Chiral Nickel(II) Complex Catalyzed Enantioselective DoyleâKirmse

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Chiral Nickel(II) Complex Catalyzed Enantioselective Doyle-Kirmse Reaction of #-Diazo Pyrazoleamides Xiaobin Lin, Yu Tang, Wei Yang, Fei Tan, Lili Lin, Xiaohua Liu, and Xiaoming Feng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 14, 2018

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Journal of the American Chemical Society

Chiral Nickel(II) Complex Catalyzed Enantioselective Doyle-Kirmse Reaction of α-Diazo Pyrazoleamides Xiaobin Lin,† Yu Tang,† Wei Yang,† Fei Tan,† Lili Lin,† Xiaohua Liu*,† and Xiaoming Feng*,†,‡ †

Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 5610064, China. ‡

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China.

Supporting Information ABSTRACT: Although high enantioselectivity of [2,3]-sigmatropic rearrangement of sulfonium ylides (Doyle-Kirmse reaction) has proven surprisingly elusive using classic chiral Rh(II) and Cu(I) catalysts, in principle it is due to the difficulty in fine discrimination of the heterotopic lone pairs of sulfur and chirality inversion at sulfur of sulfonium ylides. Here, we show that the synergistic merger of new αdiazo pyrazoleamides and chiral N,N'-dioxide-nickel(II) complex catalyst enables a highly enantioselective Doyle-Kirmse reaction. Pyrazoleamide substituent serves as both an activating and a directing group for the readily formation of metal-carbene and Lewis-acid bonded ylide intermediate in the assistant of dual tasking nickel(II) complex. An alternative chiral Lewis acid-bonded ylide pathway greatly improves the product enantiopurity even for the reaction of a symmetric diallylsulfane. A majority of transformations over a series of aryl or vinyl substituted α-diazo pyrazoleamindes and sulfides proceed rapidly (within 5-20 minutes in most cases) with excellent results (up to 99% yield and 96% ee), providing a breakthrough in enantioselective Doyle-Kirmse reaction.

INTRODUCTION The [2,3]-sigmatropic rearrangement of sulfonium ylides (known as Doyle–Kirmse reaction) is discovered by Kirmse in 19681 and modified by Doyle in 1981.2 The reaction involving of allylic or propargylic sulfides and diazo reagents represents an important method to construct new C–C and C(sp3)–S bonds with an allyl or allenyl substituted stereogenic center.3 The asymmetric catalytic version has attracted considerable attention since the initial attempt by Uemura and co-workers in 1995,4 but remains particularly challenging at present. In previous reports, the metalcarbene are typically generated in situ from diazo precursors with various transition metal salts, such as Cu(I), Rh(II), Co(III), and Fe(II) (Scheme 1a).3-8 For instances, chiral bisoxazoline-Cu(I) catalysts and the Doyle catalysts have been optimized to promote the enantioselective rearrangement of allylic arylsulfides by several research groups,6a-e but moderate enantioselectivity (78% ee) is the highest result.6c Likewise, chiral salen-Co(III) complex5 and the myoglobin variant Mb(L29S,H64V,V68F) were identified with average enantioselection.8 Until recently, the Wang group made a breakthrough in asymmetric trifluoromethylthiolation through chiral Rh(II) and Cu(I) catalysts promoted rearrangement of SCF3containing sulfonium ylides.7 In principle, chiral transition metal complexes catalyzed asymmetric Doyle-Kirmse reaction can follow two possible routes (Scheme 1a).3 Initially, metal-carbene complex generates by decomposition of diazo compound with a chiral transition metal catalyst, which then reacts with allylic sulfide via discrimination of its heterotopic lone pairs to form sulfonium ylides selectively. If chiral catalyst is bonded with ylide during the rearrangement, the chiral catalyst may further direct the enantioselective allyl shift. Otherwise, if chiral catalyst releases, free non-racemic ylide may retain the chirality during the rearrangement due to the fact that the rearrangement occurs by a concerted mechanism via an envelope

Scheme 1. Strategy for the Catalytic Asymmetric DoyleKirmse Reactions.

transition state.3-5,6a-b,7 The latter has evidenced by the experiment of Trost and Hammen that chiral sulfonium ylide generated from deprotonation of optically pure sulfonium salt could undergo [2,3]-

