Methyl bridges with

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). 

Pyridine derivatives with additional donor functions and sterically demanding substituents have been used with the intention of producing complexes of Cd (and of other metals) with low coordination number one of these ligands is the tridentate, planar-bonding 2,6-bis[(2,6-dimethyl-phenylimino)methyl] pyridine (pydim a Schiff base derived from 2,6-pyridine dialdehyde), which with Cd(BF4)2 and thiocyanate gives a dinuclear complex [(pydim)Cd(/x-NCS-S,N)]2(BF4)2 with N-dominated coordination sphere.191 As centrosymmetric Plijc, Z= 2), the complex has an antiparallel /x-1,3 NCS double bridge with Cd—N and Cd—S bonds (224.6 pm and 255.5 pm, respectively) the Cd—N(py) bond is clearly shorter than the Cd—N(imino) bonds (225.6 pm and 245.0 pm,... 

There are three possible secondary heptyl cations, 2-heptyl, 3-heptyl and 4-heptyl. Calculation at the B3LYP/6-31G level shows that the 3-heptyl cation is about lkcal/mol more stable than either the 2- or 4- heptyl cation. The 2-heptyl cation can in principle form a 1-5-p-H-bridged structure with a methyl group at each termini (isomeric structures 15 and 16) or a 1,6-p-H-bridge with a primary carbon at one end (isomeric structures 17 and 18), as shown in the following diagrams ... 

In the thiol 79 and the sulfinic acid 80, two new acidic functionalities are now present in the 2 -position of a m-terphenyl system and should be incorporable into concave structures by a double bridging of the m-terphenyl. For the acetyl-protected thiol 78, the corresponding concave thiol acetate 42 (Scheme 7) could be obtained after tetrabromination of the four methyl groups and bridging with m-phenylenedithiol (see Sect. 2.3). 

Another example is tetrameric methoxy(methyl)zinc, (MeZnOMe)4. The mechanism can be treated both as an aggregative process as well as a ligand coordination exchange process. It reveals aggregation due solely to oxygen bridging, with no direct participation of the C—Zn bond, and will be mentioned only briefly. 

Sulfonamide groups incorporated in rotaxanes enable the construction of new topological assemblies provided with mechanically and covalently bonded subunits. Methylation of a [2]rotaxane containing a sulfonamide unit in the axle revealed that the substitution reaction is not sterically hindered by the macrocycle. Similar to the synthesis of the pretzelane 96, the two sulfonamide groups of rotaxane 80m were bridged with 95 to form 100 in 71% yield (Figure 39) [46]. The additional covalent bond converts the former [2]rotaxane into a [l]rotaxane and reduces the mobility of the wheel along the axle. Rotaxanes 80m and 100 are his-... 

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