Promoting Highly Diastereoselective γ-CH Chalcogenation of α-Amino

predictively r-selective thioarylated products (9-11) in useful yields. Such thioarylated amino acids could serve as useful building blocks in peptide...
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Promoting Highly Diastereoselective #-C-H Chalcogenation of #-Amino Acids and Aliphatic Carboxylic Acids Srimanta Guin, Arghya Deb, Pravas Dolui, Souvik Chakraborty, Vikas Kumar Singh, and Debabrata Maiti ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04074 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Catalysis

Promoting Highly Diastereoselective γ-C− −H Chalcogenation of αAmino Acids and Aliphatic Carboxylic Acids Srimanta Guin,ǂ Arghya Deb,ǂ Pravas Dolui, Souvik Chakraborty, Vikas Kumar Singh, and Debabrata Maiti* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ABSTRACT: A Pd(II) catalyzed highly regioselective γ−chalcogenation, thioarylation and selenoarylation, of aliphatic carboxylic acids has been demonstrated. The present protocol provides a direct access to make structural modifications of α-amino acids such as valine, isoleucine, and tert-leucine with high diastereoselectivity (up to 52:1). Sequential hetero-bifunctionalizations have been carried out 3 at γ−(sp )C–H’s, resulting in desymmetrization of quaternary centers. The applicative potential of the chalcogenated products was exhibited by using it as a precursor for the synthesis of the biologically relevant benzothiepinone moiety. Preliminary studies were carried out to gain insights about the mechanism. KEYWORDS: γ-chalcogenation, modification, diastereoselectivity, mechanistic studies

amino acid applications,

compared to its five-membered analogue. A systematic study is thus required to explore the enormous potential of distal 3 sp C–H functionalization. The urge remained to effectuate a strategy that can promote functionalization taming the remote γ-C–H bonds, alongside dealing with the constitutional and conformational constraints arising from a six-membered metallacycle. In this context, the bidentate directing groups (DG’s) have proved beneficial in accessing γ-C–H arylation of aliphatic carboxylic 3,7-9 acids, while other functionalizations at the same γ-C–H 10 position is restricted to fewer. Hence, the pursuit continued to find out how amenable is this framework (substrate affixed with 8-aminoquinoline) to a variety of functionalizations, in particular, the C–heteroatom bond formations.

T

he advent of C–H activation strategy has streamlined the retrosynthetic disconnection approach in building up 1 complex molecular diversity. In its itinerary thus far, a formidable challenge is posed by the relative inertness of the C−H bonds in alkanes. Recognizing the subtle reactivity differences amongst a multitude of C−H bonds present in a complex aliphatic system adds up the complicacies. Further, the fluxionality in aliphatic chains renders the regioselectivity control a more complex issue. Due to these difficulties, it becomes a pressing task in executing chemical transformations at the anticipated region of alkanes, in particular, those at distal positions. Notably, reaching out to 2 a remote sp C−H bond requires several manipulations in the substrate structure or in the directing group to bring the transition metal and the desired C−H bond to close 2 proximity. The same logic, in principle, should hold true for 3 distal sp C–H bonds as well wherein, despite the aforesaid inherent shortcomings, the combination of transition metal catalyst and a tethered directing group can provide the 3 required impetus in bringing out several impressive results. A scrutiny of this genre of C–H bond activation entails that it is the Pd catalyst, among others, which is widely used to create diversification in alkanes from common substrates 4 5 6 such as aliphatic carboxylic acids, amines, and imines via five-membered organometallic intermediates. The feasibility of these regioselective functionalizations is governed primarily by the thermodynamics of the intermediate metallacycle. Pertaining to the aliphatic carboxylic acids only, substantial progress has been made towards β-C−H activation using Pd catalyst. Development of similar C–H activation reactions via six-membered metallacycle to functionalize more distal carbon centers may impart the molecule with specific structural and functional features that would render it more acceptable as a lucrative molecule. However, moving distal across an aliphatic carboxylic acid, a similar success story cannot be foretold for distal γ-C–H activation. This could be attributed to the formation of a thermodynamically less favored six-membered metallacycle

