Bond switching

These observations must be taken into account when considering the mechanism of halogen addition They force the conclusion that a simple one step bond switching process of the following type cannot be correct A process of this type requires syn addi tion It IS not consistent with the anti addition that we actually see... 

Application of symmetry rules also yields important results for the class of reactions known as sigmatropic shifts, in which a double bond switches position as an atom or a group migrates ... 

Fig. 25. Relaxation of the atoms of the hydrogen-acceptors complex shown in Fig. 24 in a (110) plane when the H bond switches from a group V atom to the other. The codes are the same as for Fig. 24.

The occurrence of the bond switch in the rearrangement has also been studied with N-labeled compounds. When 5-amino-3-methyl 1,2,4-... 

It has been argued that this rearrangement may occur via a bond-switch process in 167. In this process the sulfur acts as a nucleophilic center, which is opposite to the electrophilic behavior of the pivotal sulfur in, for example, the conversion of 149 into 150. The alternative intermediacy of a bipolar sulfur tetra-azapentalene structure or a ring-opened intermediate in... 

From studies of the kinetics of the bond switching, the equilibration, the... 

In the case of the thiadiazole 240 (R = Me), the occurrence of a bond-switch is verified by N-labeling experiments. Thus, the reaction between the imidate 243 and NH3 gives a mixture of both thiadiazoles 244 and 245, which are characterized by N in the amidine side-chain or in the... 

The condensation of 5-amino-3-methyl-l,2,4-thiadiazole (118) with aliphatic or aromatic nitriles yields 1 1 adducts, which are, according to their H NMR spectra, equilibrium mixtures of (119) and (120) (Scheme 28) <82AHC(32)285>. These adducts are produced by a bond switch at the n-hypervalent sulfur in (121). X-ray analysis of the adduct formed from the reaction of (118) with chloroacetonitrile showed the adduct to exist as (122) in the crystals <81AX(B)185>. Further examples of this type of bond switch at rc-hypervalent sulfur are observed in the reaction of 5-imino-1,2,4-thiadiazolines with various electrophilic reagents (Section 4.08.6.1). 

The synthetic approaches to 1,2,4-dithiazoles developed prior to 1980 on the basis of reaction between 1,2,3,4-thiatriazolines of type (159) and isothiocyanates have been reviewed by L abbe <82T3537>. Reactions proceeding through heterothiapentalene intermediates as discussed above have been named bond-switching rearrangements <80SRl>. 

Kekul6 proposed that the structure for benzene resonated between two alternate structures in which the position of the double and single bonds switched positions. 

Kekule proposed that the structure for benzene resonated between two alternate structures in which the position of the double and single bonds switched positions. In the figure, benzene is depicted as changing back and forth between two structures in which the position of the double bonds shifts between adjacent carbon atoms. The two structures are called resonance structures. 

Fig. 12.5 Twelve topological sets A,B, L made of symmetry-equivalent atoms of C3v C6QH18(a) hydride eight sets A,B,D,F,G,I,K,L have sjx no(jes each, whereas C,E,H,J include three nodes 13C-NMR theoretical spectrum in fact presents 12 lines with relative intensities 8(6) 4(3). Sets B(6) C(3) D(6) E(3) correspond to C-H bonds originating -NMR theoretical spectrum with four lines with relative intensities 2(6) 2(3). C3v C60H18(P) hydride has similar resonance patterns due to the fact that 12 hydrogen bonds switch from sets B and D to F and G (see Fig. 12.4)...

Unimolecular dynamics of smaller clusters has also been studied. The HF dimer provides a particularly interesting system because it involves a highly quantal degenerate rearrangement consisting of a concerted double hydrogen-bond switch (Quack and Suhm 1991 Truhlar 1990). 

Fig. 8 shows the changes in the total occupation numbers of the CH and HH bond structures along the IRC. The crossing point is located after TS, 0.42 bohr(amu)1/2. The structure at this point is given in Fig. 9. Compared to the TS, the longer and shorter CH bonds have stretched by 0.14 and 0.06 A, respectively, and the HH bond has become shorter by 0.18 A. These bond lengths are 1.03, 1.62, and 1.80 times longer than the corresponding equilibrium CH and HH bond distances. That point is the structure where the bonds switch in other words, the point is the transition state of chemical bond between the CH bonds and HH bond.

Fig. 13. Correlation diagram for degenerate bond-switch in three hydrogen molecules...

SCHEME 4 SPO platinum catalysts. Top row stoichiometric hydroformylation with 1 middle row, catalyst 4 and hydrogen bond switching in 6 bottom row heterolytic cleavage of dihydrogen (69-71). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

Fig. 4.26. Illustration of the migration of a floating bond by a bond switching mechanism (Pantelides 1987).

Fig. 6.18. Model showing the annihilation of a nearest neighbor donor-defect pair by a bond switching mechanism. The double line indicates the bond which changes position.

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