Regio- and diastereoselective intermolecular [2+2] cycloadditions photocatalysed by quantum dots

Abstract

Light-driven [2+2] cycloaddition is the most direct strategy to build tetrasubstituted cyclobutanes, core components of many lead compounds for drug development. Significant advances in the chemoselectivity and enantioselectivity of [2+2] photocycloadditions have been made, but exceptional and tunable diastereoselectivity and regioselectivity (head-to-head vs. head-to-tail adducts), required for synthesis of bioactive molecules, have not yet been achieved. Here we show that colloidal quantum dots (QDs) serve as visible-light chromophores, photocatalysts, and reusable scaffolds for homo-and hetero-intermolecular [2+2] photocycloadditions of 4vinylbenzoic acid derivatives, including aryl-conjugated alkenes, with up to 98% switchable regioselectivity and 98% diastereoselectivity for the previously minor syn-cyclobutane products, including the syn-head-to-tail cyclobutane, which has never before been accessed as the major product of a hetero-photocycloaddition. Transient absorption spectroscopy confirms that our system is the first example of catalysis triggered by triplet-triplet energy transfer from a QD. The precisely controlled triplet energy levels of QD photocatalysts facilitate efficient and selective heterocoupling, a major challenge in direct cyclobutane synthesis.Light-driven [2+2] cycloadditions 1-5 , simple routes to complex, biologically relevant tetrasubstituted cyclobutanes [6][7][8] , follow several possible excited-state routes. Those that proceed through the triplet excited state of the reagent olefin are advantageous because (i) their scope is not limited by the electrochemical potentials of the substrate, (ii) triplets have long enough lifetimes to mediate intermolecular cycloadditions, and (iii) triplets are accessible with visible light through excitation of a triplet sensitizer, such as a transition metal complex or organic chromophore 9 , followed by triplet-triplet energy transfer (TT Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:http://www.nature.com/authors/editorial_policies/license.html#termsReprints and permissions information is available at http://www.nature.com/reprints.

Fluorinating π‐Extended Molecular Acceptors Yields Highly Connected Crystal Structures and Low Reorganization Energies for Efficient Solar Cells

“…These interactions result in a large network of molecules electronically coupled to the central reference molecule, with a maximum lateral length of ≈84 Å (8.4 nm) – ≈4× larger than some photosensitizer quantum dimensions. [ 59 ] The physical size of this interaction network for just a single reference molecule shows the significant extent of the long‐range charge transfer possible within the ITzN‐F4 acceptor network, and this heretofore unrecognized characteristic may be a mechanism for efficient charge transport.…”

Colloidally Stable CdS Quantum Dots in Water with Electrostatically Stabilized Weak‐Binding, Sulfur‐Free Ligands

“…The pKao ft he first acidic protono fP O 3 H 2 within AEP is approximately 1; [31] thus it is dissociated throughout the pH range we studied (6)(7)(8)(9)(10)(11)(12). In this pH range, AEP has two protonation equilibria, PO 3 H À $PO 3 2À and NH 3 + $NH 2 .W ed etermined pH-metrically that the acid dissociation constantsa re 6.3 (pKa 1 )f or the PO 3 H À group and 11.1 (pKa 2 )f or the NH 3 + group (Supporting Information, Figure S1), and we used these valuest oc alculate the microspecies distribution curves for AEP as af unctiono fp H, Figure 1B.W eu se the notation + AEP À to indicatet he molecule in as tate with ap rotonated amine and am ono deprotonated phosphonate( NH 3 + (CH 2 ) 2 PO 3 H À ), the notation + AEP 2À to indicate the molecule in as tate with ap rotonated amine and af ully deprotonated phosphonate (NH 3 + (CH 2 ) 2 PO 3 2À ), and the notation AEP 2À to indicate the molecule in as tate with an eutral amine andafully deprotonated phosphonate (NH 2 (CH 2 ) 2 PO 3 2À ).…”

“…Water is the ultimate green solvent, and the choice of medium for many classes of reactions that utilize an emerging methodology for chemical transformations, nanoparticle photocatalysis, that is highly effective in the laboratory and potentially scalable. [1][2][3] Among the range of nanoparticles that photocatalyze aqueous reactions, semiconductor quantumd ots (QDs) stand out for their chemical and electronic tunability and versatility.Q Ds have demonstrated unprecedented activity in photocatalytic H 2 evolution, [4][5][6][7] unprecedented sensitizatione fficiency for CO 2 reduction, [8] the ability to serve as triplet exciton donors and scaffolds for stereoselective cycloadditions, [9,10] and the electrochemical potentials to act as both photo-oxidant and photo-reductant in CÀCc oupling schemes with no sacrificial reagents. [11,12] In water,p roperly functionalized colloidal QDs form colloidally stable aggregates to mimic the exciton funneling function of photosystemII, [13] and perform chemoselective alcohol oxidations.…”

“…[27,28] Here, we provide aframework for the design of stable colloidal aqueousd ispersions of semiconductor QDs using ligands that are in dynamic exchange on and off the QD surface, and therefore are potentiall igandsf or photocatalytic QDs. Our experimental system is CdS QDs with bifunctional aminoethyl phosphonic acid (AEP) ligands ( Figure 1) in water over ar ange of pH values (6)(7)(8)(9)(10)(11)(12). Changing the pH changes the protonation state of both the amino and phosphonate groups, which changes their respective charges and affinities for the QD surface.…”

Recent Progress in Engineering Metal Halide Perovskites for Efficient Visible‐Light‐Driven Photocatalysis

“…For instance, photocatalytic organic reactions with CdSe QDs, as illustrated by Weiss et al., specifically require the size of the QDs to be 1.4 nm. However, one of the clear advantages of our MHP NC catalyst is that size‐uncontrolled NCs in the range of 2–100 nm can still lead to very high reaction outcomes, as demonstrated by high conversion and high product yield (Figure a).…”