Mechanisms of CO2 Incorporation into Propargylic Amine Catalyzed

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Mechanisms of CO2 Incorporation into Propargylic Amine Catalyzed by Ag(I)/Amine Catalysts Ruming Yuan, Shuhua Xu, and Gang Fu J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01767 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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The Journal of Organic Chemistry

Mechanisms of CO2 Incorporation into Propargylic Amine Catalyzed by Ag(I)/Amine Catalysts Ruming Yuan, Shuhua Xu, Gang Fu* State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers, and Esters, and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.

Author Email Address: [email protected]

TOC Graphic

ABSTRACT Density functional theory calculations are carried out to explore the detail mechanisms of CO2 incorporation into propargylic amine catalyzed by Ag(I)/amine catalysts. Our calculations reveal that the whole reaction involves Lewis acid catalysis and Lewis base catalysis stages, and the outcomes of this reaction critically depend on the basicity of amine. Weaker base (i.e. DABCO) makes the Ag center more acidic, thus favoring the Lewis acid catalysis, resulting in benzoxazin-2-one. However, the following rearrangement of benzoxazin-2-one requires stronger base (i.e. DBU) to stabilize its deprotonated form. Thus, the product selectivity could be subtly tuned by the choice of amine and the condition control, consistent with the experimental observations.

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Introduction Carbon dioxide emission is the most important cause of global warming.1,2 However, due to its highly thermodynamic stability, only limit industrial processes have used carbon dioxide as a feedstock.3 Thus, developing efficient strategies for the catalytic utilization of CO2 could not only mitigate CO2 emission but also provide an attractive and green C1 building block.4,5 It has been well documented that Ag(I)/base systems were efficient catalysts to incorporate CO2 into alkyne, alcohols or amines.6,7 Interestingly, recent work by Yamada and coworkers demonstrated that the outcomes of CO2 incorporation into propargylic amine catalyzed by Ag(I)/amine systems critically depended on the choice of amine and the condition control. When 1,8-Diazabicyclo[5.4.0]undec-7-ene

(DBU)

was

employed,

4-hydroxyquinolin-2(1H)-one (a) was the sole product at relatively high temperature of 60°C(See eq 1);8 while when using 1,4-Diazabicyclo[2.2.2]octane (DABCO) and under the relatively low temperature of 20°C, the major product was benzoxazin-2-one (b) (See eq 2).

9

As they proposed (Scheme 1), the product

selectivity depended on the on/off status of the intramolecular rearrangement from b to a. However, the detail reaction mechanisms and the control factors are still obscure.


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Here, we present a comprehensive survey on the mechanism of carbon dioxide incorporation into amines catalyzed by Ag(I)/amine systems. We will concentrate on: (i) why different organic amines would lead to different products; and (ii) how the reaction conditions, such as temperature and the amount of amine, control the product selectivity.

Scheme 1 A brief mechanism for CO2 incorporation into propargylic amine catalyzed by Ag(I)/amine systems 8

Computational details All calculations were carried out by means of the ωB97XD functional theory.10 In our calculations, the silver atom was treated with the SDD relativistic effective core potential.11 6-311+G (d) basis sets were used for N and O atoms and the other atoms (C, H) were described by the 6-31G(d,p) basis sets.12,13 To obtain solvation-corrected relative free energies, all the structures were fully optimized in DMSO solution according to the reaction conditions with polarizable continuum model (IEFPCM).14 In addition, the XYZviewer software developed by De Marothy was used to visualize the optimized structures.15 Vibrational frequencies were calculated analytically to



ensure that a local minimum has no imaginary frequency and each transition state (TS) has only one single imaginary frequency. Intrinsic reaction coordinate (IRC) calculations were also performed to make sure the TS’ indeed connect two relevant minima. Charge analyses were also performed using the natural bond orbital (NBO) program.16 Experimentally, different silver salts, such as AgNO3 and AgOAc, were used. For consistency, herein we used AgNO3 as representative in all calculations. To reduce the overestimation of the entropy contribution of the results, we employed a correction of −2.6 (or 2.6) kcal/mol for 2:1 (or 1:2) transformations as many earlier theoretical studies did.17 Unless specifically mentioned, free energies in 298K were used in all of our discussion in this article. All of the density functional theory (DFT) calculations were carried out with the Gaussian 09 program.18 To better describe the coordination nature of Ag-N bonds, we performed the bond valence sum (BVS) calculations,19 which can be defined as:  r -r  Vi = ∑ j Sij = ∑ j exp  0 ij   B 

