A Supramolecular Chiral Auxiliary Approach: “Remote Control” of Stereochemistry at a Hierarchically Assembled Dimeric Helicate

Abstract

Dimeric hierarchically-assembled titanium(IV) helicates are in solvent-dependent equilibrium with the corresponding monomers. Statistically formed mixtures of such complexes bearing chiral stereocontrolling ligands and achiral diene-substituted ligands show high diastereoselectivity and reasonable enantioselectivity in the Diels-Alder reaction with maleimides if the reaction proceeds with the dimer but not with the monomer. Thus, solvent dependent switching between the monomer and dimer enables on/off switching of the enantioselectivity.

Catechol Thioesters: Ligands for Hierarchically Formed Lithium‐Bridged Titanium(IV) Helicates and Helicate‐Based Switches

“…In the dimer,t he stereochemistry at the complex is locked and diastereotopic protons are detected for SCH 2 . [13] This is observed for the major specieso fL i[Li 3 (1c) 6 Following remarkable aspects can be realizedf rom the collection of dimerization constantso ft he thioester-derived complexes:…”

“…[11] In the second step the methyl ethers are cleaved by reaction with BBr 3 . [12] The catechol thioester ligands 1-H 2 are reacted with titanoyl bis(acetylacetonate) in the presence of lithium carbonate to obtain dinuclear triple lithium-bridged titanium(IV) helicates Li[Li 3 (1a-p) 6 Ti 2 ]. [4] X-Ray quality crystals of the benzyl derivative Li[Li 3 (1p) 6 Ti 2 ] were obtained, the molecular structure of the anion [Li 3 (1p) 6 Ti 2 ] À is shown in Figure 1.…”

“…In more recent studies, the lithium‐dependent dimerization of carbonyl substituted titanium(IV) triscatecholate complexes has been used as a molecular switch to control the selectivity of a Diels–Alder reaction or for the development of a helicate system which is able to expand or compress depending on some external stimuli showing some spring‐type behavior…”

“…[4] Additionally,i ntersubstituent interactions as well as solvophobic/solvophilic effects do contribute to the dimer stabilization. [5] In more recent studies, the lithium-dependent dimerization of carbonyl substituted titanium(IV) triscatecholate complexes has been used as am olecular switch to control the selectivity of aD iels-Alder reaction [6] or for the development of ah elicate systemw hich is able to expand or compress depending on some externalstimulis howing some spring-type behavior. [7] In the present study as eries of thioester-substituted catechols has been synthesized and has been used to preparet he correspondingt itanium(IV) complexes.…”

From Hierarchical Helicates to Functional Supramolecular Devices

“…A final goal will be to use the switching between different species for the control of chemical movement and reactivity. In a preliminary study hierarchical helicates were used as reaction platforms to achieve a stereoselective Diels–Alder reaction in which it is possible to switch the selectivity of the reaction by changing the solvent …”