Scheme 1. Directed γ-sp Aliphatic Carboxylic Acids

3

C–H Functionalization of

Organosulfur compounds possess a prime position amongst valuable chemical entities because of their 11 prevalence in a myriad of biological systems. In addition, they are privileged members of many synthetic drugs and 12 functional materials. The use of transition metal catalyst for the direct incorporation of thio-counterpart (C–S) at an unactivated C–H bond is an extremely attractive tool, albeit it embraces certain difficulties. The crucial one being the susceptibility of sulfur species to bind metal ions which leads to a potential catalyst poisoning and thereby impeding the 13 C–H functionalization process. However, the use of dichalcogenides have proven useful to minimize such

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catalyst retarding effect. Thioarylation and selenoarylation at the γ-position of aliphatic carboxylic acid derivatives, requiring the intermediacy of a six-membered metallacycle (Scheme 1A), remained completely unexplored. Hence to accomplish chalcogenation at the γ-position of aliphatic carboxylic acid has to deal with two major difficulties: (a) constitutional and conformational constraints arising from larger metallacycle and (b) inertness of aliphatic C–H bonds. Overcoming these challenges, herein, we unveil the Pd(II) catalyzed highly regioselective γ-chalcogenation, thioarylation and selenoarylation, of aliphatic carboxylic acids employing 8-AQ as the directing group. This transformation provides a direct access to make structural modifications of α-amino acids such as valine, isoleucine, and tert-leucine. The versatility of this protocol has been demonstrated by widening the scope of our strategy to sequential γ-(sp3)C–H activation resulting in desymmetrization of quaternary centers. Table 1. Ligand 16 Thioarylation N O

+

NH

Ph

S

Optimization

γ-sp

and phenanthroline based ligands (Table 1). 2Chloroquinoline (L12) turned out to be beneficial for the present transformation. Further inclusion of NaHCO3 as base afforded the maximum possible yield (78%) achievable under 16 the present conditions. The use of other transition metal catalysts such as Ni, Co, and Cu were found to be completely 16 ineffective for the present transformation. When thiophenol was used for thioarylation of 1a, under the optimized conditions it dimerized to disulfide mostly along with the formation of the desired γ-thioarylated product 1 in 10% yield. This observation shows the superiority of disulfides over free thiols as the thioaryl source in the present transformation. O

NHQ

R

Me R2 1 R

C–H

S

Pd(OAc)2 (10 mol%) Ligand (20 mol%) Ag2CO3 (3 equiv) Ph

dry THF, 130 C, 24 h

NH

O

tBu

N L1, 25%

Me

N Me L2, 22%

N Me L3, 32%

N L4, 46%

O

N Cl L5, 63%

OMe

N L8, 40%

L7, 55%

N

Me

L9, 38%

Cl N

OMe

L11, 52%

N

Cl

L12, 71%

OMe N

N L13, 58%

L14, 35%

Me tBu N O L15, 62%

Me

O

N

N

L16, 15%

The 8-aminoquinolamide of 3,3-dimethylbutanoic acid 1a was reacted with diphenyl disulfide (5 equiv) in the presence of Pd(OAc)2 and Ag2CO3 in dioxane solvent at 130 ο C. It was intriguing to find out that an exclusively γregioselective mono-thioarylated compound 1 had formed, albeit in a lower yield. For further augmentation in yield, systematic optimizations of the reaction parameters were carried out. After extensive optimization, Ag2CO3 was found to be a useful oxidant, while Cu-based oxidants and inorganic salts gave inferior results. Of the solvents tested, the ethereal solvents were found to be fruitful for this reaction, with THF providing the best result. Since exogenous ligands play a pivotal role in facilitating and 3 accelerating the regioselective functionalizations at sp C–H bonds, a prudent choice of the ligand might allow an 3a,3b enhanced yield of the desired thioarylated product. This motivation drove us to examine various pyridine, quinoline

