Cationic alkoxycarbenes

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

Clark and co-workers have reported reactions of Ir(III) cations with terminal alkynes in methanol in which alkoxycarbene complexes are formed (60). By analogy with a more extensively studied Pt(II) system (61), it has been concluded that cationic vinylidene complexes, e.g., 35, are reaction intermediates, e.g.,... 

Acid-catalyzed dealkoxylation is particularly suitable for the preparation of highly reactive, cationic iron(IV) carbene complexes, which can be used for the cyclopropanation of alkenes [438] (Figure 3.11). Several reagents can be used to catalyze alkoxide abstraction these include tetrafluoroboric acid [457-459], trifluoroacetic acid [443,460], gaseous hydrogen chloride [452,461], trityl salts [434], or trimethylsilyl triflate [24,104,434,441,442,460], In the case of oxidizing acids (e.g. trityl salts) hydride abstraction can compete efficiently with alkoxide abstraction and lead to the formation of alkoxycarbene complexes [178,462] (see Section 2.1.7). 

The vinylidene complexes 8 also react with nucleophiles such as water and methanol. Hydration of the unsubstituted vinylidene complex 8a leads to the formation of the rj -acetyl complex 13, whereas the corresponding reactions of 8b and 8c result in the C-C bond cleavage, giving the cationic carbonyl complex 14 and organocarbonyl products R COMe(R = OMe or Me). On the other hand, the reactions of 8 with methanol afford the methoxycarbene complex 15 and the vinyl complex 16, depending upon the substituent of the vinylidene complexes 8. Intramolecular nucleophilic attack takes place in the reaction of 4 with 3-butyn-l-ol, giving the cyclic alkoxycarbene complex [Cp RuCl(p-SPr%Ru =C(CH2)30 Cp ](OTf). ... 

Alkylation of iron acyl complexes also provides access to iron carbenes. Thus, the neutral iron acyl complex will react with acid, with alkylating agents, or with trifluoromethanesul-fonic anhydride to afford cationic hydroxy- or alkoxycarbene complexes (52) and (53) or the cationic vinylidene complex (54, L = CO) (Scheme 20). The vinylidene complex can be used to prepare a more substituted analog of (51) by treatment with a thiol. The enantiomerically pure iron acyl complex (/ )-(45a) can be converted to the corresponding enantiopure methoxycarbene complex with Me30BF4 as well.7 Finally,... 

In O-deuterated methanol, no labeled product was obtained, showing that the possible mechanism involves either the formation of an ethenyl cation (9.113) rearranging to an aryl cation (9.114) or the formation of a )ff-alkoxycarbene (9.115) that rearranges to an arene. More detailed information on this unusual reaction can hardly be given. 

C-0 bond in acetals is also cleaved by a cationic Ru complex to give an alkoxycarbene complex. This reaction is explained by the initial coordination of... 

Many carbene complexes undergo nucleophilic attack at the carbene carbon. This chemistry is presented in more detail in Chapter 13 on metal-ligand multiple bonds. In brief, cationic carbene complexes tend to imdergo simple addition processes to generate neutral products (Equation 11.8), whereas neutral complexes can react by either addition (Equation 11.9) or by a sequence of addition and elimination reactions (Equation 11.10). Fischer has shown that the aminolysis of alkoxycarbene complexes occurs by initial attack at the carbene carbon by a mechanism similar to that for the aminolysis of organic esters. Although less studied than reactions of carbenes with nucleophiles, reactions of carbyne complexes with nucleophiles are also known, and these reactions generate carbene complexes. ... 

The alkoxycarbene complex, [FeGp (G0)2 =G(0Me)Me ][GF3S03], undergoes substitution of one or two carbonyl ligands to produce various mono- and disubstituted methoxycarbene complexes, providing a convenient method for the generation of cationic iron-carbene complexes. 

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