Tetrahedron Letters 55 (2014) 310–313

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Tetrahedron Letters

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Stereoselective synthesis of either (E)- or (Z)-silyl enol etherfrom the same acyclic a,b-unsaturated ketone using cationicrhodium complex-catalyzed 1,4-hydrosilylation

0040-4039/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.tetlet.2013.10.085

⇑ Corresponding author. Tel.: +81 42 759 6231; fax: +81 42 759 6493.E-mail address: takeuchi@chem.aoyama.ac.jp (R. Takeuchi).

� Present address: Division of Chemistry and Material Science, Graduate School ofEngineering, Nagasaki University, 1-14 Bunkyo, Nagasaki 852-8521, Japan.

� Present address: Department of Applied Chemistry, Faculty of Urban Environ-mental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo192-0397, Japan.

O

R1 R3

R2

+ HSiEt3Et3SiO

R1 R3

R2OSiE

R1

R3

cat.[Rh (cod)2]BF4

+

1 (Z)-2(E)-2

cat. ligand

Scheme 1. Hydrosilylation of a,b-unsaturated ketones.

Gen Onodera �, Ryosuke Hachisuka, Tomomi Noguchi, Hiroki Miura �, Toru Hashimoto, Ryo Takeuchi ⇑Department of Chemistry and Biological Science, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan

a r t i c l e i n f o

Article history:Received 12 August 2013Revised 14 October 2013Accepted 18 October 2013Available online 21 November 2013

Keywords:1,4-HydrosilylationRhodiumSilyl enol ethera,b-Unsaturated ketone

a b s t r a c t

The stereoselective synthesis of either (E)- or (Z)-silyl enol ether from the same acyclic a,b-unsaturatedketone is reported. Highly (Z)-selective conditions were the use of [Rh(cod)2]BF4/DPPE at room temper-ature with no solvent, whereas (E)-selective conditions were the use of [Rh(cod)2]BF4/P(1-Nap)3 (1-Nap = 1-naphthyl) under refluxing dichloromethane.

� 2013 Elsevier Ltd. All rights reserved.

The enolate anion is one of the most important active species incarbon–carbon bond-forming reactions. The stereoselectivity ofthe carbon–carbon double bond generated with enolate formationis primarily associated with the structures of the parent carbonylcompounds and bases used for deprotonation.1 The stereoselectiv-ity of some enolate reactions depends on whether a E- or Z-enolateis involved, even if the orientation and facial bias of the reactionare controlled. The diastereoselectivity of the aldol reaction witha boron enolate is known to be determined by the stereochemistryof the boron enolate used.2 It would be highly desirable to be ableto selectively form either a E- or Z-metal enolate from the samecarbonyl compound at will.3

Silyl enol ether is especially important among metal enolatesbecause it offers numerous opportunities for regio- and stereose-lective carbon–carbon bond formation.4 Silyl enol ether is gener-ally obtained by the generation of an enolate anion followed by areaction with chlorosilane. This method is always accompaniedby the formation of a metal salt as waste. A more atom-economicalmethod that does not form a metal salt as a by-product is the tran-sition metal-catalyzed hydrosilylation of a,b-unsaturated carbonylcompounds.5

The 1,4-hydrosilylation of acyclic a,b-unsaturated ketones givesa mixture of (E)- and (Z)-silyl enol ether.5a,b,d,f The stereoselectivityof the resulting carbon–carbon double bond is not high enough forthe stereoselective synthesis of silyl enol ether. For example, the[Rh(cod)OH]2-catalyzed hydrosilylation of (E)-3-nonen-2-one hasbeen reported to give a 35:65 mixture of (E)- and (Z)-silyl enolether.5d The highly stereoselective hydrosilylation of acyclic a,b-unsaturated ketones has not been reported. Furthermore, thehighly stereoselective formation of either (E)- or (Z)-silyl enolether, at will, from the same acyclic a,b-unsaturated ketone hasbeen desired.

We have been studying cationic rhodium complex-catalyzedhydrosilylation.6 We found that either (E)- or (Z)-silyl enol ethercould be obtained with high selectivity from the same acyclica,b-unsaturated ketone through proper selection of the reactionconditions (Scheme 1). In this Letter, we wish to report the highlystereoselective hydrosilylation of acyclic a,b-unsaturated ketones.

