Tandem energy transfer–electron transfer in the photosensitized alkylation of α,β-unsaturated ketones

Abstract

␣,␤-Unsaturated ketones are not conveniently alkylated by radicals generated from tetraalkylstannanes via photoinduced electron transfer (PET), either by direct irradiation or when a singlet sensitizer (an aromatic nitrile, a pyrilium salt) is used. However, the procedure is successful with tetramethyl pyromellitate (TMPM) as the sensitizer. TMPM is promoted to the triplet state by energy transfer from the unsaturated ketones and then sensitizes the cleavage of alkylstannanes by electron transfer. The alkyl radicals thus formed finally add to the unsaturated ketones, giving the ␤-alkyl derivatives.

Spectroscopic characterization of photoaccumulated radical anions: a litmus test to evaluate the efficiency of photoinduced electron transfer (PET) processes

“…1,3,5-TrCB [56], 1,2,4-TrCB [56], DCN [57], DCP [10], DCP-Me [10], ME [58], PME [11] and OXA [59] were obtained by known procedures. Since oxygen could interact with the photogenerated radical anions, resulting in a back oxidation [6], all of the experiments were carried out in freeze–pump–thaw deoxygenated solutions, except where otherwise noted.…”

“…As an example, the largely positive reduction potential of aromatic nitriles [5,8–9] and cyanophthalimides [10] in the singlet excited state or of aromatic esters in the triplet state [5,11] makes PET a common occurrence upon irradiation of such substrates in the presence of a variety of donors (D–X, step a). In all of the above cases, the generated radical anion (A–Y •− ) is a persistent species easily detected by laser flash photolysis.…”

Photocatalysis for the Formation of the C−C Bond

“…When the catalyst was an ester such as TMPM, active from the triplet, this limitation was overcome, since the unsaturated ketones absorbed light and, after intersystem crossing, transferred energy to the aromatic ester, which was thus able to catalyze the alkylation (see eq 2). 79 Another class of precursors of alkyl radicals is that of 2,2dialkyl-1,3-dioxolanes or 2-alkyl-2-phenyl-1,3-dioxolanes (see Scheme 19). The last compounds were easily oxidized, fragmented selectively, and gave good yields of the alkylated alkenes, while minimizing ipso substitution of TCB, a process generally competing with the desired alkylation when dialkyldioxolanes were used with this catalyst.…”

“…Photocatalysis is indeed a viable method toward this goal and occurs under mild conditions, although at present not many examples have been reported and not all of these are satisfactory as far as the products' yield/selectivity and the catalyst turnover number are concerned. In addition to C−H activation in alkanes, alkyl radicals have also been obtained by splitting a different group in other precursors, such as carboxylic acids, stannanes, ,− dioxolanes, , silanes or silyl ethers, , and halides. , …”

Photochemical Alkylation of Ketene Dithioacetal S,S-Dioxides. An Example of Captodative Olefin Functionalization

“…Recently we described the synthetic application of this process for the addition of photogenerated radicals to electron-poor olefins. , When a synthesis of this type is planned, some requirements must be met: (1) E red (A*) must be high enough to oxidize the donor; (2) E red (A) must be low enough to allow the reduction of the adduct radical by A -• (See Scheme ); (3) back electron transfer (eq 3) must be controlled. Choosing the right donor−acceptor couple according to the olefin used as a trap (α,β-unsaturated ketones, esters and nitriles , ) determines the outcome. In the present case the best results were obtained by using the DCB/Phen pair.…”