(3)

where Sij was the bond valence associated with bond length rij, and r0 and B were empirically determined parameters. For Ag-N bond, the value of r0 and B were 1.85 Å and 0.37 Å, respectively.

Results and Discussion Scheme 2 illustrates more detail mechanisms, which take into consideration of our DFT calculation results. It should be noted that there existed two separated


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catalytic stages, namely Lewis acid catalysis and Lewis base catalysis, which would proceed sequentially. In the former stage, the reaction begins with the nucleophilic attack of amine nitrogen on the carbon of CO2, following by a ring closing, leading to produce benzoxazin-2-one. In the latter stage, the resulting benzoxazin-2-one undergoes deprotonation by an amine, followed by the C-O bond cleavage to give a ring-opening

intermediate,

which

eventually

convert

into

4-hydroxyquinolin-2(1H)-one via the enolization reaction.

Scheme 2. Detailed Mechanisms for the Ag(I)/Amine-Catalyzed Carbon Dioxide Incorporation and Intramolecular Rearrangement

The structures of active Ag(I) species. In our earlier work, we proposed that Ag(I) catalysts would be four-coordinated20. In present work, not only the organic amines but also the substrate (o-alkynylaniline) would serve as potential ligands since all of them have long-pair electrons on the N atoms. Computationally, the proton affinities for DBU, DABCO and o-alkynylaniline are 187.0, 175.4 and 157.5 kcal/mol, respectively, indicating that the basicity decreases in the tendency of DBU>



DABCO>o-alkynylaniline. In order to identify the solvent structure of the Ag(I) catalysts, we compare the binding free energy of four-coordinated AgB4-xSx (x=0-4) species (see Table 1), in which B represents amines and S denotes o-alkynylaniline. According to DFT calculations, for Ag(I)/DBU, the most stable structure is Ag(DBU)3S+•NO3- which is calculated to be exothermic by 42.1 kcal/mol with respect to AgNO3, DBU and o-alkynylaniline. On the other hand, when DABCO is adopted, the calculated binding free energies are always lower than those of Ag(I) catalysts coordinated with DBU for given x. This is ascribed to the weaker basicity of DABCO compared with DBU. Interestingly, when DABCO is used, the AgB4-xSx with x=2-4 have nearly degenerated binding energies (-32.3 to -33.3 kcal/mol), indicating that the ligand exchange could readily take place. For simplicity, we use Ag(DABCO)2S2+•NO3as the initial computational model. Table 1 The binding free energies of AgB4-xSx+•NO3- (x=0-4) (Unit: kcal/mol) x DBU DABCO 0 -32.1 -26.4 1 -42.1 -29.6 2 -40.7 -33.3 3 -36.5 -32.3 4 -32.8 -32.8 Mechanisms of carbon dioxide incorporation. Figure 1 depicts the free energy profiles calculated for the incorporation of carbon dioxide into propargylic amine to give benzoxazin-2-one. The black and red lines represent the reactions catalyzed by Ag(I)/DBU and Ag(I)/DABCO systems, respectively. The optimized structures for relevant TS’ along the reaction profiles are illustrated in Figure 2.


