Carbene carbonyl olefination with

Table 3.11. Carbonyl olefinations with nucleophilic carbene complexes.

Table 3.12. Carbonyl olefinations with carbene-complex-like reagents generated in situ.

Tab. 4.16. Carbonyl olefination with zirconium carbene complexes 54. ...

The chemical properties of DPM have been probed with each of the procedures identified earlier. This carbene is known to react with alcohols to give ethers (Kirmse, 1963 Bethell et al., 1965), it adds to olefins non-stereospecifically to form cyclopropanes (Skell, 1959 Baron et al., 1973 Gaspar et al., 1980 Tomioka et al., 1984), and it is rapidly converted to a carbonyl oxide with oxygen (Werstiuk et al., 1984 Casal et al., 1984). 

In situation (a) a strong carbon-metal bond results. To this group belong the typical Schrock-type carbenes [e.g. Np3Ta=CH(7Bu)], many of which are nucleophilic at carbon. Situation (b) should also lead to nucleophilic carbene complexes, albeit with a weaker carbon-metal bond. Typical reactions of nucleophilic carbene complexes include carbonyl olefination (Section 3.2.4) and olefin metathesis (Section 3.2.5). 

One remarkable application of carbene complexes is the combination of olefin metathesis with carbonyl olefination. If a given substrate has both C-C and C-0 double bonds, it might be possible to realize with a given carbene complex olefin metathesis to yield a new carbene complex, followed by an intramolecular carbonyl olefination step. As emphasized above, because of the irreversibility of the carbonyl olefination, stoichiometric amounts of carbene complex will be required. 

A plausible intermediate of this olefination is the titanium-methylene sjtecies 4, which is formed from 1 by removal of AlMe2Cl with a Lewis base, from 2 by fragmentation with elimination of isobutene, and from 3 by a-elimination and release of methane. However, none of these three routes to titanium-carbene complexes of type 4 proved to be generally applicable. Consequently, the use of these reagents in synthesis is essentially limited to the transfer of a methylene unit 18]. From a synthetic viewpoint, a general and easy route to substituted titanium-alkylidene species and their use in carbonyl olefinations would be more desirable. 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

The utility of the Tebbe type complex in carbonyl olefination is discussed in Chapter 4. The bridged complex may be regarded as a special type of a carbene complex where the Cp2Ti=CH2 unit is masked by interaction with the AlMe2Cl entity. Formation of the Tebbe s complex suggests the occurrence of a-hydrogen elimination in the preparation of the Ziegler-Natta and Kaminsky type olefin polymerization catalysts from titanium chlorides and methylaluminum compounds. 

Recently, a novel porphyrin-based polymer, namely a poly(CO-Ru(n)-64) (with CO-Ru(n)-64 = ruthenium carbonyl spirobifluorenylporphyrin), was prepared electrochemically and used for the transfer of carbene to olefins and sulphides in a solid-state reaction. In another original study, a bimetallic porphyrin film using 65 was studied as electrode modifier with catalytic activity for molecular oxygen reduction and hydrogen peroxide reduction " . 

The carbene mechanism of COER according to our hypothesis consists of forward (Z = O, X = Y = C) and backward (X = O, Y = Z = O) Wittig-like reactions. A transition metal-carbene complex reacts with a carbonyl compound generating an olefin and transition metal oxo-complex. Then the oxo-complex reacts with another olefin generating a new carbonyl compound and regenerating the transition metal-carbene complex. It is known that oxo-alkylidene complexes can be generated via oxidative addition reaction between some tungsten complexes with carbonyl compounds. 

I 4 Carbonyl Olefination Utilizing Metal Carbene Complexes Tab. 4.1. Methylenation of ketones with the Tebbe reagent 3. 

Schrock-type carbene complexes of transition metals other than titanium are also utilized for carbonyl olefination, although their synthetic utility has not yet been fully investigated. In some cases, their reactions differ from those of titanium-carbene complexes in terms of stereo- and chemoselectivity and are complementary to olefination with titanium reagents. This section describes the use of carbene complexes of various transition metals in carbonyl olefination. 

Besides the carbene complexes, the tungsten alkyUdynes 62 [135] are also employed for carbonyl olefination. The complexes 62 react with carbonyl compounds to give 0x0 vinyl complexes 63, which are hydrolyzed with 1 N NaOH to form the... 

Scheme 5.3. Olefination of carbonyl compounds with metal-carbene complexes.

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. 

Stereoselective synthesis of tetrasubstituted alkenes remains a challenging task. A unique solution to this problem is to use torquoselectivity-controlled oletina-tion of carbonyl compounds with ynolates, which is summarized in Chapter 1. In general, the stereoselective synthesis of Z-alkenes, which are thermodynamically less favorable, is more difficult than the synthesis of corresponding -isomers. In Chapter 2, various methods for stereoselective synthesis of Z-alkenes are reviewed. Finally, the C=C double bond formation through catalytic carbene transformation has recently emerged as a new approach toward olefin synthesis. Two chapters covering olefin synthesis based on catalytic carbene transformations are included (Chapter 5 and Chapter 8). 

Formyl carbene is preparable by a copper acetylacetonate-catalyzed decomposition of diazoacetaldehyde40. This new carbenoid species formed a cyclopropane-carboxaldehyde with a substituted olefin. A novel approach to 3-lactones of above 70% optical purity involves reaction of a carbonyl compound with a ketene (or with an acyl halide convertible to a ketene) in the presence of an optically active bose. Trichloroacetaldehyde reacts with ketene in the presence of brucine to form (+)-8 in the presence of (-)-l-phenyl-l-dimethylaminoethane, (-)-8 is formed. ... 

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