Hoffman Elimination

4 Key excerpts on "Hoffman Elimination"

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  Pyrolytic Methods in Organic Chemistry

  Application of Flow and Flash Vacuum Pyrolytic Techniques

  • Roger Brown(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 4 Elimination Reactions I. INTRODUCTION In this chapter, we consider a variety of reactions which are formally related because they involve elimination of a small fragment molecule X—X or X—Y from a larger molecular framework. These reactions include thermal dehydrogenations with elimination of H—H and dealkylations with elimi-nation of H—R, as well as reactions involving loss of H—OR, H—Hal, and related species which are conventionally regarded as eliminations. Although some reactions with similar mechanisms are grouped together, the arrange-ment is primarily by formal reaction type rather than by mechanism, so that radical and radical chain reactions appear beside concerted molecular reac-tions. The coverage of particular types of reaction may seem to be, per-versely, in proportion inverse to their synthetic and general importance. The reason for this will be obvious in the case of ester pyrolysis, for which several very detailed and comprehensive reviews are already available, and which is firmly incorporated into the mainstream of organic chemistry in most advanced textbooks. II. DEHYDROGENATIONS AND DEALKYLATIONS A. Dehydrogenations Dehydrogenations are quite common in high-temperature pyrolyses, par-ticularly as the final step in a sequence leading to the formation of an aromatic system, and many examples without special comment will be found through-out this book. The mechanism of dehydrogenation and the precise structure of the penultimate species have not in most cases been established. Two typical mechanistic possibilities are shown by the kinetic work of Ellis and Frey 1 on the decomposition of the 1,3- and 1,4-cyclohexadienes. 73 Chapter 4 Elimination Reactions I. INTRODUCTION In this chapter, we consider a variety of reactions which are formally related because they involve elimination of a small fragment molecule x-x or X-Y from a larger molecular framework.

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  Organic Reaction Mechanisms 1987

  An annual survey covering the literature dated December 1986 to November 1987

  • A. C. Knipe, W. E. Watts, A. C. Knipe, W. E. Watts(Authors)
  • 2008(Publication Date)
  • Wiley

    (Publisher)

  Elimination is presumed to be preceded by oxidation of the 6- hydroxy to a keto group and followed by re-reduction to the a1c0hoi.l~ Studies of E-( 1,4) and E-( 1,6)-eliminations involving dehydration and decar- boxylation, usually in the presence of DMF dineopentyl acetal, have shown the stereochemistry of these reactions to be predominantly syn, notwithstanding geometrical constraints on the eliminating groups imposed by incorporating the reacting carbon bonds into rings [e.g. as in (l+9)].'* For the syn-thermal elimination of the dimethyl amine oxide PhCH,CD,NMe,O- a rather large a-deuterium isotope effect (1.16 per D) has been measured, which contrasts with the small value COOH H $Me + (19) ~399%) Z(1%) (1.02 per D) found for base-promoted elimination of the structurally related phenethyltrimethylammonium ions: significant C-N bond-breaking but little double-bond development (despite carbanion character at the P-carbon atom) is suggested for the transition state. There has also been further discussion of the interdependence of primary and secondary isotope effects (isotope effects upon isotope effects) which has been suggested as being a symptom of tunnelling in phenethyl elimination reactions. There have been several studies of dehydrohalogenation of a -h a l ~ -k e t o n e s ~ ' -~ ~ and -esters including isotope and Br#nsted measurements.21 The chloro- and bromo-lactones (20) have been shown to yield the corresponding butenolide (21) under typical E2C conditions,22 and there has been an abortive but interesting attempt to effect the E2C-like demethylhalogenation (22) -+ (23).25 Elimination reactions of P-halo-sulphones have been investigated as part of a search for neighbouring-group participation by the sulphone group,26 and a Hammett correlation has been reported for elimination of substituted phenols activated by a P-cyano s ~ b s t i t u e n t . ~ ~

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  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  When the Hofmann elimination is applied to 3-methylprolidine, 2-methylbutadiene is obtained. When we compare the two reactions, it is seen that the alkenes formed are different. The structure of the alkenes depends on the location of the methyl group in the pyrrolidine ring. Thus, moving back from the reaction results, it is possible to find out where the methyl group was in the ring before the Hofmann elimination. This is an important clue in the structure analysis of alkaloids with a complex structure.

