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COMMUNICATIONS. J. Connon, M. O. Senge et al. Conformational control of nonplanar free base porphyrins: towards bifunctional catalysts of tunable basicity

Volume 54Number 14 January 2018Pages 1-112

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D. Feng, S. Chen, X. Xu, L. Dang and P. N. Liu, Chem. Commun., 2020, DOI: 10.1039/D0CC02398A.

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a.Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials and School of Chemistry and Molecular Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, ChinaE-mail: liupn@ecust.edu.cn

b.Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Guangdong, 515063, China.

Electronic Supplementary Information (ESI) available: Experimental procedures and characterization data of products, spectra of products. CCDC 1956510 for 3m. For ESI and crystallographic data in CIF or other electronic format See DOI: 10.1039/x0xx00000x

Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Construction of multiple bonds via a domino reaction of trifluoroacetimidoyl nitriles with in-situ generated bis-nucleophilesDeng-Yuan Li,a Jia-Yan Chen,a Da-Fu Feng,a Shuang Chen,a Xian-Kuan Xu,a Li Dang,b and Pei-Nian Liu*,a

A transition-metal-free double addition/double rearrangement domino reaction affording CF3-substituted pyrimidines was developed, which enables the one-pot construction of five new bonds, namely three C–C bonds and two C–N bonds. The keys to achieve this highly efficient reaction include the delicate design of the bis-nucleophiles in-situ generated from the dimerization of alkyl nitriles and the use of trifluoroacetimidoyl nitriles containing C=N, C≡N, and CF3 groups as the reactant. The mechanistic studies by the experiments and DFT calculations reveal that the transformation involves two addition and two unprecedented rearrangement processes.

Efficient construction of carbon-carbon (C–C) and carbon-heteroatom (C–X) bonds is the central goal in synthetic organic chemistry, because these bonds are basic components that constitute organic compounds. In particular, the highly efficient construction of multiple C–C and C–X bonds in a single operation has attracted increasing attention because it represents a straightforward and step-economic strategy for achieving structural complexity from simple reactants.1 Domino reactions provide feasible routes for the one-pot construction of multiple bonds.2-4 For example, they have been applied successfully in the construction of various bioactive molecules,3,4 such as terpenes and steroids.3 However, the reported domino reactions could only form three or four new bonds in one operation, while one-pot reactions that form five or more new bonds remain a great challenge. In this study, a transition-metal-free domino reaction for the one-pot construction of five new bonds, namely three C–C bonds and two C–N bonds, was developed.

(a) Representative bioactive pyrimidines

N

N

NH2

O

N

CO2H

F3C

I

II

IVIII

N N

HN

N

N

S

Bz

N N

HN

Cl

NH

NN N

HN

N

HN

N

O

Cl

F

(b) Reaction of 2 with nucleophile or bis-nucleophile

Nu- ArN CF3

Nu+ CN-

Pathway I8:

+addition/elimination

nucleophile

ArN CF3

CAr

HN

N N

CF3

R

NCR

R

2

Ar

N

NN

RC R

N

CF3

Li

H

THF

n-BuLi

rearrangement

NN

RR

Li

N

F3CAr

H

C

N

rearrangement/elimination

R

NC

R

NLi H

+

bis-nucleophiledouble

addition

ArN CF3

C

Pathway II (this work):

Nu-: Ar-, n-Bu-, RC≡C-, RO-, RR'N- etc.2

2 3

1 N

N

Scheme 1 Domino reaction to synthesize CF3-substituted pyrimidines.

