Communication pubs.acs.org/Organometallics
Substituent Effects in the Cyclization of Yne-Diols Catalyzed by Gold Complexes Featuring L2/Z-Type Diphosphinoborane Ligands Fuyuhiko Inagaki,* Kenta Nakazawa, Kakeru Maeda, Tomoya Koseki, and Chisato Mukai Division of Pharmaceutical Sciences, Graduate School of Medical Sciences, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan S Supporting Information *
ABSTRACT: Gold(I) complexes featuring Z-type ligands introducing electronwithdrawing groups (EWG), Au(DPBF)Cl (4a) and Au(DPBCl)Cl (4b) (DPB = diphosphine-boron), have been synthesized and structurally characterized. These studies suggest that increasing the electron-withdrawing properties of the boron phenyl substituent only results in minor structural changes of the gold complexes. These complexes can be converted into Au(DPBF)SbF6 and Au(DPBCl)SbF6 by treatment of 4a,b with AgSbF6. The cationic complexes show interesting catalytic properties and readily promote the cyclization of yne-diols into unprecedented dioxabicyclo[3.2.2] derivatives.
O
n the basis of the definition by Green,1 there are three types of ligands: the L, X, and Z types. Both the X ligand forming a covalent bond with a metal (M−X) and L ligand providing two electrons to a metal (M←:L) are generally used for the construction of organometallic complexes and metalcatalyzed reactions. On the other hand, the σ-acceptor type Z ligand (M:→Z) metal complexes and their catalytic reactions are still limited.2−7 Therefore, the effects of the Z ligand on the neighboring metal center and the reactivity are unclear. Recent efforts in our laboratory focused on the syntheses of gold(I) complexes featuring the Z-type ligand and its applications to catalytic reactions on the basis of the alkynophilicity of gold8 for elucidating the Z-ligand effect. In a previous study,9 we succeeded in completing the X-ray crystallographic analysis of a cationic Au(I) complex featuring a Z ligand, [Au(DPB)]SbF6 (2; DPB = diphosphine-boron), which is derived from the Bourissou complex 1 (Au(DPB)Cl)10d in two steps (Scheme 1). In comparison to the distance between the Au and boron (Z ligand), the Au−B bond length of the cationic 2 (2.521 Å) was rather longer than that of the neutral 1 (2.335 Å). The extension of 0.186 Å suggests that the decreasing electron density on the gold(I) atom by cationization caused a noticeable weakening of the Au→B interaction,10 which suggests that the surrounding electronic environment of the Au−B bond would control the interaction of the Z ligand. We also found that the resulting [Au(DPB)]SbF6 (2) kept in a DCM solution in air for a long time (>7 days) formed PhB(OH)2 and [(Ph3P)2Au]SbF6 (3)11 via hydrolysis. This result indicated that the borane atom of the cationic complex 2 could work as a hard Lewis acid to react with the oxygen atom of H2O. On the basis of this information, we planned to synthesize the electron-withdrawing-group (EWG)-containing Z←M complex Au(DPBEWG)Cl (4) to achieve a stronger B← Au interaction (Z-ligand effect (a)). As the boron atom of the cationic [Au(DPBEWG)]SbF6 (5) derived from 4 would behave © XXXX American Chemical Society
Scheme 1. Gold Complexes Featuring Z-Type Ligands
as a more active Lewis acid (Z-ligand effect (b)) than 2 due to the electronic effect of the EWG, a novel reactivity between the alkyne and oxygen atom is also expected during the catalytic reactions. At the beginning of this study, Au(DPBEWG)Cl (4) was synthesized. For the introduction of the EWG, fluoride (F) and chloride (Cl) groups were selected. p-Fluorophenylboronic acid (6a) was converted into the corresponding dichloride 7a by treatment with BCl3 (Scheme 2).12 As the resulting 7a was highly unstable, product 7a was used in the next reaction without separation. The treatment of nBuLi, (2-bromophenyl)diphenylphosphine (8), and 7a formed DPBF, and then the addition of AuCl(SMe2) provided the desired Au(DPBF)Cl (4a) in 74% yield in two steps. According to the same Received: May 13, 2017
A
DOI: 10.1021/acs.organomet.7b00369 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Scheme 2. Syntheses of Au(DPBEWG)Cl (4a,b)
Our working hypothesis is shown in Figure 2. In this figure, [Au(DPBEWG)]SbF6 (5) was simplified to give a brief
procedure for the preparation of 4a, Au(DPBCl)Cl (4b) was also obtained in 97% overall yield from 8. Our continuous effort to evaluate the synthesized complexes 4a,b involved an X-ray crystallographic analysis. In both cases, recrystallization in DCM at 0 °C gave the corresponding single crystal. We anticipated that the Au−B bond length would be shortened as the electron-withdrawing effect of the para subsituent increases (σp: H [0] < F [+0.06] < Cl [+0.22]).13,14 However, the Au−B distances15 in 2 (2.335(5) Å), Au(DPBF)Cl (4a) (2.327(5) Å) ,and Au(DPBCl)Cl (4b) (2.321(2) Å) show negligible differences if one considers the accuracy of the crystallographic measurement (Figure 1). As
Figure 2. Plausible mechanisms of Z-ligand−metal-catalyzed cyclization of ynol.
