Nucleophilic ring opening of trans-2,3-disubstituted epoxides to ß-amino alcohols with catalyst-controlled regioselectivity.

We report the nucleophilic ring opening of unsymmetrical trans-epoxides to ß-amino alcohols with catalyst-controlled regioselectivity. This cationic aluminum salen catalyst, which contains bulky mesityl groups in the ortho-position of the phenoxide and a 2,2'-diamino-1,1'-binaphthalene backbone, transforms a variety of epoxides with high regioselectivity using nitrogen-containing nucleophiles. Unlike most reports, in which regioselectivity is substrate controlled, the regioselectivity in this system is catalyst controlled and allows selective nucleophilic ring opening of unbiased trans-epoxides.

Unraveling substituent effects on the glass transition temperatures of biorenewable polyesters.

Converting biomass-based feedstocks into polymers not only reduces our reliance on fossil fuels, but also furnishes multiple opportunities to design biorenewable polymers with targeted properties and functionalities. Here we report a series of high glass transition temperature (Tg up to 184 °C) polyesters derived from sugar-based furan derivatives as well as a joint experimental and theoretical study of substituent effects on their thermal properties. Surprisingly, we find that polymers with moderate steric hindrance exhibit the highest Tg values. Through a detailed Ramachandran-type analysis of the rotational flexibility of the polymer backbone, we find that additional steric hindrance does not necessarily increase chain stiffness in these polyesters. We attribute this interesting structure-property relationship to a complex interplay between methyl-induced steric strain and the concerted rotations along the polymer backbone. We believe that our findings provide key insight into the relationship between structure and thermal properties across a range of synthetic polymers.

Reversible-deactivation anionic alternating ring-opening copolymerization of epoxides and cyclic anhydrides: access to orthogonally functionalizable multiblock aliphatic polyesters.

The alternating copolymerization of epoxides and cyclic anhydrides is an increasingly popular route to aliphatic polyesters that are of interest as biodegradable replacements for petroleum-based polymers and for use in the biomedical field. However, broad and bimodal molecular weight distributions in these polymerizations continues to be an issue, limiting synthesis of multiblock copolymers. By use of a bifunctional catalytic system, the reversible-deactivation anionic alternating ring-opening copolymerization of epoxides and cyclic anhydrides gives unimodal polymers with D values generally less than 1.07. This allowed for the formation of well-defined triblock copolymers. Additionally, by incorporating both aldehyde and alkene functionalities into the polymer, orthogonal post-polymerization modification was achieved, giving access to well-defined highly modifiable aliphatic polyesters.

Mechanistic Insights into the Alternating Copolymerization of Epoxides and Cyclic Anhydrides Using a (Salph)AlCl and Iminium Salt Catalytic System.

Mechanistic studies involving synergistic experiment and theory were performed on the perfectly alternating copolymerization of 1-butene oxide and carbic anhydride using a (salph)AlCl/[PPN]Cl catalytic pair. These studies showed a first-order dependence of the polymerization rate on the epoxide, a zero-order dependence on the cyclic anhydride, and a first-order dependence on the catalyst only if the two members of the catalytic pair are treated as a single unit. Studies of model complexes showed that a mixed alkoxide/carboxylate aluminum intermediate preferentially opens cyclic anhydride over epoxide. In addition, ring-opening of epoxide by an intermediate comprising multiple carboxylates was found to be rate-determining. On the basis of the experimental results and analysis by DFT calculations, a mechanism involving two catalytic cycles is proposed wherein the alternating copolymerization proceeds via intermediates that have carboxylate ligation in common, and a secondary cycle involving a bis-alkoxide species is avoided, thus explaining the lack of side reactions until the polymerization is complete.

Substituted Diarylnorbornadienes and Quadricyclanes: Synthesis, Photochemical Properties, and Effect of Substituent on the Kinetic Stability of Quadricyclanes.

In this Article, we present a new method for the synthesis of diarylnorbornadiene derivatives. Through the use of a two-step procedure consisting of a tandem alkene insertion-Suzuki coupling reaction followed by a DDQ dehydrogenation, we have been able to synthesize derivatives with a wide variety of substituents. We also present the results of UV-visible spectroscopy studies and kinetics experiments that show the effect of substituent on light absorption properties of the norbornadienes as well as the kinetic stability of the quadricyclanes that result from their photochemical conversion. While substitution on the aromatic rings had comparatively little effect on quadricyclane lability, substitution at a bridgehead position with a methyl group produced a quadricyclane that thermally reverted to the norbornadiene at a rate that was significantly slower than that for the quadricyclane without the methyl substituent. From the results of the kinetics experiments, we determined that the reversion of the quadricyclanes occurs via a free radical mechanism with very little contribution from polar effects. This observation led us to speculate as to whether our data may form the basis for a free radical substituent constant, σQ•, analogous to the traditional Hammett σ parameter.

Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides with Discrete Metal Complexes: Structure-Property Relationships.

Polyesters synthesized through the alternating copolymerization of epoxides and cyclic anhydrides compose a growing class of polymers that exhibit an impressive array of chemical and physical properties. Because they are synthesized through the chain-growth polymerization of two variable monomers, their syntheses can be controlled by discrete metal complexes, and the resulting materials vary widely in their functionality and physical properties. This polymer-focused review gives a perspective on the current state of the field of epoxide/anhydride copolymerization mediated by discrete catalysts and the relationships between the structures and properties of these polyesters.

Electronic Effects of Aluminum Complexes in the Copolymerization of Propylene Oxide with Tricyclic Anhydrides: Access to Well-Defined, Functionalizable Aliphatic Polyesters.

The synthesis of well-defined and functionalizable aliphatic polyesters remains a key challenge in the advancement of emerging drug delivery and self-assembly technologies. Herein, we investigate the factors that influence the rates of undesirable transesterification and epimerization side reactions at high conversion in the copolymerization of tricyclic anhydrides with excess propylene oxide using aluminum salen catalysts. The structure of the tricyclic anhydride, the molar ratio of the aluminum catalyst to the nucleophilic cocatalyst, and the Lewis acidity of the aluminum catalyst all influence the rates of these side reactions. Optimal catalytic activity and selectivity against these side reactions requires a careful balance of all these factors. Effective suppression of undesirable transesterification and epimerization was achieved even with sterically unhindered monomers using a fluorinated aluminum salph complex with a substoichiometric amount of a nucleophilic cocatalyst. This process can be used to synthesize well-defined block copolymers via a sequential addition strategy.