N-Heteroaromatic skeletons are among the most significant structural components of pharmaceuticals, agrochemicals, and natural products.5 Pyrimidine is one of the most prevalent cores and exhibits a range of attractive biological activities.6 Scheme 1a shows pyrimidine derivative I, a promising inhibitor of melanoma.6a Compound II has been demonstrated to be active against Aurora A kinase in oncological studies.6b Pyrimidine derivative III was recently identified as a dual leucine zipper kinase inhibitor for the treatment of neurodegeneration.6c More importantly, inhibitor IV containing trifluoromethyl (CF3) group has displayed significantly improved diacylglycerol acyltransferase 1

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inhibition compared to that of the inhibitor without CF3, as well as better tolerance and a superior safety profile.6d However, the synthesis of CF3-substituted pyrimidines usually suffers from lengthy routes and cumbersome procedures,7 and the direct synthesis method from easily accessible starting materials is eagerly desired.

Recently, we reported an efficient method to synthesize a new type of compound, trifluoroacetimidoyl nitrile 2, which is a promising synthon containing imino (C=N), cyano (C≡N), and CF3 groups.8 The reaction of trifluoroacetimidoyl nitriles 2 with various nucleophiles such as aromatic anions, alkyl anions, terminal alkynes, alcohols and amines etc. undergoes addition/elimination process to afford imines due to the good leaving ability of cyanide anion (CN-) (Scheme 1b, Pathway I).8 To increase the atom economy and make most use of the groups in 2, we herein introduce the in-situ generated bis-nucleophile from dimerization of alkyl nitrile 1. The bis-nucleophile undergoes double addition to 2, one to C=N group and another one to C≡N group, and subsequent two unprecedented rearrangement steps to provide 2-(trifluoromethyl)pyrimidin-4-amines 3 with excellent regioselectivity and in good yields (Scheme 1b, Pathway II).

Table 1 Optimization of reaction conditionsa

base

solvent, 1 h

2a

3a

N CF3

C HN

N N

CF3

Me

Me C N

1asolvent, 1 h

Ph

PhN

MeC

NLi H

N

Entry Base Solvent T (°C) Yield (3a, %)b

1 n-BuLi MeCN −78 842 n-BuLi toluene −78 283 n-BuLi DCE −78 364 n-BuLi 1,4-dioxane −78 525 n-BuLi n-hexane −78 766 n-BuLi Et2O −78 777 n-BuLi THF −78 828c n-BuLi THF −78 519 MeLi THF −78 2710 LDA THF −78 3611 t-BuOK THF −78 ND12 n-BuLi THF −40 8013 n-BuLi THF 0 5114 - THF −78 NDa Reaction conditions: 1a (2 mmol), 2a (0.2 mmol), base (0.5 mmol), solvent (2 mL), −78 °C, under Ar. b Isolated yield. c 1a (1 mmol) was used.

Studies were initiated using nitrile 2a as a model substrate (Table 1). This substrate was treated with n-BuLi (2.5 equiv) in MeCN at −78 °C for 2 h. We are delighted that 2-(trifluoromethyl)pyrimidin-4-amine 3a was isolated in 84% yield (entry 1), suggesting that MeCN acted as not only the solvent but also a reactant. This result inspired us to investigate various sets of reaction conditions in order to optimize the process. A series of reactions were performed with the amount of MeCN reduced to 10 equiv and the results revealed that tetrahydrofuran (THF) was the best solvent among the tested solvents (entries 2–7). The reduction of the amount of alkyl nitrile instead of using as solvent is important for the reaction scope expansion. Further reducing the amount of MeCN to 5 equiv resulted in a lower yield of 3a (entry 8).

Other bases, namely MeLi, lithium diisopropylamide (LDA), and t-BuOK, did not improve the yield of 3a (entries 9–11). Raising the reaction temperature to −40 °C led to the same yield (entry 12), while an even higher temperature (0 °C) reduced the yield to 51% (entry 13). A control experiment confirmed that n-BuLi was essential for the formation of 3a (entry 14). It is noteworthy that 3a could easily convert to 1-(trifluoromethyl)benzo[4,5]imidazo[1,2-c]pyrimidine (3a’) in 61% yield through C–H activation (see Supporting Information), displaying the potential application of the domino reaction for the construction of important N-fused polycyclic skeletons.9

