Carbenes reaction with carbonyl

The only known carbene approach to dioxolane derivatives exploits car-bene reactions with carbonyl-containing compounds. The mechanism of... 

Polar X-H bonds (X=0,S,NH) undergo insertion reactions with carbonyl-carbenes in the presence of Rh-carboxylates and Cu-triflate to yield substituted ethers, thioethers or amines [76]. 

The general strategy for carrying out these MCRs starts with the generation of the NHC, which involves the deprotonation of the corresponding azolium salt by an appropriate base. Next, this carbene reacts with several electrophiles to produce reactive zwitterionic intermediates (typically from reaction with carbonyl compound or activated alkynes), followed by different rearrangements, rendering the final product. In this section, only examples based on thiazol-2-ylidenes... 

Reactions with Carbonyl Compounds and Utilization of 2-Diazo-2-(trimethyIsiIyI)ethanoIs. Diazo(trimethylsilyl)-methyllithium (TMSC(Li)N2) reacts with aldehydes and ketones to give lithium 2-diazo-2-(trimethylsilyl)ethoxides by nucleophilic addition, which produce alkylidene carbenes by expulsion of TMSOLi and nitrogen (eq 12). 

Reactions with Carbonyl Compounds Followed by Insertion Reactions. IV-Substituted -aminoketones react with TMSC(Li)N2 to give 2-pyrroUne derivatives via the alkylidene carbene intermediates by intramolecular N-H insertion, analogously to the reaction of -acyl-A(-tosylanilines with TMSC(Li)N2 (eq 21). Dehydrogenation of the 2-pyrrolines with active Mn02 (CMD, chemical manganese dioxide, produced for battery manufacture) affords the corresponding pyrroles (eq 24). ... 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

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