Enantioselective organocatalytic fluorinationinduced wagner. Wagner meerweintype rearrangements of germapolysilanes a. The rearrangement was first discovered in bicyclic terpenes for example the conversion of isoborneol to camphene. The wagner meerwein rearrangement is generally observed in. Although the wagnermeerwein rearrangement strictly involves the migration of a skeletal bond, the term is sometimes widened. Molecular orbital studies on the wagnermeerwein migration in. Synthetically useful wagnermeerwein rearrangement of. This reaction is called wagner meerwein rearrangement.
They can be described as cationic 1,2sigmatropic rearrangements, proceeding suprafacially and with stereochemical retention. Request pdf wagnermeerwein rearrangement acidcatalyzed alkyl group migration of alcohols to give more substituted olefins. Formation of the unwanted regioisomer, catechol diacetate, could thus be efficiently diminished 5. Mar 27, 2018 carbocation wagner meerwein rearrangements demystified. Wagner meerwein rearrangement was observed on silica gel during the column chromatography of isomeric tribromides. Wagner in 1899 considered rearrangements of the retropinacol type to be essential for the interpretation of the interconversions among bicyclic terpenes.
A wagner meerwein rearrangement is a class of carbocation 1,2 rearrangement reactions in which a hydrogen, alkyl oraryl group migrates from one carbon to a neighboring carbon. May 23, 2006 a synthetically useful wagner meerwein wm rearrangement is reported in this paper. Ultimately, allylic oxidation, baeyer villiger reaction, and di. Excellent yields of up to 96% of diacetate 2a were obtained. Meerwein rearrangement, and the second to a pinacol beckmann type rearrangements may also be carried out by rearrangement. A 1,2shift then occurs to form a more substituted and stabilized carbocation. A wagnermeerwein rearrangement is any reaction in which the carbon skeleton of a reactant changes due to one or more rearrangements involving carbocations, e. Pdf survey in rearrangement and migration processes itirj.
Wagner meerwein rearrangements are 1,2rearrangements of h atoms or alkyl. Carbocation wagnermeerwein rearrangements migration of. Wolff rearrangement the wolff rearrangement is an organic reaction used to convert an. A mechanism demonstrating such a rearrangement is shown below.
Pdf efficient methods for introducing various substituents into the. However, the cyclopropylmethyl cations generated are planar, which would lead to loss of chiral information in the case of chiral precursors, making an asymmetric version of such ring expansion difficult. Towards the end of his career, wagner had turned his attention to the structures. Wagner meerweintype rearrangements of germapolysilanes. A wide range of other factors, such as steric and conformational effects, play subtle.
Figure 3, who is better known in the west by the german form of his name, georg wagner. The following examples illustrate rearrangements induced by the strain in a small ring. Groups remote from a reaction centre can participate in substitution reactions neighboring group. Molecular orbital studies on the wagnermeerwein migration. Using this procedure, the wagner meerwein rearrangement of betulin afforded the title compound in high yield.
Pdf the preparation by a convenient route of 9methyl, ethyl, isopropyl, lbutyl, and benzyl phenanthrenes is described. Highly functionalized cyclopentenones can be generated by a chemoselective copperiimediated nazarov wagner meerwein rearrangement sequence of. A wagnermeerwein rearrangement is a class of carbocation 1,2 rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. Newly generated carbocation is stabilized generally by loss of a proton. Wagnermeerwein rearrangements chemistry libretexts. Michailidis department of organic chemistry, university of geneva, quai ernest ansermet 30, 1211 geneva 4 switzerland. The pinacol, wagnermeerwein, demjanov, and tiffeneau. There are probably no other examples in support of the latter statement that. Enantioselective organocatalytic fluorinationinduced.
Whereasthechlorinationand brominationinduced wagner meerwein rearrangements of electronrich cyclic enol ethers were recently shown to be. The wagnermeerwein rearrangement is an organic reaction used to convert an alcohol to an olefin using an acid catalyst. Bromination, benzobarrelene, wagner meerwein rearrangement introduction. Wagner, are permanently associated with these rearrangements. Wagnermeerwein rearrangement anthony crasto reactions. Meerwein rearrangement in oligosilanes, we found that for germylsilanes a particular preference exists for the formation of products in which the germanium atoms occupy central positions in the oligosilane frameworks.
This video simply gives the mechanism of rearrangement of alcohols under acidic condition. It is a 3,3 sigmatropic rearrangement with only carbon atoms involved in the six membered transition state why is it called 3,3. The earliest examples of wagner meerwein rearrangement was noticed in bicyclic terpenes. Example conversion of pxylene to mxylene in the presence of acid methyl group moves to meta position, which is energetically more stable. Alkyl migration occurs to give stable carbocation me me h. Mechanism studies of the retropinacol reaction by lamar. Wittig rearrangement 1,2 and 2,3wittig rearrangement format. Therefore, the fluorine shift can be considered as analogous to a wagner meerwein rearrangement. Rearrangement of anions and carboca1ons orbital theory. This is the driving force for the migration of alkyl, aryl or even hydrogen atom 3. Induced wagnermeerwein rearrangement fedor romanov. Sep 09, 20 the following other wikis use this file.
Likewise, in for example the pinacol rearrangement, the most stable cation controls the reaction pathway, as opposed to the migrating group. The chemical behavior of neopentyl bromide, 2,2dimethyl1bromopropane, is an instructive place to begin this discussion. Acidcatalyzed alkyl group migration of alcohols to give more substituted olefins. Wagner meerwein rearrangement is by far the most interestingly spread one. Competitive shifts of parasubstituted phenyl treating hydrazones with nitrous acid, as shown on the right. Wagner meerwein rearrangement rearrangements that occur with elimination of water in the dehydration of an alcohol, of hydrogen halide in the dehydrohalogenation of an alkyl halide. Dispersionenergydriven wagner meerwein rearrangements in.