explanation. There are two types of possible mechanisms: paths I and II. In case of the cationic 5, the interaction between B and Au would be weakened due to the cationization of Au. Therefore, the cationic Au of 5 might react with a typical triple bond unit like the alkynophilicity of the general Au(I)+ to form the intermediate A (path I). The recovery of the strong Au→B interaction for the coordination of Au and the alkyne should then activate the addition of OH to the alkyne unit to provide the cyclized intermediate B. Protodeauration containing other reactions could afford the product. On the other hand, a decrease in the Au→B interaction on the cationic 5 means that the boron atom also works as a hard Lewis acid. Actually, the 11 B NMR peak of Au(DPBF)Cl (4a) in CDCl3 was shifted from 29.1 to 17.9 ppm to add EtOH. Therefore, path II for the coordination of both B−OH and Au+−alkyne to afford the intermediate C would be another choice. The proximity effect might then activate the cyclization for the formation of the common cyclized intermediate B followed by the same processes of path I to give the product. In 2005, Genêt and co-workers reported the double 5-exo type cyclization of yne-diols to form dioxabicyclo[2.2.1] (5/5membered rings) derivatives with 2 mol % of AuCl or AuCl3. The larger 6/6-membered rings (dioxabicyclo[2.2.2]) were also constructed from the corresponding yne-diols with 2 mol % of AuCl via a double 6-exo type cyclization.21 However, the synthesis of the dioxabicyclo[3.2.2] derivative involving the construction of the 7-membered ring from the yne-diol was not reported.22 When we treated the yne-diol 9a with 2 mol % of Au(DPBF)SbF6, which was prepared from Au(DPBF)Cl and AgSbF6, in DCE at room temperature for 1 h, the desired dioxabicyclo[3.2.2] derivative 10a was observed in 79% yield (Table 1, entry 1). In the case of Au(DPBCl)SbF6, 10a was also provided in 32% yield (entry 2). Although the EWG-free Zligand catalysts Au(DPB)SbF6 and [Au(DPB)SbF6]2(cod) provided 10a, the chemical yields (20%, 15%) further decreased (entries 3 and 4). The addition of Au(PPh3)2SbF6 did not give the desired product (entry 5). This suggested that the reactivity for cyclization was caused by a Z-ligand effect and that the hydrolysis complex was not the active species. Reaction of Au(PPh3)SbF6 provided 10a in medium yields (48%, entry 6). Although AgSbF6 was used in a stock solution to prepare Au(DPBEWG)SbF6, the reaction using AgSbF6 hardly proceeded (entry 7). When AuCl was examined for this reaction, the
Figure 1. Pattern diagram of 2 and 4a,b: relationships among C1, B, and Au.
the electron-withdrawing properties of the boron phenyl substituent increase, a small but notable decrease in the C1− Au distance is observed (C1−Au = 3.041(3) Å for 2, 3.016(4) Å for 4a, and 2.983(3) Å for 4b). This slight contraction, which is accompanied by a minute decrease in the C1−Au−B angle (99.31(3)° for 2, 98.39(3)° for 4a, and 97.14(14)° for 4b) could indicate a direct interaction between the gold atom and the C1 carbon atom.16 With the required Au(DPBEWG)Cl complexes 4 in hand, applications to catalytic reactions were investigated next. In our previous study, [Au(DPB)]SbF6 (2) and the synthesized [Au(DPB)SbF6]2(cod) were useful catalysts for the intramolecular cycloisomerization of enynes,17−20 which indicated that the σ acceptor of the Z ligand boron activated the reactivity of the neighboring gold center. On the basis of previous studies regarding the enyne cyclizations and hydrolysis in Scheme 1, intramolecular cyclizations of the alkyne and OH were planned. B
DOI: 10.1021/acs.organomet.7b00369 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Table 1. Metal-Catalyzed Cyclization of Yne-Diol 9a
entry
catalyst
yield (%)
1 2 3 4 5 6 7 8
Au(DPBF)SbF6a Au(DPBCl)SbF6a Au(DPB)SbF6a [Au(DPB)SbF6]2(cod) Au(PPh3)2SbF6a Au(PPh3)SbF6a AgSbF6 AuCl
79 32 (58)b 20 15 n.r. 48 trace 14
Table 2. Au(I)-Catalyzed Cyclization of Yne-Diol 9
a Catalyst was prepared by mixing, filtering with Celite, and adding as a stock solution. bReacted for 48 h.