t

NHQ

7, 56% Me

NHQ SPh NPhth

O

NHQ SPh NPhth

10, 58%

NHQ SPh Me

MeO

6, 70%

NHQ Me

Me

Me

O

NHQ SPh NPhth 9, 52%

8, 60%

O

3, 73%

Bu

O

MeO

Me

NHQ SPh

2, 69%

SPh

N L10, 60%

O O

SPh

MeO N

N F L6, 47%

N

5, 75%

MeOC

Me

COMe

NPhth =

SPh Me

NHQ

F3C

Br

O

4, 63%

MeO

O

SPh

SPh Me

Me

R2

N

SPh

NHQ

Me

NHQ

SPh Me

O

Q=

NHQ

+ Ph S S Ph NaHCO3 (2 equiv) R (5 equiv) dry THF, 130 C, 24 h R1 R = H, NPhth R1= Me, Et, CH2t Bu, CH2Ar R2= H, Me

SPh Me Me 1, 78%

N O

Pd(OAc)2 (10 mol%) L12 (20 mol%) Ag2CO3 (3 equiv) O

NHQ

O

(5 equiv)

Me Me Me

for

3

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Me

O

NHQ SPh NPhth

O

NHQ SPh NPhth

11, 65%

3

Scheme 2. γ-sp C–H Thioarylation of Aliphatic Acid 16 Derivatives Given the thermodynamic lower preference of a sixmembered metallacycle intermediate compared to a fivemembered analogue, a detailed assessment of the scope of this transformation was awaited. The exploration began with the 8-aminoquinoline amide of isovaleric acid. The standard reaction condition provided 69% yield of the desired γthioarylated product 2 with an exclusive mono-selectivity (Scheme 2). The 3,5,5-trimethylhexanoic acid derivative was next explored and it provided 73% yield of the corresponding mono-thioarylated product 3. Despite having a secondary γC–H in 3,5,5-trimethylhexanoic acid, the reaction occurs specifically at the primary C–H position only. Presumably, the inability to generate the intermediate metallacycle at the secondary position due to steric hindrance imposed could be accredited for this primary selectivity. Next, we planned sequential hetero-bifunctionalization of aliphatic acids by 8 carrying out γ-arylation as the first step and utilizing the product formed for further thioarylation at other primary C– H positions (Scheme 1B). Indeed the mono-arylated products of 3,3-dimethylbutanoic acid and isovaleric acid derivatives

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ACS Catalysis underwent smooth reactions with diphenyl disulfide to give their respective products (4-8). Since α-amino acids form the basis of biological research, incorporation of a new functionality which can alternate or modify the properties of them are extremely valuable. Under the present reaction conditions, α-amino acids possessing primary γ-C–H’s viz. Ltert-leucine, L-valine, and L-isoleucine derivatives afforded predictively γ-selective thioarylated products (9-11) in useful yields. Such thioarylated amino acids could serve as useful building blocks in peptide chemistry. The diastereoselective 1 ratio of the products (10-11) was determined from HNMR 16 spectra. Also, the specific optical rotation values of these products (9-11) suggest that there is no racemization at the α-center of the amino acids under the present reaction 16 conditions. O

NHQ

R R

Me 2 1R

+

R3

Pd(OAc)2 (10 mol%) L12 (20 mol%) Ag2CO3 (3 equiv) O

S

S R (5 equiv)

