Reactions of Imines with Azazirconacyclobutenes and Generation of Electron-Deficient Imidozirconocene Complexes.

Imidozirconocene complexes react with alkynes via a [2 + 2] cycloaddition to generate highly colored azazirconacyclobutenes (1).1 We previously reported that compound 1a can then react with benzaldehyde to generate the new six-membered ring zirconacycle 2. Upon heating, this complex undergoes a retro-[4 + 2] cycloaddition to afford the α,β-unsaturated imine 3a2 and the (Cp2ZrO)n oligomer (Scheme 1).3 Recently, we became interested in related chemistry employing an imine for the metallacycle insertion reaction. A similar retro-[4 + 2] cycloaddition from the resulting zircona-cycle would afford an α,β-unsaturated imine and generate a new imidozirconocene species. We present herein our efforts to develop this imine insertion chemistry and its utility in synthesizing α,β-unsaturated imines and several new zirconium imido complexes. The imido complexes are unusual in that they bear strongly electron withdrawing groups at the nitrogen atom. Scheme 1 Azazirconacyclobutene 1a was chosen for screening purposes, because it demonstrated the fastest and cleanest aldehyde insertion reactions. Further, the resultant α,β-unsaturated imine 3a is a known compound, identified easily by 1H NMR spectroscopy.1 No reaction was observed between 1a and a series of N-arylbenzaldimine derivatives, independent of the steric and electronic properties of the N-aryl group. Upon encountering these initial difficulties, we reasoned that several fundamental differences between aldehydes and imines could be preventing the desired analogous reactivity. This observation motivated us to attempt the insertion of N-sulfonylimines 4a-c into the zirconium–carbon bond in 1a. These electron-deficient imines display less steric hindrance about nitrogen, exhibit far greater electrophilicity at the C=N carbon, and feature two oxygen atoms in the N-protective group that could themselves coordinate to zirconium. This hypothesis was confirmed on observing that the N-tosyl benzaldimine 4a4 reacted with zirconacycle 1a at 45 °C to afford the expanded metallacycle 5; a color change from dark blue to yellow accompanied this transformation. The structural assignment of 5 was based on 1H NMR correlation with 2; formation of both metallacycles causes the two Cp rings and the two methyl groups on the aniline moiety to become diastereotopic. We were pleased to find that heating 5 for 12 h at 105 °C yielded the α,β-unsaturated imine 3a in quantitative yield (Scheme 2). Related tosyl imines 4b,c reacted similarly to 4a and provided the expected α,β-unsaturated imines 7 and 8 in excellent yields (Table 1). Interestingly, the qualitative rate of imine Zr–C insertion was faster for more electron-deficient imines, such that rate4b > rate4a > rate4c; however, the rate of α,β-unsaturated imine extrusion from the corresponding expanded zirconacycles displayed the opposite trend, in which rate4b < rate4a < rate4c. Scheme 2 Table 1 Temperatures and Yields for Insertion and Extrusion Reactions Extrusion of α,β-unsaturated imines 3a, 7, and 8 should be accompanied by generation of the previously unstudied N-tosyl imido complex 6.5 We were unable to detect this imido species by NMR but observed a white semicrystalline precipitate in the reaction vessel. This solid failed to dissolve in any solvents typically compatible with zirconium complexes, and no catalytic turnover was observed when the reaction was conducted in the presence of excess imine and 1-phenyl-1-propyne. Insertion of N-(p-bromobenzenesulfonyl)benzaldimine (10),6 however, led to crystallization of the p-bromobenzenesulfonyl imido dimer 11 directly from the reaction mixture.7 An X-ray diffraction study carried out on this material showed that one sulfonyl oxygen of each monomeric unit is coordinated to each zirconium in this dimeric complex to generate an 18-electron pentacoordinate compound (Figure 1). The connectivity in this structure suggests that the sulfonyl protecting group in the starting imine may coordinate to the zirconium center to facilitate insertion but also indicates that subsequent insertion of alkyne