Manganese(I)‐Catalyzed C−H Activation: The Key Role of a 7‐Membered Manganacycle in H‐Transfer and Reductive Elimination

Abstract

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...

Aromatic C-H addition of ketones to imines enabled by manganese catalysis

“…Based on the above results and literature clues 35 , 43 , 53 , a plausible reaction mechanism was depicted in Fig. 10 .…”

“…Despite of their huge synthetic interests, considerable challenges still remain in these processes, such as the formidably competitive Mannich and/or Aldol-type reactions of α-C–H bonds of ketones, the relatively inert reactivity of aromatic C–H bonds with a weakly coordinating ketone group 26 , and the lower catalytic reactivity of base metals compared with the precious ones. To address these issues, we resort to manganese-promoted C–H activation 27 – 43 , in which the stoichiometric cyclomanganation of ketones was shown by Kaesz and Nicholson as early as in 1975 44 , 45 . However, the manganese-catalyzed aromatic C–H transformations of ketones remain elusive.…”

Polycyclization Enabled by Relay Catalysis: One‐Pot Manganese‐Catalyzed C−H Allylation and Silver‐Catalyzed Povarov Reaction

“…On the basis of the above resultsa nd preceding reports, [17] am echanistic scenariow as proposed to rationalize the reaction outcome (Scheme 5). Initially, an imine-assisted CÀHa ctivation takes place to yield intermediate B.T his process is not turnover-limiting and is reversible, and presumably following a s-complex-assisted metathesis (s-CAM) [18a] or base-assisted electrophilic substitution (BIES) [18b] mechanism.…”

Manganese Carbonyl Compounds Reveal Ultrafast Metal–Solvent Interactions

“…This method has previously proven to effectively model other aspects of Mn-carbonyl chemistry. 41,47,56 The two lowest-energy transitions were calculated to appear at 345 and 342 nm. Both had two dominant components (Figure 2c) corresponding to orbitals 19a″→ 20a″ (HOMO → LUMO largely intraligand π−π*) and 62a′ → 63a′ which is from a metal−carbonyl bonding orbital in the same plane as (en = 1,2-diaminoethane), which is CO dissociative.…”

“…Hence, we have been able to employ TRIR spectroscopy to garner structural information about this interconversion that cannot be obtained by TA. Given the role of complexes related to [1a] as versatile CO-RMs 38−40 and in Mn-catalyzed C-H functionalization reactions, 41,43,56,62 we anticipate that further TRIR studies will provide vital insight into their behavior. Importantly, when [1a] is photolyzed in neat alkynes, alkenes, and isocyanates, kinetically controlled coordination (as shown for ethers and DMSO) also occurs.…”