1,1 ′ -(Ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) as a recyclable catalyst for acylation

1,1 ′ -(Ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) catalyzes the acylation of structurally diverse alcohols, amines, thiols, and phenols with a variety of aliphatic and aromatic anhydrides. Steric factors in substrates as well as anhydrides and solvent play significant role during the formation of acylates. Chemoselective mono acetylation of symmetrical diols, primary hydroxy group over secondary and phenolic group and amines over phenols has been achieved. The compatibility of the protocol has been shown by the survival of different acid sensitive functionalities under the present reaction condition. The solvent, acetone, reacts with EDPBT giving bromoacetone and HBr, thus suppressing the bromination of substrates otherwise amenable to bromination. The reagent EDPBT being devoid of phase transfer property and owing to the high solubility of its precursor 1,1 ′ -(ethane-1,2-diyl)dipyridinium dibromide (EDPDB) in water, it was possible to isolate pure acylates by an aqueous work-up circumventing the need for further purification. The process is superior owing to the recyclability of the reagent. The spent reagent can be recovered, regenerated, and reused without any significant loss.


Introduction
2][3][4][5][6][7][8] Their exploration as a source of anhydrous HBr makes the use of tribromides an attractive alternative to conventional protic and Lewis acids. 8Notably, the acidity of the reaction medium using tribromides can be tuned by changing the polarity of the organic ˆ All authors have contributed equally medium.8e To overcome the problem of recyclability, as is the case with most organic ammonium tribromides, we have recently reported a new reagent, 1,1′-(ethane-1,2-diyl)dipyridinium bistribromide (EDPBT) containing two tribromide units per molecule. 9Unlike other tribromides, this reagent is devoid of phase-transfer property and can be recovered in significant amounts as its precursor 1,1′-(ethane-1,2-diyl)dipyridinium dibromide (EDPDB) is highly water soluble.Taking into account the potential uses of tetrabutylammonium tribromide (TBATB) for different organic functional group transformations and the efficiency and recyclability of EDPBT, make further exploration of the reagent to other functional group transformations useful and viable.
Acylation of protic nucleophiles such as alcohols, amines, and thiols is an important and commonly used reaction in organic chemistry.1][12] In general acid anhydrides are employed as the acyl sources because of their ready availability and stability.4][15][16] Because of their toxicity, flammability and unpleasant odors these bases are less attractive. 13Acylation under acid catalyzed conditions has been reported with several reagents. 17,18Metal triflates [19][20][21][22][23][24][25][26][27][28][29] and perchlorates [30][31][32][33] have been used for the purpose owing to their acidic nature.Other reagents / catalysts employed are TMSCl, 34 HClO 4 -SiO 2, 35 Sc(NTf 2 ) 3, 36 Nafion-H, 37 Yttria-Zirconia, 38 distannoxane 15,39 heteropoly acid, 40 MeSO 3 H/Al 2 O 3, 41 solid surface-Al 2 O 3 , 42 and oxomolybdenum species. 43Although metal triflates, perchlorates and other acidic catalysts are effective for the acylation reaction, their use is limited due to explosive nature of metal perchlorates and strong acidic character of triflates which results in side reactions.The drawbacks associated with some of the procedures reported in literature are arduous preparation of catalysts, difficulties in work-up and isolation, the need for an inert atmosphere, harsh reaction conditions, expensive reagents, low yields, longer reaction times, dry solvents and incompatibility with other protecting groups.In this context search for achieving general nucleophilic acyl substitution of anhydrides in a catalytic, mild fashion with integrity of existing acid and base sensitive functionalities remains in great demand.Though a plethora of reagents and procedures for acylation have been documented in the literature, need to find an efficient and mild acylation reaction still remains.Herein, we reveal a new, mild procedure for preparation of various esters, amides and thioesters with a variety of aliphatic and aromatic anhydrides in the presence of catalytic quantity of EDPBT.