With the optimized reaction conditions in hand, we explored the substrate scope of the domino reaction between trifluoroacetimidoyl nitriles and alkyl nitriles (Scheme 2). First, various alkyl mononitriles, namely butyronitrile, hexanonitrile, 2-methoxyacetonitrile, and 2-(methylthio)acetonitrile, were subjected to the domino reaction with 2a, producing corresponding products 3b–3e in 46%–78% yields. However, the use of phenylacetonitrile and methyl 2-cyanoacetate did not lead to the desired domino products, although the addition/elimination products, 3-([1,1'-biphenyl]-4-ylimino)-4,4,4-trifluoro-2-phenylbutanenitrile and methyl 3-([1,1'-biphenyl]-4-ylamino)-2-cyano-4,4,4-trifluorobut-2-enoate, were obtained in 71% and 62% yields, respectively. The results suggest that the steric hindrance at the α-C of the alkyl nitrile inhibits the domino reaction. Notably, adiponitrile, heptanedinitrile, and octanedinitrile produced single bicyclic products 3f, 3g, and 3h in good yields, while other dinitriles such as malononitrile and undecanedinitrile did not afford the desired products.

n-BuLi (2.5 equiv) THF, -78 oC, 1 h

3

ArN CF3

CAr

HN

N N

CF3

R

Scope of alkyl nitriles (1):

HN

PhN N

CF3

R1 R1 R1 = H, 3a, 82%R1 = Et, 3b, 46%

R1 = n-Bu, 3c, 52%R1 = MeO, 3d, 78%R1 = MeS, 3e, 49%

HN

PhN N

CF33f, 66%a

HN

PhN N

CF33g, 84%a

HN

PhN N

CF33h, 55%a

NCR

1THF, -78 oC, 1 h

RR

NC

R

NLi

2H

R3 = Me, 3s, 90%R3 = MeO, 3t, 82%R3 = Et2N, 3u, 76%

R3 = Cl, 3v, 63%R3 = t-Bu, 3w, 84%

R3 = Ph3C, 3x, 48%

3o, 58%

R2 = Me, 3i, 70%R2 = MeO, 3j, 71%

R2 = F, 3k, 77%R2 = Cl, 3l, 84%

R2 = Br, 3m, 79%

HN

R3

R2

HN

3n, 74%

HN

3r, 52%3q, 80%

HN

HN

3aa, 63%

HNO

Me

NEt

HN

R4

Me

Me

HN

N N

CF3

Me

N N

Me

CF3

N N

N NN N

Me

CF3 CF3

Me

Me

CF3

N N

Me

CF3

N N

CF3

Me

N N

Me

CF2CF3

Scope of trifluoroacetimidoyl nitriles (2):

3p, 60%

HN

N N

Me

CF3

HN

R4 = Me, 3y, 59%R4 = Ph, 3z, 64%

+

N

Scheme 2 Substrate scope. Reaction conditions: 1 (2 mmol), 2 (0.2 mmol), n-BuLi (0.5 mmol), THF (2 mL), -78 °C, under Ar. Isolated yields are indicated. a Dinitrile (1 mmol) was used.

Next, we investigated the domino reaction using a series of trifluoroacetimidoyl nitriles. Trifluoroacetimidoyl nitriles 2 containing electron-donating groups, namely Me and MeO, as R2 reacted with 1a, providing products 3i and 3j in yields of

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70% and 71%, respectively. Trifluoroacetimidoyl nitriles 2 bearing F, Cl, and Br groups gave products 3k–3m in moderate yields. It is noteworthy that the structure of 3m was confirmed by single-crystal X-ray diffraction analysis (Figure S3), along with the general nuclear magnetic resonance (NMR) and high-resolution mass spectrometry (HRMS) spectra.

