The overall goal of Professor Gaspar's research group is to extend the mechanistic ideas of organic chemistry to the prediction and understanding of covalent bond-making and bond-breaking throughout the periodic table.
The overall goal of the Gaspar research group is to extend the mechanistic ideas of organic chemistry to the prediction and understanding of covalent bond-making and bond-breaking throughout the periodic table. We are mechanistic organic/main group chemists, and much of our work is near the borderline between organic and inorganic chemistry.
We study heavier analogs of reactive intermediates in organic reactions such as carbenes R2C:. The reactivity of these intermediates is so distinctive that product studies can reveal similarities and differences between molecules based on heavier elements and those based on first-row elements.
The activities of our laboratory span much of experimental chemistry. Our group synthesizes precursors for reactive intermediates and authentic samples of products. We convert precursors into characterizable species that are studied by solution photolysis, by gas-phase flow pyrolysis, and through the use of trapping reactions. Solution- and gas-phase flash photolysis experiments are used to detect reactive intermediates in-situ and also to study the kinetics of their reactions. Reactive intermediates carrying positive or negative charges are studied by quadrupole ion trap mass spectrometry (QITMS).
Computational modeling of reaction systems by electronic structure calculations and by kinetic simulations plays an important role in our work and has helped us to magnify the effectiveness of our mechanistic studies.
The following problems illustrate our current research interests:
The reaction chemistry of ground-state silylenes, germylenes, and stannylenes
A facet of the chemistry of carbenes R2C: that has fascinated chemists for a half-century is the occurrence of two lowest energy electronic states of different spin multiplicity but nearly the same energy, singlet S and triplet T. Singlet carbenes are potent electrophiles and can form two new bonds in concert. Triplet carbenes have entirely different chemistries and behave as diradicals.
While many carbenes have triplet ground states, both S and T states of many carbenes have been found. An unexpected difference between heavier-element analogs of carbenes, silylenes R2Si:, germylenes R2Ge:, and stannyl-enes R2Sn:, is that, until recently, only their singlet ground states were found.
In 2001 we presented the first experimental evidence for a triplet silylene, and in 2003 a triplet ground-state silylene was observed by ESR spectroscopy in Japan. Unfortunately, these first triplet silylenes were too sterically hindered for observation of intermolecular reactions, so new types of triplet silkylenes were needed. One is shown below with an alkali-metal substituent.1
No one has previously thought about triplet germylenes, but our calculations predict that tBu3Si-Ge-Li has a triplet ground state lying a healthy 6.7 kcal/mol below the lowest singlet state, and we have designed a synthesis for a promising precursor.2
We believe that the chemistry of ground-state triplet heavy-element analogs of carbenes (R2Si:, R2Ge:, R2Sn:) will be most effectively explored employing novel, new unsaturated molecules X2M=M: (M = Si, Ge, Sn; X = N, halogen), the silylene having been suggested by Iranian theoretical chemists.3 We have carried out calculations on unsaturated stannylenes X2Sn=Sn: that support the prediction of a triplet ground state.4
Can heavy-atom heteroaromaticity surmount adverse steric factors – is phosphatropylium ion planar?
We have prepared the phosphirenylium ion and demonstrated its aromaticity through gas-phase mass-spectrometric reaction studies in our desk-top QITMS apparatus.5
Despite the mismatch in size between C 2p and P 3p atomic orbitals, the C2H2P+ ion, like the C3H3+ ion, is aromatic, with Eresonance ca. 38 kcal/mol, compared with 59 kcal/mol for C3H3+. Pi-electron delocalization in C2H2P+ reduces its carbene-like reactivity.
The phosphirenylium ion has no choice but to be planar, thus maximizing the pi-overlap needed for aromaticity, but larger rings can be nonplanar. Thus the planarity of the tropylium ion (see below) reflects the dominance of its aromaticity overcoming the steric preference for a tub-shape of the closely-related tropilidene molecule.
If the phosphatropylium ion is planar, with reduced carbene-like reactivity, then it will be a case of heavy-atom heteroaromaticity overcoming a steric preference for a tub shape.
Planarity and heteroaromaticiy are predicted for the phosphatropylium ion, and we are working on the synthesis of both substituted and unsubstituted versions of this interesting species.
To test a high temperature pyrolysis system connected to a mass spectrometer we ran a reaction reported to generate disilene:6,7
Our experiments suggested the stepwise mechanism shown below,with a diradical intermediate, noteworthy because such a mechanism suggested by Dewar for conventional Diels-Alder (DA) reactions9 was controversial for 20 years and was finally ruled out for the simplest DA reaction.10 Our mechanism explains the formation of the previously unrecognized rearrangement product H from the starting 1,2-disilacyclohex-4-ene D. The predicted reversibility of stepwise disilene extrusion has been confirmed by observation of both D and H when Me2Si=SiMe2 was independently generated in the presence of butadiene.7
We postulated that the dramatic difference between the forward and reverse [2+4] cycloaddition reactions of normal olefins and disila-olefins was due to differences in their shapes! In contrast to H2C=CH2, Me2Si=SiMe2 is pyramidal, and thus its frontier molecular orbitals lack left-right symmetry,11 discouraging concerted cycloadditions:
Our idea is that a planar dimetalla-olefin might undergo concerted cyclo-additions and retro-cycloadditions. 9,10-digerma-12 and disila13 naphthalenes are predicted to be planar, and the M=M double bonds are delocalized to a negligible extent, as indicated by the HOMO of the Ge compound, shown above. The predicted bond lengths shown below also indicate alternating single and double bonds, including electronically isolated M=M double bonds.
We plan to synthesize 9,10-disila- and digerma-naphthalene by the method of Ashe.13
Strong motivation for this synthesis was provided last summer by the computational prediction that 9,10-disilanaphthalene will undergo concerted [2+4] cycloaddition and retro-cycloaddition processes,14 which, when experimentally realized, will validate our hypothesis.
1. Sekiguchi, A.; Tanaka, T.; Ichinohe, M.; Akiyama, K.; Gaspar, P.P., J.Am. Chem.Soc.2008, 130, 426.
2. Gaspar, P.P.; Sekiguchi, A.; Solomon, A.; Yeon, H.J., Organometallics, manuscript in preparation.
3. Momeni, M.R.; Shakib, F., Organometallics, 2011, 30, 5027.
4. Bundhun, A.; Ramasami, P.; Gaspar, P.P.; Schaefer, H.F., III , manuscript in preparation.
5. Liu, X.; Ivanova, D.; Giblin, D.; Gross, M.L.; Gaspar, P.P., Organometallics, 2005, 24, 3125.
6. Marchand, A.; Gerval, P.; Duboudin, F.; Gaufyrau, M.-H.; Joanny, M.; Mazerolles, P., J.Organometal.Chem., 1984, 267, 93.
7. Zhou, D.; Nag, M.; Russell, A.M.; Read, D.; Rohrs, H.W.; Gross, M.L.; Gaspar, P.P., Silicon Chemistry, 2007, 3, 117.
8. Conlin, R.T.; Gaspar, P.P., J.Am.Chem.Soc., 1976, 98, 868.
9. Dewar, M.J.S.; Griffin, A.C.; Kirschner, S., J.Am.Chem.Soc., 1974, 96, 6225.
10. Houk, K.A.; Gonzalez, J.; Li, Y., Acc.Chem.Res., 1995, 28, 81.
11. Zhou, D., doctoral dissertation, Washington University, St. Louis, May, 2004.
12. Ghiasi, R., Theochem., 2005, 718, 225.
13. Liu, X.; Gaspar, P.P., unpublished results.
14. Ni, D., unpublished results.