Conformational interconversion cations

X-ray crystallography and variable temperature H NMR studies show that the conformation of the coordinated imidazolidin-2-ylidene, in both the neutral and cationic complexes 70, is anti, anti with respect to the Ph of the backbone of the NHC, exclusively in the solid state and predominantly in solution at lower temperatures (-75°C). At room temperature in solution, possible conformer interconversion by the rotation around the phenyl-N bond of the NHC substituent is apparent from the broadness of the peaks in the NMR spectra. Hydrosilylation of acetophenone by Ph SiH catalysed by 70 at room temperature or at -20°C results in maximum ee of 58%. However, at lower temperatures the reaction rates are much slower [55]. 

The annulation evidently proceeds much more rapidly than conformational interconversion of the intermediate cations. For example, the annulation of ( )-3-methyl-3-penten-2-one 88 with allene 89 leads to acetylcyclopentene 90 as the sole reaction product in 71% yield (Eq. 13.28) [32]. The stereochemical relationship of the two methyls is preserved in the product. When the reaction is repeated with (Z)-3-meth-yl-3-penten-2-one 91 (Eq. 13.29), acetylcyclopentene 92 is isolated as the major product, along with a small amount of 90 [32]. 

The S-trioxane radical cation was investigated in freon matrix and the changes of ESR spectra with temperature, arising from conformational interconversion of the ring were observed23. 

A cp/mas VACP 13C-NMR study was made of these three pseudopolymorphic crystals (19, 21, and 22).29 The prefix pseudo is affixed to polymorph to describe this series of nefopam methohalide crystals since the anions therein are different. The results of this solid-state 13C-NMR study are presented in Table 3 along with the solution-state 13C chemical shifts. The cp/mas 13C-NMR spectra of crystalline 19, 21, and 22 are illustrated in Fig. 5. It is seen that the C(4) peak (<5 58.51) in the cp/mas 13C-NMR spectrum of the methochloride (22) is sharp due to the sole presence of the immobile BB conformation quaternary ammonium cation in the crystal. However, the C(4) peak (V) 59.42) in the methobromide (21) spectrum was markedly lower in intensity and broad, and in the methiodide (19) spectrum it was just a broad shoulder at 5 62.29 Therefore, as the C(4) nucleus becomes more site-disordered in the crystal due to conformational interconversion, its cp/mas peaks show lower intensities and greater line-broadening. 

Pd2(dba)3 92 and 93 are generated by the reaction of Pd(OAc)2 with the ligand precursor. Aspects of their structure, fluxional behavior, and catalytic reactivity have been reported. The conformer interconversion for the cationic carbene-pyridine-carbene C-N-C) pincer 93 and the neutral C-C-C dicarbene Pd pincer 94 complexes were investigated, and it was found that, for cationic complexes, the choice of anion can affect the mechanism for... 

The tertiary-secondary 1,2-H shift O itlO is not rate-determining in the interconversion of 5 and 6, but may become so in a conformationally fixed system. It has been found for the interconversion of tertiary and secondary adamantyloxocarbonium ions that <10" sec at 70°C (Hogeveen and Roobeek, 1971a) as compared with k= 1-5 x 10 sec at 20°C for the reaction 5 6. The absence of interconversion between tertiary and secondary adamantyloxocarbonium ions is due to the circumstance that 1,2-H shifts do not occur in the tertiary adamantyl ion as a result of the effect of orbital orientation (Brouwer and Hogeveen, 1970 Schleyer etal., 1970). That the secondary adamantyloxocarbonium ion can lose CO is demonstrated by the reaction with isopropyl cation in SbFs—SO2CIF solution at 0°C with formation... 

Figure 12. Estimation of the rotational barrier for the interconversion of bisected and perpendicular conformations of the a, a-dicyclopropylcyclobutylmethyl cation.

The spectra in Figure 12.24 clearly show that the constituent ions in the liquid and in the respective solid salts vibrate rather independent of the surroundings. Therefore the liquid spectrum looks much like the sum of the solid salts. This conclusion is of course not new, but nevertheless it is still quite applicable in the evaluation of many IL Raman (and IR) spectra. However, the presence of conformational equilibria for both of the IL ions makes a closer study worth while. We therefore recommend the interested reader to study the work by Umebayashi et al. [114] in which subtle spectral band shape details, for example, around 930-880 cm are evaluated to show information on the eq-envelope trans-TT and ax-envelope trans-TT interconversion of the [C4QIm]+ ion in the liquid. Also note that the crystal structure of the [CjC4pyr][Tf2N] salt was recently solved it contained the eq-envelope trans-TT conformer of the cation [115]. Also conformers of symmetry Cj and C2 of the [Tf2N] ion show their presence hurried in the band at 400-440 cm-i [109]. 

These steric interactions become more pronounced when we consider the introduction of an additional chelate ring in those complexes containing three didentate en ligands. The A XkX form of an [M(en)3]n+ cation is estimated to be 7-8 kJ mol-1 more stable than the A 888 diastereomer. This becomes particularly important when we consider kinetically inert complex cations, such as [Co(en)3]3+, where there is a significant barrier to the interconversion of the diastereomers. In practice, the conformation of the chelate rings in [Co(en)3]3+ salts depends upon the nature of both the anions and any additional solvent molecules in the lattice which can form hydrogen bonds with the en ligands. We will return to this topic in Chapter 7, where we discuss some reactions of [Co(en)3]3+ salts in which an extraordinary steric control is exerted. 

The conformational landscapes of two ILs build from [NTf2] and from /V-propyl-and /V-butyI -A -methylpyrroIidinium were investigated from Raman spectroscopy, MD, and ab initio techniques [135], For [NTf2] the three-dimensional PES and the corresponding MD simulations confirmed the existence of two stable isomers (see also [37, 36]). The anion was calculated to be flexible, albeit hindered, but capable of interconversion between conformers in the liquid state, a result also confirmed by the Raman data. In the case of the ALV-dialkylpyrrolidinium cations, the PES showed a much more limited conformational behavior of the pyrrolidinium ring [135],... 

The preferred conformation of the cyclopropylmethyl cations is one in which the p orbital at the cationic center is oriented for maximum overlap with the cyclopropane C-C bonds (bisected conformation). It has essentially the same energy as the cyclobutyl cation, and the barrier to interconversion is very low ( 2 kcal/mol). The low energy of cyclobutyl cation is due to a strong 1,3-interaction, forming a bicyclobutyl cation. The extreme of this sort of interaction is found with the bicyclo[l.l.l]pent-l-yl cation, in which the structure is found to have a 1.54 A cross-ring distance. The ion appears to correspond to a proton attached to the nonbonded charge density near the bridgehead carbons of [l.l.ljpropellane. 

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