Synthesis of 2-Substituted 1,2-Dihydronaphthalenes and 1,2-Dihydroanthracenes Using a Recyclable Molybdenum Dearomatization Agent

Abstract

Polycyclic aromatic hydrocarbons (PAHs; e.g., naphthalene and anthracene) form stable η2-bound complexes with the dearomatizing fragment {TpMo­(NO)­(MeIm)} (where Tp = hydridotris­(pyrazolyl)­borate; MeIm = 1-methylimidazole). These complexes undergo protonation at the α carbon followed by regioselective nucleophilic addition at the adjacent β carbon. The nucleophile (a pyrrole or an enolate) adds stereoselectively, anti to the face of metal coordination. The resulting 1,2-dihydroarene ligand may be isolated via metal oxidation by iodine to provide the free 1,2-dihydroarene in moderate yield (∼60%), as well as TpMo­(NO)­(MeIm)­(I), the precursor of the original PAH complex (∼80–90%). Thus, a formal catalytic cycle for the dearomatization of naphthalene and anthracene has been demonstrated.

Molybdenum-Promoted Dearomatization of Pyridines

“…This observation is consistent with the trend noted previously with the related η 2 -PhCF 3 and η 2 -napthalene complexes of these metals. 15,25,28 Several other pyridines were also screened, including unsubstituted pyridine, 2-picoline, 3-picoline, 4-picoline, 2ethylpyridine, and 2-isopropylpyridine, all of which coordinated through nitrogen, as evidenced by a chemically reversible couple around E 1/2 = −0.9 V. 27 Of note, the reaction of bipy (2,2′-bipyridine) and MoTp(NO)(DMAP)(η 2 -PhCF 3 ) produces a complex (28) that differs significantly in appearance (deep magenta) from the other pyridine complexes (1−5, yellow-orange). Electrochemical data indicate a complex that is more reducing (E p,a = −0.42 V, NHE), and H NMR data fail to show any upfield ring-protons.…”

“…In cases where comparative data are available, each of these E p,a ’s was found to be several hundred millivolts more negative than for the analogous tungsten complexes, , which indicates that the molybdenum complexes are significantly more sensitive to oxidation. This observation is consistent with the trend noted previously with the related η 2 -PhCF 3 and η 2 -napthalene complexes of these metals. ,, …”

“…Hz, 1H, H5′), 3.06 (s, 3H, H7), 2.55 (dd, J = 10.6 Hz, 6.1 Hz, 1H, H4), 2.14 (dd, J = 10.6 Hz, 4.9 Hz, 1H, H3). 13 Synthesis of Mo(κ 2 -Tp)(NO)(κ 2 -bpy) (28). MoTp(NO)-(DMAP)(η 2 -PhCF 3 ) (720 mg, 1.18 mmol) and THF (7.0 mL) were combined in a 4 dram vial with stir pea.…”

Michael–Michael Ring-Closure Reactions for a Dihapto-Coordinated Naphthalene Complex of Molybdenum

“…In order for the desired sequence to be successful, the molybdenum complex would have to sufficiently activate the naphthalene for the initial conjugate addition reaction at C4 (Scheme ) and stabilize the 4H-naphthalenium product long enough for the second enolate to close at C3. The complex MoTp­(NO)­(DMAP)­(η 2 -naphthalene) ( 1 ) was prepared on a 10 g scale according to literature methods as a 3:1 mixture of coordination diastereomers by reducing MoTp­(NO)­(DMAP)­(I) with magnesium or sodium in the presence of naphthalene. , …”

“…We have a long-standing interest in promoting novel organic reactions of aromatic molecules through their dihapto-coordination, − and we recently investigated the ability of the complex {MoTp­(NO)­(DMAP)} (DMAP = 4-(dimethylamino)­pyridine; Tp = tris­(pyrazolyl)­borate) to dearomatize naphthalene via a 1,4-tandem addition reaction sequence (Scheme ). , We questioned whether this highly pi-basic metal complex was sufficiently electron-donating to promote a MIMIRC sequence, using this common aromatic hydrocarbon as the initial nucleophile (Scheme ). Herein we describe our attempts to effect such reactions, both where the Michael acceptors are the same (R′ = R″) and when they are different (R′ ≠ R″), using a Lewis acid to initiate the process.…”