Tebbe’s complex

Another aspect of the relevance of the carbene complex to catalysis is stabilization of the methylidene ligand by aluminum alkyls as observed in the formation of the Tebbe s complex as shown in Eq. 7.29 [125]. 

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. 

Tebbe s complex [cp2Ti(/i-CH2)(/i-Cl) AlMcj] readily reacts with alkenes and alkynes to form metallacyclobutanes (unstable) and metallacyclobutenes. It was found that there is a CH2 exchange between metallacyclobutene and CH2=CMe2 [equation (5.155)]. The /r-methylene compounds of the first series of the transition... 

There are also complexes in which the carbene bridges a transition metal and a main group element such as aluminum. For instance, Tebbe s complex is well known, because it is especially reactive, being in equilibrium with the terminal methylene complex ... 

Complexes with /i -methylidenes are also known. Complex 2.41 in which methylidene and chloride bridges are present between titanium and aluminum is known as Tebbe s complex. Note that had we not known the structure, Tebbe s complex might have been formulated as Cp TiCCH ) plus Me AlCl. As will be seen, Tebbe s complex has relevance in alkene polymerization and metathesis reactions. Complex 2.42 is a unique example of a monometallic complex with a single carbon atom as one of the ligands. From the earlier examples it is clear that representative organometallic complexes with M-CR (n = 0-3) have all been isolated and fully characterized. 

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

Carboxylic esters undergo the conversion C=0— C=CHR (R = primary or secondary alkyl) when treated with RCHBr2, Zn, and TiCl4 in the presence of A,A,A, iV -tetramethylethylenediamine. Metal carbene complexes R2C=ML (L = ligand), where M is a transition metal such as Zr, W, or Ta, have also been used to convert the C=0 of carboxylic esters and lactones to CR2. It is likely that the complex Cp2Ti=CH2 is an intermediate in the reaction with Tebbe s reagent. 

Reaction between carboxylic esters and Tebbe s reagent or metal car-bene complexes... 

A useful alternative to phosphorus ylids are the titanium reagents, such as, 71, prepared from dicyclopentadienyltitanium dichloride and trimethylaluminum. Treatment of a carbonyl compound with the titanium cyclopentadienide complex 71 Tebbe s reagent) in toluene-THF containing a small amount of pyridine " leads to the alkene. Dimethyltitanocene (Me2TiCp2), called the Petasis reagent, is a convenient and highly useful alternative to The mechanism of Petasis... 

One of the most famous Ti carbene complexes is methylidene 44. Although 44 probably occurs fleetingly as an intermediate in Schrock carbene reactions, it is too reactive to be isolated or observed spectroscopically. Two related compounds, however, have been synthesized 34 Cp2TiCH2(PEt3), in which the phosphine donates electrons to Ti to stabilize the complex, and the immediate precursor to 44, known as Tebbe s reagent (45, equation 10.1535). 

Equation 10.15 provides another example of the use of A1 compounds to form bridging alkylidenes, providing stabilization for the very reactive Group 4 carbene complexes. Later in Chapter 10 (Section 10-3-2), we shall encounter some of the synthetically useful chemistry of Tebbe s reagent. 

In Section 10-2-2, we discussed the synthesis of Tebbe s reagent, a bridged Ti methylidene complex. This reagent converts a C=0 group to an alkene in a manner analogous to that of a phosphorus ylide (Wittig reagent).56 Conversion of a carbonyl to a terminal alkene may seem to be of limited utility until one realizes that, unlike phosphorus ylides that react only with aldehydes and ketones, Tebbe s... 

Titanacyclobutanes 7 can be synthesized from Tebbe s reagent 6 with alkenes. These too give cylopropanes 8 in near quantitative yields upon treatment of the complex 7 with alkynes under photochemical conditions or thermally in the presence of oxidants (e.g. iodine, quinones, etc.), cyclopentadienones or allylpalladium complexes. ... 

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