We optimized Z-selective conditions for the reaction of (E)-3-nonen-2-one (1a) with Et3SiH (Table 1). First, we examined the ef-fect of the rhodium complex and ligand on the reaction. When PPh3

t3

R2

Table 1Optimization of Z-selective conditionsa

Entry Rh complex Ligand Solvent Time (h) Yieldb (%) E/Zc of 2a

1d [Rh(cod)Cl]2 PPh3 Neat 24 55 58/422e,f [Rh(cod)Cl]2 DPPE Neat 2 85 34/663e [Rh(cod)Cl]2 None Neat 24 Trace —4g [Rh(cod)2]BF4 PPh3 Neat 24 41 56/445h [Rh(cod)2]BF4 DPPE Neat 1 95 8/926 [Rh(cod)2]BF4 DPPM Neat 24 45 12/887 [Rh(cod)2]BF4 DPPP Neat 48 4 50/508 [Rh(cod)2]BF4 none Neat 3 86 31/699 [Rh(cod)2]BF4 DPPE DCEi 24 64 11/89

10 [Rh(cod)2]BF4 DPPE THFj 24 81 16/84

a A mixture of 1a (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), andligand (0.5 mol %) was stirred at rt under Ar.

b Isolated yield.c Determined by 1H NMR.d 0.25 mol % of [Rh(cod)Cl]2 and 1 mol % of PPh3 were used.e 0.25 mol % of [Rh(cod)Cl]2 was used.f At 70 �C.g 1 mol % of PPh3 was used.h A mixture of 1a (5 mmol), HSiEt3 (5.5 mmol), [Rh(cod)2]BF4 (0.5 mol %), and

DPPE (0.5 mol %) was stirred at rt under Ar.i DCE = 1,2-dichloroethane (5 mL).j THF (5 mL).

G. Onodera et al. / Tetrahedron Letters 55 (2014) 310–313 311

(P/Rh = 2) was used as a ligand, [Rh(cod)Cl]2 and [Rh(cod)2]BF4

gave almost equal amounts of (E)- and (Z)-silyl enol ether 2a (en-tries 1 and 4). The reaction with DPPE was more Z-selective thanthat with PPh3 (entries 2 and 5). [Rh(cod)2]BF4/DPPE gave the bestresults with regard to both yield and Z-selectivity (entry 5). Theyield and stereoselectivity were considerably decreased withDPPM and DPPP (entries 6 and 7). [Rh(cod)2]BF4/DPPE was moreselective than [Rh(cod)2]BF4 alone (entry 8). The above resultswere obtained in the absence of solvent. The use of solvent resultedin a decrease in both yield and Z-stereoselectivity (entries 9 and10). Thus, the optimal Z-selective conditions were no solvent atroom temperature with [Rh(cod)2]BF4/DPPE.

The 1,4-hydrosilylation of various a,b-unsaturated ketones wascarried out under the optimized Z-selective conditions mentionedabove. A wide range of ketones could be used in the reaction, andthe results are summarized in Table 2. Methyl ketone (1a) was con-verted to 2a in 95% yield with 92% Z-selectivity (entry 1). The reac-tion of isopropyl ketone 1b and phenyl ketone 1c gave 2b and 2c inrespective yields of 77% and 80% (entries 2 and 3). The Z-selectiv-ities were 90% and 94%. The effect of the b-substituent of methylketone was examined. The reaction of 4-phenyl-3-buten-2-one(1d) gave 2d in 68% yield with 73% Z-selectivity. 1,2-Addition

Table 2Z-Selective hydrosilylation of 1a–ja

Entry 1 R1 R2 R3

1d 1a Me H n-Pentyl2e 1b i-Pr H n-Pentyl3e 1c Ph H n-Pentyl4f,g 1d Me H Ph5 1e Me H 2-Phenylethyl6 1f Me H i-Pr7 1g Me H Cy8 1h Me H 1-Ethylpentyl9 1i Me Me Me

10f 1j Ph H Ph

a A mixture of 1 (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), DPPE (0.5 mob Isolated yield.c Determined by 1H NMR.d A mixture of 1a (5 mmol), HSiEt3 (5.5 mmol), [Rh(cod)2]BF4 (0.5 mol %), and DPPE (0e At 30 �C.f At 50 �C.g 1,2-Addition product 2d0 was obtained in 17% yield.

