Cope salts

TL2733,82TL2737). The reaction proceeds through an aza-Cope rearrangement of the initially formed iminium salt, followed by intramolecular cyclization. 

Amine oxides 2, which can be prepared by oxidation of amines 1, react upon heating to yield an olefin 3 and a hydroxylamine 4. This reaction is called the Cope elimination reaction,and as a synthetic method is a valuable alternative to the Hofmann degradation reaction of quaternary ammonium salts. 

The preparation of an alkene 3 from an amine 1 by application of a /3-elimination reaction is an important method in organic chemistry. A common procedure is the Hofmann elimination where the amine is first converted into a quaternary ammonium salt by exhaustive methylation. Another route for the conversion of amines to alkenes is offered by the Cope elimination. 

Plants are constantly subject to adverse environmental conditions such as drought, flooding, extreme temperatures, excessive salts, heavy metals, high-intensity irradiation and infection by pathogenic agents. Because of their immobility, plants have to make necessary metabolic and structural adjustments to cope with the stress conditions. To this end, the expression of the genetic programme in plants is altered by the stress stimuli to induce and/or suppress the production of specific proteins which are either structural proteins or enzymes for specific metabolic pathways. 

It is well known that chemical compo.sition of rhizosphere solution can affect plant growth. Particularly, uptake of nutrients may be considerably influenced by the ionic concentration of the rhizosphere solution (40). Despite the difficulty of defining the exact concentration of ions in the rhizosphere surrounding each root (or even root portion), it has been unequivocally demonstrated that plants have evolved mechanisms to cope with the uneven distribution of ions in the root surrounding in order to provide adequate supply of each essential nutrient (41). These mechanisms include expression of transporter genes in specific root zones or cells and synthesis of enzymes involved in the uptake and assimilation of nutrients (40,43). Interestingly, it has been shown that specific isoforms of the H -ATPase are expressed in the plasma membrane of cell roots it has been proposed that the expression of specific isoforms in specific tissues is relevant to nutrient (nitrate) acquisition (44) and salt tolerance (45). 

Several transition metal ions and complexes, especially Pd(II) salts, have been found to catalyze Cope rearrangements.207 The catalyst that has been adopted for synthetic purposes is PdCl2(CH3CN)2, and with it the rearrangement of 14 to 15 and 16 occurs at room temperature, as contrasted to 240° C in its absence.208 The catalyzed reaction shows enhanced stereoselectivity and is consistent with a chairlike TS. 

An important improvement in the oxy-Cope reaction was made when it was found that the reaction is strongly catalyzed by base.212 When the C(3) hydroxy group is converted to its alkoxide, the reaction is accelerated by a factor of 1010-1017. These base-catalyzed reactions are called anionic oxy-Cope rearrangements, and their rates depend on the degree of cation coordination at the oxy anion. The reactivity trend is K+ > Na+ > Li+. Catalytic amounts of tetra-rc-butylammonium salts lead to accelerated rates in some cases. This presumably results from the dissociation of less reactive ion pair species promoted by the tetra-rc-butylammonium ion.213... 

Trifluoromethyl imidates show similar reactivity.262 Imidate rearrangements are catalyzed by palladium salts.263 The mechanism is presumably similar to that for the Cope rearrangement (see p. 555). 

Quaternary allenylallylammonium salts, produced in situ by prototropic isomerization of propargyl precursors (see Section 7.2.2), can undergo a 3-aza-Cope rearrangement [370]. The resulting intermediates are hydrolyzed under the reaction conditions to yield 2 -methylenepent-4-enals. 

Apparently, the aromatization of the heterocyclic cation serves as a driving force of the Cope rearrangement in the transformation of the 3-formyl-4-allyl-4//-pyrane (481) into poly-substituted pyrylium salt 483 which presumably proceeds via 482 (equation 183)242. 

The behavior of tra s-[RuCl(=C=C=C=CH2)(dppm)2][PF6] towards the tertiary amine allyldimethylamine merits to be highlighted since the reaction led to the dimethylamino-allenylidene complex 32 through an Aza-Cope (or Claisen) rearrangement of the initially generated quaternary vinyl-allyl-ammonium salt [RuCl C=CC(NMe2CH2CH=CH2)=CH2 (dppm)2][PF6] (31) (Scheme 8) [47]. 

The diaminocyclopropanes and their salts have also served as precursors for the preparation of 2,3-cyclopropanodihydrodiazepines [77AG668, 77AG(E)643], which are of interest because they themselves undergo Cope rearrangements (see Section X). 

As mentioned earlier, preparation of (Z),( )-l, 5-cyclooctadiene (31) in an optically active modification 2) first demonstrated the chiral nature of ( )-cycloalkenes. In this classical experiment, Cope and coworkers obtained ( + )-31 by the Hofmann elimination of the (-l-)-ammonium salt 30. They were also successful in obtaining (+)-31 by optical resolution of racemic 31 through complexing with a chiral Pt(II)... 

Near the end of the eighteenth century the difference between the two fixed alkalies—potassium and sodium carbonates—was known sodium carbonate barilla was largely made from the ashes of sea plants, and potash from the ashes of land plants. The Arabs also had brought some natural soda into Europe, via Spain. These sources were not sufficient to cope with the demand for alkali for the manufacture of soap, glass, etc. Potash was at that time the cheaper and dominant alkali. With the steadily increasing demands for alkali and the very limited sources of supply presented by the incineration of wood, many attempts were naturally made to substitute the base of common salt, because that with a suitable method of extraction nature has provided inexhaustible, abundant, and cheap... 

Cleavage of amine oxides (Cope) 7-9 Cleavage of aliphatic diazonium salts... 

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