Alkoxy-carbene complexes

The ability of Fischer carbene complexes to transfer their carbene ligand to an electron-deficient olefin was discovered by Fischer and Dotz in 1970 [5]. Further studies have demonstrated the generality of this thermal process, which occurs between (alkyl)-, (aryl)-, and (alkenyl)(alkoxy)carbene complexes and different electron-withdrawing substituted alkenes [6] (Scheme 1). For certain substrates, a common side reaction in these processes is the insertion of the carbene ligand into an olefinic C-H bond [6, 7]. In addition, it has been ob-... 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Alkoxy carbene complexes are useful starting compounds for other organometallic complexes,1 5 particularly methylidyne (carbyne) complexes.16 By modification of the coordinated (noncarbene) ligands or of the carbene ligand, other carbene complexes can be synthesized. The use of carbene complexes in organic syntheses has been reviewed recently.17 18... 

Alkylation,4 Alkylation of anions of the usual alkoxy carbene complexes is not generally attractive because of low reactivity. However, the anion of dialkylamino chromium carbenes such as 1 (R = H) can be alkylated readily and in a useful yield. 

Scheme 9. Chiral alkoxy carbene complexes from the alcoholysis of acyloxy carbene complexes.

Although mostly alkoxy carbene complexes have been benzannulated, other types of carbene complexes are equally well suited. These include aryloxy carbene complexes as well as acyla-mino and thioalkylidene complexes, and even complexes with no heteroatoms, such as diaryl carbene complexes, are suitable (see below). Besides the commonly used methoxy and ethoxy carbene complexes, alkoxy carbene complexes with a longer alkyl chain have also been successfully reacted. The benzannulation of aryloxy carbene complexes has recently been studied to probe electronic effects [28a]. Aryloxy alkylidene complexes of type 21 have been used to prepare diaryl ethers 22, which constitute a common substructure in many important types of natural products [28b]. The benzannulation methodology provides an access to phenyl naphthyl ethers in yields of 60-93 % under mild conditions (Scheme 11). 

Arnold and co-workers also reported the deprotonation of alkoxy imi-dazolium iodides with -butyl lithium to yield lithium alkoxide carbenes (Scheme 3).14 Single crystals of one of the complexes were grown from a diethyl ether solution, and revealed a dimer of LiL with lithium iodide incorporated to form a tetramer of lithium cations (7). The lithium-NHC bond distance of 2.131(6) A is similar to that of the lithium amide carbene 4. Also as in 4 there is distortion of the lithium-NCN bond which has an angle of 152.3°. The C2 carbon resonates at 200 ppm in the 13C NMR spectrum which is a relatively high-frequency, possibly as a result of the incorporated lithium iodide. The lithium salts were able to act as ligand transfer reagents and react with copper (II) chloride or triflate to afford mono- or bis-substituted copper(II) alkoxy carbene complexes. 

Hydrazinolysis of (alkoxy)carbene complexes (CO)5M = C(OEt)R (M = Cr, Mo, W) has been studied much less than the aminolysis of such compounds.149"152 Usually, hydrazinolysis takes a nonuniform course and yields several seemingly different products. The preparation of (hydrazino)-... 

C(OMe) —CH3 with an ortho ester 175 (iii) the condensation of aryl- or alkylcarbene complexes with enolizable acid amides RCH2 —CONR2, a reaction involving insertion of the C—CN unit of the acid amide into the M=C bond 176 (iv) the insertion of an alkyne into the M=C bond of an (alkoxy)carbene complex 36,177 174 and (v) the base-catalyzed addition of an alcohol to a (l-alkynyl)carbene complex.96 168173 The last reaction has... 

Hydride abstraction in alkoxyalkyl complexes to the corresponding alkoxy carbene complex has also been shown (Equation (17)) [50]. 

Diastereoselective and enantioselective (see Enantio-selectivity) cyclopropanations of chiral alkenes can be achieved (Scheme 57). Unactivated alkenes usually do not participate in cyclopropanation reactions of Fischer carbenes. However, alkenyl- and heteroaryl-substituted group 6 alkoxy carbene complexes cyclopropanate unactivated alkenes in good yield (Scheme 58). ... 

Optically active thiazolines give rise to -lactams through the photolysis of chromium alkoxy-carbene complexes (Scheme 17). This reaction proceeds with very high stereoselectivity <94JOC496 >. 

Another interesting issue in the hydrolysis of alkoxy carbene complexes is the finding that replacement of the phenyl group with an alkyl group changes the mechanism even though the final products are the same (e.g., equation 75). The mechanism involves deprotonation of the alkyl group and hydrolytic breakdown of the anion, as discussed in Section 4. 

Region II (pH 10 to 12.5 for (CO)5Cr=C(SMe)Ph, pH 10 to 11.5 for (CO)5W=C(SMe)Ph)). Nucleophilic attack by water is rate limiting. Just as is the case for the hydrolysis of alkoxy carbene complexes, there is also general base catalysis of water attack, hence feobsd is given by equation (79) ( f [B] term not shown in Schemes 11 and 12). This rate law implies that the relationships of equations (80) and (81) hold the fe2 H+ is again negligible and omitted from equation (81). 

As it is known, the reaction of alkynylalkoxycarbene metal complexes with simple alcohols leads to conjugate addition onto the triple bond giving alkoxyvinyl(alkoxy)carbene complexes[l].However, when the alcohol has another functionality attached to the main chain at an appropriate distance, both functionalities may interact with either the triple bond and the metal carbene producing changes in the expected reaction course. 

The electrophilic nature of Fischer carbene complex of Cr and W featuring an alkoxy or a thiomethyl substituent has been assessed in the nucleophilic substitution of the carbene heteroelement by the cyanamide anion.Alkoxy carbene complexes were found to be expectedly more reactive. Tungsten carbene were found to exhibit higher kinetic constants as a consequence of the higher electronegativity of W compared to Cr. 

Conjugated chromium Fischer alkoxy carbene complex 1 is quantitatively transformed into a mixture of Z-vinyl ether 2 and ii-allyl ether 3 by NaBH4 reduction in EtOH (Scheme 7.1). 

Under these conditions, another dass of tethered biscarbene complexes such as 124-129 with the carbene centers linked by the diamine spacer are formed by reaction of alkoxy carbene complexes 117 and 118 with diamines 119-123 (Scheme 11.30) [55], In contrast, at room temperature the formation of biscarbene complexes resulting from an aminolysis-Midiael addition sequence is favored. 

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