to generate a new azazirconacyclobutene will be difficult energetically. Even highly sterically hindered N-protecting groups, such as (2,4,6-triisopropylphenyl)sulfonyl, failed to prevent dimerization of the N-sulfonyl imidozirconocene monomer. Figure 1 ORTEP and line drawings of the N-sulfonyl imido dimer 11. We next considered the case of N-sulfinyl imines for the insertion/extrusion sequence. This functional group possesses only one oxygen atom per sulfur atom, retaining the sulfonyl’s ability to coordinate to zirconium while potentially decreasing the propensity of the resulting N-sulfinyl imidozirconocene complex to dimerize. N-tert-Butylsulfinylbenzaldimine (12)8 inserted into the zirconium–carbon bond of metallacycle 1a at 105 °C. The expanded zirconacycle subsequently extruded imine 3a at 135 °C. Unfortunately, no evidence of the newly formed sulfinyl imido complex 13 was observed by 1H NMR spectroscopy, and no corresponding precipitation from the reaction mixture was noted. The starting imine was unstable above 115 °C, suggesting that the sulfinyl functional group might not survive the 135 °C temperature required for the retro-[4 + 2] cycloaddition.9 In a further exploration of N-substituents with main-group oxygen donors, we decided to pursue N-phosphinyl imines as substrates; phosphine oxides have been shown to coordinate reversibly to imidozirconocene complexes.1 N-(Diphenylphosphinyl)benzaldimine (14)10 reacted with zirconacycle 1a at room temperature to generate the expanded metallacycle 15. A crystal structure of the intermediate confirmed its six-membered-ring structure and showed that the phosphinyl oxygen is coordinated to zirconium (Figure 2a). Heating 15 to 135 °C led to extrusion of the α,β-unsaturated imine 3a in 96% yield and generated a new species bearing a single Cp resonance in the 1H spectrum and a new resonance in the 31P{1H} spectrum. There are no reports on N-phosphinylmetal–imido complexes in the literature; thus, we were unable to compare these spectral data to those of any known compounds.11 However, we were able to grow X-ray-quality crystals of this new N-phosphinyl imidozirconocene complex (16). The X-ray structure showed that this compound is an eight-membered ring, in which dimerization has occurred through the oxygen atom of the bulky phosphinyl protecting group instead of via a [2 + 2] cycloaddition as in the N-sulfonyl imido complex. Heating this soluble imidozirconcene species in the presence of external Lewis bases, such as trimethylphosphine oxide and alkynes, failed to cleave the dimer. Figure 2 ORTEP and line drawings of (a) the expanded zirconacycle 15 and (b) the N-phosphinyl imidozirconocene dimer 16. This chemistry was shown to be general with respect to the azazirconacyclobutene. Imine 14 reacted with metallacycles 1b,c at 45 and 10 °C, respectively. Heating the corresponding zirconacycles to 145 and 135 °C generated the α,β-unsaturated imines 3b,c in excellent yields.12 Only one double-bond geometrical isomer was observed for all α,β-unsaturated imines generated; however, the product 3c was generated as a 7:4 mixture of (E)- and (Z)-imine isomers, presumably due to the increased steric bulk α to the imine. In summary, we have succeeded in developing an effective method for inserting imines into azametalla-cycle zirconium–carbon bonds and generating α,β-unsaturated imines from the resulting expanded zirconacycles in high yields. The presence of an oxygen-bearing substituent on nitrogen was critical in effecting the reaction between imine and azazirconacyclobutene. However, the role of the oxygen substituent in effecting this chemistry is, at present, unknown.13 Simultaneously, we have developed reactions that access several new electron-deficient imidozirconocene complexes, two of which were shown to dimerize even in dilute solutions. Because of this dimerization process, we are not yet able to generate α,β-unsaturated imines and imidozirconocene complexes catalytically in these systems. We are continuing to explore the classes of imines best suited to reactions with azazirconacyclobutenes and to further develop the chemistry of electron-deficient imidozirconocene complexes.