Q=

NHQ

NaHCO3 (2 equiv) R dry THF, 130 C, 24 h R1

R2

SR3

O

R = H, NPhth R1= Me, Et, Ar R2= H, Me O

NHQ

Me

NPhth =

O

S

O

S

S

14, 80%

NHQ

O

Me Me

O

S

F

S

F3 C

S

O2N

S

17, 68%

18, 77% O

NHQ

NHQ

S

S

Me

OMe

19, 42%

Me

20, 56%

NHQ

21, 52%

Me S

Me

O

Me

NHQ NPhth

Cl

S

NHQ Me Me

O

NHQ Me Me

O

O

Me Me

16, 65%

The next congener of the chalcogen group is selenium. Chemical reactivity of selenium is expected to be similar to that of sulfur, but it has rarely been utilized for aliphatic C– 15a,15b Se bond formation. We thought to explore the γ-C–H selenoarylation of various aliphatic acids with diselenides (Scheme 4). Pd(OPiv)2 as the catalyst was found to be conducive for selenoarylation while other reaction conditions remained the same as that for thioarylation. When selenol was used as selenoaryl source instead of diselenide, no desired product formation was observed. Various aliphatic carboxylic acids and α-amino acids attempted for γ-selenoarylation provided respective products (25-29) with high diastereoselectivity. Interestingly, selenoarylation could be extrapolated to substituted aryl diselenides (30-31, Scheme 4).

14 (X-ray structure)

NHQ

Me Me

15, 71% mono:di 2.8:1 O

Cl

NHQ Me Me

13, 62%

NHQ

S

O

NHQ Me O2N Me

12, 70% mono:di 2.5:1

N O

Me MeO Me

Cl

N

3

CF3 and –NO2 gave synthetically useful yields of the products (16-18). The protocol was also viable with the aliphatic disulfide such as dibenzyl disulfide (19). Moreover, the aliphatic carboxamides possessing an aryl group at the βposition worked well under the present reaction conditions giving decent yields of the γ-thioarylated products (20-21) (Scheme 3). However, when butanamide was subjected to the present protocol, it afforded the β-thioarylated product owing to the preferential formation of thermodynamically 16 favored [5,5]-fused over [5,6] fused palladacycle.

S Cl

OMe O

NHQ NPhth

dr: 25:1 23, 64%

22, 60% Me S Cl

Me

O NHQ NPhth

24, 70%

S Cl

O NHQ NPhth

dr: 25:1

3

Scheme 3. γ-sp C–H Thioarylation/Thioalkylation with 16 Substituted Disulfides For a thorough evaluation of the utility of this protocol, next, we turned our attention towards disulfide variants. Irrespective of the nature of substituents and their position in arene ring, the disulfides were well suited for the present transformation affording satisfactory yields of the γthioarylated products (Scheme 3). Electron donating groups at the para position such as –Me, –OMe and electron withdrawing groups such as –Cl, –NO2 gave good yields of their respective products (Scheme 3). The structure of 14 was unambiguously characterized through X-ray crystallography. Further, the substituents at the meta position such as –F, –

3

Scheme 4. γ-sp C–H Selenoarylation of Aliphatic Acid 16 Derivatives The practicality of this transformation was established by carrying out a gram-scale reaction (4.2 mmol scale) which afforded the desired γ-thioarylated product 1 in 60% yield (Scheme 5A). To harness the applicative potential of this methodology, the γ-thioarylated product 15 was treated with p-toluenesulfonic acid. This led to the formation of the methyl ester 32 and removal of the directing group. Upon

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oxidation of 32 with oxone, the sulphenyl group was transformed to sulfone 33 (Scheme 5B). While in another approach, the thioarylated product 12 underwent base hydrolysis to give the corresponding carboxylic acid 34. An intramolecular cyclization in presence of conc. H2SO4 gave the biologically relevant, benzothiepinone moiety 35 (Scheme 5C). To investigate if a radical reaction is operative in the present case, up to three equivalent of radical scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added 16 under the standard condition. In all these cases the yield of the product remained practically unaltered; suggesting the involvement of a single-electron transfer process to be 14c,14d,14f unlikely (Scheme 6, eqn 1). Kinetic experiments were performed in two sets for the determination of the rate 16 laws. The first-order kinetics with respect to the quinolinamide was observed, while for disulfide a steady state approximation was applied. An intermolecular kinetic isotope study revealed a kH/kD value of 1.05; indicating that the C–H activation is not involved in the rate determining 16 step (Scheme 6, eqn 2). The hypothesis was further supported by the reversibility of the protodepalladation and 16 C−H activation steps, which goes in agreement with the small KIE value.