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(L) 3Ag (L) 3Ag

Ph

Ph N H H NO3

Ph

O TS2-3

17.7

NO3 N H C O H

16.9

16.7

N H

B B

15.7

10.1

9.3

9.5

Ph O

TS5-6

O

18.4

N H

17.6

TS6-1 BH+-NO 3-

3

CO2

H (L)3Ag

O

TS4-5 10.0

B

NO3

O

(L)3Ag

O

C

O

12.2 11.9 Ph

8.3 7.5

(L)3Ag 1.1

Ph

NO3

(L)3Ag

Ag(L)3

B

Ph Ph

-0.5

O

O -6.7

O

N H

4

N H

O

N H

Ph

-13.7

H

1 NO3 DBU DABCO

+B O

7

6

(L)3Ag

H

NH2

(L)3Ag

BH+-NO 3-

NO 3 N H H C O Ph

N H

Ph

0.4

0.0

Ph N

2 O

H N

C O 5

H NO 3

Ag(L) 3

H 1

Figure 1. The free energy profiles calculated for the carbon dioxide incorporation mechanism to form benzoxazin-2-one intermediate on the basis of the pathways shown in Scheme 2. The free energies are given in kcal/mol.



DBU

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DABCO

TS2-3 1.417 1.900

1.928

TS4-5 1.156 1.471

1.402 1.202

TS5-6 2.306

1.267

2.287

1.950 1.910

TS6-1 2.460

2.368

1.467

1.423

1.230

1.280

Figure 2. Optimized structures for relevant TS’ along the energy profiles shown in Figure 1. Bond lengths are given in Å. As shown in Scheme 2, CO2 incorporation would be initiated by the nucleophilic attack of amine N on the C of CO2. To expose the amine group in substrate, the Ag(I)


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catalysts should undergo isomerization from the amine coordinated species (1) to the alkyne-coordinated species (2). Such a step is predicted to be slightly endothermic by 0.4-1.1 kcal/mol. In the next step, C-N bond would be formed through the reaction of amine and CO2. The barriers of TS2-3 are predicted to be 15.6 and 17.3 kcal/mol with respect to 2 for Ag(I)/DBU and Ag(I)/DABCO, respectively. This indicates that the ligands have negligible effect on the formation of C-N bond since the reactions take place away from the Ag(I) centers and other ligands. Geometrically, we indeed find that the distances between Ag(I) centers and alkyne carbon atoms in TS2-3 are as long as 3.268-3.782 Å. With the C-N bond forming, the amine group converts into the ammonium group, which could be further deprotonated by a base to give carbamate 5. For Ag(I)/DBU catalyst, the DBU is in large excess such that the free DBU would serve as base. However, the amount of DABCO is limited (0.2 equiv.) experimentally. As above mentioned, the Ag(DABCO)2(S)2+ and Ag(DABCO)(S)3+ have nearly degenerated energies, indicating that the free DABCO could be produced from the ligand exchange. Computationally, the deprotonation processes (TS4-5) only involve small barriers of 0.1~0.2 kcal/mol whatever DBU or DABCO is used. Following the TS4-5, the reactions are downhill by 16.8 and 10.0 kcal/mol for DBU and DABCO, respectively. Next, the ring closing reactions will take place. In principle, the carboxylate could attack both of the C atoms on the alkyne group, producing six-member ring or seven-member ring products. Test calculations show that for Ag(I)/DBU, the



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seven-member ring TS is 5.1 kcal/mol less favorable than the six-member ring counterparts such that we only consider the six-member ring route hereafter. DFT calculations show that the ring closing reactions (TS5-6) exhibit barriers of 24.3 and 18.9 kcal/mol for Ag(I)/DBU and Ag(I)/DABCO, respectively. This indicates that the ring closing reactions benefit from the weaker bases (DABCO or o-alkynylaniline) serving as ligands. To understand the reason behind, TS energy decomposition analysis is performed.21 As shown in Figure 3, ∆Ed, ∆Es, cat, ∆Es, S and ∆Eint represent the dissociation energy, the strain energy of catalyst part AgL3+, the strain energy of substrate part and the interaction energy between two deformed parts, respectively. All these data can be obtained by using single point energy calculation, and all the changes of zero point energy, the thermal capacity contribution, and the entropy contribution are grouped into the ∆Gcorr. Thus, the free energy barrier of the ring closing can be decomposed into: ∆G≠= ∆Ed + ∆Es,cat +∆Es,S + ∆Eint + ∆Gcorr

(4)

Ag(L)3 + S

Es,S Ag(L)3 +S

Eint

Es,cat.

