Biomimetic Platinum-Promoted Polyene Polycyclizations: Influence of Alkene Substitution and Pre-cyclization Conformations

Abstract

Results of kinetic experiments and quantum chemical computations on a series of platinum-promoted polycyclization reactions are described. Analysis of these results reveal a reactivity model that reaches beyond the energetics of the cascade itself, incorporating an ensemble of pre-cyclization conformations of the platinum-alkene reactant complex, only a subset of which are productive for bi- (or larger) cyclization and lead to products. Similarities and differences between this scenario, including reaction coordinates for polycyclization, for platinum- and enzyme-promoted polycyclization are highlighted.

Enhancing the catalytic properties of well-defined electrophilic platinum complexes

“…54 The catalytic activity of the bis-NHC pincer complex is not limited to dien-ols but it is also efficient for the diastereoselective cascade cyclization reactions of other polyenes lacking acidic OH functional groups that are usually more difficult to cyclise (Scheme 22). 55 This system allows the synthesis of polycycles of up to four fused cycles as exemplified in Scheme 22. It is worth noting once again the lack of products arising from b-hydride elimination events.…”

Recent Developments in Polyene Cyclizations and Their Applications in Natural Product Synthesis

“…Excellent stereoselectivity was observed due to the transition state governed by the double-bond geometry of the enol ether, thioenol ether, and epoxide. Utilizing the same strategy, (-)-5epi-aureol (35) was also synthesized via alcohol 34 (91%), which was prepared from aryl enol ether 33. 10 Chang and co-workers reported that tin(II) triflate catalyzed the polyene cyclization of β-keto sulfones (Scheme 8).…”

“…The platinum(II) catalyst 143 was also used in a study on the influence of alkene substitution on pentaene cyclizations (Scheme 29). 35…”

Nuclear Magnetic Resonance and Computational Study of trans-(μ2:η2,η2-1,3-Butadiene)bis(trichloroplatinate(II))

“…Despite the formally conjugated nature of its double bonds, the extent of delocalization in free butadiene is small. A combination of rotational spectroscopy and theory showed that in s - trans -butadiene the CC bond is longer (1.338 Å) than the CC bond in ethene by only 0.007 Å, while the C–C bond is shorter by 0.016 Å (at 1.454 Å) than that of a typical sp 3 –sp 3 C–C single bond . Coordination, however, has a more profound effect on the bond lengths, and for example, a CC distance of 1.403(7) Å and a C–C distance of 1.453(7) Å were found in 1 .…”

“…For example, palladium mediates many commercial applications of the use of alkenes, while platinum was involved in very early examples of C–H activation chemistry and perhaps its most important catalytic application is in hydrosilylation chemistry, where mechanistic questions still remain . Other, more contemporary areas of platinum alkene catalysis relate to subjects such as hydroamination and cyclization reactions . As such and in the context of the utility of third-row congeners to provide mechanistic pointers, continued insights into aspects of stability and reactivity remain valuable and are of direct interest to the wide research community in the area.…”