Aluminium triflate as a Lewis acid catalyst in some epoxide and aromatic transformations
- Authors: Lawton, Michelle Claire
- Date: 2012-03-14
- Subjects: Lewis acids , Aluminum , Alcoholysis , Epoxy compounds , Catalysts , Aromatic compounds , Substitution reactions
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/368554 , uj:2163 , http://hdl.handle.net/10210/4536
- Description: M.Sc. , Lewis acids play an important role in catalysis; they are associated with mild conditions, high selectivities and unique reactivities. Traditional Lewis acids such as AlCb and BF3 successfully catalyse such well known reactions as the Friedel-Crafts acylation reaction, Aldol condensation reactions and many more. These catalysts, however, must be used in a stoichiometric amount and are destroyed during the aqueous workup procedures. Lately, there has been a lot of interest in the role of metal triflate as Lewis acid catalysts. They were found to be effective in a wide range of reactions when used in catalytic amounts. They were also found to be recyclable and reusable without the loss of activity. Most of this research has been centred around the lanthanide triflates as well as scandium, bismuth and yttrium triflates. Very little research has been done using aluminium triflate and this triflate forms the focus ofthis study. The work contained in this dissertation demonstrates that Al(OTf)3 is an efficient catalyst for the ring opening of a variety of epoxides by alcohols when present in only ppm amounts. These reactions provided products in very high yields and selectivities. Simple acyclic and cyclic epoxides readily underwent ring opening reactions with a range of alcohols, typically providing the monoglycol ethers as single compounds (from the cyclic epoxides) or as mixtures of the two possible glycol monoethers (from the acyclic epoxides). In the case of styrene oxide, essentially a single compound was isolated. In contrast, the glycidyl ethers required slightly higher catalyst loadings before similar rates and conversions to product were observed. Additionally, an interesting selectivity was observed in the orientation of the attack of the alcohol onto the epoxide, which appeared to be chelation controlled. Similarly, the Al(OTf)3 also catalysed the aminolysis of a variety of epoxides. These reactions proceeded smoothly with catalytic amounts of the triflate present, and served to nicely highlight the role that steric and electronic factors played in these reactions. A preliminary study was carried out into the efficacy of Al(OTf)3 as a catalyst for Friedel-Crafts acylation and aromatic nitration reactions. From these studies it is evident that the Al(OTf)3 is indeed an effective catalyst for these reactions when present in substoichiometric levels and further studies will be carried out in this area in the future.
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Aluminium triflate-mediated organic synthesis
- Authors: Cullen, Adam
- Date: 2012-08-20
- Subjects: Metal triflate , Organic compounds - Synthesis , Catalysts , Lewis acids
- Type: Thesis
- Identifier: uj:2736 , http://hdl.handle.net/10210/6177
- Description: Ph.D. , The work described in this thesis was directed at advancing the applications of Al(OTf)3, a metal triflate, in organic synthesis. Lewis acids play an important role in catalysis and catalyse reactions with high selectivities, unique reactivities under mild conditions. Metal triflates have become the Lewis acids of choice for acid catalysed organic transformations. A detailed literature study of metal triflates provided numerous examples of their use in organic transformations. Al(OTf)3 has been widely neglected as a Lewis acid which is contrasting to the attention the other metal triflates have received. Previous work in our laboratories had established Al(OTf)3 as an effective Lewis acid catalyst for the ring-opening of epoxides with simple alcohols and amines. The alcoholysis of epoxides provides a ready access to β-alkoxy alcohols. Whilst this reaction has been shown to occur with Al(OTf)3 as a catalyst, the established protocol calls for the use of the nucleophilic alcohol in an excess amount. Whilst this proves no problem when simple alcohols are employed as nucleophiles in the ring-opening reaction, it is a problem when more complex and expensive alcoholic nucleophiles are utilised. A modified procedure utilising Al(OTf)3 as a catalyst was developed which tolerates the use of only 1 equivalent of the nucleophilic alcohol for the ring opening reaction. The desymmetrisation of a meso-epoxide with chiral alcoholic nucleophiles was also investigated and the outcome of the diastereoselectivity of the reaction reported. The aminolysis of epoxides has been established utilising Al(OTf)3 as the Lewis acid catalyst. However, this has only been demonstrated for the ring opening of simple epoxides with simple amines. Piperazine derived β-amino alcohols with known biological activity were chosen as substrates with which to test the Al(OTf)3 catalysed aminolysis of epoxides in the synthesis of more complex β-amino alcohols. The various starting epoxides and amine nucleophiles were synthesised. During which a new approach towards the synthesis of - glycidyl amines was developed utilising a two step approach with the first step being catalysed by Al(OTf)3. It was also found that the optimal method for forming the β-amino alcohol bond was one in which the glycidyl motif was placed on the less basic heteroatom and ring opened by the more nucleophilic piperazine amine.
