Carbene complexes carbonyl group alkenation

As discussed in previous sections, high-valent carbene complexes of early transition metals have ylide-like, nucleophilic character. Some Schrock-type carbene complexes react with carbonyl compounds in the same manner as do phosphorus ylides, namely by converting the carbonyl group into an alkene. 

In traditional synthetic organic chemistry, the Wittig reaction plays an important role in carbon-carbon bond extension from the carbonyl group. CM is an attractive alternative for carbon-carbon extension from a terminal alkene. In fact, a pyrroh-dine ring of anthramycin derivative 55 has been constructed by RCM of 52, and the sidechain has been extended by CM of terminal alkene of 54 with ethyl acrylate. " In the CM, ruthenium carbene complex Ij, reported by Blechert, gives a good result since the ligand of the catalyst easily dissociated from the ruthenium metal at room temperature ... 

Wittig-type alkenation of the carbonyl group is possible with Ti carbene compounds [56], The reaction is explained by the formation of nucleophilic carbene complexes of Ti, although they are not isolated. In the carbonyl alkenation, the oxametallacyclo-butane intermediate 182 is formed by [2+2] cycloaddition of the carbene complex 181 with the carbonyl group. This intermediate is converted to the new alkene 183 and the Ti(IV) oxo species 184, which is a stable compound, and hence the carbonyl alkenation requires a stoichiometric amount of the Ti complex. Also, ester 185 is converted to the enol ether 187 via 186. 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

Carbene complexes of Fe and Co carbonyls are also prepared. Unlike the Cr carbene complexes, no cyclopropanation of alkenes occurs with these carbene complexes. Furans are formed by the reaction of alkynes involving rearrangement of methoxy group. The 2-aminofuran 323 is formed by the reaction of the dimethylaminocarbene complex 319 of Fe carbonyl, via rearrangement of the amino group. Under CO pressure, pyrone 324 is the main product [97]. In these reactions, the... 

The product from nucleophilic attack at the carbonyl carbon undergoes, besides the reverse course of the attack, some transformations useful for the purpose of utilizing the new hgand group. These include alkene insertion and reductive elimination, as in Schemes 8.3, 8.6 and 8.7. Decarboxylation of M-COOH was also described (Scheme 8.8). The acyl complex reacts with carbon electrophile to give carbene complex (Eq. 8.5) [22]. 

Exchange of the oxygen atom of a carbonyl function for the methylene group of the proposed titanium carbene complex (the Tebbe reagent) to yield terminal alkenes ... 

Bis-carbene complexes of Rh 57 were applied to the TH of alkenes and arylalkyl, cyclic and dialkyl ketones at 82 °C in NaOH/z-PrOH (Figure 13.8). Whereas TH of alkenes failed, ketones were converted almost quantitatively to the corresponding alcohols within 2h, with TON>19 000. Neopentyl-sub-stituted NHC complexes 58 were also very efficient for reductions of carbonyl groups in refluxing K2C03/z -Pr0H. For CNC-pincer-containing 59, the rate of TH was found to be faster for aromatic ketones than for aliphatic ketones. " ... 

Transition metals have been used to trap and stabilize many different types of reactive intermediates, such as carbenes. Reactive silicon intermediates have only recently yielded to this approach. In the case of alkenes, for instance, transition metal complexes are generally made by exposing the alkene to a transition metal bearing suitable leaving groups (e.g., carbonyl). Unlike carbon-based intermediates, however, silicon-based analogs have been very difficult to prepare until recently. Unless... 

The ability of chiral bis(camphorquinone-a-dioximato)cobalt(Il) complexes (Section 1.2.1.2.4.2.6.3.1.) to catalyze carbene transfer from diazocarbonyl compounds (diazoacetic esters, 2-diazo-l-phenylethan-l-one) to terminal alkenes conjugated with vinyl, aryl, carbonyl, and cyano groups, has already been mentioned. The ee-values are 75-88 /o at best, often lower. The highest values are again obtained with bulky diazoacetic esters. The significance of these catalysts, however, is their ability to promote cyclopropanation of electron-deficient alkenes, such as acrylates and acrylonitriles, in contrast to the rhodium and copper catalysts discussed above. 

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