Alphonsine Ngo Ndimba

Claudia Lalli

Gilles Argouarch

Thierry Roisnel

Comptes Rendus Chimie

Comptes RendusChimieAlphonsine Ngo Ndimba, Thierry Roisnel, Gilles Argouarchand Claudia LalliFirst application of chiral phosphotriesters in asymmetric metalcatalysis: enantioselective Zn-catalyzed hydrosilylation of ketones in thepresence of BINOL-derived phosphatesVolume 24, issue 1 (2021), p. 77-81.<https://doi.org/10.5802/crchim.67>© Académie des sciences, Paris and the authors, 2021.Some rights reserved.This article is licensed under theCreative Commons Attribution 4.0 International License.http://creativecommons.org/licenses/by/4.0/Les Comptes Rendus. Chimie sont membres duCentre Mersenne pour l’édition scientifique ouvertewww.centre-mersenne.orgComptes RendusChimie2021, 24, n 1, p. 77-81https://doi.org/10.5802/crchim.67Preliminary communication / Communication préliminaireFirst application of chiral phosphotriesters inasymmetric metal catalysis: enantioselectiveZn-catalyzed hydrosilylation of ketones in thepresence of BINOL-derived phosphatesAlphonsine Ngo Ndimbaa, Thierry Roisnela, Gilles Argouarcha and Claudia Lalli∗, aa Univ Rennes, CNRS, ISCR UMR 6226, F-35000 Rennes, FranceE-mails: alphonsinendimba@gmail.com (A. Ngo Ndimba),thierry.roisnel@univ-rennes1.fr (T. Roisnel), gilles.argouarch@univ-rennes1.fr(G. Argouarch), claudia.lalli@univ-rennes1.fr (C. Lalli)Abstract. Chiral phosphotriesters are an unexplored class of ligands in metal catalysis. We wish todisclose herein the foremost application of novel chiral BINOL-derived mono- and bisphosphates inasymmetric zinc-catalyzed hydrosilylation of ketones. Corresponding alcohols were obtained in up to92% yield and 34% ee.Keywords. Chiral phosphotriesters, BINOL, Asymmetric catalysis, Enantioselective hydrosilylation,Ketones.Manuscript received 18th December 2020, revised 26th January 2021, accepted 27th January 2021.Chiral pure phosphotriesters (phosphates) are ofgreat rarity in catalysis, compared to phosphanes [1],phosphites [2], phosphoric acids or phosphate metalsalts [3–5], and phosphoramidites [6]. Indeed, todate only two examples of their use as organocat-alysts are known, which exploit the axial chiralityof the BINOL backbone [7,8]. Ishihara’s group hasreported the N -iodolactonization of 4-arylmethyl-4-pentenoic acids catalyzed by BINOL-derived tri-aryl phosphates as nucleophilic catalysts [7]. Luoand coworkers have applied BINOL-derived tritylphosphates in carbocation catalysis, effective for∗Corresponding author.Friedel–Crafts, inverse electron-demanding hetero-Diels–Alder, and carbonyl-ene reactions [8].When we started to be interested in pure phos-photriesters as ligands for transition metal catal-ysis, their use for this purpose was almost un-known and limited to only one example, namelythe Pd/phosphate-catalyzed oxidative coupling be-tween arylboronic acids and alkynes, reported byMiura and coworkers [9]. We have recently con-tributed to this field by disclosing the versatile roleof triphenylphosphate in two racemic processes:the Ti-promoted diethylzinc addition to aldehydesand the Zn-catalyzed hydrosilylation of ketones [10].For thes latter, in a seminal contribution in 1999,Mimoun’s group has disclosed for the first time theISSN (electronic) : 1878-1543 https://comptes-rendus.academie-sciences.fr/chimie/78 Alphonsine Ngo Ndimba et al.Scheme 1. Previously reported chiral phosphotriesters A–F.use of Zn/chiral secondary diamine complexes forthe reduction of ketones in the presence of hydrosi-lanes, which enable them to reach 80% ee [11]. Sincethen, other systems still based on N -containing lig-ands have been reported [12–31], incorporating ei-ther diversified diamines or diimines, N ,S- or N ,O-chelating ligands, or Pybox, and excellent conver-sions and enantiomeric excesses on a wide range ofsubstrates were thus achieved.We have chosen to address our efforts towardthe challenge of introducing chiral phosphotriestersas ligands in the Zn-catalyzed enantioselective hy-drosilylation of prochiral ketones. To the best of ourknowledge, this is the first example of their applica-tion in the enantioselective version of this transfor-mation, and more widely no examples