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Α-Diimine Synthesis Via Titanium-Mediated Multicomponent Diimination of Alkynes with C-Nitrosos Connor W

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Α-Diimine Synthesis Via Titanium-Mediated Multicomponent Diimination of Alkynes with C-Nitrosos Connor W α-Diimine Synthesis via Titanium-Mediated Multicomponent Diimination of Alkynes with C-Nitrosos Connor W. Frye, Dominic T. Egger, Errikos Kounalis, Adam J. Pearce, Yukun Cheng, and Ian A. Tonks* Department of Chemistry, University of Minnesota−Twin Cities, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States ABSTRACT: α-Diimines are commonly used as supporting ligands for a variety of transition metal-catalyzed processes, most notably in α-olefin polymerization. They are also precursors to valuable synthetic targets, such as chiral 1,2-diamines. Their synthesis is usually performed through acid-catalyzed condensation of amines with α-diketones. Despite the simplicity of this approach, accessing unsymmetrical α-diimines is challenging. Herein, we report the Ti-mediated intermolecular diimination of alkynes to afford a variety of symmetrical and unsymmetrical α-diimines through the reaction of diazatitanacyclohexadiene intermediates with C-nitrosos. These diazatitanacycles can be readily accessed in situ via the multicomponent coupling of Ti≡NR imidos with alkynes and nitriles. The formation of α-diimines is achieved through formal [4+2]-cycloaddition of the C-nitroso to the Ti and γ- carbon of the diazatitanacyclohexadiene followed by two subsequent cycloreversion steps to eliminate nitrile and afford the α- diimine and a Ti oxo. stepwise condensation: reversible reactions Introduction A lead to intractable mono- and diimine mixtures α-Diimines (1,4-diaza-1,3-dienes) are widely used as Ph O O N ligands for organometallic complexes,1-2 perhaps most N N O notably in Brookhart-type catalysts for α-olefin Ph Me tol Me Me polymerization.3-5 The α-diimine ligand scaffold has proven first, Ph 7% Ph 8% Ph 23% PhNH2 O TsOH, PhCH tol tol to be remarkably versatile due to its facile stereoelectronic 3 N N 80 oC, 24 h 6-9 O tunability. For example, modifications to the backbone and Me 8 products! O N then, Me tol Me Ph N-aryl substituents of α-diimines can exert control over the p-tolNH 2 Ph 24% Ph 1% molecular weight and microstructure of α-olefin polymers by TsOH, PhCH3 80 oC, 24 h Ph Ph tol attenuation of chain walking processes.1, 10 α-Diimine ligands N N N N N N are also often redox non-innocent, which allows for richer Ph Me tol Me Ph Me 11-14 and more complex redox processes. α-Diimines are also Ph 2% Ph 1% Ph 2% precursors to valuable chiral 1,2-diamines through B successful stepwise condensation with bulky arylamines asymmetric hydrogenation,15-16 or to N-heterocyclic carbene (NHC) ligands through cyclization with paraformaldehyde O O and subsequent deprotonation.17-20 Ph Ph Me Me Ar2 Typically, α-diimines are synthesized through O N 2 2 (excess)Ar -NH2 Ar -NH2 condensation of α-diketones and amines. Despite the NH N N 2 Ar Me Ar Me apparent simplicity of this approach, accessing - H2O - H2O Me Me unsymmetrical α-diimines through stepwise condensations is synthetically challenging due to poor chemoselectivity.21 Ph Ph Furthermore, since imine formation is reversible, attempts at Figure 1. Challenges and current strategies for unsymmetric α- sequential selective condensations can result in complex diimine synthesis. mixtures (for example, see Figure 1A, Figures S93-S96). There are comparatively few examples of unsymmetrical Imines are also notoriously difficult to isolate because they α-diimines, and these are mostly limited to N-aryl substituent are prone to hydrolysis, making methods that generate modifications involving very sterically encumbered groups3, 9, product mixtures impractical. Highlighting this problem, 23 rather than backbone substitution (Figure 1B).11, 24-25 although amine condensation with glyoxal to afford α- Selective trapping of unsymmetrical diimines via cyclization diimines is a common route to symmetric NHCs there remain has also recently been reported.26 Given this methodology no examples of unsymmetric NHCs synthesized via gap, developing a route to these unsymmetrical α-diimines condensation with glyoxal.22 from simple feedstocks would provide a useful tool for the development of more diverse ligand scaffolds and pharmaceutically relevant building blocks.3, 15-16 Our group has reported several examples of Ti-catalyzed py py Cl Cl C D Ph Ti Ph 6 6 N oxidative functionalizations of alkynes for the synthesis of N N rt, 30 min + PhNO N multisubstituted N-heterocycles that overcome limitations -p-tolCN Ph Et 27-30 p-tol Et 2a of classical condensation reactions. We envisioned that -[Cl2Ti=O] Me Me a complementary synthetic route to unsymmetrical α- 1 3a (81%) diimines could be achieved through