product was obtained as a by-product (entry 4). A phenethyl groupat the (E)-b-position slightly decreased the Z-selectivity. The reac-tion of 1e gave product 2e in 77% yield with 88% Z-selectivity (en-try 5). The effect of steric hindrance by isopropyl, cyclohexyl, and1-ethylpentyl groups at the (E)-b-position was examined. Theseketones 1f–h gave good results with regard to both yield and Z-selectivity (entries 6–8). b,b-Disubstituted ketone 1i was smoothlyconverted to silyl enol ether 2i in 84% yield with 98% Z-selectivity(entry 9). (E)-1,3-Diphenyl-2-propen-1-one (1j) was converted to2j in 84% yield with 88% Z-selectivity (entry 10).

Encouraged by the fact that we were able to successfully opti-mize Z-selective conditions, we next tried to optimize E-selectiveconditions. The results are summarized in Table 3. We focusedon the use of a monodentate ligand to optimize E-selective condi-tions, since bidentate ligands favored the formation of Z-silyl enolether, as described in Table 1. When PPh3 was used, the reaction atroom temperature gave 2a in 41% yield (Table 1, entry 4). We per-formed the reaction at 85 �C to improve the yield. The effect ofmonodentate phosphine ligand on the reaction was examined. Al-most the same level of E-selectivity was observed when the ligandwas PPh3 or PPh2Cy (entries 1 and 2). An electron-withdrawing li-gand such as PPh2C6F5 decreased the E-selectivity to 53% (entry 3).A more-hindered phosphine ligand improved the yield and E-selec-tivity. Tris(1-naphthyl)phosphine ligand gave 2a in 86% yield with85% E-selectivity (entry 4). To improve the E-selectivity, we exam-ined other reaction variables using tris(1-naphthyl)phosphine. Wenext examined the effect of the solvent on E-selectivity. In contrastto the reaction with PPh3, the reaction with tris(1-naphthyl)phos-phine gave the product in high yield even at room temperature(entry 5). The reaction with THF at room temperature gave 2a in80% yield with 87% E-selectivity (entry 6). DCM and 1,2-dichloro-ethane gave better results than THF (entries 7 and 9). When thereaction was performed under refluxing DCM, the yield increasedfrom 85% to 90% with 93% E-selectivity (entry 8). Thus, the optimalE-selective conditions were determined to be under refluxing DCMwith [Rh(cod)2]BF4/tris(1-naphthyl)phosphine (P/Rh = 2).

The 1,4-hydrosilylation of various a,b-unsaturated ketones wascarried out under the optimized E-selective conditions describedabove (Table 4). The effect of the substituent on ketone was exam-ined, and the results showed that the E-selectivity was sensitive tothe substituent on ketone. Methyl ketone 1a was converted to silylenol ether 2a in 90% yield with 93% E-selectivity (entry 1). On theother hand, isopropyl ketone 1b and phenyl ketone 1c were less E-selective substrates than methyl ketone 1a. Isopropyl ketone 1bwas converted to 2b in 87% yield with 44% E-selectivity (entry 2).Phenyl ketone 1c was converted to 2c in 96% yield with 77% E-

Time (h) 2 Yieldb (%) E/Zc of 2

1 2a 95 8/9224 2b 77 10/9024 2c 80 6/9420 2d 68 27/734 2e 77 12/881 2f 79 3/975 2g 80 3/975 2h 82 2/986 2i 84 2/984 2j 84 12/88

l %), and solvent (5 mL) was stirred at rt under Ar.

.5 mol %) was stirred at rt under Ar.

OSiEt3

R

R'

OSiEt3

RR'

1

(Z)-2

(E)-2

HRhLn SiEt3

O

RR'

LnRh+

HSiEt3

O

R

R'LnRh

SiEt3H

++

OSiEt3

R

R'LnRh

H+

silylrhodation

OSiEt3

R

R'LnRh

H+

π-allyl Rh 5

34

OSiEt3

R

R'LnRh

H+

σ-allyl Rh 6

OSiEt3

R

R'LnRh

H+

σ-allyl Rh 7

OSiEt3

R

LnRh

H+

R'

OSiEt3

RLnRh

H

+R'

π-allyl Rh 8

OSiEt3

RLnRh

H

+R'

σ-allyl Rh 9

1

3

1

3

13

1

3

1

31

3

1

3reductive elimination

reductive elimination

syn-antiisomerization

Scheme 2. Plausible reaction pathway.

