Transition metal catalyzed reactions have an important place in synthetic chemistry, but the
mechanistic details for many of these reactions remain undetermined. Through a combination of
experimentally determined 13C kinetic isotope effects (KIEs) and density functional theory (DFT)
calculations, some of these reactions have been investigated.
The cyclopropanation of an olefin catalyzed by rhodium (II) tetrabridged complexes has been shown
to proceed through an asynchronous, but concerted mechanism. DFT does not provide an accurate
transition structure for the reaction of an unstabilized carbenoid with an olefin, but it does predict an
early, enthalpically barrierless transition state which is consistent with the reactivity of unstabilized
carbenoids. For the case of stabilized carbenoids, the theoretical structures predict the KIEs accurately
and a new model is proposed to explain the selectivity observed in Rh2(S-DOSP)4-catalyzed
cyclopropanations.
The chain-elongation step of atom transfer radical polymerization (ATRP) has been shown to be
indistinguishable from that of free radical polymerization (FRP) for the CuBr/2,2??-bipyridine system.
While DFT calculations predict an earlier transition state than observed, the calculations suggest that
with increasing levels of theory the predicted KIEs come closer to the observed KIEs.
A recently proposed [2 + 2] mechanism for the cyclopropenation of alkynes catalyzed by
Rh2(OAc)(DPTI)3 has been shown not to be a viable pathway. Rather, the experimental KIEs are
predicted well by canonical variational transition state theory employing the conventional mechanism for
cyclopropenation via a tetrabridged rhodium carbenoid. DFT calculations also suggest an alternative
explanation for the observed enantioselectivity.
The 13C KIEs for metal-catalyzed aziridination have been measured for three separate catalytic
systems. While the KIEs do not completely define the mechanism, all of the reactions exhibit similar
KIEs, implying similar mechanisms. A surprising feature of this system is the presumed nitrene
intermediate??s triplet spin state. This complicates the DFT analysis of this system.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/2308 |
| Date | 29 August 2005 |
| Creators | Nowlan, Daniel Thomas |
| Contributors | Singleton, Daniel A. |
| Publisher | Texas A&M University |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | 1200252 bytes, electronic, application/pdf, born digital |
Page generated in 0.0023 seconds