Carbenes unsaturated

Methylthio(chloro)carbene has been generated under phase transfer conditions from dichloromethyl methyl sulfide. This carbene cyclopropanates tetramethyl-ethylene in 43%yield, accompanied by a 24%yield of l,2-dichloro-l,2-Z /5-methyl-thioethylene [24]. 

A number of unsaturated carbenes have been generated under phase transfer conditions. The most extensive work has been reported for dime thy Ivinylidene carbene. Dimethylvinylidene carbene has been generated by base catalyzed 7-elimination of HCl from 3-chloro-3-methyl-l-butyne and by a-elimination of HBr from l-bromo-3-methyl-1,2-butadiene (see Eq. 4.13). Both quaternary alkylammonium salts and various crown ethers have proved to be effective catalysts for this reaction. Yields of dimethylvinylidenecyclopropane products obtained under phase transfer catalytic 

QH NaOH or crown Table 4.5. Addition of dimethylvinylidene carbene to olefins  

Number Alkene of carbons Catalyst % Yield Carbene Refer-precursor ences  

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An interesting class ot covalent Inflates are vin l and ar>/ or heteroaryl Inflates Vinyl inflates are used for the direct solvolytic generation of vinyl cations and for the generation of unsaturated carbenes via the a-elimination process [66] A triflate ester of 2-hydroxypyridine can be used as a catalyst for the acylation of aromatic compounds with carboxylic acids [109] (equation 55)... 

Scheme 9 Saturated and unsaturated carbenes, silylenes and germylenes...

Scheme 4 Access to various a,/ -unsaturated carbene complexes from alkynylcarbene complexes 23. A 1,3-Dipolar cycloaddition. B Diels-Alder reaction. C Ene reaction. D [2+2] Cycloaddition. E Michael-type addition followed by cyclization. F Michael-type additions...

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

The Diels-Alder reaction of activated olefins is considered as one of the most useful and predictable reactions in organic synthesis. The electron-acceptor character of the pentacarbonylmetal fragment makes a,/J-unsaturated carbene complexes ideal substrates for the [4S+2C] cycloaddition reaction with dienes. 

The formal [3+2+1]-cycloaddition involves an a,ft-unsaturated carbene ligand (C3-synthon),an alkyne (C2-synthon) and a carbonyl ligand (Cl-synthon) and takes place within the coordination sphere of the chromium(O), which acts as a metal template (Scheme 2). 

More recently, such vinyl cations generated by the alkaline decomposition of 3-nitroso-2-oxazolidones have been trapped by halogens to give vinyl halides as products (108). It has been suggested that unsaturated carbenes, RjC=C , may be the intermediates in the basic decomposition of 132 (109). Indeed, when 132 (Ri=R2=CH3, R3=H) was treated with lithium ethoxide in the... 

X = Br, in 50% aqueous ethanol. The observed solvent w =. 44 value for the allenyl system is comparable to the. 455 m value of the allylic system. No products were observed, as neither the expected propargyl alcohol nor acrolein was stable under the reaction conditions. In analogy with the solvolysis of trisubstituted haloallenes (203, 204) these results were interpreted in terms of an SnI mechanism and ionization to an allenyl cation. However, an alternative mechanism involving the unsaturated carbene, C=C=C , cannot be completely ruled out in the case of the parent system. Such a mechanism has been unambiguously established by a number of investigators (206-209) for the solvolysis of R2C=C=CHX or HC C—C(R)2X in aqueous solvents in the presence of a variety of bases. 

REACTIONS OF OsHCI(CO)(P Pr3)2 WITH ALKYNOLS FORMATION OF a,p-UNSATURATED CARBENE COMPLEXES... 

More recently, Schrock has reported the formation of coordinatively unsaturated Ta and W carbyne complexes (124). Like unsaturated carbene complexes, these carbyne compounds are now established as being active intermediates in a number of catalytic reactions. The discovery of acetylene metathesis reactions catalyzed by carbyne complexes (3), for example, has generated considerable interest in this class of compound. 

Complexes ligated by the unsaturated carbene at the top right of Figure 2, catalyze the reaction of aryl chlorides with a variety of amines, including primary amines at 100°C and the reactions of aryl... 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

Cyclopropanation of l,3-dienes. a,0-Unsaturated carbenes can undergo [4 + 2]cycloaddition with 1,3-dienes (12, 134), but they can also transfer the carbene ligand to an isolated double bond to form cyclopropanes. Exclusive cyclopropanation of a 1,3-diene is observed in the reaction of the a,(3-unsaturated chromium carbene 1 with the diene 2, which results in a frans-divinylcyclopropane (3) and a seven-membered silyl enol ether (4), which can be formed from 3 by a Cope rearrangement. However, the tungsten carbene corresponding to 1 undergoes exclusive [4 + 2]cycIoaddition with the diene 2. 

In the laboratory 254 has inter alia been obtained in an argon matrix by irradiation of diazopropyne (253) at 10 K [106], Matrix isolation ESR and IR studies and ab initio calculations show it to be a 1,3-diradical with C2 symmetry, as indicated in Scheme 5.39 [107]. Irradiation with shorter wavelength light induces a 1,3-hydrogen shift by which the triplet propynylidene 254 is converted into the singlet propadienyli-dene 255, the parent system of unsaturated carbenes such as 251 (Scheme 5.39) [108]. 

Addition of electrophiles to diynyl complexes is expected to occur at either or C, the latter being favored if sterically demanding ligands shielding C and are present. The products are butatrienylidenes and the chemistry of these species is closely related to the chemistry of the related unsaturated carbene ligands (Section VIILB). " ... 

Telluro ketene acetals are also accessible by the insertion of unsaturated carbenes into Te-Te bond. 

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

Going one step beyond, the reaction of these n-donor-substituted Group 6 allenylidenes with bifunctional N,N- or W, 5-dinucleophiles opened up a fruitful route for the synthesis of an extensive family of N- or 5-heterocyclic carbenes. Thus, treatment of complex [Cr =C=C=C(NMe2)Ph (CO)5] with benzamidine, guanidine or thioacetamide has been reported to yield the a,(3-unsaturated carbenes 54 (Scheme 16) [62], arising from nitrogen attack at Cy, subsequent HNMe2... 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

The electronic structure of these carbenes was investigated by early theoretical studies [36,40-48] to elucidate the reasons for the surprising stability, which came to different conclusions concerning the importance of the sta-bihzing effect of the /r-delocalization. While early studies predicted that the C-N 7r-interaction does not play a major role [33], others found that the pM population at the carbene carbon atom is 30% higher for the unsaturated case, indicating that cychc delocalization is clearly enhanced in the unsaturated carbene [48]. 

Preparation of allenic acetals from unsaturated carbenes [137]. 

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