Bioinspired Metal‐Free Formal Decarbonylation of α‐Branched Aliphatic Aldehydes at Ambient Temperature

Abstract

A sequence of a Baeyer–Villiger oxidation and a Lewis acid‐promoted reduction of the resulting formate with Et3SiH enabled the metal‐free formal decarbonylation of tertiary and secondary aliphatic aldehydes. The new methodology mimics the biosynthetic decarbonylation pathway through oxidative C−C bond cleavage rather than the C(O)−H bond activation known from conventional Tsuji–Wilkinson‐type reactions. The substrate scope is complementary to existing transition‐metal‐catalyzed protocols.

Chemoselective Deoxygenation of 2° Benzylic Alcohols through a Sequence of Formylation and B(C6F5)3‐Catalyzed Reduction

“…In analogy to the widely applied Barton‐McCombie reaction, [8] we planned to modify the hydroxy group in the first step to alter the selectivity in the subsequent B(C 6 F 5 ) 3 ‐catalyzed reduction. We anticipated the aforementioned change in mechanism from S N 2 to S N 1 for this reduction with hydrosilanes based on our previous study on the deoxygenation of benzylic formates in the context of a formal decarbonylation strategy [9] …”

“…Et 3 SiOMe ( 11 ) is ultimately reduced to methane ( 12 ) along with the corresponding disiloxane [12] . The deoxygenation of the secondary formate 6 a follows an S N 1 mechanism [9] . Silylcarboxonium ion 9 a dissociates into the benzylic carbenium ion 14 a and the silylated formate 13 .…”

“…We anticipated the aforementioned change in mechanism from S N 2 to S N 1 for this reduction with hydrosilanes based on our previous study on the deoxygenation of benzylic formates in the context of a formal decarbonylation strategy. [9] We began our investigation by studying the B(C 6 F 5 ) 3catalyzed reduction of selected secondary benzylic alcohol derivatives with Et 3 SiH (1 a-6 a!7a; Table 1). As a general trend, it was observed that acylated substrates react faster to the corresponding hydrocarbon 7 a than the archetypal triethylsilyl ether 1 a (entry 1 versus entries 2-6).…”

“…[12] The deoxygenation of the secondary formate 6 a follows an S N 1 mechanism. [9] Silylcarboxonium ion 9 a dissociates into the benzylic carbenium ion 14 a and the silylated formate 13. Hydride transfer from the borohydride to the carbenium ion affords the hydrocarbon 7 a while formate 13 converts further into bis-silyl acetal 15, silylated methanol 11, methane (12), and the disiloxane by B(C 6 F 5 ) 3 -catalyzed hydrosilylation.…”

Defunctionalisation catalysed by boron Lewis acids

“…A formal decarbonylation of aliphatic aldehydes 29a-d via a sequence of Baeyer-Villiger oxidation and B(C 6 F 5 ) 3 -or BF 3 -$OEt 2 -catalysed deoxygenation of the resulting formate with Et 3 SiH as the reductant was developed by Richter and Oestreich (Scheme 27). 56 Mechanistic investigations suggested that an S N 1 mechanism is involved for the deoxygenation process.…”