Naphthyl trifluoroacetimidoyl nitriles underwent the domino reaction to provide corresponding products 3n and 3o in yields of 74% and 58%, respectively. Moreover, heterocyclic trifluoroacetimidoyl nitriles derived from 1H-indole, 9-ethyl-9H-carbazole, and dibenzo[b,d]furan participated in the domino reaction under the optimized conditions to produce corresponding products 3p–3r in yields of 52%–80%. Next, the suitability of phenyl trifluoroacetimidoyl nitriles bearing various substituents on the benzene ring for the domino reaction was evaluated. Reactions with nitriles bearing Me, MeO, Et2N, and Cl groups at the para-position of the benzene ring underwent this reaction to produce desired products 3s–3v. t-Bu- and trityl-substituted phenyl trifluoroacetimidoyl nitriles also successfully participated in the reaction to produce 3w and 3x. Multi-substituted trifluoroacetimidoyl nitriles performed well in the reaction, affording products 3y and 3z in 59% and 64% yields, respectively.

To verify the versatility of the domino reaction, pentafluoropropanimidoyl nitrile was synthesized and subjected to a reaction with MeCN under the optimized conditions. Desired product 3aa was isolated in 63% yield, indicating that the current method is a powerful tool for the production of perfluoroalkyl pyrimidin-4-amines.

In order to clarify the mechanism of the domino reaction, β-cyanoenamine 4 was isolated in 30% yield through the dimerization of alkyl nitrile 1d (Scheme 3, eqn 1).10 Moreover, when the reaction of 2a with 4 (1.5 equiv) was performed in the presence of 0.25, 1.5 or 2.5 equiv of n-BuLi, 3d was isolated in 18% and similar 56%, 58% yields, respectively (eqn 2). The results demonstrate that the in-situ generated bis-

nucleophile, lithium β-cyanovinylamide 5, is the reaction intermediate in the domino reaction.

When CD3CN was reacted with 2a under the standard conditions, [D]3a was isolated in 87% yield. Deuteration rates of >98% in the methyl group and 84% in the pyrimidine ring of [D]3a were observed (eqn 3). The slight loss of deuterium rate in the pyimidine ring of [D]3a might attribute to the slight H/D exchange with exogenous proton source such as H2O. The results confirm the elimination of CN- from the bis-nucleophile intermediate generated from the dimerization of the alkyl nitriles.

HN

PhN N

CD3

CF3

D

(>98% D)

(84% D)

[D]3a, 87%

THF, -78 oC, 1 hMeO CN OMe

OMe

NC

NH2

n-BuLi (0.25 equiv)2

4, 30%

N

Ph

CF3

C +

2a (1 equiv)

THF, -78 oC, 1 h

HN

PhN N

CF3

OMe

n-BuLi (2.5 equiv): 3d, 58%n-BuLi (1.5 equiv): 3d, 56%

n-BuLi (0.25 equiv): 3d, 18%

OMen-BuLi

CD3CN

n-BuLi (2.5 equiv)THF, -78 oC, 1 h

then, 2a (1 equiv)THF, -78 oC, 1 h

n-BuLi (2.5 equiv)MeCN (10 equiv)THF, -78 oC, 1 h

3l, 70%

+

N CF3

C

Cl

N CF2CF3

C

Me3aa, 63%

2l (0.5 equiv) 2aa (0.5 equiv)

without n-BuLi: 3d, ND

OMe

OMe

NC

NH2

then 2l and 2aaTHF, -78 oC, 1 h

(1)

(2)

(3)

(4)

4 (1.5 equiv)

OMe

OMe

NC

N

bis-nucleophile intermediate 51d (1 equiv) Li H

N

N N

[D]1a (10 equiv)

Scheme 3 Preliminary mechanistic studies.

In addition, a crossover experiment involving substrates 2l, with Cl and CF3 substituents, and 2aa, with Me and CF2CF3 substituents, was carried out under the standard conditions, and corresponding products 3l and 3aa were obtained without crossover products (eqn 4).11a The result indicates that the rearrangement of CF3 group is an intramolecular process.

