Boron-mercury bonds

Little experimental data on the photochemistry of compounds with B—0 bonds have been reported. Boron-hydride/oxygen mixtures are explosive. These explosions can be initiated by photochemical processes. Grimm and Porter have studied the photochemical decomposition of H2B2O3 using a low pressure mercury lamp. The UV spectrum of this compound is illustrated in Fig. 13. The rate of the reaction was increased when mercury was present as a photosensitizer. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

Positively charged or neutral electron-deficient groups may serve as interaction sites for anion binding. Ammonium and guanidinium units, which form +N-H" X bonds, have mainly been used, but neutral polar hydrogen bonds (e.g., with -NHCO- or -COOH functions), electron-deficient centers (boron, tin, mercury, [3.6, 3.7] as well as perfluoro crown ethers and cryptands [3.8], etc.), or metal-ion centres in complexes also interact with anions. 

The allyl-metal bond is usually broken very readily by electrophilic reagents and allyl groups are cleaved from mercury, boron, silicon, germanium, tin, and lead much more easily than are the saturated alkyl groups. Kinetic studies, however, have not been very numerous such studies are reviewed in this chapter. 

Lewis acids based on boronic acid derivatives or main group elements such as mercury, tin and silicon form strong bonds to anions with considerable covalency, exemplified by hydride sponge and the anticrowns. 

While the mercury compound shows localized N—Hg bonds, the TADB-system (which is isoelectronic with cyclopentadiene) forms a low-spin complex with iron. This was the first identified example of a sandwich-compounds of a boron-nitrogen ring. The complex is prepared by adding an etheric solution of TADBLi to a suspension of FeCl2 in tetra-hydrofuran. 

Although we have assigned a structure to the transition state so as to obtain the observed inversion, the coordination of solvent or chloride at boron and mercury is actually unknown. Ingold (1962) and Thorpe (1966) have suggested that SB2 reactions which go with inversion probably involve transition states in which bond formation lags bond 9... 

These reactions usually proceed more slowly than the reactions of the corresponding alcohols or phenols with boron halides. In some cases, where direct reaction is too slow, the lead or mercury derivatives of thiols are used. Attempts to use the reactions of thiols with diboron trisulfide to prepare trialkylthioboranes have usually been unsuccessful, although analogous reactions with oxygen compounds are commonly used to prepare trialkoxyboranes. Formation of B-S bonds... 

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