Preparative semiconductor photoredox catalysis: An emerging theme in organic synthesis

Abstract

Heterogeneous semiconductor photoredox catalysis (SCPC), particularly with TiO2, is evolving to provide radically new synthetic applications. In this review we describe how photoactivated SCPCs can either (i) interact with a precursor that donates an electron to the semiconductor thus generating a radical cation; or (ii) interact with an acceptor precursor that picks up an electron with production of a radical anion. The radical cations of appropriate donors convert to neutral radicals usually by loss of a proton. The most efficient donors for synthetic purposes contain adjacent functional groups such that the neutral radicals are resonance stabilized. Thus, ET from allylic alkenes and enol ethers generated allyl type radicals that reacted with 1,2-diazine or imine co-reactants to yield functionalized hydrazones or benzylanilines. SCPC with tertiary amines enabled electron-deficient alkenes to be alkylated and furoquinolinones to be accessed. Primary amines on their own led to self-reactions involving C-N coupling and, with terminal diamines, cyclic amines were produced. Carboxylic acids were particularly fruitful affording C-centered radicals that alkylated alkenes and took part in tandem addition cyclizations producing chromenopyrroles; decarboxylative homo-dimerizations were also observed. Acceptors initially yielding radical anions included nitroaromatics and aromatic iodides. The latter led to hydrodehalogenations and cyclizations with suitable precursors. Reductive SCPC also enabled electron-deficient alkenes and aromatic aldehydes to be hydrogenated without the need for hydrogen gas.

Keywords: carboxylic acids; free radicals; organic synthesis; photocatalysis; titania.

Figure 1

Production and utilization of h⁺ and e^– by photoactivation of a semiconductor.

Figure 2

Photoredox activity of TiO₂ with moist air.

Scheme 1

TiO₂ promoted oxidation of phenanthrene [29].

Scheme 2

SCPC assisted additions of allylic compounds to diazines and imines [–42].

Scheme 3

TiO₂ promoted addition and addition–cyclization reactions of tert-amines with electron-deficient alkenes [–47].

Scheme 4

Reactions of amines promoted by Pt-TiO₂ [–49].

Scheme 5

P25 Promoted alkylations of N-phenylmaleimide with diverse carboxylic acids [–54]. ^aAccompanied by R–R dimers. ^bObtained as 1:1 mixtures of two diastereoisomers.

Scheme 6

SCPC cyclizations of aryloxyacetic acids with suitably sited alkene acceptors [54]. ^aYields in brackets are maximum yields from NMR monitoring of reactions in sol–gel TiO₂ coated tubes.

Scheme 7

TiO₂ promoted reactions of aryloxyacetic acids with maleic anhydride and maleimides [–54].

Scheme 8

Photoredox addition–cyclization reactions of aryloxyacetic and related acids promoted by maleimide [63].

Scheme 9

SCPC promoted homo-couplings and macrocyclizations with carboxylic acids [64].

Scheme 10

TiO₂ promoted alkylations of alkenes with silanes [66] and thiols [67].

Scheme 11

TiO₂ reduction of a nitrochromenone derivative [70].

Scheme 12

TiO₂ mediated hydrodehalogenations and cyclizations of organic iodides [71].

Scheme 13

TiO₂ promoted hydrogenations of maleimides, maleic anhydride and aromatic aldehydes [79].

Scheme 14

Mechanistic sketch of SCPC hydrogenation of aryl aldehydes.