(1)–Si(1) 116.6(2), Si(1)–C(1)–C(2) 169.3(4).

Glycosylation reactions catalyzed by complex 1

With the shelf-stable Au(I) complex 1 in hand, we investigated its catalytic activity in a few representative glycosylation reactions (Scheme 2 and Table 1). Using 2,3,4,6-tetra-O-benzoyl-D-glucopyranosyl ortho-cyclopropylethynylbenzoate (7)^2b as donor and 1,2;3,4-di-O-isopropylidene-D-galactoside (10) as acceptor, the condensation in the presence of complex 1 (0.1 eq., CH₂Cl₂, 5 Å molecular sieves (MS), RT) proceeded smoothly, providing the coupled β-disaccharide 14 in nearly quantitative yield (97%; entry 1) within 10 h. In comparison, the completion of a similar reaction with Ph₃PAuOTf (6) as the catalyst required 3 h (entry 2). This phenomenon could be rationalized assuming the reversible alkyne→[Ph₃PAu]⁺ coordination was in action, resulting in a lower concentration of the catalytically active species [Ph₃PAu]⁺ (than in Ph₃PAuOTf) which reduces the rate.

Scheme 2 The Au(I)-catalyzed glycosylation reactions.

Table 1 Glycosylation reactions catalyzed by complex 1, Ph₃PAuOTf (6), or PPh₃AuNTf₂ (13)

Entry	Au(I)	Donor	Acceptor	Product	Yield^a (α/β ratio)^b
a Isolated yields. b The α/β ratio was determined by ^1H NMR spectroscopic measurements.
1	1	7	10	14	97% (β only)
2	6	7	10	14	99% (β only)
3	1	8	10	15	92% (α/β = 1.0 : 1)
4	6	8	10	15	99% (α/β = 1.2 : 1)
5	1	9	11	16	94% (α/β = 6.8 : 1)
6	13	9	11	16	95% (α/β = 6.3 : 1)
7	1	9	10	17	96% (α/β = 7.4 : 1)
8	1	9	12	18	91% (α/β = 13.0 : 1)

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We then explored the catalytic activity of complex 1 in the glycosidation of armed donors, which are more active than those with a participating acyl group at 2-OH (such as 7). Not surprisingly, coupling of sugar alcohol 10 with 2,3,4,6-tetra-O-benzyl-D-glucopyranosyl cyclopropylethynylbenzoate (8) in the presence of complex 1 (0.1 eq., CH₂Cl₂, 5 Å MS, RT) completed within 10 h, providing the coupled glycoside 15 in 92% yield with no stereoselectivity (entry 3).¹⁰ These results are comparable to those obtained with Ph₃PAuOTf (6) as the catalyst (entry 4). With 3,4,6-tri-O-benzoyl-2-deoxy-D-glucopyranosyl cyclopropylethynyl benzoate 9 as donor,^11,12 the glycosylation of sugar alcohol 10 in the presence of complex 1 led to the coupled disaccharide 17 in excellent yield in favour of the thermodynamically more stable α anomer (α/β = 7.4/1; entry 7). Similar coupling of 9 with the simple alcohol pent-4-en-1-ol (11) provided the coupled glycoside 16 in high yield and α selectivity (entry 5). In comparison, similar results were obtained with PPh₃AuNTf₂ (13) as the catalyst (entry 6). Again, coupling of the 2-deoxy-sugar donor 9 with cholesterol (12) afforded the coupled glycoside 18 in high yield (91%) and excellent α selectivity (α/β = 13 : 1; entry 8).

Other reactions catalyzed by complex 1

To further demonstrate the catalytic profile of the Au(I) π-bis(tert-butyldimethylsilyl)acetylene triphenylphosphine complex (1), four well documented gold(I)-catalyzed alkyne π activation reactions were examined briefly under the catalysis of complex 1 to compare with the conventional gold(I) pre-catalyst PPh₃AuCl/AgSbF₆ (4), i.e. alkyne hydration,¹³ intramolecular rearrangement of alkynyl furan,¹⁴ 1,6-enyne rearrangement,¹⁵ and a tandem 3,3-rearrangement–Nazarov reaction.¹⁶

Thus, the alkyne substrate 19 in the presence of complex 1 (1 mol%) in wet 1,4-dioxane at RT, was activated and converted smoothly to the corresponding ketone 20 in a satisfactory yield of 93%, which is comparable to the reaction under catalysis of PPh₃AuCl/AgSbF₆ (4) (1 mol%) (Scheme 3). When the loading of the gold(I) pre-catalyst, either 1 or 4, was reduced to 0.1 mol%, the hydration of alkyne 19 proceeded sluggishly, leading to 93% recovery of the starting alkyne.

