Halodehydroxylation of alcohols to yield benzylic and alkyl halides in ionic liquids
© Petten et al. 2015
Received: 26 May 2015
Accepted: 2 October 2015
Published: 13 October 2015
Alcohols are widely used, and sometimes renewable, reagents but the hydroxyl moiety is a relatively poor leaving group under mild conditions. Direct nucleophilic substitution of alcohols is a desirable reaction for synthetic and process chemists.
Synthesis of twelve alkyl and benzyl halides was achieved in [Bmim]PF6 (Bmim = 1-butyl-3-methylimidazolium) from their parent alcohols using ammonium halides as the halogenating agents. Trends in reactivity based on the alcohol and halide were discovered. Mechanistic evidence suggests that the reaction proceeds via SN2 substitution of the hydroxyl group, which is activated via hydrogen-bonding with the acidic proton of the imidazolium cation. Also, for benzyl substrates, equilibria involving formation of dibenzyl ether complicate the reactions and reduce optimum yields.
KeywordsHalogenation Ionic liquid Catalysis Substitution Solid acid Microwave Dehydration
Alcohols are widely encountered chemicals and are useful intermediates in modern organic synthesis due to their ability to be transformed into a wide range of products. Furthermore, many renewable feedstocks contain hydroxyl moieties. Unfortunately, hydroxyl groups are not favourable leaving groups under mild conditions. This usually means, for nucleophilic substitution reactions, that the alcohol must be activated and subsequently displaced in order to produce the desired product. This sort of procedure violates the principles of green chemistry as the activation step can be considered unnecessary derivatization. Therefore, direct nucleophilic substitution of alcohols is desired and is one of the key green chemistry research areas previously identified by pharmaceutical manufacturers . Catalytic methods for these reactions remain relatively underexplored and were the focus of a recent review .
In particular, halogenation of alcohols is a useful reaction as alcohol-starting materials are cheap and large varieties are commercially available. Many procedures are known that convert alcohols to halides however, they can involve harsh conditions and hazardous chemicals. For example, the chlorination of alcohols is traditionally performed using HCl gas or thionyl chloride, both of which are hazardous to human health and often produce many side reactions. Although improvements have been made upon these halogenation reactions, further green methodology is desired. On a laboratory scale, ammonium halide salts (NH4X) are solid reagents, which means they are easy to handle and measure.
Discovering a mild, neutral, highly selective and environmentally friendly system for catalytic dehydroxyhalogenation is desirable but it is not a trivial task. Compromises in the journey towards an ideal sustainable process have to be made. This is exhibited by some of the trends found in the literature regarding chlorination of alcohols. Some reports involve the use of triphenylphosphine (PPh3) as a superstoichiometric additive in dehydroxychlorination reactions [3, 4]. Although these systems use mild conditions (often performed at room temperature with neutral chlorinating agents), exhibit high selectivity, and possess a large and diverse substrate scope, a large amount of PPh3 oxide is formed in the reaction, which causes purification to be troublesome. An interesting reagent that has been researched in dehydroxychlorination reactions is 2,4,6-trichloro-1,3,5-triazine (TCT), which is generally considered non-toxic. Systems involving this reagent, in combination with either DMSO or DMF, give very good yields in short reaction times with no heating required [5, 6]. A drawback with these systems is that only two chlorine atoms of TCT are used in the reaction, and therefore atom efficiency is compromised. Another type of chlorination system involves the use of silicon-based chlorinating agents in the presence of Group 13 chlorides (e.g. InCl3) as catalysts [7, 8].
Recently reported methodologies concerning bromination and iodination of alcohols are similar in substance to the reports of chlorination processes; some are truly ‘green’ systems whereas others would not be suitable for scale-up [9–12]. One particular system produces alkyl iodides starting from the corresponding ketone with the use of a ruthenium-based catalyst that is active towards both reduction and halogenation . This system appears to be extremely useful as it can be used for a number of substrates and uses a mild iodinating agent, NaI. However, while crucial to organic synthesis, precious metal catalysis is often not economically feasible on a large scale.
