Electrophilic Aromatic Substitution: New Insights into an Old Class of Reactions.

The classic SEAr mechanism of electrophilic aromatic substitution (EAS) reactions described in textbooks, monographs, and reviews comprises the obligatory formation of arenium ion intermediates (σ complexes) in a two-stage process. Our findings from several studies of EAS reactions challenge the generality of this mechanistic paradigm. This Account focuses on recent computational and experimental results for three types of EAS reactions: halogenation with molecular chlorine and bromine, nitration by mixed acid (mixture of nitric and sulfuric acids), and sulfonation with SO3.

An Experimentally Established Key Intermediate in Benzene Nitration with Mixed Acid.

Experimental evidence is reported for the first intermediate in the classic SEAr reaction of benzene nitration with mixed acid. The UV/Vis spectroscopic investigation of the reaction showed an intense absorption at 320 nm (appearing as a band shoulder) arising from a reaction intermediate. Our theoretical modeling shows that the interaction between the two principal reactants with solvent (H2SO4) molecules significantly affects the structure of the initial complex.

Stabilization of elusive silicon oxides.

Molecular SiO2 and other simple silicon oxides have remained elusive despite the indispensable use of silicon dioxide materials in advanced electronic devices. Owing to the great reactivity of silicon-oxygen double bonds, as well as the low oxidation state of silicon atoms, the chemistry of simple silicon oxides is essentially unknown. We now report that the soluble disilicon compound, L:Si=Si:L (where L: = :C{N(2,6-(i)Pr2C6H3)CH}2), can be directly oxidized by N2O and O2 to give the carbene-stabilized Si2O3 and Si2O4 moieties, respectively.

Covalent hypercoordination: can carbon bind five methyl ligands?

C(CH3)5(+) is the first reported example of a five-coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis-violating "electron-deficient bonds" between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)5(+) fleeting existence near 0 K.

Dynamic complexation of copper(I) chloride by carbene-stabilized disilicon.

Reaction of N-heterocyclic-carbene (NHC)-stabilized disilicon (1) with CuCl gave a carbene-stabilized disilicon-copper(I) chloride complex (2). The nature of the structure and bonding in 2 has been investigated by crystallographic, spectroscopic, and computational methods. The dynamic complexation behavior of 2 was experimentally explored by variable-temperature NMR analysis.

Splitting molecular oxygen en route to a stable molecule containing diphosphorus tetroxide.

In contrast to stable phosphorus oxides such as P4O6 and P4O10 that possess iconic adamantane-like cage structures, highly reactive phosphorus oxides such as PO, PO2, and P2Ox (x = 1-5) only have been studied in the gas phase or by matrix isolation techniques. Elusive diphosphorus tetroxide, the long sought phosphorus analogue of N2O4, is particularly noteworthy. Computations predict that the oxo-bridged O2POPO form of P2O4 is energetically more favored than the P-P bonded O2P-PO2 isomer.

Electrophilic aromatic sulfonation with SO3: concerted or classic S(E)Ar mechanism?

The electrophilic sulfonation of several arenes with SO(3) was examined by electronic structure computations at the M06-2X/6-311+G(2d,2p) and SCS-MP2/6-311+G(2d,2p) levels of theory. In contrast to the usual interpretations, the results provide clear evidence that in nonpolar media and in the absence of catalysts the mechanism of aromatic sulfonation with a single SO(3) is concerted and does not involve the conventionally depicted 1:1 σ complex (Wheland) intermediate. Moreover, the computed activation energy for the 1:1 process is unrealistically high; barriers for alternative 2:1 mechanisms involving attack by two SO(3) molecules are 12-20 kcal/mol lower! A direct 2:1 sulfonation mechanism, involving a single essential transition state, but no Wheland type intermediate, is preferred generally at MP2 as well as at M06-2X in isolation (gas phase) or in noncomplexing solvents (such as CCl(4) or CFCl(3)).

What is the preferred structure of the Meisenheimer-Wheland complex between sym-triaminobenzene and 4,6-dinitrobenzofuroxan?

The geometries, energies, and electronic properties of possible configurations of Meisenheimer-Wheland (M-W) complexes of sym-triaminobenzenes and 4,6-dinitrobenzofuroxan (DNBF) were investigated theoretically by MP2 and a variety of DFT methods. The pi-pi complex is preferred thermodynamically by more than 15 kcal/mol over the sigma-complexes for the unsubstituted species. However, the N-substituents of the 1,3,5-triaminobenzenes influence the relative stabilities of the alternative configurations significantly.

Electrophile affinity: a reactivity measure for aromatic substitution.

The reactivity and regioselectivity of the electrophilic chlorination, nitration, and alkylation of benzene derivatives were rationalized by comparing literature data for the partial rate factors (ln f) for these S(E)Ar processes with theoretical reactivity parameters. The Electrophile Affinity (Ealpha), a new quantity, is introduced to characterize reactivity and positional selectivity. Ealpha is evaluated theoretically by the energy change associated with formation of an arenium ion by attachment of a model electrophile to the aromatic ring.

A stable silicon(0) compound with a Si=Si double bond.

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.

Strain and reactivity: electrophilic addition of bromine and tribromide salts to cyclic allenes.

The kinetics and the products of the bromination of several cyclic allenes, from C(9) to C(13) (1 a-e), with tetrabutylammonium tribromide (TBAT) and Br(2) have been investigated in 1,2-dichloroethane (DCE) and methanol. The first product of the interaction between the allene and Br(2) is a 1:1 pi complex. The stability constant of this complex, determined at 25 degrees C for allene 1 a, is 7.