Olefins cleavage of electron-rich

The access to NHC complexes is mainly based on three routes the in situ deprotonation of ligand precursors, the complexation of the free, preisolated NHCs, and the cleavage of electron-rich olefins (Scheme 5). A variety of other methods, mainly of importance in special cases, will be presented at the end of this chapter. 

In a variation of this method, a dimethylamine adduct can be used in the same way as the methanol adduct described previously [Eq. (20)]. Nickel(II) and palladium(II) complexes with allyl-substituted NHCs are accessible by this route. These compounds cannot be prepared by the cleavage of an electron-rich olefin vide infra) because of an amino Claisen rearrangement of the tetramino-substituted olefin. However, [(NHC)M(CO)4] (M = Cr or Mo) were accessible via cleavage of electron-rich olefins with [M(CO)6] as the precursors but for the very same NHC. ... 

The reaction of thiourea derivatives with a metal complex to form NHC complexes is a combination of the NHC formation from thioureas with potassium or sodium [Eq. (23)] and the cleavage of electron rich olefins. For example, a lO-S-3-tetraazapentalene derivative is cleaved by Pd(PPh3)4 and [(Ph3P)3RhCl], respectively [Eq. (35)]. Other substitution patterns in the carbene precursor, including selenium instead of sulfur can also be used. ... 

These were found to produce formamidines on treatment with isonitriles or cyanide ion. The cleavage of electron-rich olefins in the presence of [AuCl(PPh3)] also led (145, 146) directly to biscarbene complexes [Eq. (40)]. 

Reaction of triethylsilyl hydrotrfoxide with electron-rich olefins to gh/e dioxetanes that react IntrarTMlecularly with a keto group in the presence of t-txrtyidimethyl silyl triflateto afford 1,2,4 Inoxanes also oxydatnre cleavage ol alkenes Also used in cleavage ol olefins... 

The quantum yields for oxetane formation have not been determined in every case, and only a few relative rate constants are known. The reactivities of singlet and triplet states of alkyl ketones are very nearly equal in attack on electron rich olefins. 72> However, acetone singlets are about an order of magnitude more reactive in nucleophilic attack on electron-deficient olefins. 61 > Oxetane formation is competitive with a-cleavage, hydrogen abstraction and energy-transfer reactions 60 64> so the absolute rates must be reasonably high. Aryl aldehydes and ketones add to olefins with lower quantum yields, 66> and 3n-n states are particularly unreactive. 76>... 

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. ... 

Finally, an additional synthesis of recent times (1971) comes from Lappert and his group (48). They treated an electron-rich olefinic system, such as 1, l, 3,3 -tetraphenyl-2,2 -biimidazolidinylidene, with a suitable complex compound. In this manner, they attained cleavage of the C=C double bond and attachment of the carbene fragment to the metal ... 

The 2,4,6-triphenylpyrylium terafluoroborate (TPT)-sensitized electron transfer of aryl-substituted epoxides such as 250 leads to ring opening via selective C—O bond cleavage, while subsequent [3 + 2]-cycloaddition of the resultant carbonyl ylide with electron-rich olefins 251 leads to the synthesis of substituted THF derivatives 252 and 253 (Scheme 8.69) [109]. 

More recently, two types of Ru complexes were obtained by the reaction of mesityl-substituted electron-rich olefins with [RuCl2(p-cymene)]2 [27]. Cleavage of the chlorine bridges occurs first to yield the expected (NHC)(p-cymene)Ru(II) complex 9. Under harsher reaction conditions (140 °C in p-xylene) further arene displacement takes place to yield the chelated ( 6-mesityl-NHC - Ru complex 10 (Scheme 6). The olefin 8 was easily obtained by deprotonation of the corresponding dihydroimidazolium salt. 

Another example for methoxy functionalised imidazolium salts comes from the group of Cetinkaya [185,186] featuring an -alkyl tether. Cetinkaya etal.me the traditional route to transition metal carbene complexes employing the electron-rich olefins as carbene source [57-59], Thermal cleavage of the olefinic double bond in the presaice of the metal precursor complex yields the desired transition metal carbene complex (see Figure 3.66). Using this method, Cetinkaya et al. synthesised rhodium(l) [185,186] and ruthenium(ll) [185] complexes. 

Instead, only two reaction pathways are used to generate transition metal NHC complexes with thiazol-2-ylidene orbenzothiazol-2-ylidene (i) thermal cleavage of an electron-rich olefin in the presence of a suitable transition metal (see Figure 6.12) [40,41] and (ii) deprotonation of a thiazolium or benzothiazolium salt in the presence of a transition metal (see Figure 6.13) [40,42-44]. 

Figure 6.12 Thermal cleavage of an electron-rich olefin to form a transition metal benzothiazol-...

Ru(Tp)(Ph)(NCMe)CO] reacts with the electron-rich olefins, ethyl vinyl sulfide and 2,3-dihydrofuran, yielding [Ru(Tp)(CO)( x-SEt)]2 and [Ru(Tp)(CO)(NCMe)(C=CCH2CH2OH)] through a transformation that involves stoichiometric C-S and C-H/C-O bond cleavage, respectively.326 [Ru(Tp)(Me)(CO)(NCMe)] reacts with pyrrole forming [Ru(Tp)(CO) K2- V, V-(H)N=C(Me)(NC4H3) ]. Mechanistic studies indicate that the most likely reaction pathway involves metal-mediated N-H activation of pyrrole to form [Ru(Tp)(CO)(A/-pyrrolyl)(NCMe)], followed by C-C bond formation and proton transfer (Fig. 2.71).327... 

Scheme 1.24B shows the other possibility of catalytic hydrogenation of olefin driven by a transition metal monohydride. The monohydride can be sometimes obtained by cleavage of dinuclear transition metal complexes such as Co2(CO)g on treatment with dihydrogen or it can be generated by protonation of an electron rich metal complex [71]. Insertion of an olefin into the M-H bond to form an alkyl may proceed similarly to Scheme 1.24A. 

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