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Tetraphosphacubane: An Unexpectedly Strong Base in the Gas Phase
José-Luis M. Abboud1, Marta Herreros, Rafael Notario
1Instituto de Química Médica (CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain.
The Journal of Organic Chemistry
|November 1, 1996
Summary
Tetra-tert-butylphosphocubane (TtBuPC) exhibits surprisingly high gas-phase basicity. Steric hindrance from tert-butyl groups directs protonation to phosphorus, unlike its parent compound, phosphacubane (PC).
Area of Science:
- Organophosphorus chemistry
- Computational chemistry
- Physical organic chemistry
Background:
- Tetra-tert-butylphosphocubane (TtBuPC) is a strained organophosphorus molecule with unique structural and electronic properties.
- Understanding the reactivity and basicity of TtBuPC is crucial for its application in various chemical processes.
- Previous studies suggested potential carbon basicity in related phosphacubane systems.
Purpose of the Study:
- To experimentally determine the gas-phase basicity of TtBuPC.
- To investigate the site of protonation (phosphorus vs. carbon) in TtBuPC and related compounds.
- To elucidate the influence of tert-butyl substituents on the reactivity and stability of phosphacubane derivatives.
Main Methods:
- Gas-phase basicity measurements using Fourier Transform Ion Cyclotron Resonance (FT-ICR) spectroscopy.
- Molecular orbital calculations incorporating electron correlation effects.
- Theoretical prediction of protonation sites and relative stabilities of protonated species.
Main Results:
- TtBuPC shows a high gas-phase basicity (GB = 221.8 kcal mol(-1), PA = 230.5 kcal mol(-1)).
- Experimental and theoretical data indicate that TtBuPC is a phosphorus base, with protonation occurring at the phosphorus atom.
- Computational studies predict carbon basicity for the parent phosphacubane (PC) due to P-C bond fission alleviating strain.
- Steric repulsion between tert-butyl groups in TtBuPC destabilizes the C-protonated form, favoring P-protonation.
- Protonation site degeneracy is observed when one tert-butyl group is removed.
Conclusions:
- The tert-butyl substituents in TtBuPC play a critical role in directing protonation to the phosphorus atom.
- The steric effects of bulky substituents can override inherent electronic preferences in strained systems.
- Accurate prediction of proton affinities for strained systems like PC and TtBuPC requires advanced computational methods and basis sets (e.g., G2-type [6-311+G(3df,2p)]).