Wagner meerwein shifts these 1,2alkyl shifts occur if it leads to the new carbocation being more stabilised. The wagnermeerwein rearrangement involves the generation of a carbocation on a bicyclic system followed by the 1,2shift of an adjacent skeletal c c bond to generate a new carbocation. Wagner meerwein rearrangement rearrangement reactions. There are probably no other examples in support of the latter statement that are. How to choose which hydride or alkyl group will migrate and why. Bromination of 2,3dihydrobenzobarrelene and synthesis of. Plausible mechanisms of the wagner meerwein rearrangement of diepoxyisoindoles. Asymmetric synthesis of azepinefused cyclobutanes from yne. As such it gives rise to a new bicyclic or polycyclic ring system, and so can be a powerful technique for the generation of complicated structures, with c c bonds in positions difficult to.
Experimental and theoretical studies on the nazarov. The earliest examples of wagnermeerwein rearrangement was noticed in bicyclic terpenes. A skeleton rearrangement of carbonium ion formed from highly branched alcohol or alkyl halide or alkene through migration of alkyl group is known as wagner m. After the work of zelinsky and zelikow had extended the rearrangement beyond the bicyclic field to the 6 simple alicyclics, meerwein demonstrated its application to a simple monocyclic case. Dispersionenergydriven wagner meerwein rearrangements. This mechanism requires a subsequent 1,2alkyl migration or wagner meerwein rearrangement in order to produce the c2 normal prenylated structure of tryprostatin b. The investigation on the design and optimization involved the substrates quaternary bromovinyl methyl ethers as well as the reaction conditions. Meerwein rearrangements in oligosilanes, in contrast to the classical rearrangement in hydrocarbon systems, which are dominated by electronic substituent e. Two such examples, already noted, are the addition of hcl to 3,3dimethyl1butene and forced hydrolysis of neopentyl. Wagnermeerwein rearrangement an overview sciencedirect. A rearrangement is not well represented by simple and discrete electron transfers represented by curved arrows in organic chemistry texts. Find, read and cite all the research you need on researchgate.
The mechanism of the wolff rearrangement is essentially one step which is initiated by the catalyst. In such situations the question of which group migrates depends on several factors such as the geometry of molecule. The story of the rearrangement reveals that many scientists were. However, the cyclopropylmethyl cations generated are planar, which would lead to loss of chiral information in the case of chiral precursors, making an asymmetric version of such ring expansion di. The semiempirical pm3 scfmo method is used to investigate the wagner meerwein migration of various groups during the pinacolpinacolone rearrangement of some acyclic systems. The mechanism begins with protonation of the alcohol by the acid which is then released as water to forms a carbocation. The wagnermeerwein rearrangement is a nucleophilic rearrangement involving 1,2migration of a ring carbon atom in a bicyclic or polycyclic molecule. A wagnermeerwein rearrangement is a class of carbocation 1,2rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to a neighboring carbon. Ring expansion of in situ generated cyclopropylmethyl cations via wagner meerwein rearrangement to cyclobutanes is widely used in synthesis.
Learn how to spot how and when a carbocation will form and how it will rearrange. The migration of an alkyl group to a cationic centre is known as a wagner meerwein rearrangement or wagner meerwein shift. This intermediate was further elaborated to amide g. Recently, we have reported the use of bismuthiii salts as catalysts for the wagner meerwein rearrangement of lupane derivatives with expansion of ring e and formation of an additional ocontaining ring salvador et al. Stevens rearrangement description of the reaction and detail at the mechanism. Wagner meerwein rearrangement mechanism, scope, application.
Sep 12, 2020 wagnermeerwein rearrangement last updated. Now i showed this to my teacher, but he said that in this particular case the carbocation b wont convert to carbocation c. Rearrangements to edeficient o baeyervilliger rearrangement this is a rearrangement to electron deficient oxygen. Mechanism involves rearrangement of the carbocation intermediate. Pdf survey in rearrangement and migration processes. Meerwein rearrangement to cyclobutanes is widely used in synthesis. Wagnermeerwein rearrangements arylgermanes as linkers for. The following examples illustrate rearrangements induced by the. Wagner meerwein rearrangement description and reaction mechanism.
Sulfonation and subsequent rections of ethylidene norbornane and dihydrodicyclopentadiene. The actual mechanism of alkyl groups moving, as in wagner meerwein rearrangement, probably involves transfer of the moving alkyl group fluidly along a bond, not ionic bondbreaking and forming. Herein, we report the results of the synthe sis, and the xray crystal structures and the possible mechanism of processes are discussed. Pinacol first protonates and dehydrates to form a carbocation that undergoes a 1,2migration to form a protonated ketone, which then deprotonates to yield the pinacolone product. Bromination of 2,3dihydrobenzobarrelene and synthesis of its. Hofmann rearrangement similar net result to the above 3 rearrangements, but the formation of the isocyanate again differs. Wagner meerwein rearrangement is a class of carbocation 1,2 rearrangement reactions in which a hydrogen, alkyl oraryl group migrates from one carbon to a neighboring carbon. As already discussed, these reactions are usually summarized under the category of semipinacol rearrangement. A wagner meerwein rearrangement is a class of carbocation 1,2rearrangement reactions in which a hydrogen, alkyl or aryl group migrates from one carbon to. Asymmetric synthesis of azepinefused cyclobutanes from. Carbocation wagnermeerwein rearrangements migration. Stieglitz rearrangement nucleophilic migration from carbon to nitrogen. A wagnermeerwein rearrangement can be part of the isomerization of an.
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