Nucleophiles metal carbene synthesis

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

There is a variety of reagents that undergo carbene-related transformations that do not fit into the categories of nucleophilic and electrophilic metal carbenes described earlier. Those that are the most versatile for organic synthesis, like the Tebbe... 

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with tirne. " There are two types of transition metal carbene complexes those which have electrophilic carbene carbons and which are typified by the pentacarbonylchro-mium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as carbene and alkylidene complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume. ... 

This chapter will focus on the nucleophilic addition reactions of transiton metal carbene and carbyne complexes with Grignard reagents. The synthesis and some general reactions of these carbene and carbyne complexes will be presented. A more detailed description of the chemistry of these complexes can be found in the literature [1]. This chapter, although not exhaustive, is descriptive of the prototypical nucleophilic addition reactions of metal-carbon (M-C) multiple bonds with Grignard reagents. 

Various elementary processes such as oxidative addition, reductive elimination, olefin and CO insertion into the metal-to-carbon bond have found extensive applications in organic synthesis. Other processes such as attack of nucleophiles on metal-bound CO and olefins, unique reactions of metal carbene complexes, and a-bond metatheses are among the topics of special interest to organometalhc chemists as well as to synthetic organic chemists. Our aim is to provide the reader with detailed accounts of elementary processes with the hope that the information provided here is used for further development of molecular catalysis. 

In addition to the syntheses of metal-carbene complexes from compounds not containing the carbene ligand, a variety of methods for modifying the structure of carbene complexes have been developed. Three different reactive centers in a transition metal-carbene complex can be utilized in designing a synthesis of a new carbene complex (see Scheme 4). For example, (CO)s-CrC(OCH3)CH3 can be converted into a wide range of carbene complexes by (a) nucleophilic attack at the carbene carbon atom followed by loss of... 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

Metal-carbene and carbyne complexes occupy a central place in synthesis and catalysis. Metal-carbene or alkylidene eomplexes can have an eleetrophilic singlet carbene (Pettit type) or a nucleophilic triplet carbene (Sehrock type, often d ). 

In 1964, Fischer prepared and characterized unambiguously the first metal carbene complex 3 obtained by nucleophilic attack of phenyl lithium at tungsten hexacarbonyl followed by 0-alkylation. This was followed by Schrock s synthesis of a high oxidation state metal alkylidene complex 4 obtained by a-hydrogen abstraction from tris(neopentyl) tantalumfv) dichloride (Scheme 1.1)/ ... 

The manifold chiral information readily available in customary carbohydrates provides an attractive approach to a chiral modification of metal carbenes that may be applied to either metal-mediated stereoselective organic synthesis via sugar auxiliaries or to carbohydrate synthesis via organometallic methodologies [107, 108]. The sugar moiety can be incorporated into metal carbenes by well-established procedures such as nucleophilic addition to the metal-coordinated carbene carbon atom or conjugate addition to the vinylogous position in alkenyl or alkynyl car-... 

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

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