Methyl bridges

Fig. 5.9. Energy profile for the scrambling and rearrangement of 4119 cation. A H-bridged B methyl-bridged C Edge protonated methycyclopropane D classical secondary E classical primary F tertiary. Adapted from refs 120 and 121.

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

The effect of methyl substituents is interesting the classical linear geometry still is preferred over the hydrogen or methyl bridged structure. Furthermore, 3 substitution of a methyl group provides very little or no extra stabilization over the unsubstituted parent system, 172a, R = H. On the other... 

The 2-butyl cation is the smallest secondary cation that can be stabilized either by C-C or C-H hyperconjugation. Experimental results give evidence for two equilibrating isomers.33 MP2/6-311G(d,p) calculations show that the symmetrically hydrido-bridged structure 11 is marginally more stable than the partially methyl-bridged structure 10.34 35... 

A comparison of IGLO/DZ//MP2/6-31G(d) calculated and measured 13C NMR chemical shifts demonstrates that the partially methyl bridged isomer is the preferred species.36 It was demonstrated that the calculated 13C chemical shifts are highly sensitive toward hyperconjugational distortion, i.e. the degree of bridging (ca. 6ppm/deg from 68° to 98°). 

The methyl group of the free [MeB(C5F5)3] anion tends to give NMR signals at 8 0.5 ( H) and 5 = 10 ( C), whereas the methyl-bridged anion is found at about... 

The importance of solution equilibria was further demonstrated by NMR studies on the system Cp2ZrMe2/B(C5F5)3, where it could be shown that in the presence of excess Cp2ZrMe2 the methyl-bridged binuclear species (Al) dominates [44]. 

No such compounds have been characterized by structural studies, but both the mixed cyclopropyl-methyl bridged dimer and the phenyl-or tolyl-methyl bridged dimer (57, 110) have been well studied by NMR. A simple analysis of the stability of this type of species suggests an equilibrium among the following dimeric species ... 

Of course, 57 must lie on the reaction path connecting the two open ions, but it is evidently a transition state and not an intermediate. However, evidence from x-ray photoelectron spectroscopy (ESCA) has shown that the 2-butyl cation is substantially methyl bridged.172... 

In order to visualize the reaction path of Eq. (21), the reader should be reminded that intramolecular methyl bridges between the two aluminum atoms in 35 need to be invoked (see Section III,D). 

Fig. 12. Five fused four-membered cycles with magnesium atoms as spiro centers and methyl bridges in the dimeric [(MeSi)(NrBu)3(AlMe2)(H)MgMe]2 (75).

The bridging chloride ligands in 47b can be replaced by methyl groups upon treatment with two equivalents of MeLi in the presence of TMEDA ( = A,A,A, A -tetramethylethylenediamine) to afford the methyl-bridged complex [PhC(NSiMe3)2](C5Me5)Y(jt-Me)2Li(TMEDA) (48b). This product was structurally characterized by an X-ray diffraction study (Fig. 15) [27]. The... 

From IR data it appears that in the solid state the magnesium atoms are associated via methyl bridge bonds and that the cyclopentadienyl rings are approximately pentahapto bonded as indicated in structure XXIVb. When dissolved in benzene, the methyl bridge bonds are retained. 

The CpjYb unit can be coordinated by saturated hydrocarbons, as in MeBeCp. The positions of the hydrogens on the bridging methyl group show that this is not a methyl bridge having a 3c-2e bond between Yb-C-Be, as shown in Fig. 18.4.3(b). 

The titanium complexes 29 and 30 were found to polymerize ethylene after the addition of the Lewis add AlEtCl2- The reaction of 10 with methyl alumoxane resulted in the m-methyl bridged bimetallic compound 31, Eq. (31), which was able to polymerize ethylene in the absence of a Lewis acid cocatalyst, when dichloromethane was used as solvent... 

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