A.3. Present work - A dissertation submitted in partial fulfillment for the degree of Doctor o

C HAPTER I

II. A.3. Present work

oxypalladation of alkyne.²¹ Imidoylpalladium generated by isocyanide insertion is thought to be a key intermediate in this reaction (Scheme II.A.2.7).

Scheme II.A.2.7. Synthesis of 3-aroylindoles from o-alkynylisocyanides and aryl iodides

(c)From o-alkynylated-N,N-dialkylamines:

o-Alkynylated-N,N-dialkylamines has been realized to be a promising starting material for the synthesis of 3-aroylindoles through the sp³ C−H functionalization of amines α to nitrogen atom under oxidative conditions. A Pd–Cu co-catalyzed oxidative intramolecular Mannich type reaction for the formation of 3-acylindoles has been reported by Liang group using o-alkynylated-N,N-dialkylamines in which tert-butyl hydroperoxide serve as an oxidant.^22a Zhou et al developed a photocatalytic pathway for their synthesis by adopting the same strategy (Scheme II.A.2.8).^22b

Scheme II.A.2.8. Synthesis of 3-aroylindoles from o-alkynyl-N,N-dialkylamines

Table II.A.3.1. Screening of reaction conditions^a,b

Entry Catalyst (mol %) Solvent Oxidant (equiv) Temp (^oC) Yield (%)^b

1 CuBr (10) Toluene TBHP^c (1) 80 45

2 CuBr (10) Toluene TBHP^d (1) 80 55

3 CuBr (10) Toluene TBHP^d (2) 80 62

4 CuBr (10) Toluene TBHP^d (3) 80 70

5 CuBr2 (10) Toluene TBHP^d (3) 80 58

6 CuCl (10) Toluene TBHP^d (3) 80 55

7 CuI (10) Toluene TBHP^d (3) 80 68

8 Cu(OAc)2 (10) Toluene TBHP^d (3) 80 40

9 CuCl2 (10) Toluene TBHP^d (3) 80 25

10 CuBr (5) Toluene TBHP^d (3) 80 52

11 CuBr (10) DMSO TBHP^d (3) 80 77

12 CuBr (10) o−xylene TBHP^d (3) 80 66

13 CuBr (10) dioxane TBHP^d (3) 80 62

14 CuBr (10) DMF TBHP (3) 80 25

15 CuBr (10) THF TBHP^d (3) 80 10

16 CuBr (10) DMSO TBHP^d (3) 100 60

17 CuBr (10) DMSO TBHP^d (3) 60 43

18 Nil DMSO TBHP^d (3) 80 0

19 CuBr (10) DMSO Nil 80 0

20 Pd(OAc)2 (10) DMSO TBHP^d (3) 80 35

21 CoCl2 (10) DMSO TBHP^d (3) 80 50

22 FeCl3 (10) DMSO TBHP^d (3) 80 45

aReaction conditions: o-alkynyl-N,N-dimethylamine (1a), (0.25 mmol), time 2 h. ^bIsolated yield.

cDecane solution (5−6 M). ^d70% aqueous solution.

Optimization of reaction conditions. To check the feasibility of the intended intramolecular coupling of o-alkynylated-N,N-dimethyl amines an initial reaction was carried out by treating (1a) with CuBr (10 mol %) as the catalyst and TBHP (5-6 M in decane) (1 equiv) as the oxidant in toluene solvent at 80 ^oC, which yielded 3-aroylindole (1ʹa) in 45% isolated yield (entry 1, Table II.A.3.1). Interestingly, the use of an aqueous TBHP (70% wt in water) instead of TBHP in decane, gave a superior yield of 55% (entry 2, Table II.A.3.1). Further, increasing the amount of TBHP to 2 equiv and 3 equiv improved the yield to 62% and 70% respectively (entries 3−4, Table II.A.3.1). Copper(I)

bromide was found to be superior to the other Cu (I and II) salts such as CuCl, CuI, Cu(OAc)2, CuCl2 and CuBr2 employed (entries 5−9, Table II.A.3.1). Decreasing the catalyst loading to 5 mol %, significantly lowered the product yield (52%) (entry 10, Table II.A.3.1). Further, when the reaction was carried out in DMSO solvent instead of toluene, an improved yield of 77% was obtained (entry 11, Table II.A.3.1). The reaction in other solvents such as o-xylene, 1,4-dioxane, DMF and THF did not give any satisfactory yields compared to DMSO (entries 12−15, Table II.A.3.1). Both increase (100 ôC) and decrease (60 ôC) in the reaction temperature had an adverse effect on the product yields (entries 16−17, Table II.A.3.1). The control experiments carried out with either the copper salt or TBHP alone did not give any trace of product (entries 18−19, Table II.A.3.1). The use of other catalyst such as Pd(OAc)2, CoCl2 and FeCl3 in combination with aq TBHP gave inferior yields compared to CuBr (entries 20−22, Table Table II.A.3.1). Thus, CuBr (10 mol %), aq. TBHP (70%) (3 equiv) at 80 ôC in DMSO solvent was found to be the most suitable condition for this transformation and all other reactions were carried out adopting this condition.

Substrate scope for 3-aroylindoles. The scope of this unique intramolecular strategy was next explored towards the synthesis of various 3-aroylated indoles from their respective o- alkynylated N,N-dimethylamines under the optimized reaction conditions. As shown in Scheme II.A.3.1, a variety of 2-alkynyl-N,N-dimethyl anilines could be transformed to their corresponding 3-aroylindoles. Initially, the effects of various substituents on the aryl ring (R²) were examined. Irrespective of the nature of the substituents such as p-Me (1b), p-^tBu (1c), and p-NO2 (1d) all provided their corresponding 3-aroylindoles (1ʹb), (1ʹc), and (1ʹd) with yields ranging from 73−80% (Scheme II.A.3.1). The structure of the 3- aroylindole (1ʹb) has been unequivocally confirmed by X-ray crystallography (Figure II.A.3.1).

