What is the Nature of the First‐Formed Intermediates in the Electrophilic Halogenation of Alkenes, Alkynes, and Allenes?

Abstract

The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f). Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K(f) value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of the bromonium ion. The effect of various polar groups situated in equatorial homoallyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series.

Charge-transfer character of halogen bonding: Molecular structures and electronic spectroscopy of carbon tetrabromide and bromoform complexes with organic σ- and π-donors

“…[2][3][4][5][6][7]. Furthermore, such intermolecular interactions are critical for the reactivity of halogen derivatives [1,8,9] and play important roles in biochemical systems [10]. Thus the nature of halogen bonding has become the focus of numerous experimental and quantummechanical studies which concentrate on the relative roles of electrostatic forces, polarization, dispersion, etc.…”

The Diiodine Basicity Scale: Toward a General Halogen‐Bond Basicity Scale

“…The respective contributions of these interactions are still a matter of debate. [23,25,27,29,[33][34][35][36][37][38][39] Halogen bonding has recently been recognised as a significant interaction in molecular recognition and self-assembly processes, [40,41] and is therefore employed for a wide range of applications in materials, [42][43][44][45] synthetic [46][47][48] and medicinal chemistry. [49][50][51][52] Concerning the fundamentals of halogen bonding, recent studies have focused mainly on structural determinations [18][19][20][21][22] and on quantum mechanical calculations of the bond energy in vacuo at 0 K. [25][26][27][28][29][30][31][53][54][55][56][57][58][59][60] The experimental knowledge of the thermodynamics of the halogen bond at room temperature in solution shows a marked contrast between inorganic and organic halogen-bond donors.…”

Cooperativity between the Halogen Bond and the Hydrogen Bond in H3N⋅⋅⋅XY⋅⋅⋅HF Complexes (X, Y=F, Cl, Br)

“…[10,11] Moreover, in many cases halogen-bonded complexes are intermediates in chemical reactions. Well-known examples are the 1:1 complexes of dihalogen molecules with alkenes and aromatics, [12] which lie on the way to formation of addition/substitution products.…”