Results and Discussion
Initially 3-phenyl propanol 3 (5 mmol) was taken as the model substrate for acetylation and was reacted with acetic anhydride (6.25 mmol) in a donor solvent such as acetonitrile (10 mL) in the presence of catalytic quantity of EDPBT (0.25 mmol).Progress of the reaction was monitored using thin layer chromatography, which showed complete conversion of the alcohol to its acetate within 35 min.Earlier we have reported that the acidity of the reaction medium employing tribromides can be tuned by changing the polarity of the solvent. 8So using this fact; the reaction was performed in different solvents such as toluene, methylene chloride, chloroform and acetone.When 3-phenyl propanol 3 was reacted with acetic anhydride in the presence of EDPBT in above solvents separately, it was observed that the reaction proceeded much faster in acetone compared to other solvents.This could be due to the reaction of acetone with EDPBT forming bromoacetone and thereby generating anhydrous HBr in situ, which catalyzes the reaction.In a control experiment treatment of the reaction mixture with a catalytic quantity of bromine and aq.HBr instead of EDPBT in acetone yielded 95% and 88% of acylated product respectively within 10 minutes.The summary of the solvent dependent study is shown in Table 1.This result prompted us to use acetone as the reaction medium.

GC yield
The reagent and the methodology are superior with respect to other conventional reagents as it results in a highly efficient acetylation of 3-phenyl propanol 3 (Table 2).Summary of the acetylation using 0.05 equivalents of different reagents employing acetone as the solvent is shown in Table 2.It may be noted that the yields reported in Table 2 are much lower than those has been reported in the literature, a possible reason could be the use of different amounts of catalyst, acetic anhydride and solvent system.
Under the present optimized reaction condition, diverse arrays of alcohols were converted to their respective acetates (Table 3).Aliphatic primary alcohols 1-3 were transformed to their corresponding acetates in excellent yields in a short time.Benzylic alcohols with deactivated substituents in the aromatic ring 4-5 were also acetylated efficiently.By employing this reagent we carried out acetylation of hindered and deactivated benzylic alcohol 6 in excellent yield but with a longer reaction time 24 h.However, other hindered alcohols 7-8 underwent acetylation smoothly giving product in excellent yield.Primary aliphatic diols 9-12 were diacetylated completely with acetic anhydride (2.5 equiv.).When pentaerythritol 13, a substrate containing four symmetrical hydroxyl groups, was reacted with acetic anhydride (5 equiv.) it gave a diisopropylidine derivative 13″a along with the isopropylidine diacetate 13′a in the ratio (30:70).It is worth noting that a change of solvent from acetone to acetonitrile resulted in the formation of pentaerythritol tetraacetate 13a as the sole product in nearly quantitative yield (96%), Scheme 1.
Table 2. Acetylation of 3-phenyl propanol 3 in acetone using 0.05 equiv. of the reagents and acetic anhydride (1.25  Secondary alicyclic alcohol such as cyclohexanol 14 was converted to its acetate in good yield.It was gratifying to observe that menthol 15 was completely converted to menthyl acetate 15a with 1.25 equiv of Ac 2 O within 10 min, an earlier work has reported that it was acetylated in 1.5 h at 0 o C with 5 equivalents of Ac 2 O in the presence of 1 mol% of Bi(OTf) 3. 21 Benzylic secondary alcohols such as 16-19 underwent acetylation smoothly but with a slightly longer reaction time as compared to their primary analogues.However, the scope of acetylation is somewhat limited for hindered aromatic secondary alcohol 9-fluorenyl alcohol 20, which took 5 h for complete conversion under reflux.The results obtained have been summarized in Table 3.This methodology was also successfully applied to a representative variety of functionalized alcohols (Table 4).Diacylated product was obtained without affecting the multiple bonds in substrates, but-2-ene-1,4-diol 21 and but-2-yne-1,4-diol 22 with acetic anhydride (2.5 equiv.).Cholesterol 23 took comparatively longer reaction time for the transformation.A substrate containing two hydroxyls and a NHBoc functionality 26 gave monoacylated product without affecting the NHBoc group when reacted with acetic anhydride (1.25 equiv.).The present method is also effective for acylation of α,β-unsaturated alcohol such as cinnamyl 24.Conversion of 4-allyloxy benzyl alcohol 25 to its acetate also occurred smoothly without affecting the double bond.Moreover, substrates bearing acid sensitive groups such as NHBoc 26, OMe 27, THP 28, and TBS ether 31, and base sensitive groups such as benzoate 29-30 remained intact under the described reaction condition revealing the functional group compatibility.The pK a s of HBr, HClO 4 and