Nucleophilic carbenes structural studies

Fischer-type complexes such as 1 were first prepared in 1964 and their chemical properties studied [1], Schrock-type nucleophilic complexes such as 2 were prepared later [2], They are formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7i-acccptcr ligand to metals of high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and the structures may be considered as ylides (eq. 8.1)... 

In cooperation with Teles and colleagues, our research group has studied the triazole heterocycle as an alternative core structure of nucleophilic carbenes. First, the triazol-5-ylidene 12 (Fig. 9.3 see also Scheme 9.2) was synthesized and shown to be stable at temperatures up to 150 °C in the absence of air and moisture [22]. Compound 12 exhibited the typical behavior of a nucleophilic N-heterocyclic car-bene, and was found to be sufficiently stable to become the first commercially available carbene [23]. As shown in Scheme 9.2, the crystalline carbene was obtained from the corresponding triazolium salt precursor 13 by the addition of methanolate and subsequent thermal decomposition of the adduct 14 in vacuo via a-elimination of methanol [24]. 

I have been pursuing enzyme mimics, artificial enzymes that perform biomimetic chemistry, since starting my independent career in 1956. In the first work [52-59] my co-workers and I studied models for the function of thiamine pyrophosphate 1 as a coenzyme in enzymes such as carboxylase. We discovered the mechanism by which it acts, by forming an anion 2 that we also described as a stabilized carbene, one of its resonance forms. We examined the related anions from imidazolium cations and oxazolium cations, which produce anions 3 and 4 that can also be described as nucleophilic carbenes. We were able to explain the structure-activity relationships in this series, and the reasons why the thiazolium ring is best suited to act as a biological... 

Reactions of carbene complexes with phosphorous compounds are topics of current research.164-167 (l-Alkynyl)carbene complexes (CO)5M= C(OEt) —C=CR 1 (M = Cr, W R = Ph, r-Bu, SiMe3) readily add tertiary phosphanes PPhR2 (R = Ph, Me) and form stable zwitterionic phosphonium allenide complexes (CO)5M —C(OEt)=C=C(R)—P+PhR2 145, which have been characterized by X-ray diffraction (Scheme 57).55 In previous studies on Michael-type additions of nitrogen and oxygen nucleophiles to (l-alkynyl)carbene complexes 1, zwitterionic allenide complexes of type 145 have been postulated as intermediates.21,23,503 62 112,168,169 Unambiguous structural evidence of the tendency to form such intermediates is given by isolation of a stable zwitterionic adduct, such as compound 145. Zwitterionic... 

C2a-carbanion or enamine as suggested 35 years ago by Breslow. After considerable research by many groups " , the true structure of the former is still unknown, as its existence is too fleeting for direct detection. Suffice it to say that more recent kinetic studies suggest that it is formed with a pA" near 17-19 and its behavior is consistent with a localized carbanion structure similar to that of cyanide ion. While there were some reports favoring a carbene °, rather than nucleophilic reactivity for the C2-carbanion vis-a-vis electrophiles, more recently there was evidence presented reconfirming that the reactivity of this C2-carbanion is best reflected by nucleophilic pathways, and the mechanistic probes developed provided evidence inconsistent with carbene insertion into C—H single bonds or addition across C=C double bonds ... 

Structural Isomerization Reactions.—In earlier work Kropp and his co-workers19 suggested that the unsaturated and cyclopropane products formed by the irradiation of olefins in solvents of low nucleophilicity were produced by initial rearrangement to carbenes. In a later study Kropp and Fields20 have demonstrated that carbene intermediates are indeed formed in the irradiation of olefins... 

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