Bis(imino)pyridine iron complexes for catalytic carbene transfer reactions

The bis(imino)pyridine iron complex, for the first time, is developed as an effective metal carbene catalyst for carbene transfer reactions of donor–acceptor diazo compounds.


Introduction
Transition-metal-catalyzed carbene transfer reactions occurring through metal carbene intermediates encompass a vast array of reactants and catalysts to achieve novel and selective strategies for organic synthesis. 1 The reactive carbenoid intermediates can be catalytically generated from diazo compounds by metalcatalyzed dinitrogen extrusion, 2 and their reactions extend from addition and insertion to cycloaddition and ylide formation. 3 Dirhodium complexes have been established as the most successful catalysts for carbene transfer reactions of diazo compounds; 4 great achievements have also been accomplished recently by copper and other precious metal catalysts (e.g. ruthenium, palladium, gold). 5 Iron, the second most abundant metal, with its particular biological relevance, is emerging as an important metal for catalytic metal carbene reactions. 6 However, iron catalysis is comparatively underdeveloped, with the enduring dominance of precious metal catalysis in metal carbene chemistry.
Since the launch of iron porphyrin-catalyzed cyclopropanation by Woo, 7 various carbene transfer processes of diazo compounds, including cyclopropanation, heteroatomhydrogen bond insertions, and intramolecular C-H inversion, have been achieved by porphyrin and related macrocyclic iron complexes; however, these generally occur with active ahydrogen-diazocarbonyl compounds, diazoalkanes, or the corresponding precursors. 8 The spiro-bisoxazoline iron complexes developed by Zhou's group have exhibited high catalytic activities and selectivities for heteroatom-hydrogen bond insertions and intramolecular cyclopropanation reactions of a-diazoesters. 9 Despite these achievements, iron has not been developed as a catalyst to the same extent as other late transition metals, particularly for usage in metal carbene reactions. The advancement of iron catalysis for general carbene transfer reactions with broad substrate schemes, especially asymmetric processes and under mild reaction conditions, remains a wideopen eld for discovery and innovation. We report here, for the rst time, bis(imino)pyridine iron complexes serving as effective catalysts for a range of metal carbene reactions under mild reaction conditions (at room temperature or 40 C), including cyclopropanation/cyclopropenation, epoxidation, Doyle-Kirmse reaction, O-H insertion, N-H insertion, and C-H insertion (Scheme 1). To the best of our knowledge, this bis(imino)pyridine iron catalyst represents the most broad-ranging catalytic activity towards metal carbene reactions of diazo compounds over the previously reported iron catalysis system. 6 The bis(imino)pyridine iron-catalyzed cyclopropanation proceeds on a wide range of aryldiazoacetates, vinyldiazoacetates, styrenes and phenylacetylene. Notably, a new chiral bis(imino)pyridine ligand derivatized from L-valine methyl ester has been synthesized, and the corresponding enantiopure, C 1 -symmetric iron catalyst enabled the asymmetric cyclopropanation of styrene and phenyl diazoacetate.
In the past decade, bis(imino)pyridine chelated iron complexes have emerged as an effective class of catalysts for ethylene polymerization, olen hydrogenation, hydrosilation, and [2p + 2p]-cycloaddition reactions. 10 Owing to its ease of preparation, the bis(imino)pyridine ligand is easily modiable, allowing versatility in ligand design, synthesis, and screening. 10a,b However, catalytic metal carbene reactions by bis(imino)pyridine iron complexes have not been achieved. Recently, Chirik reported the formation of a bis(imino)pyridine iron carbene complex B from a stoichiometric amount of bis(imino)pyridine iron dinitrogen complex A and diphenyldiazomethane by dinitrogen extrusion (Fig. 1). 11 However, the attempts towards metal carbene reactions, such as cyclopropanation and C-H insertion, were unsuccessful with this bis(imino)pyridine iron carbene complex. 11,12 We hypothesized that one reason for the lack of reactivity for bis(imino)pyridine iron carbene complex B in the carbene transfer process is due to the charge delocalization induced by the diphenyl group. To address this issue, we predicted that augmenting the electrophilicity of the disubstituted diazo compound would increase the reactivity of the corresponding iron carbene; thus, it could more readily engage in carbene transfer reactions. 2b,c It has been documented that the donoracceptor metal carbene, which can be produced from donoracceptor diazo compound by metal-catalyzed dinitrogen extrusion, exhibited higher reactivity than the one from diphenyldiazomethane due to its stronger electrophilicity. 1c,3c,4a Herein, a donor-acceptor diazo compound, aryldiazoacetate, was selected as the carbene precursor to investigate the bis(imino)pyridine iron-catalyzed metal carbene reactions. Additionally, recent computational studies of bis(imino)pyridine iron complexes for C-H functionalization of donor-acceptor diazo compound also suggest feasibility. 13 The catalytic cycle for the conversion of a diazo compound to a metal-stabilized carbene intermediate is initiated from the metal-catalyzed dinitrogen extrusion of nucleophilic diazo compound. We predicted that compared to the formal iron(0) complex A, the more electrophilic bis(imino)pyridine iron(II) complexes would exhibit higher reactivity towards the nucleophilic diazo compound and facilitate the subsequent metal carbene transfer. Therefore, we aimed to electronically and sterically tune the bis(imino)pyridine iron(II) complexes to achieve the carbene transfer reactions of the donor-acceptor diazo compound under mild reaction conditions.

