Transition Metal Olefination Reagents

Sommelet-Hauser product 678. Small amounts of the para rearrangement product (679) and the direct displacement product (680) are also present. The ammonium salt precursor to the ylid (674) can generate either ylid (675 or 681), hut formation of 681 is sterically inhibited relative to 675. The Stevens product of 681 (amine 682) is more sterically crowded than 676 or 677. In polar aprotic solvents the major product is the Sommelet-Hauser product 678, hut in nonpolar solvents (hexane) the Stevens product 676 predominates. In DMSO, the enhanced nucleophilicity of the base causes the displacement product 680 (base attacks the methyl carbon and displaces the amine) to be formed in high yield. Alkoxide bases are not strong enough to generate the ylid from 674, and the displacement reaction (to give 680) dominates in those cases. 

It is noted that a nitrogen ylid has been invoked in the Hofmann elimination reaction (sec. 2.9.C.i). 4 This ylid mechanism is highly questionable for many substrates that proceed via 3-elimination with a coordinated hydroxide.In those cases, removal of the hydrogen directly by hydroxide appears more likely. The ylid mechanism probably operates when PhLi is used as a base and when there is steric inhibition of the usual (3-elimination process. 

There are other organometallic derivatives that give Wittig-type olefination reactions, with titanium derivatives being the most commonly used. The initial addition reaction to the carbonyl gives a transient metal-ated alcohol that leads to an alkene. An example is the Tebbe reagent, which exists as a bridged methylene species (see 683), where Cp is cyclopentadienyl. 9 The aluminum species can be varied to include chloride 

Chapter 8. Nucleophilic Species That Form Carbon-Carbon Bonds 

The Tebbe reagent is quite useful in that it reacts with the carbonyl of esters to give vinyl ethers. Pine showed that 683 reacted with methyl benzoate to give an 81% yield of 1-methoxy-l-phenylethene. O Similar reaction of lactone 686 with 683 gave the exo-methylene vinyl ether 687 in Holmes synthesis of (+)-laurencin.571 The Tebbe reagent also reacts with conjugated esters via 1,2-addition to give the vinyl ether.  

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Zinc compounds have recently been used as pre-catalysts for the polymerization of lactides and the co-polymerization of epoxides and carbon dioxide (see Sections 2.06.8-2.06.12). The active catalysts in these reactions are not organozinc compounds, but their protonolyzed products. A few well-defined organozinc compounds, however, have been used as co-catalysts and chain-transfer reagents in the transition metal-catalyzed polymerization of olefins. 

The transition metal-catalyzed cyclopropanation of alkenes with diazomethane is a valuable alternative to Simmons-Smith methodology [645]. Because of the mild reaction conditions under which this reaction takes place, diazomethane is the reagent of choice if sensitive olefins are to be cyclopropanated [646-648]. 

A series of reagents have been developed which are prepared in situ from a geminal dihalide or a dithioacetal [635,730] and a transition metal complex. Titanium-based reagents of this type olefinate a broad range of carbonyl compounds, including carboxylic acid derivatives (Table 3.12), and are a practical alternative to the use of isolated carbene complexes. 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

The Wittig reaction efficiently olefinates aldehydes and ketones, but not esters or amides. Several early-transition-metal approaches have been taken to this problem. Recently, Takeshi Takeda of the Tokyo University of Agriculture and Technology reported (Tetrahedron Lett. 44 5571,2003) that the titanocene reagent can effect the condensation of an amide 10 with a thioacetal 11 to give the enamine 12. On hydrolysis, 12 is converted into the ketone 13. When the reaction is intramolecular, reduction proceeds all the way, to give the pyrrolidine IS. 

A recent review of olefin metathesis is found in J. Tsuji, Transition Metal Reagents and Catalysts, Innovations in Organic Synthesis, Wiley, New York, 2000, Chapter 8, pp. 305-322. An excellent discussion of RCM is found in R. H. Grubbs and S. Chang, Recent Advances in Olefin Metathesis and Its Applications in Organic Synthesis, Tetrahedron, 1998, 54, 4413-4450. 

As discussed in connection with olefin-coupling reactions and shown in Fig. 4, the coupling of vinyl Grignard reagents is stereospecific and dependent upon the transition metal catalyst used (32, 33). The dimerization of ethylene, shown in Fig. 6, was also shown to produce primarily the terminal olefin 1-butene (35). The size of the metal has also been shown to influence the course of the catalyzed oligomerization reactions of butadiene. When bis-(ir-allyl) metal complexes are used as... 

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