Acyloxy carbenes

Nucleophilic acyl complexes can be 0-alkylated with hard electrophiles to yield the corresponding alkoxy- or (acyloxy)carbene complexes. The first carbene complex ever isolated [61] was prepared by this route the intermediate, anionic acyl complex was generated by addition of phenyllithium to tungsten hexacarbonyl (Figure 2.3). 

The new class of carbene complexes, acyloxy-carbenes [(CO)sCrC(X)OAc] (X = 2-furyl or CH2SiMe3) have been isolated. These compounds extend the synthetic range of these carbene systems and afford the nitrile [(CO)5CrNCC4-H3O] and isonitrile [(COijCrCNMe], respectively, with HN3. ... 

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

Besides alkyloxy carbene complexes, acyloxy carbene complexes have also been proven to be effective in the benzannulation reaction. For example, Yamashita and Ohishi have reported the synthesis of various naphthoquinones in yields of 47-71 % using acetyloxy phenyl chromium carbene complex and terminal alkynes with different alkyl chain lengths [29]. 

Fischer-type (alkylthio)- and (arylthio)carbene complexes187 are available by thiolysis of the corresponding alkoxycarbene complexes in presence of a base catalyst,188 or by thiolysis of (acyloxy)carbene complexes generated in situ.10c-10d-189 192 Addition of thiols R SH (R1 = alkyl, aryl)... 

A study of aryl(acyloxy)carbenes, generated photolytically from diazirine... 

Alkoxy(l-alkynyl)carbene complexes also are obtained by condensation of (l-ethoxy)methylcarbene complexes with bulky acyl chlorides as side products (10-12% yield) together with [(2-acyloxy)ethenyl]carbene complexes (50-78% yield),36 e.g., (CO)5M=C(OEt)—CH3 + 3 RCOC1 + 3 Et3N — (CO)5M=C(OEt) - CR (M = Cr, W R = r-Bu, c-Pr). 

Photolysis of [(Z)-2-(acyloxy)ethenyl]carbene complexes (= 4-acyloxy-1-chroma-l,3-dienes) (CO)sCr=C(OEt)-CH=C(OCOR1)R (Z)-169 (R = C6H5, t-Bu, c-Pr R1 = C6H5, c-Pr, c-C7H7CH2, f-Bu, Me2C=CH) with UV light of >300 nm affords 2-butene 1,4-diones R CO —C(OEt)=CH — COR (E)-170 (60-68%) by a novel metal-mediated acyl migration. Compounds (Z)-171 and furans 172 are formed as minor products (Scheme 72).4g... 

Scheme 72. Butene 1,4-diones by photolysis of [2-(acyloxy)ethenyl]carbene complexes.

Acyloxy)alkenyl carbene complexes, synthesis, 223-224 Alkenes, bishydroxylation cycloreversion of rhenium diolates, 148-156... 

As pointed out earlier, propargylic esters coordinate to gold to form complexes that can undergo 1,2- or 1,3-acyl migrations to form a-acyloxy-o ,jS-unsaturated carbenes or allene-gold complexes (equation 23). ... 

Preparation of 2-acyloxy-l,l-dichlorocyclopropanes via the addition of dichlorocarbene to acyloxyalkenes depends on the structure of the alkene and on the mode of generation of the carbene. Thus, ketone enol esters give cyclopropanes irrespective of the method used for the generation of dichlorocarbene. 

I.2.I.6. Oxygen-Substituted Carbenes or Equivalents 1.2.1.6.1. Acyloxy(organo)carbenes... 

Organo-substituted 1-acyloxycyclopropanes 2 are obtained by the reaction of acyloxy chromium carbene complexes 1, generated in situ from tetraalkylammonium chromium acylates and acyl bromides, with enol ethers (alkyl or trialkylsilyl). These complexes are much more reactive than the corresponding alkoxy-complexes (see Section 1.2.1.6.3.). 

Barluenga et al. demonstrated a copper-catalyzed furan formation starting from bis-propargylic esters 136 (Scheme 19.33) [52]. The multistep process is consistent with (fur-2-yl)copper(I) carbene complex intermediates 137, which allow for 1,1-insertion into X—H bonds (X = Ge or Si). Generation of the carbenes 137 can be rationalized by copper-catalyzed 1,3-acyloxy migration from 136 followed by stereoselective formation of (Z)-enynones 141 via 140 [53]. 

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