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Aluminium triflate-mediated reactions of glycals: towards chiral multicyclic products
- Authors: Simelane, Sandile Bongani
- Date: 2015-04-23
- Subjects: Substitution reactions , Organic compounds - Synthesis , Enantioselective catalysis , Lewis acids
- Type: Thesis
- Identifier: uj:13557 , http://hdl.handle.net/10210/13698
- Description: Ph.D. (Chemistry) , The bridged chiral benzopyrans were strategically ring opened via acetolysis to yield either galactose based chromenes or chromans, depending on the reaction conditions. A proposal relating to the mechanism of this selective ring opening acetolysis is discussed. The benzopyrans (chromenes, chromans and bridged chiral benzopyrans) were de-acetylated via triethyl amine catalysed transesterification. Interestingly, the chromenes did not yield the anticipated hydrolysis product (triol) but a new class of bridged chiral benzopyrans which were a result of intramolecular oxa-Michael addition. A chromene that formed during the selective ring opening of the bridged chiral benzopyrans was employed to develop a method for the synthesis of a carbohydrate derived oxepane. The oxepane synthesis was achieved, albeit in the face of numerous challenges from side reactions. The difficulties encountered in the synthesis are discussed...
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Applications of metal triflates and assisted acids as catalysts for organic transformations
- Authors: Sibiya, Mike Sbonelo
- Date: 2012-11-05
- Subjects: Metal triflates , Organic compounds , Catalysts , Lewis acids , Organic reaction mechanisms , Organic chemistry
- Type: Thesis
- Identifier: uj:7340 , http://hdl.handle.net/10210/8089
- Description: Ph.D. , The research contained in this thesis was aimed at the applications of Lewis acids (metal triflate salts in particular) and Brønsted acids as catalysts for various organic synthesis reactions. The ultimate objective was to prepare combinations of the Lewis and Brønsted acids to form assisted acids. The assisted acids yield to the formation of highly acidic assisted acids which exhibit high activity as compared to the individual Lewis and Brønsted acids. A detailed literature study was undertaken, with emphasis on the applications of metal triflate salts as catalysts for various organic reactions and the applications of assisted acids. The study was motivated by the fact that metal triflate Lewis acids are thermally stable, non corrosive and water tolerant catalysts, hence can be used industrially to replace the corrosive, moisture sensitive acids as catalysts. However, metal triflates have not yet been recognised and utilised in the chemical industry. On the other hand, the active Brønsted acids such as triflic acid, H2SO4 etc. are corrosive, which restricts the type of construction material to hastelloy. However, the assisted acids composed of less corrosive Brønsted acids and metal triflate Lewis acid is desirable to address the corrosion and safety challenges. The metal triflate salts and Brønsted acids were evaluated as catalysts for etherification reactions of alcohols and olefins, Friedel-Crafts alkylation reactions phenolic substrates with isobutylene. The study showed that some dependence of the charge density to the activity, i.e. metal triflate salts such as Al(OTf)3, Zr(OTf)4 and Sc(OTf)3 with relatively high charge density were more effective in catalysing the reactions than those with relatively smaller charge density such as lanthanides, which were virtually active. The activity of Brønsted acids showed a clear dependence on the acid strength pKa, with H3PO4 giving the least activity. The assisted acids formed via a combination of metal triflate salts with mineral Brønsted acids showed a significant enhancement of the reaction rates as compared to the individual acids. This set of new combined acids was proven to be excellent catalysts for the etherification reactions, Friedel-Crafts alkylation reactions and also for the synthesis of biologically active compounds called chromans. The assisted acids as well as Al(OTf)3, and Zr(OTf)4 could be recycled at least four times without significant loss of activity. The study also showed that assisted acids could be recycled for both etherification and Friedel-Crafts reactions.