Solvents are required in most organic synthetic processes to produce homogeneous mixtures, provide a means for energy and mass transfer, stabilize transition states, and/or provide a means for separation and purification of products . In the substitution reactions described above, dipolar aprotic solvents (e.g. DMF, DMSO) are often required to dissolve ionic reagents and stabilize intermediates. Ionic liquids are one class of alternative reaction media, which are suitable replacements for such solvents . The field of catalytic reactions performed in ionic liquids has been reviewed extensively and the sheer volume of transformations performed with ionic liquids is outstanding [14–16]. In surveying these reactions, an interesting observation is warranted. Depending on its properties, an ionic liquid can be used as an ‘innocent’ solvent, a ligand precursor, a co-catalyst, or even the catalyst itself. Ionic liquids have been used as “reagent-solvents” in bromination and iodination of alcohols, where the active nucleophile originates as the anion of the ionic liquid [17–19]. These systems have commonly been used with long-chain alcohols , which have previously been problematic to halogenate due to their poor solubility in most solvents. The greenness of these systems is enhanced as the ionic liquid can be regenerated and reused a number of times simply by stirring, with a NaX salt (X = Br, I), at an elevated temperature over a short period of time . This allows a number of reactions to be performed in the same solvent, which is generally unheard of in reactions involving traditional organic solvents.
Results and discussion
Reaction screening: effect of reaction medium, halogenating agent and alcohol on dehydroxyhalogenation reactions
Mechanistic and equilibrium considerations
In this particular system, there are three acidic species that could be proposed as catalysts in this reaction: NH4 +, HF and the acidic proton of [Bmim]PF6. As [Bmim]Cl could also be used as the chlorinating reagent, one can conclude that NH4 + is not the major catalyst as the reaction proceeds even when NH4 + is not present. Also, we have already hypothesized that HF is not the catalytic species, as significantly different results were obtained when [Bmim]PF6 and [BMmim]PF6 were used, which should produce the same amount of HF if anion decomposition was occurring. Therefore, we propose that the major catalytic species in the presented reaction system is the acidic proton of [Bmim]PF6. The involvement of this proton is supported by the large decrease in yield (from 68 to 18 %) when [BMmim]PF6, which does not contain this acidic proton, is used as the solvent instead of [Bmim]PF6 in the chlorination of benzyl alcohol. The proposal that this acidic proton is involved is based on precedent seen in the literature. Despite having a relatively high pK a of 21–24 , this proton has been observed to participate in a number of interactions. In one particular case, where Michael addition reactions were studied, results obtained suggested that the acidic proton of [Bmim]PF6 formed a hydrogen bond with a basic catalyst, resulting in a decrease of activity .
In nucleophilic substitution reactions, there are two general mechanisms: SN1 and SN2. For SN1 reactions, the leaving group (in this case -OH) is ejected, forming a carbocation intermediate, which is attacked by the nucleophile. In general, SN1 reactions are favored by good leaving groups and highly branched systems that can form a stable carbocation intermediate (e.g. 3° alkyl reagents). For SN2 reactions, an incoming nucleophile attacks from the backside of the substrate and ejects the leaving group. SN2 reactions are favored by good nucleophiles and unhindered C-X bonds such as primary alcohols. In order to determine whether an SN1 or SN2 process dominates, competition studies were performed. Reaction of one equiv. NH4Cl with one equiv. of both benzyl alcohol and 1-phenyl ethanol was performed. This afforded benzyl chloride in yields identical to those performed in the absence of 1-phenyl ethanol. Under the reaction conditions explored, there was no evidence of 1-phenyl ethanol reacting. This lack of reactivity suggests that the reaction does not proceed in an SN1 fashion, as the more stable carbocation should afford higher yields of 1-phenyl-chloroethane than benzyl chloride.
The examination of this proposed mechanism allows for rationalization of some of the previously described results. For the reaction performed in the phosphonium based ionic liquid, [P66614]DBS, a very low yield of product was observed. Since this ionic liquid does not contain an acidic site similar to [Bmim]PF6, without this acidic site, activation of the alcohol could not be achieved, leading to very poor conversions.
