Manganese‐catalyzed C−H bond activation chemistry is emerging as a powerful and complementary method for molecular functionalization. A highly reactive seven‐membered MnI intermediate is detected and characterized that is effective for H‐transfer or reductive elimination to deliver alkenylated or pyridinium products, respectively. The two pathways are determined at MnI by judicious choice of an electron‐deficient 2‐pyrone substrate containing a 2‐pyridyl directing group, which undergoes regioselective C−H bond activation, serving as a valuable system for probing the mechanistic features of Mn C−H bond activation chemistry.
Manganese-catalyzed C À Hbond activation chemistry is emerging as ap owerful and complementary method for molecular functionalization. Ah ighly reactive seven-membered Mn I intermediate is detected and characterized that is effective for H-transfer or reductive elimination to deliver alkenylated or pyridinium products,r espectively.T he two pathwaysa re determined at Mn I by judicious choice of an electron-deficient 2-pyrone substrate containing a2 -pyridyl directing group,w hichu ndergoes regioselective C À Hb ond activation, serving as av aluable system for probing the mechanistic features of Mn CÀHb ond activation chemistry.C À Hb ond activation-functionalization chemistry is ac entral arena for catalyst development and synthetic application.[1] Tr ansition metals mediate the efficient and selective activation of CÀHb onds,w ith recent attention focusing on environmentally benign and sustainable metals,f or example, Mn, Co,F e, and Cu.[2] Mn I promotes CÀHa ctivation of substrates containing nitrogen-directing groups.[3] Fore xample, 1 gives cyclomanganated complex 2,w ith subsequent reaction with alkyne 3 forming aproposed 7-membered ring intermediate 4 (Scheme 1).[4] Formation of either 5, 6,o r7 results from reductive elimination, H-transfer,o rd ehydrogenative annulation, respectively.Processes utilizing Mn I ,p articularly [Mn(C^N)(CO) 4 ] 2, [5, 6] have been of broad interest. Them echanistic features of the remarkable synthetic work of Ackermann and Wang, [3,4] where intermediates 4a-c have been proposed, prompted us to examine whether they could be detected and characterized and then subsequently be shown to deliver organic products such as 5-7.C omplexes 4d-f,f ormed by insertion of internal alkynes are known, [6,7] but their competence in terms of af ully connected reaction system, affording organic products,h as not been examined. As 18-electron species containing four CO ligands,p ossessing high thermodynamic stability,t hey are unlikely to be directly involved in the catalytic cycle.[8]Herein we describe asuitable reaction system (1g!4g! 5g or 6g,S cheme 1) that takes advantage of the exquisite reactivity of an electron-deficient 2-pyrone ring system containing a2 -pyridyl directing group (1g). We recognized that the 2-pyrone could act as ah emilabile ligand in 7-membered manganacycle 4g,p otentially providing sufficient stabilisation for observation of this key intermediate.O ur findings demonstrate that 4g acts as ac entral manifold to reductive elimination and H-transfer, giving products 5g and 6g,respectively,w ith details described herein.Our study began with the reaction of 2-pyrone 1g with BnMn(CO) 5 in hexane at 75 8 8C, which gave cyclometalated 2g cleanly and in quantitative yield (Scheme 2). Complex 2g was fully characterized (see the Supporting Information);asingle crystal X-ray structure confirmed that regioselective CÀH activation occurred at C3, in keeping with Pd II -direct arylations of 2-pyrones, [9] albeit most likely by a s-CAM-type process. [10] We hypothesized tha...
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