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sigmatropic rearrangement to yield the product with 94% ee (Scheme 1b).9 Nevertheless, in fact three issues will affect the stereochemistry of this type of rearrangement. One is that it is difficult to fine discrimination of the heterotopic lone pairs of sulfur with chiral metal complex. Second, chiral metal-bonded ylide is in equilibrium with free ylide and free ylide is more likely to undergo the following rearrangement.3c-i,10 A solid evidence of these is that nearly racemic product generated from chiral Rh(II) and Cu(I) catalyzed reaction of diallylsulfane (Scheme 1c).6c,7 In addition, if the inversion at sulfur of free non-racemic ylide is much faster than sigmatropic rearrangement, enantioselectivity of the reaction is thus poor. For all these reasons, further development of novel

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approaches to achieve highly enantioselective Doyle-Kirmse reaction is highly desirable and challenging. Our strategy towards enantioselective Doyle-Kirmse reaction stems from development of new α-diazo carbonyl compounds and employment of novel dual-tasking chiral metal complex catalyst (Scheme 1d). We get a new donor–acceptor α-diazo compound 1 by introducing a pyrazoleamide group instead of ester group in view of two points. The reactivity of α-diazo compound could be defined by this electron-withdrawing substituent that adorns the related carbene intermediate. Pyrazoleamide group is an excellent functional unit which accepts a strong bidentate coordination to a

Table 1. Optimization of the Reaction Conditionsa

entry

1

metal salt(s)

ligand

T (oC)

time

solvent

yield (%)b

ee (%)c

1

1a

Sc(OTf)3 or Yb(OTf)3

L3-PiPr2

35

10 h

CH2Cl2

N.R.

--

2d

1a

Rh2(OAc)4

L3-PiPr2

35

7h

CH2Cl2

--

--

3

1a

AgOTf

L3-PiPr2

35

10 h

CH2Cl2

19:1 dr and 86% ee.

Scheme 3. Control Experiments

Figure 1. Proposed Catalytic Model for Stereoinduction

Scheme 2. Gram-Scale Reaction and Transformations of 3b

The feasibility of chiral Ni(II) complex catalyzed enantioselective Doyle–Kirmse reaction was assessed through a gram-scale reaction between phenylvinyl α-diazo pyrazoleamide 1b and allyl(phenyl)sulfane 2a, yielding the product 3b in 93% yield and 91% ee within 5 minutes (Scheme 2a). Furthermore, the pyrazoleamide group easily underwent transformations11,16. As mentioned in Scheme 2b, the product 3b could be converted to alcohol 8 in 93% yield through reduction, as well as the corresponding ester 9 and amide 10 with the enantiomeric excess maintained (for details see Supporting Information). To figure out the characters of our newly designed diazo pyrazoleamides and to gain insights to the mechanism of [2,3]sigmatropic arrangement in current catalytic system, control experiments were carried out (Scheme 3; for details see Supporting Information). Ester 2-diazo-2-phenylacetate 11 and (E)-2-diazo-5phenyl-1-(1H-pyrrol-1-yl)pent-4-en-1-one 13 instead of diazo substrate 1b were examined in this catalytic system, and the related products 12 and 14 were obtained with little enantioselectivity after the diazo compound consumed (6% ee for 12; 4% ee for 14, Scheme 3). The diazo pyrroleamide 13 showed higher reactivity than the diazo ester 11 (15 min vs 10 h). These results demonstrated the necessity of pyrazolyl group because the amide has a higher affinity for Lewis acid than the corresponding ester. The stronger

electron-withdrawing property of pyrazoleamide makes α-diazo pyrazoleamide 1 higher electrophilicity than the related α-diazo ester and pyrroleamide. The nitrogens of pyrazole unit played important role for enantioselection. Moreover, high enantioselectivity obtained from the reaction of diallylsulfane (6j and 6k). These experiments elucidated the basis of the reactivity and enantioselectivity trends, showing that the coordination of chiral Ni(II) complex to the ylide intermediates facilitates the enantioselective [2,3]-sigmatropic arrangement. The relationship between the enantioselectivity of the ligand and the product was studied (see Supporting Information for details), and there is no non-linear effect in this catalytic system. The X-ray diffraction of L2-PiPr2-Ni(BF4)2•6H2O complex shows that chiral ligand L2-PiPr2 readily coordinates to Ni(NTf)2 to form an octahedral 20-electon structure with two amine oxide oxygens, two amide oxygens, and two solvent molecules (Figure 1). Based on these information, a model for the enantiocontrol observed in chiral N,N′-dioxide-Ni(II) complex promoted Doyle–Kirmse reaction is shown in Figure 1. The mixture of NiCl2 and AgNTf2 generates Ni(NTf)2 in situ as the dominate metal precursor. The coordination of the ligand L2-PiPr2 yield the desired chiral Ni(II) complex catalyst. When diazo pyraozleamide is added, the chiral nickel(II)



