Schrock reagent

F.N. Tebbe (1978 [footnote 20]) and R.R. Schrock (1976) have shown that electrophilic titanium or tantalum ylides can alkylidenate the carbonyl group of esters. Vinyl ethers are obtained in high yields with Tebbe s reagent, p-chlorobis(ri -2,4-cyclopentadien-l-ylXdime-thylaluminum)- 4-methylenetitanium (S.H. Pine, 1980 A.G.M. Barrett, 1989). 

Most experimental data suggest that the actual methylenating agent derived from the Tebbe reagent upon treatment with a weak base, is the highly reactive carbene complex Cp2Ti=CH2 [709]. This complex is a typical Schrock-type carbene, because it is high-valent [Ti(IV)], electron-deficient (16 valence electrons) and nucleophilic at carbon. 

Apart from the tandem metathesis/carbonyl o[efination reaction mediated by the Tebbe reagent (Section 3.2.4.2), few examples of the use of stoichiometric amounts of Schrock-type carbene complexes have been reported. A stoichiometric variant of cross metathesis has been described by Takeda in 1998 [634]. Titanium carbene complexes, generated in situ from dithioacetals, Cp2TiCl2, magnesium, and triethylphosphite (see Experimental Procedures 3.2.2 and 3.2.6), were found to undergo stoichiometric cross-metathesis reactions with allylsilanes [634]. The scope of this reaction remains to be explored. 

In many of their reactions, these carbenes behave like the familiar Wittig reagent, PhjPCH,. Schrock carbenes are important intermediates in olefin metathesis.68... 

In cases where an alkyl has no (5 hydrogens (or no accessible ]8 hydrogen), an important alternative process, a elimination, can occur. M. L. H. Green40 has proposed the process shown in Scheme 3 to explain the formation of the ylide complex shown. Another interesting example (Scheme 4) is due to Shaw.6 It is not known whether Schrock s62 remarkable chemistry (equation 45), which led to the first examples of carbenes not stabilized by heteroatoms, also goes via a elimination or, perhaps more likely, by deprotonation of an alkyl at the a position by an organolithium reagent or other base. 

Methylidene-rare earth complexes (27) have been synthesized and fully characterized.32 In these complexes, the methylene should be seen as a doubly charged negative ligand. Nevertheless, these methylidene complexes react as Schrock-type nucleophilic carbenes, (27) therefore being analogous to the Tebbe reagent. 

The stereoselective synthesis of 1,4-disubstituted-l,3-dienes proceeds by head-to-head oxidative coupling of two alkynes with formation of an isolable metallacyclic biscarbene ruthenium complex [23], as shown in Scheme 6. Several key experiments involving labeled reagents and stoichiometric reactions and theoretical studies support the formation of a mixed Fischer-Schrock-type biscarbene complex which undergoes protonation at one carbene carbon atom whereas the other becomes accessible to nucleophilic addition of the carboxylate anion (Scheme 6) [23]. 

Schrock, Gibson et al. [52d] found that styrene and 1,3-pentadiene could be used as chain transfer reagents for the living ring-opening olefin metathesis polymerization of norbornene with molybdenum based catalyst 35a. Renewed norbornene addition to a polymerization mixture containing initiator 35a and 30 equivalents of styrene resulted in the formation of polynorbomene with a low polydispersity and a molecular weight controlled by the number of norbornene equivalents in each of the individual monomer solutions, Eq. (38). This method allows a more efficient use of the catalyst. 

Beyond the use of the Fischer-type chromium carbenes as stoichiometric reagents and the Grubbs-type ruthenium carbenes as versatile catalysts for the preparation of organic compounds, Schrock s molybdenum and tungsten complexes of the general composition M(CHR)(NAr)(OR )2 (and derivatives thereof) and... 

Introduction. Homogeneous catalytic hydrogenation with cationic rhodium catalysts has been extensively explored by Schrock and Osborn. Use of these complexes in stereoselective organic synthesis has been a topic of more recent interest, and has been recently reviewed. The reagent of choice for many of these directed hydrogenations has continued to be [Rh(nbd)(dppb)]BF4 (1). 

Grignard reagents have also been reported to be used in the synthesis of some carbene complexes. For example, Schrock and co-workers [14] reported the... 

The reactions of Fischer and Schrock carbyne complexes are of interest because they may act as intermediates in chemical synthesis. Typical reactivity of carbyne complexes with nucleophiles (i.e., alkyl lithium reagents and metal(I) alkoxides) are consistent with the electronics and molecular orbital calculations for these types of complexes [Eq. (9) 22]. The nucleophile adds to the carbyne-carbon, resulting in the formation of a carbene complex. The reaction of a Grignard reagent with a carbyne complex is expected to demonstrate similar reactivity. 

Schrock discovered that the r-butylalkylidene complex of tantalum (and in lower yield, niobium) was a structural analog to the phosphorus ylide. The complex proved to be a reagent for r-butylalkene formation. Of the Wittig-type reactions tried, most notable was the ability of the complex to react with esters and amides to form the corresponding r-butylalkenes in good yields. 

The active species in the Tebbe olefination is believed to be the nucleophilic (Schrock-type) titanocene methylidene, which is formed from the Tebbe reagent upon coordination of the aluminum with a Lewis base (e.g., pyridine). This methylidene in its uncomplexed form, however, has never been isolated or observed spectroscopically owing to its extreme reactivity. The same intermediate can also be generated by other means." The titanocene methylidene reacts with the carbonyl group to form an oxatitanacyclobutane intermediate that breaks down to titanocene oxide and the desired methenylated compound (alkene). The driving force is the formation of the very strong titanium-oxygen bond. 

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). 

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