Bond-switching reactions

Another transformation of one aromatic compound to another is the Stone-Wales rearrangement of pyracyclene (113), which is a bond-switching reaction. The rearrangement of bifluorenylidene (114) to dibenzo[g,p] chrysene (115) occurs at temperatures as low as 400° C and is accelerated in the presence of decomposing iodomethane, a convenient source of methyl radicals. This result suggested a... 

Solvation plays a key role because the magnitude of the electrostatic ion-polar molecule interaction is large, say 10-20 kcal mol , comparable to the barriers for concerted bond-switching reactions. Consider the Sn2 reaction OH + CH3CI Cl -h CH3OH in the gas phase. The reaction is concerted so the new (CH3-OH) bond forms as the old (CH3-CI) bond is stretched. This process has an activation barrier but it must be significantly lower than the energy needed to break the old bond. On the other hand, as the reactants approach, the first interaction they experience... 

Organo derivatives of transition metals are also believed to be involved in a fascinating series of reactions aptly described as bond-switching reactions. These reactions are typical of highly strained hydrocarbons. Scheme 5.9 lists a number of such reactions. 

It was noted that the isomeric ion species was unlikely to be the weakly bonded CH3-0-0+ since switching reactions such as,... 

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 ... 

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>. 

Following reactions (55) and (56), the weakly bonded ions 02 02 and 02 N2 undergo fast binary switching reactions with other abundant atmospheric gases e.g. 

A detailed study of these switching reactions has been made by Adams et al.76 who determined the relative bond strengths of a series of molecules to both 02 and 02 and recognised that the 0 J NO ion formed in reaction (59) is not the stable nitrate ion, NOJ, but the more reactive peroxy isomer which reacts with NO ... 

Solution As shown, the double bond is ruptured making the oxygen end negative and single bonded. The double bond switches to become a carbon-to-carbon double bond. As indicated in subsequent reactions, this flip-flopping of the double bond continues until the formation of chloroform. Ans... 

Fig. 9. Bond distances (see inset) vs. time, in fs, for the old and new bonds in the H2 + I2 —> 2 HI reaction in an Xei2s cluster for an impact velocity of 6 km/s. The Iruge disparity in the vibrational periods of the two reactants make for somewhat more complex dynamics because it takes a rather long time for the very heavy iodine molecule to move. It is therefore mostly the H atoms that move during the bond switching. Also, it is necessary for the reactants to be rotationally excited and the fast rotation of H2 is very evident in the oscillation of the H-I bond distances before the collision. Similarly, the rotational excitation of the product HI molecules is evident in the oscillation of the H-H bond distance ruound the I-I bond distance after the collision.

Like in the dissociation process, no caging of the four-center reaction products was found in our simulation. On the other hand, the cluster can provide a cage for the reactants, i.e. early on when the cluster is very compressed. The role of the caging depends on whether the collision of the two reactants did or did not lead to bond switching. If reaction occurred, the repulsion between the products causes them to rapidly recoil from one another. The role of the cluster atoms is to cool the internal excitation, as shown above. If at the first try bond rearrangement did not occur, the cage insures a second chance (cf. Fig. 18). 

Four-Center Reactions A Computational Study of Collisional Activation, Converted Bond Switching, and Collisional Stabilization in Impact Heated Clusters, T. Raz and R. D. Levine, J. Phys- Chem. 99, 7495 (1995). 

Although boron is more accurately described as a metalloid rather than a metal, this section is concluded by two papers that describe the structures and bonding in several organoboron/organophosphorus compounds that display ylidic character. The X-ray structure of 9-borylanthracene (88) shows that only one of the diisopropylphosphine moieties is bonded to the boron in the solid state. However, H NMR evidence shows that an intramolecular bond-switching process takes place very rapidly in solution. The structures of a series of borabenzene adducts of phosphorus ylides, iminophosphoranes and tertiary phosphines have also been determined. Treatment of l-chloro-3,5-dimethyl-2-(trimethylsilyl)-l,2-dihydroborinine (89) with methylenetriphenylphosphorane (90) produces (triphenylphosphonio)methanide-3,5-dimethylborabenzene (91). However, if the reaction sequence is reversed and (90) is treated with (89), then (trimethylsilyl)(triphenylphosphonio)methanide-3,5-dimethylborabenzene (92) is obtained (Scheme 26). Treatment of an isomeric mixture of l-chloro(trimethyl-silyl)dihydroborinines (93) with N-(triphenylphosphoranylidene)aniline (94) produces iV-(triphenylphosphonio)anilide-borabenzene (95) (Scheme 27). Crystal structures of (91), (92) and (95) show that the P-C or P-N bonds are... 

Clearly, a bond-switching mechanism of the type in Equation 1.2 is unrelated to the electronic structure of the excited state and does not provide a basis for understanding the reaction or predicting further excited-state processes. 

The strain energy associated with the small-ring systems in the substrates provides the driving force for these rearrangement reactions. Unstrained hydrocarbons do not react with the various transition-metal species that cause bond-switching in strained hydrocarbons. 

---