Since benzyl chloride is the desired product in the reactions that have been presented thus far, it would be desirable to control the selectivity of these reactions and improve the yield of benzyl chloride. In the dehydration reaction of benzyl alcohol to produce dibenzyl ether, water is produced as a by-product. However, based on the mechanism proposed in Fig. 6 for the formation of benzyl chloride from benzyl alcohol, hydroxide is formally produced as a by-product. While the majority of the hydroxide produced may undergo rapid proton transfer with NH4 + to produce H2O, the highly ionic nature of the solvent may allow a substantial amount of the hydroxide to exist as a charged species in solution. Based on this argument, we hypothesized that the presence of a small amount of water added to the reaction mixture would hinder the production of dibenzyl ether more than it would suppress the production of benzyl chloride. In order to investigate this prediction, the best yielding reaction presented above (68 % yield of benzyl chloride) was attempted in the presence of a half molar equivalent of water with respect to benzyl alcohol to see what effect this had on the yield. It was found that this reaction yielded 75 % benzyl chloride, improving the yield slightly from what was previously observed. This result is in agreement with the rationale presented above. However, the amount of water added needs to be carefully controlled. When three molar equivalents of H2O are added, only a 39 % yield of benzyl chloride was obtained. Nevertheless, these findings show that dehydroxylation reactions need to be explored carefully in terms of the water content of the reaction medium and methods for removing water to enhance yields.
In summary, it has been discovered that an equilibrium exists between benzyl chloride and dibenzyl ether in the reactions that have been studied and that some control over this equilibrium can be acquired with the addition of a small amount of water to the reaction mixture. However, in order to improve the yield of benzyl chloride, it is necessary to gain further control of the production of dibenzyl ether, either through an additive or catalytic species.
Extension of this reaction to bromide, iodide and other alcohols
Using the optimized reaction conditions for the conversion of benzyl alcohol to benzyl chloride, attempts were made to halogenate benzyl alcohol with NH4Br and NH4I. The results for these reactions are summarized in Table 1. The yield of benzyl iodide is quite similar to the yield of benzyl chloride under the same conditions (72 versus 68 %), but the yield of benzyl bromide (50 %) is significantly lower. Based on the proposed mechanism, it would be expected that the best nucleophile, iodide, would give the best yield followed by bromide and chloride. Since dibenzyl ether was also observed as a product in the reactions involving NH4Br and NH4I, it is expected that similar equilibria exist in these halogenation reactions, as was the case for the chlorination reaction. Therefore, these yields are not completely comparable as the time and temperature dependence of the bromination and iodination reactions have not been fully investigated. Fluorination of benzyl alcohol was attempted with NH4F using a number of reaction conditions (80–180 °C, 15–40 min). For all of these reactions, it was not possible to identify any fluorinated product, although all of the starting material had reacted, in most cases. This result is not surprising based on the properties of the fluoride anion, which is a poor nucleophile but extremely basic and rarely reacts neatly in nucleophilic displacement reactions.
Using the optimized conditions for the chlorination of benzyl alcohol, attempts were made to halogenate a variety of other alcohols. These included substituted benzyl alcohols, both functionalized and non-functionalized aliphatic alcohols, and a secondary alcohol. The results of these studies are discussed below.
In order to investigate substrate scope, chlorination was attempted using 2-phenylethanol as the substrate. Using the optimum reaction conditions for chlorination of benzyl alcohol, 2-phenylethyl chloride was obtained in 60 % yield. From the GC–MS analysis, it was observed that only a very small amount of the etherification product was obtained and that no other by-products were produced. This result suggests that benzyl alcohols are much more reactive in this system, leading to unwanted side-products. Interestingly, the chlorination of this substrate was studied with an indium-catalysed system that used chlorodimethylsilane as the chlorinating agent, and although this system was successful in chlorinating a wide range of alcohols, no yield was obtained for this particular substrate . This result was rationalized with the prediction that a carbocation intermediate was being formed and since 2-phenylethanol is a primary alcohol, a primary carbocation would have to be formed, which is unflavored . The fact that the system presented herein is effective in the transformation of a primary alcohol to a primary chloride further supports the SN2-like mechanism described above.