(L)3Ag

Ag(L)3 +S

Ph Gcorr

O N H

O

Ed G

(L)3Ag

(L)3Ag

Ph

Ph N O

C

H

O Ag(L)3-S

N H

O

5

O

6

Figure 3. Diagram of energy decomposition of ring closing TS’.


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Table 2 Energy decomposition analysis for the ring closing TS’. (Unit: kcal/mol) Energies

Ag(I)/DBU

Ag(I)/DABCO

∆Ed ∆Es,cat. ∆Es, S ∆Eint ∆Gcorr

28.4 7.1 29.0 -38.0 -2.2

30.5 2.4 27.9 -43.9 2.0

From Table 2, it is clear to see that different amines have virtually no effect on both the ∆Ed and the ∆Es,S. However, the ∆Es,cat, i.e. the deforming energy of AgL3+, is predicted to be endothermic by 7.1 kcal/mol for Ag(I)/DBU catalyst, which is 4.7 kcal/mol higher than that of Ag(I)/DABCO. This can be rationalized in terms of the competition among the ligands coordinated to Ag(I). Upon the ring closing, the Ag-C bond is going to form, while the Ag-N bonds are going to elongate. Geometrically, two of the Ag-N bonds would elongate from 2.222-2.240 Å to 2.415-2.421 Å for Ag(I)/DBU, while for Ag(I)/DABCO, only one of the Ag-N bonds

would

significantly stretch from 2.445 Å to 2.670 Å. To better describe the changes of the Ag-N bonds, BVS calculations are carried out. We show that during the process, the sum of the Ag-N bond orders decreases from 0.923 to 0.639 for Ag(I)/DBU, and 0.691 to 0.533 for Ag(I)/DABCO. In this regard, stronger basicity of the amine, more loss in the Ag-N bond order, and higher cost for the ∆Es,cat. In contrast, less compact structure of the TS5-6 would be compensated by the entropy increase. Indeed, our calculations demonstrate that the ∆Gcorr for Ag(I)/DBU is 4.2 kcal/mol lower than that of Ag(I)/DABCO. Interestingly, the ∆Es,cat. and the ∆Gcorr can nearly be offset each



other such that the difference (5.4 kcal/mol) in TS5-6 could be well captured by the difference in the ∆Eint (5.9 kcal/mol ). By definition, the ∆Eint describes the interaction between AgL3+ and the alkyne group. From this, we expected that the ∆Eint would be associated with Lewis acidity of Ag(I). Indeed, NBO charge analysis shows that Ag in Ag(DBU)3+ is positively charged by +0.462 a.u., while in the Ag(DABCO)(S)2+, Ag carries a charge of +0.606 a.u. All these indicates that weaker bases would make the Ag center more acidity, thus enhancing the interaction between AgL3+and the alkyne group, resulting in the lower barrier of the ring closing (TS5-6). In the final step of Lewis acid catalysis, proton transfer reactions will occur to give benzoxazin-2-one 7 (See Figure 1). Based on our calculations, such steps (TS6-1) involve small barriers of 3.6-4.7 kcal/mol. From above discussion, the rate-determining step for the Lewis acid catalysis cycles is TS5-6 which is calculated to be 24.3 kcal/mol ( or 18.9 kcal/mol) for Ag(I)/DBU (or Ag(I)/DABCO). Mechanisms for the intramolecular rearrangement. Figure 4 illustrates the free energy profiles for the Lewis base catalysis from benzoxazin-2-one intermediate 7 to 4-hydroxyquinolin-2(1H)-one product 15. And the optimized TS’ along the energy profiles are demonstrated in Figure 5. As shown in Figure 4, the intramolecular rearrangements begin with the deprotonation by bases (TS8-9). Clearly, DBU exhibits higher reactivity than DABCO (2.1 kcal/mol vs. 5.4 kcal/mol), consistent with the tendency of basicity. Following TS8-9, we can obtain the charge separated intermediates 9. Computationally, when DBU is used, 9 is 1.3 kcal/mol more favorable than 8; however, for DABCO, 9 is


