Carbene titanium

Reactions of Titanium Carbene Complexes with Carbon Carbon Double Bonds... 

Another approach to synthetically useful olefin metathesis involves the utilization of higher homologues of titanium-methylidene 15, as shown in Scheme 14.11. If the resulting titanium carbene complex 20 is more stable than the starting alkylidene complex 15, this reaction can be employed for the generation of various titanocene-alkylidenes and as a method for the preparation of unsaturated compounds. 

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],... 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

Since the hybridization and structure of the nitrile group resemble those of alkynes, titanium carbene complexes react with nitriles in a similar fashion. Titanocene-methylidene generated from titanacyclobutane or dimethyltitanocene reacts with two equivalents of a nitrile to form a 1,3-diazatitanacyclohexadiene 81. Hydrolysis of 81 affords p-ketoena-mines 82 or 4-amino-l-azadienes 83 (Scheme 14.35) [65,78]. The formation of the azati-tanacyclobutene by the reaction of methylidene/zinc halide complex with benzonitrile has also been studied [44]. 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

The Reaction of Titanium Carbene Complexes with Nitriles 495 ... 

Fig. 3.34. Cyclopropanation with titanium carbene complexes generated in situ [33].

Experimental Procedure 3.2.2. Cyclopropanation with a Titanium Carbene Complex (E)-l-Hexyl-2-(2-phenylethenyl)cyclopropane [33]... 

It should be mentioned here that nucleophilic titanium carbene complexes, generated in situ from dithioacetals and Cp2Ti[P(OEt)3]2 (see e.g. Sections 3.2.2.1 and 3.2.4.2), can undergo M-H insertions with silanes, germanes and stannanes [694]. This reaction represents an interesting alternative procedure for the che-... 

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. 

A novel route to 2,3-dihydrothiophenes involved a titanocene-promoted carbene formation and subsequenct intramolecular cyclization onto a thiol ester <99SL1029>. Treatment of thioacetal 9 with the low-valent titanium complex 10 gave 2,3-dihydrothiophene 12 by intramolecular olefination of the thiol ester of titanium-carbene intermediate 11. Another metal-mediated cyclization onto the thiophene ring system involved the palladium-catalyzed cyclization of 1,6-diynes <99T485>. For example, treatment of thioether 1,6-diyne 13 with Pdlj in the presence of CO and Oj in methanol followed by treatment with base gave 14. 

A review covering the use of titanium carbene complexes in organic synthesis has appeared.8 Special emphasis is placed on titanium carbene generated from titanocene bis(triethylphosphate) by action on thioacetals or gem-dichlorides. 

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