Cyclic aminocarbene complexes

The reactions of TpRuCl(COD) with the bidentate ligands 2-aminopyridine (apy), 2-amino-4-picoKne (apic) and 2-(methylamino)pyridine (mapy) in the presence of terminal alkynes HC=CR (R = Ph, "Bu, CeHg) afford the cyclic aminocarbene complexes TpRuCl = C(CH2R)apy, TpRuCl = C(CH2R)apic) and TpRuCl = C(CH2R)mapy. This reaction proceeds most likely via the intermediacy of both a reactive TpRu complex containing a strained, and thus labile, k -KN -coordinated aminopyridine Ugand and a vinylidene intermediate. 

Pyridine-based aldimines and aminals were shown to act as chelating ligands towards the RuTpCl fragment giving K -7V,A -coordinated cyclic imine complexes which rearrange into cyclic aminocarbene complexes. TpRu(COD)Cl readily reacts with the imines py-N = CHR (R = Ph, -Ph-OMe, Np) at elevated temperatures to yield the aminocarbene complexes TpRu( = C(R)NH-py)Cl, which is a new synthetic route for obtaining carbene complexes. The mechanism of this novel imine-aminocarbene conversion was analyzed by DFT/B3LYP calculations. 

The chemistry of iron vinylidene complexes is dominated by the electrophilicity of the carbon atom adjacent to the iron organometallic unit. While addition of water leads to an acyl complex (i.e., the reverse of the dehydration shown in equation 10), addition of an alcohol leads to a vinyl ether complex. Similarly, other iron vinyl complexes can be prepared by the addition of thiolate, hydride, or an organocuprate (Scheme 33). " The nucleophilic addition of imines gave enaminoiron intermediates that could be further elaborated into cyclic aminocarbenes. This methodology has been used to provide access to /3-lactams and ultimately penicillin analogs, and good diastereoselectivities were observed (6 1-15 1) (Scheme 34). 04 Iso, vinylidene complexes are intermediates in cyclizations of alkynyl irons with substituted ketenes, acid chlorides, and related electrophiles an example is shown (equation 11). These cyclizations led to the formation of a series of isolable and characterizable cyclic vinyl iron complexes. 

Cyclic aminocarbene thioether chelate complexes containing Cr-5 or W-S bonds arise in complicated reactions of... 

Electrochemical measurements permit also a quantitative comparison of two types of Fischer carbene complexes with different stabilizing heteroatom the aminocarbene complexes of Cr and W (data from Fig. 48.1 Table 48.1) and some analogous alkoxycarbene complexes of Cr and W (data published in Reference 26), whose reduction potentials were measured by cyclic voltammetry under similar conditions (in nonaqueous acetonitrile vs SCE). 

A series of new metal aminocarbene-substituted Fp complexes have also been prepared starting from the above ethoxycarbene complex. Some examples of these binuclear aminocarbene complexes are shown in Scheme 38. The complexes were characterized by analytical and spectroscopic methods. Cyclic voltammetry was also used to investigate these binuclear aminocarbene compounds, and two complexes were structurally characterized by X-ray crystallography. 

Chromium aminocarbenes [39] are readily available from the reaction of K2Cr(CO)5 with iminium chlorides [40] or amides and trimethylsilyl chloride [41]. Those from formamides (H on carbene carbon) readily underwent photoreaction with a variety of imines to produce /J-lactams, while those having R-groups (e.g.,Me) on the carbene carbon produced little or no /J-lactam products [13]. The dibenzylaminocarbene complex underwent reaction with high diastereoselectivity (Table 4). As previously observed, cyclic, optically active imines produced /J-lactams with high enantioselectivity, while acyclic, optically active imines induced little asymmetry. An intramolecular version produced an unusual anti-Bredt lactam rather than the expected /J-lactam (Eq. 8) [44]. 

Recent research on aminocarbenes has led to the development of a very fruitful field. The synthesis of relevant complexes (Scheme 19) such as aminobis(yhde) carbene species (69) [147], cyclic C-amino P-ylides (70) (easily transformed into carbenes) [148] and their corresponding complexes (71) [149], and special ylides (72), which also transform very easily into carbenes by loss of pyridinium group, has been reported. Emphasis has been made on the transformation between ylides and carbenes and on the donor properties of the ylides. From the results obtained the ylides have shown a stronger a-donor behavior compared with the carbenes. 

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