(o‐Phenylenediamino)borylstannanes: Efficient Reagents for Borylation of Various Alkyl Radical Precursors

Abstract

DedicatedtoProfessor Ilhyong Ryu on the occasion of his 70th birthday Abstract: (o-Phenylenediamino)borylstannanes were newly synthesizedt oa chieve radical boryl substitutions of av ariety of alkyl radical precursors. Dehalogenative, deaminative, decharcogenative,a nd decarboxylative borylationsp roceeded in the presenceo faradical initiator to give the corresponding organic boronc ompounds. Radical clock experiments and computational studies have provided insights into the mechanism of the homolytic substitution (S H 2) of the borylstannanes with alkyl radical intermediates. DFT calculation disclosed that the phenylenediamino structure lowered the LUMO level including the vacant p-orbital on the boron atom to enhance the reactivity to alkyl radicals in S H 2. Moreover,C (sp 3)-H borylation of THF was accomplished using the triplet state of xanthone.

anti‐Selective Borylstannylation of Alkynes with (o‐Phenylenediaminato)borylstannanes by a Radical Mechanism

“…[10,11] Borylstannane 1 b captures vinyl radical B by its vacant p-orbital on the boron atom, the LUMO of which is lowered and expanded by conjugation with the o-phenylenediaminato structure. [7] The anti-addition path (the activation energy of TS3: + 23 kcal mol À 1 ) is more favored than the syn-addition path (the activation energy of TS4: + 36 kcal mol À 1 ) due to steric repulsion between the stannyl group and 1 b. In the case of aliphatic alkynes, the same regio-and stereoselectivity is also supported by DFT calculation (see the Supporting Information).…”

“…Optimized vinyl radical B is slightly‐bended π‐radical, and this configuration between the Sn−C( α ) bond and the SOMO on the C( β ) center dominantly leads to the anti ‐selectivity, which is supported by Alabugin's and Hale's reports [10, 11] . Borylstannane 1 b captures vinyl radical B by its vacant p ‐orbital on the boron atom, the LUMO of which is lowered and expanded by conjugation with the o ‐phenylenediaminato structure [7] . The anti ‐addition path (the activation energy of TS3 : +23 kcal mol −1 ) is more favored than the syn ‐addition path (the activation energy of TS4 : +36 kcal mol −1 ) due to steric repulsion between the stannyl group and 1 b .…”

“…Then, 1 b reacts with the vinyl radical B via a homolytic substitution mechanism to afford borylstannylation product 3 while regenerating stannyl radical A [Eq. (3)] [7] . The reversible addition of A to a triple bond of 2 a regioselectively gives more thermodynamically‐stable vinyl radical B than C , in which the reversibility was suggested by Hale [10] .…”

“…UV irradiation conditions without a radical initiator resulted in 19 % yield (entry 4). A Et 3 B/air system [7,8] afforded the desired product 3 a-anti, but the yield was decreased (entry 5). The addition of TEMPO (2,2,6,6tetramethylpiperidine 1-oxyl) completely hampered the borylstannylation (entry 6), which indicates this reaction proceeds via a radical mechanism.…”

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

“…[193] Also, Suzuki, Nishimoto, and Yasuda developed boronation of alkyl halides using (o-phenylenediamino)borylstannane (154) as the boron source (Table 6, entry 22). [189] This reagent had a dual role in the reaction (Scheme 44). At the initiation step, it produced a Sn-centered radical capable of attack at the alkyl chloride.…”