The Cu-catalyzed methoxylation and 17 benzyloxylation, introduced by the Buchwald group, at the 6,6'-positions furnished modified diols S -4a and S -4b, respectively. Isopropoxy substituted diol S -4c was obtained by debenzylation of S -4b and etherification.
Double alkylation of cyclopentadiene 7c,d,h followed by thermal rearrangement yielded a mixture of the precursors of cyclopentadienyl ligands S -6a-c and S 6a'-c'. Analogue complex S -K3d with 6,6'-unsubstituted SCp was obtained via a similar procedure from known dichloride S 15 5d.
Page 2 of 5 tion of the asymmetric environment of C2-symmetric Cp ligands. Encouraged by the above structural information, we next tested the catalytic activity of Rh-complexes S -K3.
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Since axially chiral biaryls are of high importance in natural products as well as asymmetric synthesis, we have previously demonstrated that the axially chiral biaryls could be obtained through an oxidative coupling from 1- naphthalenx 1-yl benzo[h]isoquinoline with olefins by Cramer Cp Rh catalyst R -K2a. However the average enantioselectivity 7g could only reach around er entry 1, Table 1. It was found that the steric bulkiness of the substituents at the 6,6'positions of 1,1'-spirobiindane scaffold exerts great influence on the reaction outcomes.
According to the steric 7d model suggested by Cramer and coworkers, relatively extended sidewalls are incorporated in SCp. Therefore, a better chiral environment around the Rh center in S -K3a might be expected. Determined by HPLC analysis on a chiral d e stationary phase. The results are taken from ref. Reaction was performed on 0. Figure 3. Ethylene moieties are omitted for clarity.
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To our delight, ACS Paragon Plus Environment Page 3 of 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Journal of the American Chemical Society Scheme 2. Notably, the reaction proceeded well with the challenging isoquinoline-derived substrate providing 10ja with good yield, albeit in moderate enantioselectivity.
The scope of olefins was also examined Scheme 3.
Almost for all the olefins tested, the oxidative coupling reaction worked with uniformly better enantioselectivity than that 7g obtained using R -K2a as the catalyst. In summary, we have developed a series of novel cyclopentadienyl ligands SCps based on 1,1'-spirobiindane scaffold and one of their corresponding Rh I complexes behaved as a superior catalyst in asymmetric oxidative coupling of biaryl derivatives with olefins.
Notably, excellent enantioselectivity could be achieved in most cases and the reaction could be run even at room temperature.
Expanding the applications of SCp ligands to other synthetically useful enantioselective transformations is currently under investigation in our laboratory. Substrate Scope: Olefinsa Supporting Information Experimental procedures and analysis data for all new compounds. Tetrahedron Lett. Organometallics , 15, Organometallics , 22, Organometallics , 26, Organometallics , 7, Organometallics , 23, Dalton Trans.
Organometallics , 33, Science , , Scope, limitations, and mechanism. Schmidt et al. Catalytic asymmetric approaches towards enantiomerically enriched diarylmethanols and diarylmethylamines.
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Scriban et al. Platinum-catalyzed asymmetric alkylation of secondary phosphines: enantioselective synthesis of P-stereogenic phosphines. Schwink et al. Douglass et al. USREE1 en. Clavier et al. Design and synthesis of new bidentate alkoxy-NHC ligands for enantioselective copper-catalyzed conjugate addition. Hilgraf et al. Hayashi et al.
Spiro skeletons: a class of privileged structure for chiral ligand design.
Asymmetric synthesis catalyzed by chiral ferrocenylphosphine-transition-metal complexes. Palladium-catalyzed asymmetric allylic amination. EPB1 en.
Brunel et al. Schaarschmidt et al.
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