Scheme 3 Complex 1 catalyzed reactions A and B.

Complex 1 also proved to be effective in promoting the intramolecular conversion of alkynyl furan to phenol (Scheme 3).^14a,b In the presence of 3 mol% or 1 mol% of the pre-catalyst 1 (CH₂Cl₂, RT), alkynyl furan 21 was converted to the corresponding isoindoline 22 in 87% and 67% yield, respectively. These results demonstrate that complex 1 is comparable or even more effective than the Schmidbaur–Bayler binuclear gold(I) complex ([(Mes₃PAu)₂Cl]BF₄), 2.5 mol%, 65% yield) and PPh₃AuCl/AgSbF₆ (4) (3 mol%, 89%) in this phenol synthesis.

Echavarren et al. reported that N-propargyl-N-isoamylene toluene-4-sulfonylamine 23 underwent endo rearrangement to provide diene 24 in 96% yield under catalysis of PPh₃AuCl/AgSbF₆ (4) (2 mol%) in dry CH₂Cl₂ at RT (Scheme 4).^15a,b With 2 mol% of the Au(I) complex 1, a similar reaction proceeded smoothly to afford diene 24 in 95% yield. When the catalyst loading was decreased to 1 mol% or even 0.1 mol%, the reaction still performed cleanly and afforded the desired product in 83% and 50% (44% starting material recovered) yield, respectively. In this case, the shelf-stable Au(I) complex 1 showed excellent catalytic efficiency comparable to the freshly prepared PPh₃AuSbF₆ (4).

Scheme 4 Complex 1 catalyzed reactions C and D.

Zhang et al. reported the preparation of cyclopentenone 26 from enynyl acetate 25via a Au(I)-catalyzed tandem 3,3-rearrangement–Nazarov reaction (Scheme 4).¹⁶ In the presence of Au(I) complex 1 (1 mol%) in wet CH₂Cl₂ at RT, however, we were surprised to find that the conversion of 25 to 26 did not occur at all, whereas a similar transformation proceeded well in the presence of PPh₃AuCl/AgSbF₆ (1 mol%) to give cyclopentenone 26 in 95% yield. Our previous studies with a polystyrene-supported Au(I) catalyst showed that a Brönsted acid was crucial to facilitate this reaction.^15c In fact, upon addition of 1.0 eq. HSbF₆ (65% aq.) into the inert mixture of 25 and complex 1 (1 mol%) in CH₂Cl₂ at RT, 26 was obtained smoothly in 93% yield. In comparison, the reaction did not take place in the absence of complex 1. The present results imply that the release of catalytic [Ph₃PAu]⁺ from the coordinated complex 1 is much milder than from PPh₃AuCl/AgSbF₆ (4).

Conclusion

A linear η²-alkyne π complex of [Ph₃PAu]⁺, namely Au(I) π-bis(tert-butyldimethylsilyl)acetylene triphenylphosphine complex (1), was readily prepared and characterized. This shelf-stable complex 1 is convenient to prepare, store, and handle, while it has been shown to be an effective pre-catalyst for a variety of the Au(I)-catalyzed alkyne π activation reactions, including the glycosidation of ortho-alkynylbenzoates, the hydration of alkynes, the intramolecular rearrangement of alkynyl furan, the 1,6-enyne rearrangement, and the tandem 3,3-rearrangement–Nazarov reaction. The mild release of catalytic [Ph₃PAu]⁺ species in complex 1 might be beneficial in catalyzing certain transformations involving selectivities, and this is a topic of our current interest.

Experimental section

The preparation of complex 1

To a 5 mL flask containing the commercially available TBS-C≡CH (0.5 mL) and dry THF (freshly distilled with Na, 3 mL) was added n-BuLi solution (2.5 M in hexane, 1.2 mL) under argon at −15 °C. After stirring for 1 h, a solution of TBSCl (484 mg in 3 mL dry THF) was introduced into the mixture. The mixture was slowly warmed to room temperature and stirred for another 1 h, and was then quenched by H₂O (1 mL). The resulting mixture was extracted with hexane. The organic layer was dried over Na₂SO₄ and concentrated. The residue was recrystallized from MeOH to give the desired compound 5 as a white solid (444.7 mg, 65%): ¹H NMR (400 MHz, CDCl₃) δ 0.94 (s, 18H), 0.10 (s, 12H); ¹³C NMR (126 MHz, CDCl₃) δ 112.9, 26.0, 16.5, −4.7; MS-EI⁺ 197 (100%), 155 (30%), 198 (22%), 199 (9%), 73 (8%), 254 (7%), 141 (6%), 156 (6%); HRMS-EI⁺ calcd for C₁₄H₃₀Si₂ ([M]⁺) 254.1886, found 254.1885; Raman (532 nm, cm⁻¹) ν_C≡C 2103.

