Ortho Self-Bromination of Octaphenyl POSS: A Modeling Study
Bromination of the phenyl rings of octaphenyloctasilsesquioxane proceeds in the ortho position without any Lewis acid catalyst, in contradiction to the regiochemical rules that govern isolated aromatic substrates. The study titled “Why Do the [PhSiO1.5]8,10,12 Cages Self-Brominate Primarily in the Ortho Position? Modeling Reveals a Strong Cage Influence on the Mechanism” by M. Bahrami, H. Hashemi, X. Ma, J. Kieffer, and R. M. Laine, published in Physical Chemistry Chemical Physics in 2014, accounts for this outcome. The modeling places the bromine molecule 2.7 Å above a cage face, tilted by 30°, with a Br−Br separation of 2.32 Å and an induced dipole moment of 0.33 D, an arrangement that delivers Brδ+ to the ortho carbon.
The premise of the present entry, namely that the Si8O12 framework acts as an electron withdrawing center that stabilizes anionic guests and polarizes neutral molecules approaching its faces, is treated in full in the companion entry on the electronic properties of octameric silsesquioxanes, which covers the endohedral fluoride preference and the trapping of atomic hydrogen. What follows here concerns only the regiochemical consequence of that property in the substitution chemistry of the pendant phenyl rings, and the computational analysis that establishes it.
Uncatalyzed Ortho Bromination of Octaphenyloctasilsesquioxane
Bromination of the pendant aromatic rings of [PhSiO1.5]8 occurs at the ortho position in the absence of any catalyst, a result that is difficult to reconcile with ordinary electrophilic aromatic substitution, where a Lewis acid is normally required to activate the halogen and where steric considerations favor the para position. Continued bromination delivers crystalline [2,5-Br2PhSiO1.5]8, a material with a density of 2.32 g cm−3 and a calculated refractive index of 1.7, and, under more forcing conditions, the tetraicosabromo compound [Br3PhSiO1.5]8. That the cage promotes its own bromination was reported by Roll and co-workers in the Journal of Materials Chemistry, and the substitution pattern is consistent with the structural analysis of phenylsilsesquioxane cage and ladder architectures.
Approach Geometry of the Br2 Molecule and the Induced Dipole Moment
The modeling study resolves the regiochemical puzzle by examining what happens to a bromine molecule as it approaches the cage. The Br−Br bond of a Br2 molecule situated in the vicinity of the octaphenyloctasilsesquioxane core becomes polarized, and the induced dipole moment reaches 0.33 D at the optimal approach geometry. The bromine atom carrying the partial negative charge is directed toward the walls of the silsesquioxane, at a separation of 2.7 Å from the cage face, while the bromine atom carrying the partial positive charge points toward the ortho position of the phenyl ring with respect to the silicon atom. The internuclear Br−Br distance in this arrangement is 2.32 Å and the molecular axis is inclined by 30° to the normal of the face. This orientation delivers the electrophilic terminus of the halogen directly to the carbon that is substituted, so that electrophilic substitution at the ortho position becomes the geometrically accessible pathway.
Fig. 1. (a) Proposed orientation of the Br2 molecule relative to the POSS cage in the bromination of the phenyl ring. The bromine molecule sits 2.7 Å above the siloxane face, its Br−Br internuclear distance is 2.32 Å, the molecular axis is inclined by 30° to the face normal, and the induced dipole moment amounts to 0.33 D. Atom colors are brown for Br, yellow for Si, red for O, gray for C, and white for H. (b) Contour map of the electron density for the same arrangement, showing Brδ− directed toward the cage and Brδ+ directed toward the ortho carbon of the phenyl ring. Atom colors in panel (b) are yellow for C, teal for Si, red for O, and gray for H. Reproduced from Bahrami et al., Physical Chemistry Chemical Physics 2014, 16, 25760 (Royal Society of Chemistry).
Electron Density Map and the Cage as an In Situ Lewis Acid
The electron density contour shown in panel (b) makes the same point in a different representation. The isosurface enclosing the cage and its eight phenyl substituents is continuous, and the two bromine centers sit in regions of visibly different density, with the atom closer to the siloxane wall carrying the accumulated charge. What the calculation supplies is therefore not a qualitative statement but a specific geometry, a specific separation, and a specific dipole moment that together define the approach to the transition state. The cage performs the function normally assigned to a Lewis acid catalyst, polarizing the halogen and generating the electrophile in situ, and it does so in a fixed spatial relationship to the aromatic ring that leaves only the ortho carbon within reach.
Persistence of the Ortho Preference Across the T8, T10 and T12 Series
The conclusion that self-bromination occurs primarily in the ortho position holds across the octamer, the decamer, and the dodecamer of the [PhSiO1.5]8,10,12 series. Persistence of the preference over cages of different size and curvature indicates that the governing interaction is local, involving one face of the siloxane framework and the phenyl ring attached at its vertex, rather than a global property of a particular polyhedron. The practical consequence for synthetic work is that functionalization of phenyl substituted silsesquioxanes does not follow the regiochemical rules established for isolated aromatic substrates, and that the same reactivity supplies a direct route to densely halogenated cages such as [2,5-Br2PhSiO1.5]8, whose high density and high calculated refractive index arise from the substitution pattern that the cage imposes on itself.
DOI: 10.1039/C4CP03997A
Full text: Physical Chemistry Chemical Physics → Publisher
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