triflic acid respectively are -9, -10, and -13 showing that triflic and perchloric acids have stronger acidic character compared to HBr.It may be noted that strong Lewis acid character of metal triflates makes them unsuitable for acidsensitive substrates.In scandium triflate catalyzed acetylation, rearranged products of allylic alcohols have been observed.24d Results obtained for differential functionalized alcohols have been summarized in Table 4.The acetylation of a wide range of structurally varied aliphatic, benzylic, allylic alcohols and phenols highlight the fact that the method is capable of generalization.However, phenols were sluggish under the present reaction condition and took comparatively longer reaction time for completion (Table 5).This result was attributed to the differential nucleophilicities of phenols and aliphatic alcohols under the reaction condition.It is noteworthy to quote that phenols are less nucleophilic than aliphatic alcohols under acidic condition but more nucleophilic under basic condition.24b Phenols containing electron donating groups in the aromatic ring 33-35 were acetylated with ease in comparison to those with electron-withdrawing groups 36-37 (Table 5).
Numerous procedures have been reported in literature for acylation of amines.V(O)(OTf) 2. 44 The versatility of the described procedure can be observed from its successful application to N-and S-acylation of structurally different amines and thiols.Primary aromatic amines 38-40 and anilines with various substituents 41-44 were converted to their corresponding amides in short time, whereas secondary amine 45 took hours for completion.This result is not surprising since N-acylation of primary amines has been carried out without the use of any acidic or basic catalyst. 50Thioacetal and hemithioacetal were obtained as side products when acylation of dodecanethiol 46 was carried out in acetone.However, the corresponding thioacetate was obtained by changing the solvent to acetonitrile.The results have been summarized in a Reactions were monitored by TLC.b Isolated yield.c CH 3 CN was used as the solvent.
The marginal difference in acylation rate between primary and secondary alcohols, particularly for diols containing both types of hydroxyl yields substantial amount of diacylate.21e,23b,25,27,39-42,51 High selectivity was obtained for unsymmetrical aliphatic diol possessing both primary and secondary hydroxyl groups such as 1,3-butanediol 47 when the present method was employed.The primary hydroxyl group was selectively acetylated prior to secondary one with lot wise addition of acetic anhydride (1.2 equiv.) in the presence of EDPBT (0.05 equiv) in acetone.However, lower selectivity was observed for symmetrical primary diols 9 and 21 even with lot wise addition of the anhydride.The ratios of mono and diacetylated product obtained for substrates pentane-1,5diol 9 and but-2-ene-1,4-diol 21 were respectively 65:25 and 75:20.The poor reactivity of phenols with acetic anhydride in presence of EDPBT raised a genuine possibility of selective acylation of aliphatic alcohols over phenols.For substrate 48 containing both primary and phenolic hydroxyl group, selective monoacetylation occurred at the aliphatic hydroxyl giving exclusively monoacetylated product 48a.Substrate p-aminophenol 49 produced the corresponding acetamide; without affecting the phenolic group.Selective N-acetylation is of significant interest for the preparation of the antipyretic and analgesic drug paracetamol 49a.There are few reports of pivaloylation, benzoylation and acylation using other anhydride and alcohols. 19,21,24,43It is worth noting that none of the procedures reported in literature has focused on the acylation of alcohols, amines, and thiols with isobutyric anhydride.In order to extend the scope of the methodology, acylation of alcohols, amines, and thiols with other anhydrides was carried out under the identical condition as described using acetic anhydride.A variety of aliphatic and aromatic alcohols (1, 3, 16, 24), amine 41 and thiol 46 were propionylated using propionic anhydride.Chemoselective propionylation of primary alcohol over phenol, and amine over phenol could be observed as demonstrated for substrates 48 and 49 respectively as shown in Table 7. Benzoic anhydride, an aromatic anhydride reacted with alcohols, amines, and thiol to give corresponding benzoates (1e, 3e, 24e, 41e, 46e, 48e and 49e).The reaction of benzoic anhydride with various alcohols, amine and thiol is summarized in Table 10.In general, the more hindered the anhydride; the slower is the acylation rate.Notably, under the present reaction condition there is not much difference in the acylation rates of alcohols, amines and thiols with acetic, propionic and isobutyric anhydride but the reaction is slower for pivalic and benzoic anhydride.This observation is consistent with the observations made by others.21a,b