NC N

Me

H

F3C

N

MeNC

Li

0.0

F3C CN

MeNC

NLi

0.6

-16.4

2.6

-1.6

-14.0

4.82.8

-9.0

-0.7

-38.7

LiCN

-14.1

PhN

LiNC

NHN

CF3

Me

10.6

-18.9

PhN

LiC

N

NHN

CF3

Me

Ts6

N

Me

HNCF3

NPh

C

N

CF3NPh

A

3

N

Me

NCF3

HNPh

NCMe

1a

n-BuLi

2

Thorpe reaction

tautomerization

H I

Ts5

G

Ts4

F

E

Ts2

D

C

Ts1

B

2

C

addition

Li

N

MeNC

N CF3

N

Ph LiH

N

MeNC

HN

N

CF3

Ph

Li

N

MeNC

N

Ph NHLiF

CF2

N

MeNC

NLi

NH

F3CPh

N

MeNC

NLi

N

F3CPh

H

HN

N

Ph

HNPh N

MeNC

N

NHLiF

CF2 N

MeNC

N

Ph NHLi

FF2C

N

MeNC

CF3

NPh

Li NH

N

MeNC

HN

N

CF3

Li

Ph

Ph

Ts3

Scheme 4 Proposed reaction mechanism based on DFT calculation.

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To further elucidate the mechanism of the domino reaction, we performed density functional theory (DFT) calculations on domino reaciton starting from intemediate B and establised the intramolecular addition and subsequent two rearrangement pathway as the lowest energy route (Scheme 4). Intermediate B obtained from the doubtless addition of A to 2 undergoes intramolecular addition of the lithium imino species to the C≡N group to generate cyclization intermediate C, with the extreamly low barrier of 0.6 kcal/mol. The proton exchange in C forms the active zwitterion intermediate D, which undergoes Li-promoted rearrangement of CF3 to afford intermediate F via elimination11 and addition process, with respective reaction barrier of 16.7 kcal/mol and 6.4 kcal/mol. The next rearrangement of F occurs through the intramolecular addition of lithium amide species to the imino group (reaction barrier: 16.8 kcal/mol) and the following cleavage of C–C bond (reaction barrier: 19.6 kcal/mol), which accomplish the ring expansion to afford the zwitterion intermediate H. The elimination of LiCN from H with the reaction barrier of 18.2 kcal/mol gives intermediate I, which can readily tautomerize to afford the final product 3. In the calculated energy landscape, the C–C cleavege step from azabane intermediate G to six-membered ring intermediate H demonstrates highest reaction barrier, and the value of 19.6 kcal/mol is coherent with the low reaction temperature of the domino reaction in the experiments.

In summary, a novel transition-metal-free double addition/double rearrangement domino reaction, which can construct three C–C bonds and two C–N bonds in a single operation, has been achieved. This highly efficient transformation affords CF3-substituted pyrimidines from trifluoroacetimidoyl nitriles and alkyl nitriles with excellent regioselectivity and in good yields. Mechanistic experimental studies and DFT calculations indicate that the reaction involves the Thorpe reaction of alkyl nitrile to form a lithium β-cyanovinylamide, which undergoes subsequent addition to a trifluoroacetimidoyl nitrile. The product forms after subsequent addition cyclization, unprecedented rearrangement of CF3 group and another rearrangement to accomplish ring expansion. As the first example of the direct synthesis of CF3-substituted pyrimidines, this protocol shines light on the step-economic construction of multiple bonds and may provide an efficient approach to achieve structural diversity in N-heteroaromatics.

This work was supported by the National Natural Science Foundation of China (Project No. 91845110, 21672059 and 21925201), the Program of the Shanghai Committee of Science and Technology (Project No. 18520760700) and the Program for Eastern Scholar Distinguished Professor.

Conflicts of interestThe authors declare that they have no conflict of interest.

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transition-metal-freedomino reaction

ArN CF3

CAr

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one-step constructionof three C-C and twoC-N bonds

Page 5 of 5 ChemComm

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