Chemical Science Blog

Researchers at Monash University in Australia have described the first example of a combined Brønsted/Lewis base cascade catalysis using an N-heterocyclic carbene (NHC) catalyst. David Lupton’s group have reported the synthesis of a variety of different dihydropyranones (2) in a single step from cyclopropyl enol esters (1).

Lisa Candish conducted mechanistic studies, which have enabled the group to suggest a possible catalytic cycle for the transformation. Firstly, catalytic deprotonation of 1a generates the α- or γ-enolate I which undergoes proton transfer to form the cyclopropyl anion II, primed for an electrocyclic ring opening to give allyl anion III. Protonation of III serves to regenerate catalyst 3, enabling its involvement in a second step of the mechanism. Addition of catalyst 3 to ester IV forms hemiacetal V, which undergoes a Claisen rearrangement to generate the final product 2a.

Exploitation of this dual Brønsted and Lewis base activation may enable the development of other interesting transformations. Additionally, the use of homochiral NHCs may enable enantioselective transformations in the future.

Lupton’s Chemical Science Edge article is free to download – read it today to find out more.

Posted on behalf of Alice E. Williamson, Chemical Science web writer.

Patrick Steel’s group at the University of Durham have used computational modelling in combination with experimental studies to generate silene equivalents.

Silenes (2) are compounds containing a silicon–carbon double bond, which are generally so reactive that they are only transient in existence and rapidly rearrange to other products (3). Additionally, methods for the preparation of silenes are often incompatible with a range of functional groups. Photochemical or thermal decomposition of α-silyldiazocarbonyl compounds (1) is an established method for the formation of silenes (2); however, the Wolff rearrangement product (4) is also often formed under these conditions.

The Steel group aimed to generate a silene equivalent via a rhodium-catalysed decomposition of α-silyldiazocarbonyls. Computational studies enabled the group to determine that silene formation was favoured when electron-donating groups were attached to the carbonyl groups. With these results in hand, they synthesised α-hypersilyl diazoesters (5) and found that the resultant silene equivalents (6) were unusually stable and could also undergo reactions with α,β-unsaturated carbonyl compounds (7) to form cyclic silyl enol ethers (8). A subsequent fluoride-mediated fragmentation of the cycloadducts (8) gave 1,5-dicarbonyl products (9 and 10) in good yields.

The group successfully used results from their computational modelling to design more stable and synthetically useful silene equivalents. The mild reaction conditions developed represent an important advance in the use of silene reagents for organic synthesis.

Find out more – download the Edge article for free.

Posted on behalf of Alice E. Williamson, Chemical Science web writer.