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predicted to be less favorable than 8 by 4.8 kcal/mol. This indicates that strong base could stabilize the deprotonated form of benzoxazin-2-one, which shifts the equilibrium to the right side. In the next step, 9 undergoes isomerization via C-O bond breaking to form the isocyanate species 10. We show that whatever the amine is adopted, the calculated barriers of TS9-10 with respect to 9 are nearly the same (~13 kcal/mol). However, the distance of the breaking C-O bond in TS9-10 with DBU is 0.065 Å longer than that with DABCO. Subsequently, the carbon atom of the isocyanate will be attacked by a carbon atom from the C=C double bond via a six-member ring TS (TS10-11) to make a C-C bond, resulting in 1,3-diketone ion species 11. Interestingly, the bond lengths of the C-C, C-N and C-O in TS10-11 are nearly the same for DBU and DABCO. From 10 to 11, the reaction is strongly exothermic by 17.6~21.2 kcal/mol, which pays back the cost of forming the unstable isocyanate species 10. Next, the proton will transfer back to the N atom to produce 1,3-diketone intermediate 13 by overcoming barriers of 1.6-3.8 kcal/mol (TS12-13). Finally,

the

enolization

reactions

occur

via

TS14-15

to

make

4-hydroxyquinolin-2(1H)-one product 15. According to our calculations, only 2.8-6.2 kcal/mol barriers need to be overcome from 14 to TS14-15. From above discussion, the effective barrier for the Lewis base catalysis is corresponding to the energy gap between 9 (or 8) and TS10-11, which is calculated as 13.9 kcal/mol (or 20.6 kcal/mol) for DBU (or DABCO).



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B H

Ph

N Ph

O

H B

O

Ph

O

N C O

TS9-10

TS10-11 4.7

O N

2.2

O

H

0.3

-13.7 Ph

-14.6

B

-15.9

O N C O

-18.0 Ph

O

H H B

10

N H

O

N

-21.3

-26.5

-27.5 -28.2

O

B

-29.7

-30.3

H Ph

DABCO

N

C O

11

N

Ph Ph

Ph

DBU

O H

O

O

O

B

-27.8

-27.4

H 9

TS14-15

-21.8

-25.0

-23.4

B B 8

N C O H

B TS12-13

-17.3

O O

O H Ph

Ph

-11.1

Ph

7

C O N

-6.2

-16.7

O N H

Ph

-5.2

-10.5 B

O

-4.1

B TS8-9

C O

N H

14 -43.6 B H

H B 12

C O

-32.4

N C O H

B

O Ph

13 N H 15

C O

Figure 4. Free energy profiles calculated for the intramolecular rearrangement from benzoxazin-2-one intermediate to 4-hydroxyquinolin-2(1H)-one product assisted by DBU or DABCO. The free energies are given in kcal/mol. DMSO was used as the solvent in the PCM geometry and energy calculations.

From above discussion, we infer that different bases would exhibit different reaction manners. The Lewis acid catalysis would be enhanced when the weaker bases serve as ligands, while the Lewis base catalysis would be promoted by using the stronger bases. For Ag(I)/DBU, the effective barriers for the Lewis acid catalysis and the Lewis base catalysis are predicted to be 24.3 and 13.9 kcal/mol, respectively. That is to say, once benzoxazin-2-one 7 has been formed, the following rearrangement to 4-hydroxyquinolin-2(1H)-one 15 could be readily accomplished. Thus, when the target product is 15, it requires relative high temperature and excess DBU, in according with the experimental conditions (60°C and 1.0 equiv. DBU). On the other