A 5 mL flask containing TBS-C≡C-TBS (79.5 mg, 0.31 mmol), PPh₃AuCl (76.5 mg, 0.15 mmol) and AgSbF₆ (52.8 mg, 0.15 mmol) was cooled to −70 °C under argon. Then dry CH₂Cl₂ (4 mL, freshly distilled with CaH₂) was introduced via a syringe. After stirring for 5 h, the mixture was warmed to room temperature slowly. The resulting AgCl was removed by cotton filtration and the filtrate was concentrated under reduced pressure. The residue was dissolved in CH₂Cl₂ (l mL) and then hexane (5 mL) was added. The white precipitate was collected to give the desired complex 1 as a white solid (127.9 mg, 87%). The sample for X-ray diffraction analysis was obtained by recrystallization from CH₂Cl₂–hexane via vapor diffusion at 0 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.69–7.55 (m, 9H), 7.49 (dd, J = 13.8, 7.7 Hz, 6H), 1.01 (s, 18H), 0.32 (s, 12H); ¹³C NMR (101 MHz, CDCl₃) δ 133.8 (d, J = 2.7 Hz), 133.3 (d, J = 2.7 Hz), 130.1 (d, J = 12.3 Hz), 125.9 (d, J = 64.0 Hz), 122.8 (s), 23.9 (s), 16.9 (s), −4.4 (s); ³¹P NMR (400 MHz, CDCl₃) δ 39.0; HRMS (MALDI) calcd C₃₂H₄₅AuPSi₂ ([M − SbF₆]⁺) 713.2457, found 713.2449; Raman (532 nm, cm⁻¹) ν_C≡C 1986.

General procedure for the glycosylation reactions (14–18)

To a 25 mL flask containing the donor (0.10 mmol, 1.0 eq.), the acceptor (0.12 mmol, 1.2 eq.) and activated 5 Å molecular sieves (weight equal to the combined weight of the donor and the acceptor), was added dry CH₂Cl₂ to maintain a concentration of 0.05 M under Ar. The gold(I) catalyst 1 (0.0102 mmol, 0.1 eq.) was added 30 minutes later. The mixture was stirred until TLC indicated disappearance of the donor. The molecular sieves were filtered off over Celite and washed with CH₂Cl₂ twice. The filtrate was concentrated under reduced pressure and chromatographed with a gradient of petroleum ether–EtOAc to afford the coupled glycosides (14–18).

The hydration of alkyne 19

To a 5 mL Schlenk tube was added a solution of gold(I) catalyst 1 (2.1 mg in 0.5 mL CH₂Cl₂, 0.45 mL), and the solvent was removed under reduced pressure. Then compound 19 (35.5 mg, 2.0 mmol) was added, and the reaction system was protected with Ar. Wet 1,4-dioxane (36 μL H₂O in 1 mL dry 1,4-dioxane, 0.3 mL) was added to the mixture. After stirring for 12 h, the mixture was concentrated in vacuo. The residue was chromatographed (petroleum ether–EtOAc, 8 : 1) to yield compound 20 as a colorless oil (36.6 mg, 93%): ¹H NMR (500 MHz, CDCl₃) δ 7.42–7.39 (m, 5H), 5.98 (s, 1H), 2.19 (s, 3H), 2.11 (s, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 201.59, 170.16, 133.10, 129.30, 129.01, 127.99, 80.87, 26.04, 20.63; MS-EI⁺ 149 (100%), 107 (96%), 43 (32%), 79 (21%), 77 (14%), 150 (11%), 105 (11%), 108 (10%); HRMS-EI⁺ calcd for C₁₁H₁₂O₃ ([M]⁺) 192.0786, found 192.0788.