Table 4E-Selective hydrosilylation of 1a–ja

Entry 1 R1 R2 R3 Time (h) 2 Yieldb (%) E/Zc of 2

1 1a Me H n-Pentyl 1 2a 90 93/72 1b i-Pr H n-Pentyl 24 2b 87 44/563 1c Ph H n-Pentyl 24 2c 96 77/234d 1d Me H Ph 24 2d 77 86/145 1e Me H 2-Phenylethyl 1 2e 87 94/66 1f Me H i-Pr 2 2f 68 93/77 1g Me H Cy 5 2g 79 93/78e 1h Me H 1-Ethylpentyl 24 2h 74 91/99f 1i Me Me Me 24 2i 63 92/8

10 1j Ph H Ph 24 2j 90 72/28

a A mixture of 1 (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), P(1-Nap)3 (1 mol %, 1-Nap = 1-naphthyl), and CH2Cl2 (5 mL) was stirred at reflux under Ar.b Isolated yield.c Determined by 1H NMR.d 1,2-Addition product 2d0 was obtained in 9% yield.e 1 mol % of [Rh(cod)2]BF4 and 2 mol % of P(1-Nap)3 (1-Nap = 1-naphthyl) were used.f A mixture of 1i (5 mmol), HSiEt3 (5.5 mmol), [Rh(cod)2]BF4 (0.5 mol %), P(1-Nap)3 (1 mol %, 1-Nap = 1-naphthyl), and CH2Cl2 (8 mL) was stirred at reflux under Ar.

Table 3Optimization of E-selective conditionsa

Entry Ligand Solvent Temperature Time (h) Yieldb (%) E/Zc of 2a

1 PPh3 Neat 85 �C 2 63 75/252 PPh2Cy Neat 85 �C 1 76 77/233 PPh2(C6F5) Neat 85 �C 2 70 53/474 P(1-Nap)3

d Neat 85 �C 2 86 85/155 P(1-Nap)3

d Neat rt 2 86 84/166 P(1-Nap)3

d THF rt 3 80 87/137 P(1-Nap)3

d CH2Cl2 rt 3 85 93/78 P(1-Nap)3

d CH2Cl2 Reflux 1 90 93/79 P(1-Nap)3

d DCEe rt 1 89 91/9

a A mixture of 1a (3 mmol), HSiEt3 (3.3 mmol), [Rh(cod)2]BF4 (0.5 mol %), ligand (1 mol %), and solvent (5 mL) was stirred under Ar.b Isolated yield.c Determined by 1H NMR.d 1-Nap = 1-naphthyl.e DCE = 1,2-dichloroethane.

312 G. Onodera et al. / Tetrahedron Letters 55 (2014) 310–313

selectivity (entry 3). We next examined the effect of the (E)-b-sub-stituent on E-selectivity. (E)-4-Phenyl-3-buten-2-one (1d) wasconverted to 2d in 77% yield with 86% E-selectivity (entry 4). (E)-4-Phenyl-3-buten-2-one (1d) was a less (E)-selective substratethan (E)-3-nonen-2-one (1a). Methyl ketones with (E)-b-primaryand secondary alkyl substituents gave good results with regardto both yield and E-selectivity. Ketone 1e was converted to silylenol ether 2e in 87% yield with 94% E-selectivity (entry 5). Thereaction of 1f-h gave the products in 68–79% yields with 91–93%E-selectivity (entries 6–8). b,b-Disubstituted ketone 1i was con-verted to 2i in 63% yield with 92% E-selectivity (entry 9). (E)-1,3-Diphenyl-2-propen-1-one (1j) was converted to 2j in 90% yieldwith 72% E-selectivity (entry 10).

