Carbon Nitride‐Catalyzed Photoredox Sakurai Reactions and Allylborations

Abstract

Within the last couple of years, the photocatalytic, oxidative CC coupling has grown to a well‐established methodology, especially for the modification of aryl‐substituted tetrahydroisoquinoline derivatives. However, until now, this reaction is mainly restricted to the usage of strong nucleophiles like nitroalkanes, cyanides, enamines, phosphonates and malonates. Within this publication we present an extension of such a method towards the application of weaker nucleophiles, namely allylstannanes, allylsilanes and allylboranes. Therefore, a very simple protocol was invented, using heterogeneous mesoporous graphitic carbon nitride (mpg‐C3N4) as catalyst and air as oxidizing agent, leading to high yields for a broad range of substrates.

One-pot functionalisation of N-substituted tetrahydroisoquinolines by photooxidation and tunable organometallic trapping of iminium intermediates

“…We reasoned that use of a less reactive organometallic reagent would supress undesirable pathways. However, allyltrimethylsilane does not react with 5a ( Table 1 , entry 12) [ 16 , 22 ]. As organometallics of intermediate reactivity, allylzinc halides were explored.…”

“…Murthy and Blechert and their respective co-workers reported allylation of THIQs under aerobic conditions using allyltrialkylstannanes [ 12 , 16 ] (Blechert’s studies also included success with allylboron reagents). Whilst our reactions are carried out under N 2 , the indium metal used, allylindium reagents generated and indium trihalide salt byproducts are non-toxic [ 43 ].…”

“…Whilst our reactions are carried out under N 2 , the indium metal used, allylindium reagents generated and indium trihalide salt byproducts are non-toxic [ 43 ]. Our conditions benefit from the absence of amide side-products typically effected by peroxide intermediates in aerobic photoactivation of THIQs [ 16 , 22 ] and so our methodology serves to complement existing strategies in the literature.…”

“…Alternatively, the α-amino radicals can be trapped by electrophiles [ 7 – 9 ]. Oxidative C–H functionalisation of THIQs is reported using Cu(I) [ 10 – 11 ], Fe(III) [ 12 ], V(IV) [ 13 ] and I 2 [ 14 – 15 ] catalysts, but also with heterogeneous [ 16 ], metal-organic [ 17 – 18 ] and organic [ 19 – 20 ] photocatalysts. Such catalysts are used in combination with various stoichiometric oxidants including oxygen [ 16 , 21 ].…”

Photoredox Catalytic Organic Transformations using Heterogeneous Carbon Nitrides

“…In this case, methanol was used as a solvent and the weak nucleophiles, such as allyl-or allenyl-stannanes, reacted selectively with the photoredox chemically generated iminium ions to furnish the desired cross-coupling products (Scheme 9). [40] Scheme 9. mpg-CN assisted photoredox catalytic oxidation of Narylisoquinolines coupled with the Sakurai reaction. Reaction was performed on 0.25 mmol scale.…”