Stoichiometric olefin metathesis

Scheme 11.33 Tungsten carbenes in stoichiometric olefin metathesis.

Scheme 11.63 Synthesis of metal furanosylidenes based on stoichiometric olefin metathesis.

Although the molybdenum and ruthenium complexes 1-3 have gained widespread popularity as initiators of RCM, the cydopentadienyl titanium derivative 93 (Tebbe reagent) [28,29] can also be used to promote olefin metathesis processes (Scheme 13) [28]. In a stoichiometric sense, 93 can be also used to promote the conversion of carbonyls into olefins [28b, 29]. Both transformations are thought to proceed via the reactive titanocene methylidene 94, which is released from the Tebbe reagent 93 on treatment with base. Subsequent reaction of 94 with olefins produces metallacyclobutanes 95 and 97. Isolation of these adducts, and extensive kinetic and labeling studies, have aided in the eluddation of the mechanism of metathesis processes [28]. 

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. 

Complex 17 shows versatile reactivity in both catalytic and stoichiometric reactions. It is a catalyst for olefin metathesis, in which a metallacyclobutane is proposed to be a key intermediate [52]. Grubbs et al. successfully isolated titanacyclobutanes 18 by treatment of 17 with alkenes in the presence of a Lewis base such as dimethylaminopy-ridine [56,57]. When cyclic olefins such as norbornene are treated with a catalytic amount of 18, ring opening metathesis polymerization occurs [58]. 

The chemistry of metal carbene and carbyne complexes has undergone very rapid development since the sixties and olefin metathesis reactions now form part of a much larger family of [2 + 2] metathesis reactions between multiply bonded compounds. In Section 4.2 we shall briefly consider this wider context. Some examples will be given of the use of stoichiometric metathesis reactions of such complexes in organic synthesis (Dotz 1984 Schubert 1989). 

H. L. Krauss, E. Amberger, N. Arfsten, P. Blumel, W. Hammon, R. Hopfl, W. Riederer Stoichiometric Reactions with Reduced Phillips Catalysts, in Y. Imamoglu (ed.) Olefin Metathesis and Polymerization Catalyszts, Kluwer Academic Publishers, Dordrecht, 359 (1990)... 

The metal-carbene complexes are nowadays extensively used as reagents in organic synthesis. Fischer carbene complexes have become valuable building blocks in various stoichiometric reactions such as the aldol reaction, benzannu-lation, cycloaddition, cyclopropanation, and Michael reaction. In particular, Fischer alkenyl carbene complexes have been widely used in cycloadditions [2c,2f,5]. On the other hand, a typical reaction of Schrock carbene complexes is olefin metathesis (Figure 5.2) [6]. Schrock s olefin metathesis catalyst is a high reactivity, but air- and moisture-sensitive reagent. 

Cyclopropanes in low yield were first noted in 1964 by Banks and Bailey (12) during the disproportionation of ethylene, but little significance was attached to that observation until recently, because such products had no obvious relevance to early mechanistic concepts based on pairwise rearrangements of bisolefin complexes. However, the subsequent adoption of carbenelike species as metathesis intermediates (4) provided a foundation for later development of cyclopropanation concepts. The notable results of Casey and Burkhardt (5) made an impact which seemed rather neatly to unify mechanistically the interconversion of cyclopropanes and metathesis olefins, although the reactions which they observed were stoichiometric rather than catalytic [see Eq. (4)]. Nevertheless, their work indicated a net redistribution of =CPh2 and =CH2 from (CO)5W=CPh2 and isobutylene, respectively, to form CH2=CPh2. Dissociation and transfer of CO yielded W(CO)6. Unfortunately, the fate of the isopropylidene moiety remained unknown. In 1976,... 

The course of decomposition of confirmed or presumed metallocyclo-butane intermediates is important, but most results reported deal with stoichiometric rather than catalytic processes. Retention of the 3-carbon skeleton via pathways d or f in Eq. (26) occurs much more frequently than does cleavage to metathesis-related products. For example, thermolysis of phenyl-substituted platinocyclobutanes yields propenylben-zenes and phenyl-cyclopropane, but no styrene or ethylene (77). Similarly, the decomposition of tantalum carbene adducts (8) with olefins... 

Titanium-catalyzed cyclization/hydrosilylation of 6-hepten-2-one was proposed to occur via / -migratory insertion of the G=G bond into the titanium-carbon bond of the 77 -ketone olefin complex c/iatr-lj to form titanacycle cis-ll] (Scheme 16). cr-Bond metathesis of the Ti-O bond of cis- iij with the Si-H bond of the silane followed by G-H reductive elimination would release the silylated cyclopentanol and regenerate the Ti(0) catalyst. Under stoichiometric conditions, each of the steps that converts the enone to the titanacycle is reversible, leading to selective formation of the more stable m-fused metallacycle." For this reason, the diastereoselective cyclization of 6-hepten-2-one under catalytic conditions was proposed to occur via non-selective, reversible formation of 77 -ketotitanium olefin complexes chair-1) and boat-1), followed by preferential cyclization of chair-1) to form cis-11) (Scheme 16). 

The complex 8W (R = Me) can also be used in a stoichiometric metathesis sequence to effect the ring closure of unsaturated ketones so as to form 1-substituted cyclopentenes, cyclohexenes and cycloheptenes in good yield, e.g. equation 24. The C=C bond reacts first to give [W]=CH(CH2)3C0(CH2)0(CH2)3Ph, which then undergoes an internal carbonyl-olefination reaction13. 

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