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Ethers and acetals in aluminium triflatepromoted reactions
- Authors: Henning, Hendrik
- Date: 2013-07-24
- Subjects: Aluminum , Ethers , Lewis acids , Metal catalysts
- Type: Thesis
- Identifier: uj:7695 , http://hdl.handle.net/10210/8561
- Description: M.Sc. (Chemistry) , The work in this thesis describes some catalytic reactions of aluminium triflate. This Lewis acid has been shown to be of great value in the catalysis of several types of organic transformations. These include but are not limited to the alcoholysis and aminolysis of epoxides, uses in methoxycarbonylation catalysis and in esterification reactions. Therefore, aluminium triflate holds promise for application in both the bulk and fine chemistry industries. Aluminium triflate has received little interest compared to some other, more costly triflates such the lanthanide triflates. Oxetanes were used in various catalysis experiments during this study, yet few are commercially available. Therefore, various synthetic approaches to the synthesis of 2-phenyl-oxetane were explored. There were many failed attempts, but the preparation was eventually successful using a sulfur ylide route with good yield. The next part of this thesis explores the boundaries of ring-opening reactions of oxetanes. Lewis acid ring-opening of oxetanes has been neglected compared to that of epoxides, likely because it is a more difficult undertaking. Yet, alcoholysis reactions showed good yields with a variety of different alcohol substrates. Some of the alcohols contained functionality which could provide access to further modification. Furthermore, the aminolysis and thiolysis of oxetanes were explored, essentially without success. However, with an activated oxetane it was shown that oxetanes could be opened with good yield under mild conditions with amines and thiols...
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Metal triflate catalysed organic transformations
- Authors: Lawton, Michelle Claire
- Date: 2010-10-28T08:11:00Z
- Subjects: Organic compounds synthesis , Catalysts , Lewis acids , Metal triflate , Trifluoromethanesulfonate
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/388178 , uj:6946 , http://hdl.handle.net/10210/3456
- Description: Ph.D. , The research described in this thesis was directed at advancing the application of metal triflates, Al(OTf)3 in particular, in organic synthesis, on the one hand and to contribute to the understanding of the underlying basis for their catalytic activity. The study was undertaken against the background and on the bases of a detailed literature study of metal triflates, their chemical and catalytic properties and applications thereof. Amongst others, it deals with the possible role of metal-bound water that give rise to Brønsted type acidity and that this induced Brønsted acidity may be responsible for the catalytic activity that is observed. The study was prompted by the realisation that Al(OTf)3 was largely neglected as a potential reusable catalyst. This is in marked contrast to the attention paid to other metal triflates, the rare earth metals in particular. Earlier work in this laboratory has shown that Al(OTf)3 is stable in water from which it can be recovered easily for reuse. In addition it showed promise as a Lewis acid catalyst and is relatively soluble in several organic solvents. New applications for the use of Al(OTf)3 have now been demonstrated. These include the efficient formation of acetals from aldehydes and ketones. The conversions can be carried out in an alcohol/orthoester mixture or preferably in neat orthoester. Other metal triflates, notably Sc(OTf)3 and In(OTf)3, are useful alternative catalysts. Al(OTf)3 can be easily recycled without loss of activity. This methodology also can be applied to aldehydes and ketones containing TBDMS groups without effecting deprotection of the ethers. In view of the sensitivity of the TBDMS groups to hydrolysis in the presence of triflic acid the results suggest little hydrolysis (or alcoholysis) of the metal triflates in the protic solvents used, which would generate trifluoromethanesulfonic acid as a consequence of such metal based hydrolysis. Al(OTf)3 was also found to be a good catalyst for the formation of THP ethers. It proved to be excellent for Friedel Crafts reactions using alkynes as substrates. Al(OTf)3 together with other triflates offers a mild alternative to the more traditional water sensitive Lewis acids, e.g. BF3, AlCl3 and TiCl4, which are difficult to recover and require the use of extremely dry solvents.
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