Conclusions
The reagent EDPBT serves as an excellent source of anhydrous HBr in acetone, which catalyzes acylation of structurally diverse alcohols, amines, thiols, and phenols with different anhydrides.No bromination was observed for substrates susceptible to bromination due to consumption of active bromine in EDPBT by acetone giving bromoacetone and HBr.Solvent and steric factors in substrate as well as anhydride play a significant role during the formation of acylates.Chemoselective acylation of symmetrical diols, primary hydroxyl over secondary and phenolic, and amines over phenols has been achieved.Compared to the existing methods, which uses various acidic and basic catalysts, this method is very general, simple, gives high yield, has shorter reaction time, and is environmental friendly.In terms of compatibility and selectivity this method is superior to many of the reported methods.Due to the mild reaction conditions a number of functional groups remain intact, in spite of being capable of reacting with tribromides.The reagent EDPBT being devoid of phase transfer property and owing to the high solubility of its precursor 1,1′-(ethane-1,2-diyl)dipyridinium dibromide (EDPDB) in water, isolation of pure acylated product can be achieved only by an aqueous work-up circumventing the need of chromatographic purification.Further, the superiority of the process lies in the recyclability of the reagent.The extensive waste of EDPBT reagent is avoided by regeneration of the reagent from the aqueous layer.It can be used without any loss of its activity for further transformations which is of significant interest.
General procedure for benzoylation of thiols with benzoic anhydride.Similar to the acetylation with acetic anhydride but 5 mmol of benzoic anhydride was used per 5 mmol of the substrate in acetonitrile.
General procedure for selective monoacetylation of diols.Similar to the acetylation of alcohols with acetic anhydride except 6 mmol of acetic anhydride was used per 5 mmol of the substrate with lot wise addition of anhydride over a period of 30 minutes.The products were purified over a column of silica gel, using a mixture of hexane and ethyl acetate as eluent.
In principle aqueous layers of several reactions were combined and kept for several days to allow the water to evaporate and proportionate amounts of KBr and Oxone® were added to regenerate the reagent.

Table 4 .
EDPBT catalyzed acetylation a of functionalized alcohols in acetone a Reactions were monitored by TLC.b Isolated yield.c 2.5 equivs of Ac 2 O were used.

Table 5 .
Acetylation a of phenols, amines and thiols with Ac 2 O catalyzed by EDPBT

Table 6 .
Selective monoacetylation a of diols with Ac 2 O catalyzed by EDPBT in acetone a Reactions were monitored by TLC.b Isolated yield

Table 7 .
Propionylation a of alcohols, thiol and amines with (EtCO) 2 O catalyzed by EDPBT CH 3 CN was used as the solvent Other anhydrides such as isobutyric and pivalic reacted successfully as shown in Table8and 9 respectively.It is needless to mention that chemoselective isobutyrylation and ARKAT pivaloylation of primary alcohol over phenol was observed as demonstrated for substrate 48 (Table8 and 9).
a Reactions were monitored by TLC.b Isolated yield.c

Table 8 .
Isobutyrylation a of alcohols, thiol and amines with (iPrCO) 2 O catalyzed by EDPBT a Reactions were monitored by TLC.b Isolated yield.c CH 3 CN was used as the solvent

Table 9 .
Pivaloylation a of alcohols and amines with (t-BuCO) 2 O catalyzed by EDPBT a Reactions were monitored by TLC.b Isolated yield