MeCAAC=N−: A Cyclic (Alkyl)(Amino)Carbene Imino Ligand

Abstract A cyclic (alkyl)(amino)carbene (CAAC) has been shown to react with a covalent azide similar to the Staudinger reaction. The reaction of MeCAAC with trimethylsilyl azide afforded the N‐silylated 2‐iminopyrrolidine (MeCAAC=NSiMe3), which was fully characterized. This compound undergoes hydrolysis to afford the 2‐iminopyrrolidine and trimethylsiloxane which co‐crystallize as a hydrogen‐bonded adduct. The N‐silylated 2‐iminopyrrolidine was used to transfer the novel pyrrolidine‐2‐iminato ligand onto both main‐group and transition‐metal centers. The reaction of the tetrabromodiborane bis(dimethyl sulfide) adduct with two equivalents of MeCAAC=NSiMe3 afforded the disubstituted diborane. The reaction of MeCAAC=NSiMe3 with TiCl4 and CpTiCl3 afforded MeCAAC=NTiCl3 and MeCAAC=NTiCl2Cp, respectively.


Introduction
In 1919, Staudinger and Meyer first reported their seminal work on the reaction of triaryl phosphanes with covalent azides to form phosphinimides. [1] Since that time, the Staudinger reactionh as played important roles in organic chemistry and led to the development of the Staudinger ligation, which is used extensively in chemical biology. [2] In certain instances, an intermediate in the Staudinger reaction, ap hosphazide ( Figure 1), can be isolated. [3] Similarly,N -heterocyclicc arbenes have been shown to react with covalent azides to form stable triazenes, [4] which couldt hen be thermally decomposed to the imidazole imides. Phosphorane-iminato (phosphinimide) ligands have been widely used in both transition-metal complexes [5] and main-group compounds. [6] Similar to phosphorane iminato ligands, imidazolin-2-iminato ligands [7] have been used in aw ide variety of transition metal complexes. [8] These monoanionic ligands( ImN À )a re highly basic and can act as a2 s-, 2p-o ra s2 s-, 4p-electron donors. N-heterocyclic olefins (NHOs) share similar strong electron donating properties and have seen aresurgence in recent years. [9] Imidazolin-2-iminato ligandsh ave recently been used in fblock chemistry [10] andt he resulting complexes have shownr e-markable catalytic activity. [11] The ligands have also started to find use in stabilizing av ariety of interesting main-group species. For example, due to their strong p-donor and s-donor properties, these ligandsw ere able to stabilizea nd allow isolation of as inglet phosphinonitrene. [12] More recently,t his ligand type has been used to obtain av ariety of unusual germanium, [13] aluminum, [14] silicon, [15] tin, [16] and other compounds. [17] Several imidazolin-2-iminato-supported boron compounds have been shown to act as catalysts for the metal-free dehydrogenation of amine-boranes. [18] Very recently,I noue and coworkers alsou sed ab ulky imidazolin-2-iminato ligand to stabilize ab orasilene, [19] and Dielmann and co-workersw ere able to isolatea nd characterize three-coordinate phosphorus dications. [20] This class of ligands has also been used in the synthesis of superbasic phosphines, which were subsequently shown to split CO 2 and activate SF 6 . [21] Cyclic (alkyl)(amino)carbenes (CAACs) are stronger s-donors and p-acceptorst han N-heterocyclic carbenes (NHCs). [22] They have been used to stabilize aw ide range of highly reactive speciesa nd have been shown to be useful ligands for transition-metal catalysis. [23] Despite their extensive use in maingroup chemistry,r eactions between CAACs and covalent azides have not been reported and we were unable to find exampleso ft he pyrrolidine-2-iminato ligand in main-group or transition-metal chemistry.I nl ight of the success of andi nterest in imidazolin-2-iminato ligands, we sought to explore the possibility of synthesizingapyrrolidine-2-iminato transfer reagent and its use as al igand in both main-group and transition-metal chemistry.
Herein, we report the facile room temperature reactiono f Me CAAC with Me 3 SiN 3 ,w hich readily affords Me CAAC=NSiMe 3 under mild conditions. In simple procedures, the anionic pyrrolidine-2-iminato ligand derived therefrom was subsequently bound to the boron and titaniumc enters of an umber of compounds, and these were characterizedb yN MR spectroscopy and X-ray crystallography.

Results and Discussion
The N-silylated 2-iminopyrrolidine Me CAAC=NSiMe 3 (1)w as synthesized by the reaction of Me CAAC with TMSN 3 (trimethylsilyl azide) at room temperature with the loss of N 2 (Scheme 1).
Evaporation of the solvent afforded pure 1 as ah ighly crystalline, colorless solid in essentially quantitative yield, with no discernable side-products. An excess of TMSN 3 helped ensure that the reaction proceeded to completion and did not result in any byproducts. In contrast to N-heterocyclic carbenes, which generallyrequireh igh temperatures and long reactiontimes to convertt he triazene to the imine, [8] the imine 1 is formed rapidly with minimal heating. At room temperature, the mixture of Me CAAC and TMSN 3 in benzene develops as light shade of yellow,w hich dissipates after the reactioni sc omplete. This is corroborated by the observation of new weak signals in the 1 HNMR spectrum of the mixture, which disappear after the reaction is complete and suggest the formationo ft he triazene intermediate. Therefore, Me CAAC=N 3 ÀSiMe 3 remains elusive at room temperature (RT). The formation of dinitrogen was observed by 14 NNMR spectroscopy.
The crystalso f3 are highly soluble in CDCl 3 and their 1 HNMR spectrum contains abroad signalat6.2 ppm, which integrates to six relative to the dimeric unit, suggestingt hat the iminium and water protons are in rapid exchange with the azide anions in solutiona tr oomt emperature. The imine 13 CNMR signal is at 175.7 ppm, which is downfield relative to 1 (167.1 ppm).
In order to synthesize ad iiminodiborane, Me CAAC=NSiMe 3 was mixed with half an equivalent of B 2 Br 4 ·(SMe 2 ) 2 in C 6 D 6 .T he reactions olution was stirred at room temperature and monitored by 1 Ha nd 11 BNMR spectroscopy,w hich indicated that no reaction occurreda tr oom temperature even after three days. However,a fter heating the reactions olution at 80 8Cf or 48 h, 1 HNMR spectroscopy showed the formationo fT MSBr (0.30 ppm),w hereas 11 BNMR spectroscopy provided an ew signala td = 27.9 ppm and the signal attributable to the starting material, B 2 Br 4 ·(SMe 2 ) 2 (d = À0.3 ppm), had disappeared. This suggestedt hat the reactionh ad proceededt oc ompletion (Scheme 3).