Alkoxycarbenes complexes

For chromium alkoxycarbene complexes the MLCT and the lowest energy LF bands overlap. Irradiation at A>385 nm led to anti-syn isomerization... 

Tungsten alkoxycarbene complexes underwent similar anti-syn rearrangements but were much less prone to undergo CO loss [5-10]. 

A few heterocyclic imines reacted poorly if at all with chromium alkoxycarbene complexes. Oxazines required the use of the more reactive (and less stable) molybdenum alkoxycarbenes, producing oxacephams in -40% yield. Oxazo-lines gave low yields (-12%) of the oxapenam system, along with similar amounts of oxazinone, resulting from incorporation of two equivalents of ketene (Eq. 3) [20]. 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

In contrast to alkoxycarbene complexes, most aminocarbene complexes appear too electron-rich to undergo photodriven reaction with olefins. By replacing aliphatic amino groups with the substantially less basic aryl amino groups, modest yields of cyclobutanones were achieved (Table 10) [63], (Table 11) [64]. Both reacted with dihydropyran to give modest yields of cyclobutanone. Thio-carbene complexes appeared to enjoy reactivity similar to that of alkoxycar-benes (Eq. 15) [59]. 

Photolysis of chromium alkoxycarbene complexes with aldehydes in the presence of Lewis acids produced /J-lactones [83]. Intermolecular reactions were slow, low-yielding, and nonstereoselective, while intramolecular reactions were more efficient (Eqs. 19 and 20). Subsequent studies showed that amines, particularly DMAP, could also catalyze this process (Table 13) [84], resulting in reasonable yields and diastereoselectivity in intermolecular cases. 

This photodriven benzannulation was used in the synthesis of indolocar-bazoles (Eq. 22) [96] and calphostins (Eq. 23) [97]. The thermal insertion of isonitriles into these same classes of carbenes provided a complementary approach to similar benzannulations [98-100]. Manganese alkoxycarbene complexes underwent both inter- [101] and intramolecular [102] photodriven benzannulation reactions with alkynes (Eqs. 24 and 25). 

Photolysis of alkoxycarbene complexes in the presence of stabilized ylides produced allenes having a donating group at one terminus and an accepting group at the other. These were highly reactive and rearranged to 1,3-dienes under mildly acidic conditions and hydrolyzed to y-keto-a,/J-unsaturated esters (Eq.31) [117]. 

The dinuclear isonitrile and alkoxycarbene complexes 6 and 7 were obtained from [Fe(CO)3( X-dppm) Tl2- j,2-Si(OMe)2(OMe))PtCl] by chloride substitution with isonitriles and 3-butyne-l-ol (or ( )-4-pentyne-2-ol), respectively [2], In these complexes, the organic ligand bound to Pt only occupies one coordination site, thus allowing the trimethoxysilyl ligand to display a T 2- i2-Si-0 bonding mode. 

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.,... 

Among the synthetically useful reactions of Fischer carbenes, the benzannulations are certainly the most prominent. In particular, the so-called Dotz reaction, first reported by Dotz in 1975 [3], is an efficient synthetic method that starting from aryl- or alkenyl-substituted alkoxycarbene complexes of chromium affords p-alkoxyphcnol derivatives by successive insertion of the alkyne and one CO ligand in an a,/Tunsaturated carbene, and subsequent electrocyclic ring closure (see Figure 1). 

Generally phenol formation is the major reaction path however, relatively minor modifications to the structure of the carbene complex, the alkyne, or the reaction conditions can dramatically alter the outcome of the reaction [7]. Depending on reaction conditions and starting reactants roughly a dozen different products have been so far isolated, in addition to phenol derivatives [7-12], In particular, there is an important difference between the products of alkyne insertion into amino or alkoxycarbene complexes. The electron richer aminocarbene complexes give indanones 8 as the major product due to failure to incorporate a carbon monoxide ligand from the metal, while the latter tend to favor phenol products 7 (see Figure 2). 

Fig. 2.5. Preparation of hydroxy- and alkoxycarbene complexes from alkyl complexes by 1,1-insertion of carbon monoxide [106-108].

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

In addition to the benzannulation reactions discussed in Section 2.2.5, other reactions of heteroatom-substituted carbene complexes are known which lead to the formation of six-membered rings. Alkoxycarbene complexes have a reactivity... 

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). 

Aminocarbene complexes usually are less sensitive towards oxygen than the corresponding alkoxycarbene complexes, but nevertheless should be handled in an inert atmosphere. Their preparation is carried out under nitrogen free from oxygen and moisture, using dry, nitrogen-saturated solvents. 

It is noteworthy that the methylplatinum(II)-vinylidene intermediates are four-coordinate square-planar complexes, with essentially no hindrance to attack from above or below the square-plane. Only alkoxycarbene complexes have ever been isolated from the reactions in alcohol solvents (10). 

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