With the discovery that this chlorination system was effective in the chlorination of 2-phenylethanol, investigations were performed to see if this system could be expanded to simple aliphatic alcohols. Since it is known that the [Bmim]+ cation interacts favorably with aromatic systems , the halogenation of 1-butanol (Table 1) was investigated to see if the presence of the aromatic ring was required for the halogenation to occur. The yield of the butyl halide increases with increasing nucleophilicity of the anion reacted (best yield obtained with NH4I). This is the trend expected when studying substitution reactions that follow SN2 mechanisms. This mechanism is also supported by the observation that no 2-butyl halides or rearrangement products were obtained based on analysis of the 1H NMR spectra for the reactions involving 1-butanol. One would expect that if a SN1 mechanism existed in these reactions, rearrangement products would be observed, as is the case with the indium-catalysed system . As well, it should be noted that no ether by-products were observed in these reactions, suggesting that etherification under these reaction conditions is limited primarily to compounds containing aromatic groups. Given the selectivity of these reactions, ionic liquid recycling studies were performed for the n-butanol to 1-chlorobutane process. The reaction was performed (150 °C, 17 min) and after completion, the product could be isolated via distillation. The diluted ionic liquid phase containing some unreacted alcohol was dried over anhydrous sodium sulfate, decanted, concentrated and then successfully re-used for the same reaction four times. Yields obtained were 50–57 % (based on the amount of alcohol added before each reaction).
Although it was observed that halogenation could be performed on aliphatic alcohols, the effect of functional groups on the halogenation process had not yet been determined. For this study, the renewable alcohol citronellol was chosen (Table 1) as it contains a double bond that could be reactive under the acidic conditions contained in this chlorination system. As before, the optimized conditions obtained from the study of benzyl alcohol were used. From the GC–MS analysis of this reaction, it was found that the desired chlorinated product was obtained as thea major product. A number of other by-products were obtained in small amounts such as the terminal alkene generated from an elimination reaction involving either the hydroxyl of the starting material or the generated chloride. This suggests that for the most part, the double bond functionality of citronellol is insensitive to the reaction conditions and similar compounds could be halogenated. However, if more acid-sensitive functional groups were present in the starting material, such as terminal alkenes or esters, it is predicted these groups would not be able to withstand the conditions of this halogenation system.
Overall, it has been shown that NH4X (X = Cl, Br, I) in [Bmim]PF6 under microwave irradiation can be used to halogenate a number of alcohols. Fluorination was not possible under the conditions explored. With respect to para-substituted benzyl alcohols, the halogenation process is highly dependent upon the nature of the substituent. For aliphatic alcohols, the results obtained support a SN2 mechanism, as the yield is directly correlated to the nucleophilicity of the anion used and no rearrangement products are observed. It has also been shown that it is possible to chlorinate unsaturated aliphatic alcohols. However, it is predicted that this system would not be suitable to halogenate alcohols containing acid-sensitive functional groups. For 1-indanol, a secondary alcohol, an unusual coupled product was obtained upon attempting chlorination. Therefore, further investigation into the reactivity of secondary and tertiary alcohols in this reaction system may be warranted.
Chemicals and instrumentation
[Bmim]Cl, [Bmim]Br, [Bmim]Tf2N, [Bmim]BF4 and [Bmim]PF6 were prepared according to literature procedures [26–28]. [BMmim]PF6 (BMmim, 1-butyl-2,3-dimethylimidazolium) was purchased from Alfa Aesar, while [P66614]DBS was received as a gift from Cytec Inc. Benzyl alcohol was distilled under vacuum and stored under a blanket of nitrogen in contact with 4 Å activated molecular sieves. Butanol was distilled from sodium hydroxide (NaOH) and stored, with 4 Å molecular sieves, under nitrogen. All other reagents were purchased from either Alfa Aesar or Sigma Aldrich and used without further purification.