are providedon the use of chiral phosphotriesters as ligands intransition metal catalysis.To achieve this, the BINOL-based phosphates A–F (Scheme 1), which have been published elsewhereby us [32], were employed, together with the newlyprepared compounds G–I (Scheme 2).The reaction of 3-I-(R)-BINOL [33] (1) with com-mercially available phenyl phosphorodichloridate(2) in the presence of Et3N smoothly led to the forma-tion of a 50:50 mixture of two diastereomers G and Hin 90% yield, separable by column chromatographyon silica gel. Single-crystal X-ray analyses confirmedtheir structures and unambiguously established thecentral chirality at the P-atom to be (Sp) and (Rp),for G and H, respectively (Figure 1) 1. Following adifferent synthetic pathway [32], pre-mixing in basicconditions, PCl3 with (S)-BINOL (3), delivered thecorresponding phosphorochloridite, which in turnreacted with ethylene glycol to give the desired bis-phosphite. The latter underwent in situ oxidationunder mild conditions with a 5% NaOCl aqueoussolution to afford the corresponding phosphotriesterI in 58% overall yield.With these chiral ligands in hand, we turnedour attention toward their application in the Zn-catalyzed hydrosilylation of acetophenone (4a), cho-sen as the model substrate (Table 1). Using the re-action conditions optimized with non-chiral phos-phates [10], namely 1.5 equiv. of diethoxymethylsi-lane (DEMS), 5 mol% of ZnEt2, and 5 mol% of lig-and in THF, a rapid screening of ligands A–I was con-ducted. It has to be underlined that the use of anyother hydrosilylating agent, in particular PMHS, wasdetrimental for the reactivity leading to the desired1CCDC deposition numbers are as follow: 2012272 for G and2012273 for H.C. R. Chimie, 2021, 24, n 1, 77-81Alphonsine Ngo Ndimba et al. 79Scheme 2. Synthesis of phosphates G, H, and I.1-phenylethanol (5a) in only 69% conversion. Whenthe monophosphates A and B derived from (S)-BINOL and (S)-H8-BINOL, respectively, were used asdonors for diethylzinc, 4a was reduced to 5a almostwithout stereochemical control (Table 1, entries 1and 2). When ligands G and H possessing both axialand central chiralities were used, the reactivity wasmaintained, since 4a was fully converted into 5a in79% and 84% yield, respectively (Table 1, entries 3and 4). Moreover, a kind of matched/mismatched ef-fect was observed regarding the enantioselectivity:5a was obtained in 18% ee with ligand G of abso-lute configuration (R, Sp), whereas a racemate wasformed with its diastereomer H of absolute config-uration (R, Rp). The use of bisphosphates changedthe scenario; when BINOPHAT C was employed, 4awas completely consumed and 5a was recovered in92% yield and 34% ee (Table 1, entry 5). We thentried to decrease the catalytic amount of ligand Cto 2.5 mol%, but unfortunately, even if the conver-sion was still high, the enantiomeric excess droppedto 27% (Table 1, entry 6). Comparison of our setof data for this compound with those reported inthe literature [13], and with chiral HPLC analysis ofcommercially available enantiopure (S) and (R) 1-phenylethanol, confirmed the (S) stereochemistry atthe carbon stereocenter. We tried to better controlthe enantioselectivity by using the symmetric H8-BINOPHAT D (Table 1, entry 7), but Unfortunately, inthis case no reaction occurred. This is probably dueto the difference in the dihedral angle values of thechiral axis between the aromatic and the partially hy-Figure 1. ORTEP representations of com-pounds G (left) and H (right) at 50% thermalellipsoids.drogenated ligands, preventing the Zn-center to ap-proach the P=O coordination sites of D. Therefore,other structures containing BINOL such as the un-symmetrical non-homo-BINOL ligands E and F weretested. For both, the reactivity (5a was isolated in 88%and 80% yield, respectively) as well as the enantiose-lectivity was recovered, although thes latter was de-creased when compared with C (18% and 16% ee, re-spectively) as shown in entries 8 and 9 of Table 1. Fi-nally, with bisphosphate I the desired product wasobtained in 69% yield but in racemic form (Table 1,entry 10). This can be attributed to the flexibility ofthe ethyl linker, which precludes the