N O

Ph

+

NH

S

S

Pd(OAc)2 (10 mol%) Ligand (20 mol%) Ag2CO3 (3 equiv) Ph

(5 equiv)

Me Me Me (4.2 mmol,1.017 g)

Pd to sequester the organosulfide byproduct along with the concomitant regeneration of Pd catalyst for the further catalytic cycle. A similar mechanistic pathway is expected to operate for γ-selenoarylation. The experimental observation reveals that C−H activation is not the rate determining step as it is reversible. Hence, other steps as proposed in the mechanism i.e. either the oxidative addition or the reductive elimination is likely to be the rate-determining step for this 10d transformation.

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Scheme 6. Control Experiments and Kinetic Studies Me

5A. Gram-scale reaction

Page 4 of 6

Me

O N H

SPh

N

dry THF, 130 C, 24 h

L= Pd(OAc)2

N L

AcO Pd

[Ag] + OAc

SPh Me

Cl

PhS [Ag]

NH

O

N

+

PhS

Me Me O

AcO

HN

Me

(1, 60%)

L

(A)

OAc Me Me

Me

O

Na N N

5B. Synthesis of sulfone O

O

NHQ

O2N

Me Me

PTSA (3 equiv)

O2N

Me Me

MeOH, 120 C, 24 h

S

Oxone (2 equiv)

PhS (E)

N

dioxane: H2O (2:1) 0 C, 1 h

S

15

NaOAc + L

Pd

OMe

Me Me AcO

Reductive Elimination

32, 92% OMe

O2N

(B)

S O 5C. Synthesis of benzothiepinone derivative NHQ O O Me

Me Me

NaOH

Me

O

Pd N (D)

Me

Me Me

S

S

12

34, 87%

Oxidative Addition

PhS-SPh

O H2SO4

Me S 35, 75%

Me Me

16

Scheme 5. Applications of γ-Thioarylated Products

Based on the above mechanistic studies and related literature reports a plausible mechanism is proposed 10d,14,15 (Scheme 7). Pd(II) undergoes complexation with 2chloroquinoline to generate the pre-catalyst (A) which binds with the amidate forming the active catalyst-substrate chelate complex (B). A selective γ-C−H activation via the concerted metalation–deprotonation (CMD) pathway leads to the formation of six-membered palladacycle intermediate (C). Subsequently, a Pd(IV) species (D) is formed by the 17 oxidative addition of Pd(II) center to the disulfide bond. The intermediate (D) then undergoes reductive elimination to afford the γ-thioarylated product. During the release of the product, the Ag species can undergo a ligand exchange with

N L

SPh

OH

EtOH

C H Activation (CMD)

N

PhS

33, 85%

N

AcOH

O

Me

Me Me

N

Pd L

AcOH

O

O Me

Scheme 7. Plausible Chalcogenation

Pd Me

N

O

Me (C)

Mechanistic

Cycle

for

γ-

In summary, we have developed a palladium catalyzed highly regioselective γ-chalcogenation (thioarylation and selenoarylation) of aliphatic acids via six-membered metallacycle. The protocol is tolerant of a variety of carboxylic acids as well as disulfides and diselenides giving synthetically useful yields of thioarylated and selenoarylated products with high diastereoselectivity. Sequential heterobifunctionalizations and structural modifications of α-amino acids such as valine, isoleucine, and tert-leucine were also carried out. The thioarylated products were utilized for the synthesis of sulfone and benzothiepinone. Control experiments and kinetic studies shed light on the mechanism.

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ACS Catalysis ACKNOWLEDGEMENT This activity is supported by DST, India {(SR/NM/NS1065/2015 (G)}. Fellowships from DST-NPDF (File No. PDF/2015/000127) (S.G.), CSIR-New Delhi (A.D.) and UGC India (for P.D. and S.C.) are gratefully acknowledged. SG acknowledges Prof. S. M. Mobin (IIT Indore), Mr. Prabir (IIT Bombay), and Mr. Nishikant (IIT Bombay) for their help.

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AUTHOR INFORMATION Corresponding Authors: dmaiti@iitb.ac.in (6)

AUTHOR CONTRIBUTIONS ǂ

These authors contributed equally

Supporting Information Available: Experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org

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