Figure II.A.3.1. ORTEP view of (1ʹb)

When the N,N-dimethyl substituted aryl ring (R¹) of 2-alkynyl-N,N-dimethyl aniline is substituted with an electron donating group such as p-Me and the other aryl moiety (R²) without a substituent (2a) and with substituents such as p-Me (2b) and p-^tBu (2c) all yielded their 3-aroylated products (2ʹa), (2ʹb) and (2ʹc) respectively in excellent yields.

Scheme II.A.3.1. Substrate scope for 3-aroylindoles^a,b

N Me

Me R²

R¹

N O

Me CuBr (10 mol %)

80^oC, 2.5 4 h aq TBHP (3 equiv)

N O

Me ^tBu NO₂

Me Me

tBu

Me Me

Br Br

Me ^tBu

tBu 1'a

(77%, 3.5 h)

1'b (80%, 3 h)

1'c

(78%, 3 h) 1'd

(73%, 3.5 h)

2'a (80%, 3 h)

2'b (82%, 3.5 h)

2'c (80%, 2.5 h)

3'a (74%, 3 h)

3'b (78%, 2.5 h)

3'c (76%, 2.5 h)

4'a (60%, 4.5 h)

4'b (70%, 4 h)

4'c (62%, 4 h)

R¹

R²

1 (a-d), 2-4 (a-c) 1' (a-d), 2'-4' (a-c)

aReaction conditions: o-alkynyl-N,N-dimethylamines 1 (a-d), 2-4 (a-c) (0.5 mmol), CuBr (0.05 mmol), 70% aqueous TBHP (1.5 mmol) in DMSO (2 mL) at 80 ^oC. ^bIsolated yield.

Similarly, when the N,N-dimethyl substituted aryl ring (R¹) of 2-alkynyl-N,N-dimethyl aniline is substituted with 3,4-dimethyl group and the other aryl moiety (R²) without a

substituent (3a) and with substituents such as p-Me (3b) and p-^tBu (3c) all gave their expected aroylated products (3ʹa), (3ʹb) and (3ʹc) respectively but in a slightly lesser yields (Scheme II.A.3.1). However, a significant drop in the yields were observed for substrates possessing a moderately electron withdrawing group such as p-Br as has been demonstrated with substrates (4a), (4b) and (4c) all giving their expected products (4ʹa), (4ʹb) and (4ʹc) in the yields ranging from 60−70% as shown in Scheme II.A.3.1.

Mechanistic studies. To investigate the nature of the mechanism, when a standard reaction was carried out in the presence of radical scavenger TEMPO (3 equiv), the reaction gave many other side products and only a trace of the desired product (<10%), indicating the possibility a radical mechanism (Scheme II.A.3.2). As a further support to the radical path of the mechanism, a reaction was carried out with CuBr alone without the use of aq TBHP. The reaction failed to give any traces of the desired product and the starting material was recovered completely. These control experiments suggest that Cu(I) alone is not sufficient to form the iminium cation and the radical initiator TBHP is essential for this transformation.

N Me

Me N

Me H

(1a) (1'a)

N O.

CuBr

tBuOOH

< 10 %

Scheme II.A.3.2. Reaction in presence of radical scavenger TEMPO

Based on literature reports,^4,23observations of the control reactions and yields of the products obtained from substituted substrates, a probable mechanism is proposed for this transformation as depicted in Scheme II.A.3.3. Copper salt in combination with peroxide serve as a single electron oxidant.^22a This active oxidant accepts an electron from the nitrogen atom forming an aminyl radical cation species (A) (Scheme II.A.3.3). Abstraction of a proton radical α to the nitrogen atom gives an iminium intermediate (B). The intramolecular attack of the alkynyl group onto the iminium carbon in the intermediate (B)

results in C−C bond formation with concurrent formation of C−O bond by the attack of either water or TBHP onto the alkynyl moiety to give intermediate (C). Ketonization of the intermediate (C) provides 3-aroylindoline (D) which undergoes further oxidation to give 3- aroylindole (1ʹa) (Scheme II.A.3.3). Although Liang et al have proposed the nucleophilic attack of TBHP to the alkyne intermediate (B) but in our case the attack of water as the nucleophile cannot be completely ruled out since the reaction proceeds better with aqueous TBHP. The proposed mechanism (Scheme II.A.3.3) is supported from the fact that substrates containing electron withdrawing group such as bromo (4a, 4b and 4c) in the N,N−dimethyl substituted aryl ring (R¹) gave poor yields because of the instability of the aminyl radical cation (A). Further, when both the rings are substituted with electron donating groups as in the case of 2a, 2b and 2c the product yields were found to be better which is partly because of the enhanced stability of the intermediate (B).

Scheme II.A.3.3. Plausible mechanism for Cu(I) catalyzed formation of 3-aroylindoles In conclusion, we have developed an elegant method for the synthesis of 3- aroylindoles through a copper catalyzed oxidative process involving o-alkynylated N,N- dimethylamines in the presence of aqueous TBHP as the oxidant. This protocol simultaneously installs C−C and C−O bonds through an intramolecular oxidative path in the construction of 3-aroylindoles. This method uses inexpensive Cu catalyst without the requirement of any co-catalyst and additives and proceeds at a relatively lower temperature. Besides being a one pot and atom economical process, judging the practical utility makes the present method the best alternative synthesis for 3-aroylindoles.

Dalam dokumen A dissertation submitted in partial fulfillment for the degree of Doctor of Philosophy (Halaman 69-75)