bisphosphotri-ester to act as bidentate ligand, giving therefore com-parable results to those of monophosphates.At this point it seemed that a rigid bisphosphatederived from BINOL was preferable to observe theformation of enantioenriched (S)-1-phenylethanolwith modest enantiomeric excesses, up to 34% eefor BINOPHAT C. The reduction of 4a in the pres-C. R. Chimie, 2021, 24, n 1, 77-8180 Alphonsine Ngo Ndimba et al.Table 1. Study for the asymmetric hydrosilyla-tion of ketonesaEntry Product Ligand Conv. (%)b e.e. (%)c1 A >99 (86) 6 (R)2 B 70 6 (S)3 5a G >99 (79) 18 (S)4 5a H >99 (84) 05 5a C >99 (92) 34 (S)6d 5a C 99 (86) 27 (S)7 5a D 08 5a E >99 (88) 18 (S)9 5a F 91 (80) 16 (S)10 5a I 88 (69) 011 C >99 (95) 16 (S)12 C 94 (75) 10 (S)13 C 98 (89) 8 (R)14 C >90 (88) 20 (S)15 C >99 (90) 4 (S)a Reaction conditions: ketone (1 mmol), DEMS (1.5mmol), THF (5 mL), ZnEt2 (5 mol%), ligand (5mol%).b Conversion determined by 1H NMR spectroscopyafter basic hydrolysis, in parenthesis, isolated yieldafter purification by column chromatography.c Enantiomeric excess determined by chiral HPLCanalysis.d Reaction performed with 2.5 mol% of C in other-wise identical conditions.ence of this ligand was thus further investigated bychanging the solvent or the temperature: unfortu-nately, performing the reaction in toluene, CH2Cl2,or CH3CN, as well as decreasing the temperature inTHF to 0 °C, turned out to be detrimental for the reac-tivity. Therefore, a series of representative acetophe-none derivatives were next hydrosilylated with thecombination of ZnEt2 and C in THF at room tem-perature. The substitution in the meta position of thearomatic ring did not affect the reactivity; indeed m-chloroacetophenone (4b) was completely convertedinto alcohol 5b, which was isolated in 95% yield withan ee of 16% (Table 1, entry 11). The more encum-bered ortho-substituted substrates 4c and 4d alsoparticipated to the reaction leading to 5c and 5d ingood yields, and in 10% and 8% ee, respectively (Ta-ble 1, entries 12 and 13). Alpha-tetralone (4e) as wellwas reduced to the corresponding alcohol 5e in 88%yield and 20% ee (Table 1, entry 14). Finally, the ac-tivated 2,2,2-trifluoroacetophenone (4f), which is re-ported to give very low enantiomeric excesses in thistransformation [12,13,17,20,29], was completely con-verted into 5f which was recovered in 90% yield andin only 4% ee (Table 1, entry 15).In summary, we have reported the synthesis ofthree new chiral BINOL-derived mono- and bis-phosphotriester architectures and we have providedthe proof of concept that phosphates, which arean underexplored class of ligands, can be useful inasymmetric metal-catalyzed transformations, as ex-emplified herein by the Zn-catalyzed hydrosilylationof prochiral ketones. It is certain that, since the appli-cation of such readily available compounds remainsunderexamined in the field of catalysis, our reportopens up an avenue for further achievements. In-deed, the synthesis of a larger panel of ligands aswell as the exploration of different reaction condi-tions and new catalytic transformations are under-way in our laboratory and will be reported in duecourse.AcknowledgmentsThe authors thank Université de Rennes 1, CNRS,Rennes Métropole and Région Bretagne for finan-cial support. The CRMPO of the Institut des SciencesChimiques de Rennes for mass measurement, theCentre de Diffractométrie X for X-ray analyses andC. R. Chimie, 2021, 24, n 1, 77-81Alphonsine Ngo Ndimba et al. 81FEDER funds for acquisition of single-crystal equip-ment are gratefully acknowledged. Part of this workhas been performed using the PRISM core facility(Biogenouest©, UMS Biosit, Université de Rennes35000 RENNES, FRANCE). 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First application of chiral phosphotriesters in asymmetric metal catalysis: enantioselective Zn-catalyzed hydrosilylation of ketones in the presence of BINOL-derived phosphates

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First application of chiral phosphotriesters in asymmetric metal catalysis: enantioselective Zn-catalyzed hydrosilylation of ketones in the presence of BINOL-derived phosphates

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