The reversal of stereoselectivity observed here can be explainedbased on mechanistic considerations (Scheme 2). The catalytic cy-cle of the reaction involves a p-allyl rhodium complex as a keyintermediate. The 1,4-hydrosilylation of enones begins with theoxidative addition of hydrosilane to a rhodium center to giveRh(III) species 3. Enone coordinates to 3 in an s-cis form.7 Silylrho-dation of the carbon-oxygen double bond gives intermediate 4.5h,i,8

Coordination of the carbon–carbon double bond to the rhodiumcenter gives p-allyl intermediate 5. This p-allyl intermediate iso-merizes to a r-allyl intermediate when reductive elimination oc-curs. To avoid steric repulsion between the triethylsiloxy moietyand the rhodium moiety, reductive elimination from r-allyl inter-mediate 6 occurs at the C-3 position to give (Z)-silyl enol ether 2.Syn–anti isomerization of p-allyl intermediate9 5 plays a decisiverole in providing (E)-silyl enol ether 2. Syn–anti isomerization ofp-allyl intermediate 5 to 8 followed by reductive elimination gives

(E)-silyl enol ether 2. The relative rates of syn–anti isomerization ofp-allyl intermediate 5 via a p–r–p process and reductive elimina-

G. Onodera et al. / Tetrahedron Letters 55 (2014) 310–313 313

tion from 5 determine the stereoselectivity. This proposed mecha-nism is similar to that for the hydrosilylation of 1,3-dienes to giveallylsilanes.10

The proposed mechanism can explain the results describedabove. Z-Selective conditions require the use of DPPE and no sol-vent. (Z)-Silyl enol ether 2 is obtained when reductive eliminationfrom 5 is faster than the syn–anti isomerization of 5 to 8. It is wellknown that organic groups that are to be eliminated from a metalcenter have to take a cis-geometry.11 A strongly cis-chelatingbidentate diphosphine such as DPPE would be more suitable thana monodentate ligand for a cis-geometry leading to rapid reductiveelimination. Another important factor for high Z-selectivity is theabsence of solvent. It has been suggested that reductive elimina-tion to give a hydrosilylation product is a bimolecular process withan incoming hydrosilane.12 Thus, reductive elimination requiresthe participation of another molecule of hydrosilane. Therefore, ahigh concentration of hydrosilane accelerates reductive elimina-tion from 5 to increase the Z-selectivity. In contrast, dilution by asolvent slows reductive elimination from 5 to decrease the Z-selectivity.

E-Selective conditions require the use of P(1-Nap)3 and a sol-vent. (E)-Silyl enol ether 2 is obtained when the syn–anti isomeri-zation of 5 to 8 is faster than reductive elimination from 5. Thesteric bulkiness of P(1-Nap)3 promotes the syn–anti isomerizationof p-allyl intermediate 5. Silylrhodation of the carbon–oxygen dou-ble bond of enone coordinated in an s-cis form gives p-allyl rho-dium intermediate 5, in which a triethylsiloxy substituent islocated at an anti position with respect to the central hydrogen.Steric repulsion between an anti triethylsiloxy substituent andthe P(1-Nap)3-ligated rhodium moiety promotes the syn-anti isom-erization of 5 to 8 via a p–r–p process to the more stable p-allylrhodium intermediate 8 in which the triethylsiloxy substituent islocated at a syn position.13 In addition, dilution by a solvent slowsreductive elimination from 5 to increase the E-selectivity.

The ketone substituent affected the stereoselectivity only underE-selective conditions (Table 4, entries 1–3). In contrast, the ketonesubstituent had no effect under Z-selective conditions (Table 2, en-tries 1–3). This difference can be explained by the proposed mech-anism. When the ketone substituent is bulky, such as an isopropylor phenyl group, the difference in the steric bulkiness of the ketonesubstituent and the triethylsiloxy substituent is small. With p-allylintermediate 8, the location of the triethylsiloxy substituent at thesyn position does not always lead to a more stable p-allyl interme-diate. Syn–anti isomerization in such cases is not promoted asmuch as it is with a methyl ketone. Consequently, E-selectivity de-creases. Under Z-selective conditions, the reductive elimination of5 is faster than syn–anti isomerization from 5 to 8. Consequently,the ketone substituent has no effect on Z-selectivity.

In summary, we could obtain either (E)- or (Z)-silyl enol etherwith high selectivity from the same acyclic a,b-unsaturated ketoneby choosing the conditions for the 1,4-hydrosilylation reaction.Further examinations of the scope of the reaction and mechanisticstudies are underway.

Acknowledgments

We thank Mr. Naoki Tomita for the assistance of experiments.We thank Prof. Nishibayashi and Dr. Nakajima for measuringNMR and HRMS spectra.

A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.tetlet.201310.085.

References and notes

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