The Hashmi transformation of alkynyl furan 21 to phenol 5-methyl-2-tosylisoindolin-4-ol (22)

To a 25 mL flask containing compound 21 (60.7 mg, 0.2 mmol) and gold(I) catalyst 1 (5.5 mg, 0.006 mmol) was added CH₂Cl₂ (2 mL) at room temperature. After stirring for 1 h, the mixture was concentrated in vacuo. The residue was purified by silica gel chromatography (petroleum ether–EtOAc–CH₂Cl₂, 5 : 1 : 1) to give compound 22 as a white solid (52.8 mg, 87%): ¹H NMR (400 MHz, d₆-DMSO) δ 8.97 (s, 1H), 7.74 (d, J = 8.2 Hz, 2H), 7.43 (d, J = 8.0 Hz, 2H), 6.96 (d, J = 7.6 Hz, 1H), 6.59 (d, J = 7.5 Hz, 1H), 4.49 (s, 2H), 4.46 (s, 2H), 2.36 (s, 3H), 2.08 (s, 3H); ¹³C NMR (101 MHz, d₆-DMSO) δ 149.8, 143.7, 134.8, 132.9, 130.5, 123.0, 127.4, 123.4, 122.1, 113.3, 53.7, 51.9, 20.9, 15.7; HRMS-ESI⁺ calcd for C₁₆H₁₇NO₃SNa ([M + Na]⁺) 326.0821, found 326.0827.

Synthesis of 3-(propan-2-ylidene)-1-tosyl-1,2,3,6-tetrahydro-pyridine (24) via the 1,6-enyne rearrangement of 23

To a 25 mL flask containing compound 23 (54.7 mg, 0.2 mmol) and gold(I) catalyst 1 (3.79 mg, 0.004 mmol) was added dry CH₂Cl₂ (2 mL) at room temperature under Ar. After stirring for 30 minutes, the solution was concentrated in vacuo. The mixture was purified by silica gel chromatography (petroleum ether–EtOAc–CH₂Cl₂, 10 : 1 : 1) to give compound 24 as a white solid (52.2 mg, 95%): ¹H NMR (300 MHz, CDCl₃) δ 7.65 (d, J = 8.0 Hz, 2H), 7.28 (d, J = 6.7 Hz, 2H), 6.34 (d, J = 10.2 Hz, 1H), 5.53 (d, J = 10.0 Hz, 1H), 3.90 (s, 2H), 3.77 (s, 2H), 2.42 (s, 3H), 1.77 (s, 3H), 1.68 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ 143.3, 134.1, 129.9, 129.3, 127.6, 124.5, 122.3, 120.9, 45.03, 45.00, 21.5, 20.3, 19.6; HRMS-ESI⁺ calcd for C₁₅H₁₉NO₂SNa ([M + Na]⁺) 300.1029, found 300.1034.

The synthesis of 3-cyclohexyl-5-methylcyclopent-2-enone (26) via a tandem 3,3-rearrangement–Nazarov reaction of 25

To a 25 mL flask containing compound 25 (46.3 mg, 0.2 mmol), HSbF₆ (∼65% aq., 76.5 mg), and gold(I) catalyst 1 (1.86 mg, 0.002 mmol), was added wet CH₂Cl₂ (2 mL) at room temperature. After being stirred for 2 h, the mixture was concentrated in vacuo. The residue was purified by silica gel chromatography (petroleum ether–EtOAc, 8 : 1) to give compound 26 as a colorless oil (35.0 mg, 93%): ¹H NMR (400 MHz, CDCl₃) δ 5.88 (s, 1H), 2.85 (dd, J = 18.3, 6.7 Hz, 1H), 2.40 (pd, J = 7.3, 2.4 Hz, 1H), 2.28 (t, J = 11.1 Hz, 1H), 2.19 (d, J = 18.4 Hz, 1H), 1.89 (d, J = 12.6 Hz, 2H), 1.81 (d, J = 12.5 Hz, 2H), 1.73 (d, J = 12.7 Hz, 1H), 1.43–1.19 (m, 5H), 1.17 (d, J = 7.5 Hz, 3H); ¹³C NMR (126 MHz, CDCl₃) δ 212.74, 185.52, 126.47, 41.75, 40.35, 38.41, 31.17, 31.14, 25.96, 25.95, 25.90, 16.42. MS-EI⁺: 178 (100%), 122 (97%), 136 (38%), 107 (37%), 163 (26%), 79 (24%), 95 (20%), 93 (19%); HRMS-EI⁺ calcd for C₁₂H₁₈O ([M]⁺) 178.1358, found 178.1356.

Acknowledgements

Financial support from the Ministry of Science and Technology of China (2012ZX09502-002) and the National Natural Science Foundation of China (21432012 and 21102169) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1043329. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5qo00023h

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