  It is well established that the leaving groups in the Hofmann elimination must be in the anti-periplanar conformation. Otherwise, elimination cannot take place. For example, in trans-4-t-butylcyclohexyltrimethylammonium hydroxide, substituents prefer the equatorial–equatorial conformation [35 , 36] . In this conformation, the molecule cannot have the required anti-periplanar conformation. In an anti-periplanar conformation, both groups (t-butyl and trimethylamine) will be in axial positions, which will increase the energy of the molecule. Therefore, the trans-isomer does not produce an elimination product. The situation is different in the cis-isomer. An essential part of the molecule will be in the conformation shown below. Because the hydrogen and an amine group to be eliminated are in the anti-periplanar conformation, elimination takes place to form a cyclohexene ring in 92% yield.

  The Hofmann degradation has been successfully applied to the synthesis of some compounds. Cyclooctatetrane was first synthesized by Richard Willstaetter in 1905. He recognized its potential as a starting material for the synthesis of a carbocyclic eight-membered ring. The synthesis consists of progressive degradation of an alkaloid named pseudopelletierine obtained from the root bark of the pomegranate tree [37 –39] . The synthetic steps are given below.

  Hofmann eliminations are not limited to tetraalkylammonium salts. Elimination reactions take place with phosphonium and sulfonium salts of a similar structure. Base-promoted elimination of dimethyl-sec-butyl sulfonium salt gives 1-butene as the major product in 74% yield and 2-butenes (cis- and trans-mixture) in 26% yield. Electron withdrawal increases here the acidity of β-hydrogen atoms. The proton Ha from the methyl groups is more acidic than the proton Hb from the methylene group. Therefore, the base abstracts one of the acidic protons, Ha

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  eBook - PDF

  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  Therefore, it is desired to have a good leaving group. We have presented examples where the leaving groups were halides and a hydroxyl group. Elimination reactions can also be performed with sulfonium and ammonium salts. Thioethers can also undergo elimination reactions. However, the thioether functionality first must be converted into a good leaving group, such as sul- fonium salts. If the substrate can form a carbocation after removal of the thioether group, an elimination product will be formed [4]. C H 3 C CH 3 CH 3 CH 3 CH 3 S CH 3 CH 3 CH 3 CH 3 C CH 3 CH 3 C CH 2 H 3 C C H 2 C H 3 C H 3 C Zaitsev product 87% Minor product 13% –H + C H 3 C H 3 C S + H 3 C CH 2 CH 2 EtOH/H 2 O (4 : 1) EtOH/H 2 O (4 : 1) C H 3 C C H CH 3 CH 3 –S(CH 3 ) 2 –S(CH 3 ) 2 94 3 Elimination Reactions In general, the following conclusions can be drawn from the E1 elimination mechanism described above: 1. Reaction kinetics is first order, 2. The rate-determining step is the formation of a carbocation, as in the case of S N 1 reactions, E1 and S N 1 reactions compete, 3. Rearranged products can be formed, 4. If there is the possibility of forming more than one elimination product, the double bond to which the most substituent is attached is formed as the major product (Zaitsev product). Rearrangement of the carbocation and its detailed mechanism will be discussed in Section 7.1. 3.2 Bimolecular Elimination Reactions, E2 Elimination reactions can also proceed under second-order conditions with a strong base. In S N 2 reactions, the transition state is formed between the substrate and the nucleophile. Therefore, the reaction is a second-order reaction. E2 reactions are also second-order reactions and the transition state is formed between the substrate and the base. Like the S N 2 reaction, the E2 reaction also proceeds in one step with one transition state and without the formation of any intermediates.

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