carbene intermediate A generated from α-diazo pyrazoleamide 1b after the release of N2. The IR experiment verifies Ni(II)-assisted decomposition of diazo pyrazoleamide 1b (for details see Supporting Information). The attack of allyl(2chlorophenyl)sulfane generates the sulfonium ylide species, but metal-bonded ylide B is unstable and chiral Ni(II) complex releases, which is likely to transfer to bond the pyrazoleamide unit securely via a bidentate chelation. It is well established that the [2,3]sigmatropic arrangement of sulfonium ylide typically favors in an envelope transition state3-5,6a-b,7. We propose that transition structure of chiral Ni(II) Lewis acid-bonded ylide C’ is disfavored due to steric interactions between the five-membered sulfur-cycle and the left amide unit (block 1) of the N,N′-dioxide ligand. Alternatively, chiral Ni(II) Lewis acid-bonded ylide C therefore proceeds through a concert anticlockwise [2,3]-sigmatropic arrangement to give R-6c with high enantiocontrol. Moreover, when diallyl sulfide 2x is used, the nucleophilic step is controlled by the barrier shielding of the block 1, then the enantiocontrol of the rearrangement step arises from the steric hindrance between the envelope structure of the reactive allyl group and the other amide (block 2) of the ligand. Therefore, the corresponding chiral metal complex bonded ylide intermediate will yield the product in an enantioselective manner.

CONCLUSION We have developed a new asymmetric Ni(II) complex-mediated Doyle–Kirmse reaction. The introducing donor–acceptor α-diazo pyrazoleamides furnishes a novel strategy for both readily formation of sulfonium ylides and enantioselective [2,3]sigmatropic arrangement. The study reveals that chiral N,N′dioxide-Ni(II) catalyst triggers two steps of the reaction highly efficiently which completes within 5-20 minutes in most cases. Reactions typically proceed with excellent yields (up to 99%) and enantioselectivities (up to 96% ee) over a series of aryl or arylvinyl substituted α-diazo pyrazoleamindes and sulfides under mild reaction conditions. Another discovery is that pyrazoleamide group may elevate ee by channeling pathways of metal-bonded ylide or free non-racemic ylide towards a new chiral Lewis acid-bonded ylide. Thus, product enantiopurity may be improved even a symmetric diallylsulfane could undergo Doyle–Kirmse reaction in a highly enantioselective manner. Additionally, the products could be conveniently transformed into the corresponding alcohol, ester, and amide without loss of the enantiomeric excess. The model for enantioselection in Doyle–Kirmse reaction were proposed according to the X-ray diffraction of L2-PiPr2-Ni(II) complex and the control experiments. These investigations shed light on several factors that directly impact the efficiency and enantioselectivity of Doyle–Kirmse reaction, enabling reasonable strategies for chirality transfer. The newly synthesized α-diazo pyrazoleamides and novel dual-functionality of N,N′-dioxide metal complex catalyst are likely to be instrumental in the success of future endeavors.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, full spectroscopic data for all new compounds, and copies of 1H, 13C{1H} NMR, and HPLC spectra (PDF) X-ray crystallographic data for L2-PiPr2-Ni(II) complex (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]

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ORCID Xiaoming Feng: 0000-0003-4507-0478 Xiaohua Liu: 0000-0001-9555-0555

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors acknowledgements financial support from the National Natural Science Foundation of China (grant nos. 21625205, and 21432006), and the National Program for Support of Top-Notch Young Professionals, and the Fundamental Research Funds for the Central Universities. This paper is dedicated to Professor Jin-Pei Cheng on the occasion of his 70th birthday.

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