Multiply bonded

An atom that is multiply bonded to another atom IS considered to be replicated as a substituent on that atom... 

In azoles containing at least two annular nitrogen atoms, one of which is an NH group and the other a multiply-bonded nitrogen atom, electrophilic attack occurs at the latter nitrogen. Such an attack is frequently followed by proton loss from the NH group, e.g. (66) (67). If the electrophilic reagent is a proton, this reaction sequence simply means... 

Since the electrophilic reagent attacks the multiply-bonded nitrogen atom, as shown for (68) and (69), the orientation of the reaction product is related to the tautomeric structure of the starting material. However, any conclusion regarding tautomeric equilibria from chemical reactivity can be misleading since a minor component can react preferentially and then be continually replenished by isomerization of the major component. 

Substituents are expected to alter the electron density at the multiply-bonded nitrogen atom, and therefore the basicity, in a manner similar to that found in the pyridine series. The rather limited data available appear to bear out these assumptions. The additional ring nitrogen atoms in triazoles, oxadiazoles, etc. are quite strongly base-weakening this is as... 

Azoles containing a free NH group react comparatively readily with acyl halides. N-Acyl-pyrazoles, -imidazoles, etc. can be prepared by reaction sequences of either type (66) -> (67) or type (70)->(71) or (72). Such reactions have been carried out with benzoyl halides, sulfonyl halides, isocyanates, isothiocyanates and chloroformates. Reactions occur under Schotten-Baumann conditions or in inert solvents. When two isomeric products could result, only the thermodynamically stable one is usually obtained because the acylation reactions are reversible and the products interconvert readily. Thus benzotriazole forms 1-acyl derivatives (99) which preserve the Kekule resonance of the benzene ring and are therefore more stable than the isomeric 2-acyl derivatives. Acylation of pyrazoles also usually gives the more stable isomer as the sole product (66AHCi6)347). The imidazole-catalyzed hydrolysis of esters can be classified as an electrophilic attack on the multiply bonded imidazole nitrogen. 

A multiply bonded nitrogen atom deactivates carbon atoms a or y to it toward electrophilic attack thus initial substitution in 1,2- and 1,3-dihetero compounds should be as shown in structures (110) and (111). Pyrazoles (110 Z = NH), isoxazoles (110 Z = 0), isothiazoles (110 Z = S), imidazoles (111 Z = NH, tautomerism can make the 4- and 5-positions equivalent) and thiazoles (111 Z = S) do indeed undergo electrophilic substitution as expected. Little is known of the electrophilic substitution reactions of oxazoles (111 Z = O) and compounds containing three or more heteroatoms in one ring. Deactivation of the 4-position in 1,3-dihetero compounds (111) is less effective because of considerable double bond fixation (cf. Sections 4.01.3.2.1 and 4.02.3.1.7), and if the 5-position of imidazoles or thiazoles is blocked, substitution can occur in the 4-position (112). 

Substituents in the 4-position of these compounds are also a to a multiply-bonded nitrogen atom, but because of bond fixation they are relatively little influenced by this nitrogen atom even when it is quaternized (333). This is similar to the situation for 3-substituents in isoquinolines, cf. Chapter 2.02. In general, substituents in the 4- and 5-positions of imidazoles, thiazoles and oxazoles show much the same reactivity of the same substituents on benzeneoid compounds (but see Section 4.02.3.9.1). 

The transition metal catalysed addition of a hydridosilane to a multiply-bonded system is known as hydrosilylation (1). Under such conditions, alkynes undergo clear cis-addition, so providing one of the most direct routes to vinylsilanes (Chapter 3). Hydridosilanes also add to the carbonyl group of saturated aldehydes and ketones, to produce alkyl silyl ethers. Fot example, under suitable conditions, 4-t-butylcyclohexanone (2) can be reduced with a high degree of stereoselectivity. 

Stereochemical aspects of reactions of complexes of d° transition metals with multiply bonded ligands. Y. V. Kokunov and Y. A. Buslaev, Coord. Chem. Rev., 1982, 47,15-40 (120). 

Phosphorus, arsenic, antimony and bismuth multiply bonded systems with low coordination number — their role as complex ligands. O. J. Scherer, Angew. Chem., Int. Ed. Engl., 1984, 24, 924 (85). 

We can generalize from these examples to the description of a multiply bonded species according to valence-bond theory ... 

The first stable silaallene, 56, was synthesized in 1993 " " by the intramolecular attack of an organolithium reagent at the /f-carbon of a fluoroalkynylsilane (Scheme 16). Addition of two equivalents of r-butyllithium in toluene at O C to compound 54 gave intermediate 55. The a-lithiofluorosilane then eliminated lithium fluoride at room temperature to form the 1-silaallene 56, which was so sterically hindered that it did not react with ethanol even at reflux temperatures. 1-Silaallene 56 was the first, and so far the only, multiply bonded silicon species to be unreactive toward air and water. The X-ray crystal structure and NMR spectra of 56 is discussed in Sect. IVA. 

Figure 12.20 Potential-dependent SFG spectra from atop and multiply bonded CO on a Pt(lll)/Ru electrode in 0.1 M H2SO4 at Im V/s (see Fig. 12.18). The scan potential for each spectrum is shown on the right. Data show disappearance of atop CO at lower potentials than multiply bonded CO.

Figure 12.21 BB-SFG spectra of CO on a mixed metal electrode (in a CO-saturated 0.1 M H2SO4 solution) made by depositing Ru nanoislands on Pt(lll) at a coverage of 0.35 ML. The electrolyte is a 25 p-m thick solution layer (see Fig. 12.1). The acquisition time was 40 s data were obtained at = 0.1 V. As in Fig. 12.11b(II), the BBIR pulses are tuned to optimize the multiply bonded spectra, suppressing the atop intensity.

The other reactant in a dipolar cycloaddition, usually an alkene or alkyne, is referred to as the dipolarophile. Other multiply bonded functional groups such as imine, azo, and nitroso can also act as dipolarophiles. The 1,3-dipolar cycloadditions involve four it electrons from the 1,3-dipole and two from the dipolarophile. As in the D-A reaction, the reactants approach one another in parallel planes to permit interaction between the tt and tt orbitals. 

Consistent with the involvement of the boron 2pz-orbital in this transition is the almost total reduction of the long-wavelength absorption of (PhCH2)3B above 250 nm by its complexation with ammonia. Strong chemical support for a multiply bonded boron in the excited state (48b) can be deduced from photolysis results with tribenzylborane (49).48 While photolysis of... 

Multiply Bonded Main Group Metals and Metalloids... 

Bond paths are observed between bonded atoms in a molecule and only between these atoms. They are usually consistent with the bonds as defined by the Lewis structure and by experiment. There are, however, differences. There is only a single bond path between atoms that are multiply bonded in a Lewis structure because the electron density is always a maximum along the internuclear axis even in a Lewis multiple bond. The value of pb does, however, increase with increasing Lewis bond order, as is shown by the values for ethane (0.249 au), ethene (0.356 au), and ethyne (0.427 au), which indicate, as expected, an increasing amount of electron density in the bonding region. 

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