Pyrrolidinium ions

Metal ion and halide impurities are an issue in ionic liquids with discrete anions. As we have demonstrated in Chapter 11.5 Li+ (and K+) are common cationic impurities, especially in the bis(trifluoromethylsulfonyl)amides which typically contain 100 ppm of these ions from the metathesis reaction. Although Li and K are only electrodeposited in the bulk phase at electrode potentials close to the decomposition potential of the pyrrolidinium ions, there is evidence for the underpotential deposition of Li and K on gold and on other rather noble metals. For a technical process to deposit nickel or cobalt from ionic liquids the codeposition of Li and/or K, even in the underpotential deposition regime, has to be expected. 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

Concomitant to the enhancement of the signal for C-3 in the C NMR spectrum, the signals for H2 and H4 decreased in the H-NMR spectrum of all the tropine moieties compared with those in authentic samples. This indicates the incorporation of deuterium at C2 and C4 derived from C Hj CXX)". As the diminution of the signals for H2 and H4 was not identical at both positions, we suppose a sequential incorporation of labeled acetate (example given in Fig. 15b). From these data we assume that the biosynthesis from N-methyl-pyrrolidinium ion to tropine is a two-step process which does not involve a four carbon unit (acetoacetyl coenzyme A) but two units of acetyl coenzyme A, which were added sequentially as has been suggested for the biosynthesis of cocaine [28, 37]. On the other hand it has been reported that l,2- C2-acetate was incorporated with an equal efficiency at C2 and C4 by non-transformed root cultures of H. albus [36]. Further investigations are required to clarify this matter. 

Fold alternating axis of symmetry, n even, n 2 5 Tetramethyl spiro-pyrrolidinium ion ( 4)... 

Ionic liquids (ILs) are a class of organic salts that are liquid at or near room temperature. The cation parts of ILs are tetravalent ammonium ions with asymmetric substitution tetralkyl ammonium ion, imidazolium ion, or pyrrolidinium ion (O Fig. 23.8). The most widely used ILs have positive ions as asymmetric derivatives of N,N -dialkylimidazolium. As the anion counterpart a great number of negative ions are used including BFp, FFg"", or N(CN)2". 

The cyclization process can be promoted by using a single electron transfer mediator. Electron transfer from the mediator generates the carbonyl radical-ion away from the electrode surface so that cyclization can occur before there is opportunity for a second electron transfer. Thus reduction of 16, R = Me, in dimethyl-forraaraide at mercury in the presence of tetraethylammonium fluoroborate leads only to conversion of the ketone function to the secondaiy alcohol. However addition of a low concentration of N,N-dimethyl pyrrolidinium fluoroborate alters the course of reaction and the cyclized tertiary alcohol is now formed. This pyrrolidinium salt is reduced at -2.7 V vs. see at mercuiy to yield a complex DMP(Hg5) which is thought to act as a single electron transfer mediator [94]. Cyclization can... 

Figure 12.23 Structure and conformations of N-butyl-N-methylpyrrolidinium cation [CjC4pyr]+. In [P14]+, P denotes pyrrolidinium and the digits denote the number of carbon atoms in radicals and R. Also, the ion is commonly called [bmpy]+ (N is atom number 1). The ring of the (CH2)4NR R pyrimidinium ion is not planar and has two stable (twist and envelope) forms.

Ion-exchange and ring-opening reactions of telechelic poly(DMS) having pyrrolidinium salt groups... 

Thus a quantitative ion-exchange reaction was confirmed to take place by a simple precipitation of a THF solution of lc and l c into aqueous solution containing a benzoate anion, and the subsequent ring-opening reaction of the pyrrolidinium salt groups was found to proceed by the heating treatment at 100°C. 

Pyrrolidinium tetrafluoroborate (105) serves as an organocatalyst for the reaction of benzaldehyde with A-methyl pyrrole, to give the corresponding dipyrromethanes (106) under mild conditions.301 Initial formation of an iminium ion by condensation of the aldehyde with the catalyst is proposed. 

The size and nature of the ionic liquid ions are expected to influence the extent to which they are incorporated into the polymer during growth or electrochemical cycling. The size of the cation can be easily tailored by modifying the length of the alkyl substituent, and choice of a planar aromatic cation, such as imidazolium, rather than non-planar aliphatic species such as pyrrolidinium, may also enhance cation intercalation. These are also particularly important considerations with respect to their use in electromechanical actuators, where the magnitude of displacement is a direct result of ion movement into and out of the film. 

The lifetimes of hypervalent radicals have been found to depend rather dramatically on isotope substitution. For example, dimethyloxonium, (CH3)2OH, dissociates completely on a 1 -ps time scale when formed by collisional reduction of the stable cation (CH3)2OH+. By contrast, (CH3)2OD furnishes an abundant survivor ion in the +NR+ mass spectrum that is evidence that the deuterated hypervalent radical is metastable [178,179]. From the time scale of the NR measurements and the survivor ion relative intensities one can estimate that (CH3)2OH dissociates >5 times faster than (CH3)2OD. Similar isotope effects have been observed for CH3OH [180], C2H5OH [181], and hypervalent ammonium radicals, e.g., CH3NH [182], (CH3)2NH [60], (CH3)3NH [183], and [pyrrolidinium] [184], which are metastable only as deuterated species. 

Compound I must be activated by the strongly acidic medium. One possible protonation site is the basic pyrrolidinium nitrogen atom (structure X), which would favor the formation of carbenium ion XI (see Scheme 16.3). This intermediate would gain additional stabilization from the polar solvent. Conditions for an SnI substitution would prevail to yield XII, whose fragmentation would resemble, formally speaking, the retro cycloaddition process used in the synthesis of I (i.e., VIII - I). The resulting ketone (XIII) would contain the elements required for the construction of a five-membered carbocycle with the desired incorporation of the functional carbon of one of the nitrile groups, as... 

The first enantioselective organocatalytic 1,3-dipolar cycloaddition of acyclic nitrones with acrolein and crotonal-dehyde has been reported <2000JA9874>. In particular, the reversible formation of iminium ions from a,/3-unsatu-rated aldehydes and the enantiopure imidazolidinone 535 provided ( A-4-formylisoxazolidines in high yields and ees (Equation 86). A polymer-supported version of catalyst 535 was also prepared <2004EJ0567>. The catalytic performance of various chiral pyrrolidinium salts in the cycloaddition of 1-cycloalkene-l-carboxaldehydes was also evaluated <2003EJO2782>. 

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