Substitution reactions Tebbe reaction

Sn2 substitution, 225 StilL-Wittig rearrangement, 227 Tebbe reaction, 231 types, 208... 

Substitution reactions may be effected on saturated analogues as well as on aromatic species. An interesting reaction of Tebbe s reagent with succinic thioanhydride derivatives 9 rapidly leads to monoolefination and. more slowly, to bisolefination. Acidic isomerization of the bis>adduct leads to 2.5-dimethyl thiophene derivatives (Eq. 3) <94JOC494>. 

Titanacyclobutenes, prepared readily from Tebbe reagent and alkynes, react with aldehydes and ketones to form insertion products which undergo facile retro-Diels-Alder reaction to afford substituted 1,3-dienes (equation 107)185. 

The alkyl-substituted titanium carbene complex 18 reacts with norbornene 24 to form a new titanacycle 25, which can be employed for the ROMP of 24 (Scheme 14.13). The titanacycle generated by the reaction of the Tebbe reagent with 24 is also used as an initiator for the same polymerization [23]. These preformed titanacyclobutanes also initiate ROMP of various other strained olefin monomers [24],... 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

A limitation of the methylenation of carboxylic acid derivatives with Tebbe-type reagents such as 3 and 11 is that highly electrophilic acid anhydrides and acid halides cannot be transformed to the corresponding enol esters or alkenyl halides. The reaction of titanocene-methylidene generated from 11 with add chlorides results in the formation of acylation products via titanium enolates 12 [44] rather than methylenation. Since the titanium enolates 12 are stable and do not isomerize to the more highly substituted enolates, they are useful for regioselective aldol reactions (Scheme 4.11) [44a]. Although similar titanium enolates are also produced... 

Carbene Chemistry. Isobutene reacts readily with car-benes and metallocarbenes to form substituted cyclopropanes (eqs 25-27). Reaction of Ethyl Diazoacetate with isobutene in the presence of a chiral copper catalyst affords the cyclopropane in high optical purity (eq 28). Isobutene has also been used to form titanocyclobutanes that are more stable forms of the Tebbe reagent (see L-Chlorobis(cyclopentadienyl)(dimethylaluminum)- L-methylenetitanium) (eq 29). ... 

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