Alkoxycarbene

The benzannulation reaction of ethynylferrocene 120 with the diterpenoid chromium alkoxycarbene 119 leads to novel diterpenoid ferrocenyl quinones 121 which, due to their electron-transfer properties, are regarded as potential candidates for non-linear optical materials [71] (Scheme 52). 

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]. 

Table 1 Reaction of chromium alkoxycarbenes with acyclic aryl aldimines...

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]. 

Other miscellaneous imines that underwent photoreaction with chromium alkoxycarbenes include iminodithiocarbonates [33],the mono-N-phenylimine of benzil and the bis-JV-phenyl imine of acetoin [20]. By preparing the chromium carbene complex from 13CO-labeled chromium hexacarbonyl, /J-lactams with two adjacent 13C labels were synthesized [34]. 

Table 3 Reaction of chromium alkoxycarbenes with chiral heterocyclic imines ...

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]. 

Table 6 Reaction of alkoxycarbenes with monosubstituted alkenes...

Table 8 Reaction of alkoxycarbenes with cyclic dienes...

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]. 

Of perhaps greater use for organic synthesis was the observation that photo-driven reactions of alkoxycarbenes with unsubstituted optically active ene carbamates [65] produced aminocyclobutanones in fair yield with high dia-stereoselectivity (Table 12) [66]. In contrast, with a gem-disubstituted ene carbamate, the syn-anti selectivity was low but high asymmetric induction a to nitrogen was observed (Eq. 16). Trans-monosubstituted ene carbamates failed to react, as did a,/J-unsaturated chromium carbene complexes. 

Table 12 Reaction of alkoxycarbenes with optically active ene carbamates...

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. 

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

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]. 

A narrow range of 2-phenyl-1-azirines underwent photodriven reactions with alkoxycarbenes to give AT-vinylimidates, in a process probably related to the above reactions (Table 24) [126]. 

Table 22 Photo-driven reaction of sulfur-stabilized ylides with alkoxycarbenes...

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.,... 

The species responsible for alkyne polymerization, which is kinetically more facile than eyelotrimerization since only a small fraction of the added alkyne is converted to benzenes, is not yet known. Carbene-metal complexes, both mononuclear (54) and binuclear (y2-CR2) complexes (55,56), have been shown to act as alkyne polymerization initiators and several years ago it was shown that terminal alkynes and alcohols can react to give alkoxycarbene ligands (57) As yet, we have no evidence... 

Although we are not specifically concerned here with kpp and the kinedcs of carbene-pyridine ylide formation, we note that the magnitude of is directly related to the structure and reactivity of the carbene. fcpyr ranges from 105 M s-1 for ambiphilic alkoxycarbenes to 109-10I° M-1 s 1 for electrophilic halocarbenes or alkylcarbenes. Very nucleophilic carbenes (MeOCOMe) do not react with pyridine.13... 

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). 

Because of n-electron donation by the heteroatom, these carbene complexes are generally less electrophilic at C than the corresponding non-heteroatom-substituted complexes (Chapter 3). This effect is even more pronounced in bis-heteroatom-substituted carbenes, which are very weak Tt-acceptors and towards low-valent transition metals show binding properties similar to those of phosphines or pyridine. Alkoxycarbenes, on the other hand, have electronic properties similar to those of carbon monoxide, and stable heteroatom-monosubstituted carbene complexes are also usually formed from metals which form stable carbonyl complexes. 

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. 

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]. 

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