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hand, when DABCO is employed, the effective barrier for the Lewis acid catalysis is calculated as 18.9 kcal/mol, 1.7 kcal/mol lower than that of the Lewis base catalysis. This indicates that to obtain 7 with high selectivity, relative low temperature and limited amount of DABCO are preferred, also consistent with experiment conditions (20°C and 0.2 equiv. DABCO). Recently, we have systematically investigated the roles of amines in both heterogeneous and homogeneous transition-metal catalysts. For the heterogeneous selective hydrogenation, we demonstrated that the amine pre-coating on the surface would create a unique environment which only allows specific groups or specific substrates to interact with the catalytically active sites, thus raising the chemoselectivity.22 For the homogeneous CO2 incorporation, we have demonstrated that the five-member ring TS become more favorable than the six-member ring counterpart when Ag(I) catalyst was saturated with stronger base, such as DBU.20 And in this contribution, we showed that the weaker base enhances the reactivity of Lewis acid catalysis but slows down the following Lewis base catalysis, and vice versa. All these examples are complementary, which greatly enrich our knowledge on how to design transition-metal/ amine combined catalysts with higher selectivity.



Figure 5. Optimized structures for relevant TS’ along the energy profiles shown in Figure 4. Bond lengths are given in Å.

Conclusions The detailed mechanisms of the Ag(I)/amine-catalyzed CO2 incorporation into propargylic amine and the following rearrangement have been systematically


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investigated with the aid of DFT calculations. The main conclusions are summarized as follows: (1) The whole reaction might involve two stages, namely Lewis acid catalysis and Lewis base catalysis. In the former stage, Ag(I) serves as the Lewis acid center, which plays key roles to bind the reactants and stabilize the TS’ for CO2 incorporation and the following ring closing, leading to formation of benzoxazin-2-one. In the latter case, amine itself would serve as the base catalyst, which promotes the intermolecular rearrangement of benzoxazin-2-one to 4-hydroxyquinolin-2(1H)-one. (2) For the Lewis acid catalysis stage, the rate determining step is corresponding to the ring-closing, whose reactivity is closely associated with the acidity of the Ag center. Our calculations show that DABCO, which is less basic than DBU, makes the Ag center more acidic, thus facilitating the ring closing step. (3) The reactivity tendency of the Lewis base catalysis is opposite to that of Lewis acid catalysis. In this case, the amine with the stronger basicity could stabilize the deprotonated form of benzoxazin-2-one, which enhances the following rearrangement. (4) Since the Lewis acid catalysis and the Lewis base catalysis would proceed sequentially, the final product selectivity critically depends on the basicity of the amine.

When DBU is employed, the effective barriers for the Lewis acid catalysis

significantly higher than the Lewis base catalysis (24.3 v.s. 13.9 kcal/mol), indicating that once benzoxazin-2-one has been formed, the following rearrangement could be readily accomplished. Thus, elevating the reaction as well as increasing the amount of



DBU would improve the selectivity of 4-hydroxyquinolin-2(1H)-one. On the other hand, when the DABCO is used, the Lewis acid catalysis has lower effective barrier than that of the Lewis base catalysis (18.9 v.s. 20.6 kcal/mol). In this case, lowering the reaction temperature as well as controlling the amount of DABCO would stop the reaction at the intermediate stage. ASSOCIATE CONTENTS Supporting Information The following file is available free of charge on the ACS Publications website at DOI: 10.1021 The Cartesian coordinates and electronic energies for all of the calculated structures (PDF)

ACKNOWLEDGMENT We acknowledge the financial support from the MOST of China (2017YFA0207303), the National Nature Science Foundation of China (21373167, 21573178, 21773192), the Fundamental Research Funds for the Central Universities (20720160046 and 20720150043) and the Program for Innovative Research Team in Chinese Universities (IRT_14R31).

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Hydrogenation by Using Molecular Modification Strategies: Experiment and Theory. Catal. Today 2017, 279, 36-44.


Mechanisms of CO2 Incorporation into Propargylic Amine Catalyzed

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