Carbonyls, olefination

The most useful methods for the formation of C-C bonds are based on the addition of C-nucleophiles to carbonyl compounds. Among the many variations of this basic scheme phosphorus ylides, capable of olefinating aldehydes or ketones in a single step, have proven to be exceedingly valuable reagents in organic synthesis. 

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. 

It is particularly interesting, that some titanium and tantalum carbene complexes olefinate derivatives of carboxylic acids. These reagents are, moreover, much less basic than phosphorus ylides, and thus enable the olefination of strongly C-H acidic carbonyl compounds. 

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The Corey-Chaykovsky reaction entails the reaction of a sulfur ylide, either dimethylsulfoxonium methylide (1, Corey s ylide, sometimes known as DMSY) or dimethylsulfonium methylide (2), with electrophile 3 such as carbonyl, olefin, imine, or thiocarbonyl, to offer 4 as the corresponding epoxide, cyclopropane, aziridine, or thiirane. ... 

Tlie interest in the preparation and use of dithiolium salts in connection with the synthesis of TTF derivatives led to the development of a new uses of heteroaromatic cations in organic synthesis. Based on that, a new carbonyl olefination for the synthesis of numerous heterofulvalenes was developed (77S861). For example, 2-dimethoxyphosphinyl-l,3-benzodithiole was deprotonated with butyllithium in THF at -78°C and the resulting phosphonate carbanion reacted with 9-alkyl-acridones to give the dithia-azafulvalenes of type 45 (78BCJ2674) (Scheme 15). 

Scheme 2.19. Carbonyl Olefination Using Trimethylsilyl-Substituted Organo-... 

Aryl ketones are often used to effect cis and tram isomerization of olefins.(118-ia0) Although this, in some cases, can be viewed as an energy transfer process where the ketone triplet transfers its energy to the olefin, which then isomerizes, the failure of noncarbonyl sensitizers of comparable triplet energy to isomerize the olefins suggests that a process other than energy transfer may be involved. Schenck and Steinmetz<118) suggested that isomerization results from decomposition of a biradical carbonyl-olefin adduct similar to that involved in oxetane formation ... 

Oxetanes are the cycloadducts from a carbonyl compound and an olefin. This one step photochemical formation of a four membered ring heterocycle has been named the Paterno-Buchi reaction 489a> b). Oxetanes are important synthetic intermediates as they can fragment into the carbonyl-olefin pair by which they were not formed (a so termed carbonyl-olefin metathesis). Two examples of such oxetan cracking reactions are shown below in (4.76)490) and in (4.77)491) in this last example the oxetane was used as a precursor for the pheromone E-6-nonenol,... 

Polycyclic oxetanes are obtained in good yields in intramolecular carbonyl-olefin cycloadditions, in an analogous way as the corresponding alicyclic systems are formed in intramolecular enone-olefin additions. Two applications are given in (4.78)492) and in (4.79)493). 

Cydopentadienyl Titanium Derivatives for Carbonyl Olefination/Olefin Metathesis... 

Intermolecular Carbonyl Olefination and Subsequent Ring-Closing Metathesis... 

Recently, Nicolaou and coworkers have devised a novel, one-pot strategy for the direct transformation of acyclic olefinic esters to cyclic enol ethers [34]. Unlike the molybdenum alkylidene 1 (see Sect. 3.2), initial reaction between the Tebbe reagent 93 and an olefinic ester results in rapid carbonyl olefination to afford a diene intermediate. Subsequent heating initiates RCM to afford the desired cyclic product (Scheme 17). 

An alternative approach involves a two-step procedure, in which carbonyl olefination, using the Tebbe reagent 93, generates an acyclic enol ether-olefin (Scheme 16). In this case, subsequent RCM using molybdenum alkylidene 1 proceeds to give cyclic enol ethers. An efficient, one-pot carbonyl olefination-RCM approach has been developed by Nicolaou et al. for the formation of cyclic enol... 

Scheme 14.25. Carbonyl olefination utilizing a CH2X2/Zn/TiCI4 system.

Table 14.7. Carbonyl olefination utilizing titanocene-alkylidenes.

Table 14.8. Carbonyl olefination utilizing a thioacetal-titanocene(l I) system.

Scheme 14.27. Intramolecular carbonyl olefination utilizing a thioacetal-titanocene(ll) system.

Abstract Aldehydes obtained from olefins under hydroformylation conditions can be converted to more complex reaction products in one-pot reaction sequences. These involve heterofunctionalization of aldehydes to form acetals, aminals, imines and enamines, including reduction products of the latter in an overall hydroaminomethylation. Furthermore, numerous conversions of oxo aldehydes with additional C.C-bond formation are conceivable such as aldol reactions, allylations, carbonyl olefinations, ene reactions and electrophilic aromatic substitutions, including Fischer indole syntheses. 

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). 

Schrock type carbene complexes are usually high-valent, electron-deficient complexes without 7t-accepting ligands. These complexes often behave as C-nucleophiles and typical reactions include carbonyl olefination and olefin metathesis. 

The carbonyl olefination is mechanistically closely related to olefin metathesis (Figure 3.40). [2 + 2] Cycloaddition of the carbene complex to the carbonyl... 

Similarly, neither zirconium, tantalum, molybdenum, nor tungsten carbene complexes have been applied extensively by organic chemists for carbonyl olefination [609,727-729], probably because of the difficulty of their preparation and the high price of some of these compounds. These reagents can, however, have appealing chemo- and stereo-selectivity (Table 3.11). 

Table 3.10. Carbonyl olefination with Tebbe-type titanium reagents.

Table 3.11. Carbonyl olefinations with nucleophilic carbene complexes.

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

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. 

Phosphoniutn ylides can be generated by treatment of diazoacetates and triphe-nylphosphine or triethyl phosphite with catalytic amounts of RuCl2(PPh3)2 [ 1343] or ReOCl3(PPh3)2 [1344]. If these reactions are conducted in the presence of aldehydes, carbonyl olefination takes place in high yields. 

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