Results and discussion
As a starting point, we focused on evaluating a series of bis(imino)pyridine iron(II) catalysts for the cyclopropanation reaction of styrene 2a with methyl phenyldiazoacetate 1a ( Table 1). As proposed, in the presence of 5 mol% of bis(arylimino)pyridine iron(II) dichloride complexes (entries 1 and 2), the reaction of 2a and phenyldiazoacetate 1a afforded the cyclopropanation product 3a, however, in low yields with predominately recovered starting material. To improve the catalytic activity of the iron complexes, examination of the noncoordinating counterions was performed. The employment of more electrophilic iron complexes with hexauoroantimonate (SbF 6 À ) as counterions (entries 3 and 4) led to a marked increase in yield. The combination of ( iPr PDI)FeCl 2 and NaBAr F 4 also delivered 3a with enhanced yield (48%, To reveal the imino-substituents' effect on the catalyst, iron complexes containing N-alkyl substituents were examined. However, they resulted in low catalytic activity with recovery of starting material (entries 6 and 7), which could be due to electronic and/or steric constraints from the imino-alkyl groups. Additionally, neither iron complexes of pyridine bis(oxazoline) ligands nor oxazoline iminopyridine iron complexes were effective catalysts for this transformation (entries 8 to 10). These results demonstrate the indispensability of the imino-aryl substituent in the ligand frame to conduct active iron catalysis in metal carbene reactions of donor-acceptor diazo compound.
Under the optimized condition, we investigated the scope of this bis(arylimino)pyridine iron-catalyzed cyclopropanation across a range of aryldiazoacetates and styrene derivatives ( Table 2). As indicated by entries 1 to 5, aryldiazoacetates with electron-rich, halogen para-substituents and 2-naphthyl group all reacted smoothly with styrene, generating the corresponding cyclopropanes in good yields (81-88%, 3b-3f) with excellent diastereoselectivities (dr > 20 : 1). However, no reaction occurred with the electron-decient system, even at 40 C (1g, entry 6). Reactions of aryldiazoacetates 1h and 1j bearing orthosubstituents on the aromatic ring resulted in lower yields (entries 7 and 8). We rationalize that such lower reactivity can be attributed to a higher kinetic barrier for the generation of corresponding iron carbene intermediate, which is caused by the increased steric hindrance between the ortho-substituent and the bulky bis(imino)pyridine ligand frame. The cyclopropanes 3j-3l derived from styrene derivatives 2b-2d were obtained in yields ranging from 88 to 91%, whereas moderate yield (67%, entry 12) was obtained with 4-(triuoromethyl)styrene 2e. Disubstituted styrenes, including a-phenylstyrene 2f and transb-methylstyrene 2g, were also ideal reagents for this ironcatalyzed cyclopropanation, producing products 3n and 3o in good yields.
In addition to the styrene derivatives, the reaction of 1,3cyclohexadiene and 1a was also effectively catalyzed by [( iPr PDI) Fe(CH 3 CN) 2 ](SbF 6 ) 2 , affording the cyclopropane product 3p in 80% yield with dr > 20 : 1 (eqn (1)). Furthermore, as shown in eqn (2), [( iPr PDI)Fe(CH 3 CN) 2 ](SbF 6 ) 2 catalyzed the cyclopropanation of cyclohexene, and 1a was also successfully achieved, affording the desired product 3q in 72% yield. To further probe the diazo substrate generality, vinyl-diazoacetate 1j was subjected to bis(imino)pyridine iron-catalyzed cyclopropanation with styrene (eqn (3)). Gratifyingly, the cyclopropane product 3r was obtained in 84% yield, which demonstrates the catalytic capability of bis(imino)pyridine iron for a broader scope of donor-acceptor diazo compounds. Remarkably, [( iPr PDI)Fe(CH 3 CN) 2 ](SbF 6 ) 2 was also capable of catalyzing the cyclopropenation of 1a and phenylacetylene, furnishing the product 3s in 61% yield at 40 C (eqn (4)), which has not been achieved by other reported iron catalysts. (1) With the accomplishment of achiral bis(arylimino)pyridine iron-catalyzed cyclopropanation, we have sought to modify the ligand architecture to generate a chiral iron catalyst for asymmetric cyclopropanation. Our catalyst screening (Table 1) indicated that the N-aryl substituent in bis(imino)pyridine ligand is indispensable for the effective catalytic activity of iron complexes. Guided by these experimental results and Bianchini's original design of chiral bis(imino)pyridine ligand, 15 we synthesized an enantiopure, C 1 -symmetric chiral bis(imino) pyridine ligand [(S)-VME PDI] (Scheme 2), in which one imine is "anchored" by a 2,6-diisopropylphenyl group (activating element) and the other is prepared from L-valine methyl ester (chiral element). To our delight, the asymmetric cyclopropanation reaction of 1a and styrene was successfully achieved by in situ prepared chiral iron catalyst from (S)-VME PDI, FeCl 2 , and AgSbF 6 at room temperature. The cyclopropane product 1a was isolated in 78% yield with 67% enantiomeric excess. 