Ti-mediated oxidative [4+2] retro- -py -[Cl Ti=O] diimination of alkynes. Examples of alkyne diamination or cycloaddition [4+2] 2 31-34 diimination are scarce despite many reports of alkene Ph Cl 35-40 py Cl diamination. To the best of our knowledge, there is only N retro- Me [4+2] Ti Ph a single example of a multicomponent intermolecular alkyne p-tol O Et O N 41 -p-tolCN N diamination and no examples of alkyne diimination. N Ti N a Ph Et py Ph Nevertheless, there are several elegant examples of alkyne Cl Cl Me difunctionalizations42 using Ti including alkyne A B carboamination43-45 and iminoamination46 that provide motivation for further exploring Ti-catalyzed or -mediated Figure 3. Reaction of a C-nitroso with 1 yields an α-diimine 3a via diamination/diimination. a sequence of cycloaddition/retrocycloaddition steps. Previously, we reported that diazatitanacyclohexadienes Table 1. Substrate scope of the addition of C-nitrosos to 1. (prepared from the multicomponent coupling of Ti imidos, py py Ph alkynes, and nitriles) could undergo oxidation-induced N-N Cl Cl C6H6 Ti Ph N coupling to yield pyrazoles (Figure 2, top).30 In the interest of N N rt, 30 min + RNO N further expanding the utility of this unique intermediate, we -p-tolCN R Et p-tol Et 2a-n -[Cl2Ti=O] Me have begun examining its reactivity with various group Me transfer reagents. Herein, we report the intermolecular 1 3a-o diimination of alkynes by Ti imidos and C-nitrosos to afford Ph Ph Ph unsymmetrical α-diimines (Figure 2, bottom). This N N N N N N diimination reaction proceeds through a cascading sequence Et Et Et of formal cycloaddition and retrocycloaddition reactions Me Me Me from this key diazatitanacyclohexadiene intermediate. This 3a (82%) 3b (77%) 3c (76%) approach is a useful strategy for synthesizing unsymmetrical Ph Ph Ph α-diimines, which are challenging to access through N N N N N N traditional condensation reactions. Et Et Et R3 prior work: pyrazole formation then, Me Me Me 2 TEMPO N F3C Br EtO2C N 2 3d (77%) 3e (85%) R 3f (71%) 50 oC, 2 h py py a R Ph Ph Ph 0.5 equiv. N N N 3 R1 [py2TiCl2(NR )]2 Cl Cl R2 Ti 3 1 equiv. MeCN R N N N N N Et Et Et C H Br a Me Me Me R1 6 5 Me R2 O N 115 °C, 4 h R3 3i (49%)c R1 then, 3g (84%) 3h (82%) a 4 N in situ generated R NO Ph Ph Ph diazatitanacyclohexadiene N CF N N O N 4 2 3 rt, 30 min R R N N N -MeCN R1 Et Et Et this work: alkyne diimination -[Cl2Ti=O] a Me Me Me b 3j (85%) 3k (74%) 3l (74%) Figure 2. In situ generated diazatitanacycloehexadiene Ph Ph N Ph intermediates in synthesis. Top: previous work on oxidative N-N N N N N coupling to pyrazoles (top), and intermolecular diimination of Et N Et Et alkynes (this work, bottom). N Me Me Me c b b 3m (20%) 3n (71%) 3o (70%) Addition of C-nitrosos to Diazatitanacyclohexadienes. a Conditions: 1 (0.2 mmol), 2a-o (0.2 mmol, 1 equiv.), 3 mL C6H6. Isolated Addition of PhNO (2a) to diazatitanacycle 1 resulted in rapid, yields. bMixture of imine/enamine tautomers; see SI for ratios. cNot 1 near-quantitative formation of α-diimine 3a (81% by H isolated, 1H NMR yield vs. 1,3,5-trimethoxybenzene internal standard. NMR) with the concomitant formation of p-tolunitrile (Figure 3). It is proposed that this occurs through [4+2]-cycloaddition The scope of the C-nitroso reactants examined is shown in of PhNO to the Ti and γ-carbon of the ligand backbone, Table 1. Conveniently, nitrosoarenes can be readily prepared followed by [4+2]-retrocycloaddition to eliminate p- from the corresponding aniline via Oxone® oxidation.47 In tolunitrile, and a second cycloreversion to afford 3a and a most cases, the α-diimine products were obtained in Ti=O species (vide infra). excellent isolated yields (70% to 85%). Use of sterically demanding nitrosos 2b and 2c resulted in good yields of 3b and 3c (77% and 76%), which are commonly used as bulky α- diimine substituents.3 This method tolerates both electron- poor (3d-3f) and electron-rich (3g-3i) para-substituted (3d- species from competing side reactions also formed that were 3i) nitrosoarenes, as well as ortho-substituted (3j-3l) difficult to separate from the desired product given the nitrosoarenes (3l is formed in a 6:1 ratio with its enamine instability of diimines towards hydrolysis: pyrrole (D, 5%) tautomer). The reaction also proceeds cleanly with aliphatic from insertion of a second equivalent of alkyne;27 imine from nitrosos: 2-methyl-2-nitrosopropane 2n gives a mixture of the hydroamination of 3-hexyne (E, 6%); and azobenzene (F, tert-butyl substituted α-diimine 3n and its enamine tautomer 12%) from direct metathesis of PhNO with leftover Ti in a 3.75:1 ratio (71%). Similarly, 1-nitrosoadamantane 2o imido.48-49 Using excess MeCN (10 equiv.) suppressed these afforded α-diimine 3o and its enamine tautomer in a 3.13:1 competing side reactions (Figure 4B) and leads to cleaner ratio (70%).
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