A Biotage microwave reactor was used to run the experiments under microwave (MW) irradiation. The ‘very high’ absorption level setting was used each time to ensure controlled heating of the reaction. 1H NMR spectra were acquired on a Bruker AVANCE 500 MHz spectrometer with TMS as the internal standard. GC–MS spectra were recorded on an Agilent 7890A GC system coupled with an Agilent 5975C MS detector that was equipped with a capillary column DB-5 (column length: 30.0 m and column diameter: 0.25 mm). All 1H NMR experiments were performed with acetone-d6 as the solvent, except when [P66614]DBS was used, in which case the 1H NMR experiments were performed in chloroform-d. In all cases, samples were injected into the GC–MS instrument with use of diethyl ether as the solvent. All yields were determined by 1H NMR with use of acetophenone as the internal standard and reported in reference to the limiting reagent. Products could be isolated via vacuum distillation or flash chromatography.
General procedure for dehydroxyhalogenation reactions
Ammonium salt (1.10 mmol), alcohol (1.10 mmol), and ionic liquid (0.50 g) were placed in a 2 mL microwave vial equipped with a stir-bar. The vial was sealed under nitrogen and the reaction mixture was stirred at room temperature for 15 min to ensure homogeneity. The reaction was then subjected to microwave irradiation for a set period of time at a pre-selected temperature, both of which are noted in the results section above. Upon cooling, a sample of known mass was taken from the reaction mixture, added to a known amount of acetophenone (internal standard) and dissolved in acetone-d6 to be analyzed by 1H NMR. For analysis by GC-MS, a 0.1 g sample of the reaction mixture was extracted with 2.5 mL of dry diethyl ether. For product isolation, the ionic liquid was extracted with 3 × 5 mL diethyl ether. The combined extracts were concentrated and flash chromatography was performed using a Biotage Isolera system to separate any unreacted alcohol and ether by-products from the desired halogenated product.
Ionic liquid recycling was performed as follows. Ammonium chloride (4.40 mmol), n-butanol (4.40 mmol), and [Bmim]PF6 (2.00 g) were placed in a microwave vial. The mixture was microwave-heated to 150 °C for 17 min whilst stirring. Upon cooling, the vial was transferred to a distillation apparatus and the chlorobutane isolated . The ionic liquid phase was then treated as follows prior to re-use. The contents of the microwave vial were dissolved in anhydrous acetone (10 mL) and dried over anhydrous sodium sulfate. The mixture was decanted and the acetone removed under vacuum. The resulting ionic liquid was then re-used. Yields of chlorobutane were 51, 57, 56, 54, and 52 % (based on the amount of alcohol added before each reaction).
Further investigation of catalytic chlorodehydroxylation of benzyl alcohol in ionic liquids led to a number of interesting observations. Firstly, although it was initially thought that this transformation was palladium-catalysed , it has been proven that this transformation is acid-catalysed and that the presence of palladium has a negative effect on the yield of the desired product. As well, when benzyl alcohols, with the exception of 4-nitrobenzyl alcohol, are employed in this reaction system, an equilibrium exists between the chlorinated product and the corresponding ether that is highly dependent upon reaction temperature, time and substituent. Furthermore, based on experimentally obtained data, it appears that the dehydroxyhalogenation reaction proceeds through a SN2 mechanism and that the acidic proton of [Bmim]PF6 plays a crucial role in this process. It has also been shown that this reaction system can be expanded for use with other ammonium halide salts, with the exception of NH4F, and both functionalized and non-functionalized primary alcohols. However, when 1-indanol, a secondary alcohol, was used, no chlorinated product was observed and an interesting biindenylidene product was formed in moderate yields. It is known that biindenylidenes have interesting photochromic properties [1, 29–31]. Overall, NH4X (X = Cl, Br, I) in [Bmim]PF6 under microwave irradiation is a safe, green method of transforming several alcohols into the corresponding halides.
- PPh3 :
gas chromatography-mass spectrometry
CP performed the majority (80 %) of reactions and analyses with the remaining portion performed by HK (20 %). FK conceived of the study, and participated in its design and coordination and wrote the manuscript. All authors read and approved the final manuscript.
We thank Memorial University, the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI) and Research and Development Corporation (RDC) of Newfoundland and Labrador for funding.
Contribution for special issue on “Green and sustainable solvents”.
The authors declare that they have no competing interests.
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