chemical yield was 14% (entry 8). On the basis of these results, Au(DPBF)SbF6 was the best catalyst for the cyclization of 9a, and Au(DPBCl)SbF6 gave a lesser result. Judging from the mechanisms in Figure 2, the electron-withdrawing effect might occur during the process of the intermediates A and C, which enabled us to construct the 7-membered ring. However, the stronger acceptor of Au(DPBCl)SbF6 might inhibit the other steps, such as intermediate B to product, which would lead to the lesser result. Indeed, a prolonged reaction time of 48 h improved the chemical yield to 58% (entry 2). Next, other yne-diol derivatives were examined for the syntheses of dioxabicyclo[3.2.2] derivatives. The results are described in Table 2. From the preliminary examinations in Table 1, both Au(DPBF)SbF6 and Au(DPBCl)SbF6 were applied in all cases. When the substrate 9b had an allyl group at R, Au(DPBF)SbF6 gave the desired product 10b in 69% yield (entry 1), and Au(DPBCl)SbF6 led to a low yield (19%, entry 2). In case of the methallyl group on R (9c) using both catalysts, the chemical yields of the cyclized 10c were observed to be similar (55 and 57%, entries 3 and 4). In the case of the cinnamyl substrate 9d, it was treated with catalysts, and the yield of 10d (91%) using Au(DPBCl)SbF6 was rather better than the result (81%) for Au(DPBF)SbF6 (entries 5 and 6). When we used the NHC-type catalyst Au(IPr)SbF6, the reaction time (1 h) was same as that for Au(DPBF)SbF6. However, the yield of 10d (65%) was rather lower than the results for Z-ligand catalysts presumably due to side reactions (e.g., hydration etc.). The butyl product 10e (78%) was also observed from the reaction with Au(DPBCl)SbF6 in a better yield (entry 7 vs entry 8). The substrates 9f−h bearing Bn, PMB, and p-CF3Bn groups on R provided the desired products 10f−h in 50−89% yields (entries 9−14). While the reactions of Au(DPBF)SbF6 were faster than those of Au(DPBCl)SbF6 in most cases, the results of Au(DPBCl)SbF6 gave better chemical yields of the products in comparison to those of Au(DPBF)SbF6 in some cases. In conclusion, we succeeded in synthesizing gold(I) complexes Au(DPBF)Cl (4a) and Au(DPBCl)Cl (4b) featuring Z-type ligands bearing electron-withdrawing phenyl substituents. The cationic Au(DPB F )SbF 6 and Au(DPB Cl )SbF 6 prepared from 4a,b and AgSbF6 were effective catalysts for the construction of 7-membered-ring systems. The yne-diols 9 were converted into dioxabicyclo[3.2.2] derivatives 10 in good
Catalyst was prepared by mixing, filtering with Celite, and adding as a stock solution. a
yields. Although the substituent constants σp of p-F (+0.06) and p-Cl (+0.22) were quite lower than those for any other EWG (e.g., CO2Et (+0.44), CF3 (+0.53), CN (+0.71)), the catalytic reactivity based on Table 1 was dramatically changed. Further investigations dealing with the introduction of a stronger EWG on the Z ligand are ongoing.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00369. Experimental procedures and characterization data for all products and crystallographic data for 4a,b (PDF) Accession Codes
CCDC 1542880 and 1542975 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The C
DOI: 10.1021/acs.organomet.7b00369 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
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Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*fi
[email protected]. ORCID
Fuyuhiko Inagaki: 0000-0003-2999-083X Chisato Mukai: 0000-0001-8621-1068 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the support of a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (Japan).
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DOI: 10.1021/acs.organomet.7b00369 Organometallics XXXX, XXX, XXX−XXX