16 Although with moderate enantioselectivity, the success of this asymmetric cyclopropanation reaction provides a strong basis for the development of a new chiral bis(imino)pyridine iron catalyst for metal carbene reactions. Encouraged by the success of bis(arylimino)pyridine iron(II)catalyzed cyclopropanation, we then sought to examine the generality of this iron catalyst for metal carbene reactions. As depicted in Scheme 3, a range of metal carbene reactions of phenyldiazoacetate 1a, including epoxidation, Doyle-Kirmse reaction, N-H insertion, C-H insertion, and O-H insertion, were all successfully catalyzed by [( iPr PDI)Fe(CH 3 CN) 2 ](SbF 6 ) 2 . The bis(arylimino)pyridine iron-catalyzed reaction of 1a and benzaldehyde yielded the epoxide product 4 in 80% yield with excellent diastereoselectivity at room temperature (Scheme 3a). Under the same condition (Scheme 3b), allyl phenyl sulde reacted with 1a smoothly to form the Doyle-Kirmse product 5 in 91% yield. [( iPr PDI)Fe(CH 3 CN) 2 ](SbF 6 ) 2 was also able to catalyze the N-H insertion of aniline and C-H insertion of N,N-dimethylaniline, although higher reaction temperatures were required (Scheme 3c and d). Furthermore, in the presence of 5 mol% [( iPr PDI)Fe(CH 3 CN) 2 ](SbF 6 ) 2 , O-H insertion reactions of 1a with methanol, n-butanol, and water were achieved, furnishing the corresponding products 8a-8c in good to moderate yields (Scheme 3e).
As documented, bis(imino)pyridines have been recognized as radical-based, redox non-innocent ligands that can directly participate in the electronic structure of metal complexes. 10d,f,17 Chirik's study demonstrated that a carbene radical is engaged in bis(imino)pyridine iron carbene complex A, which is obtained from a formal iron(0) complex (Scheme 2). 11 Therefore, considering the redox activity of the bis(imino)pyridine ligand, radical tapping experiments were conducted to address whether a radical carbene involved in this bis(arylimino)pyridine iron(II) catalyzes carbene transfer reactions. 18 As shown in Scheme 4a, the addition of the radical scavenger TEMPO (2,2,6,6-tetramethylpiperidine N-oxide) did not harm the [( iPr PDI)Fe(CH 3 -CN) 2 ](SbF 6 ) 2 -catalyzed cyclopropanation reactions of 1a or vinyldiazoacetate 1j, and the corresponding products were isolated with similar yields to those from the reactions in the absence of TEMPO. These results reveal the unlikely involvement of the carbene radical intermediate in [( iPr PDI)Fe(CH 3 CN) 2 ](SbF 6 ) 2catalyzed cyclopropanation reactions. Moreover, the achievement of C-H insertion reaction of 1a with N,N-dimethylaniline Scheme 2 Chiral bis(imino)pyridine iron-catalyzed cyclopropanation. (Scheme 3d) implies the likely generation of donor-acceptor iron(II) carbene intermediate. 1c,6 Based on the obtained experimental results and mechanism study, we propose that the donor-acceptor diazo compound was decomposed by the bis(arylimino)pyridine iron(II) catalyst to generate an iron(II) carbene intermediate, which readily undergoes cyclopropanation of olens to afford the cyclopropane product (Scheme 4b).

Conclusions
In summary, the effective catalytic activity of bis(arylimino) pyridine iron(II) complexes for carbene transfer reactions of donor-acceptor diazo compounds has been demonstrated by a range of metal carbene transformations from cyclopropanation and insertions to ylide formation. Notably, the asymmetric cyclopropanation of methyl phenyldiazoacetate and styrene has been achieved by a new chiral iron catalyst based on the bis(imino)pyridine ligand derivatized from L-valine methyl ester. Future studies will be aimed at developing new asymmetric bis(imino)pyridine iron catalysts for highly enantioselective metal carbene reactions, as well as elucidating the mechanism of such process and the nature of the iron carbene intermediate.

Conflicts of interest
There are no conicts to declare.