Titanium complexes acetylene

Titanium-acetylene complexes 29 generated in situ from acetylenes, Ti(0-i-Pr)4 and /-PrMgX react with imines to form azatitanacyclopentenes 30 which then react with carbon monoxide under atmospheric pressure to provide pyrroles 31 <96TL7787>. This reaction, which utilizes commercially available reagents is an improvement over a related procedure via the corresponding zirconium complexes under 1500 psi CO <89JA776>. 

Although terminal acetylenes themselves do not form stable titanium—acetylene complexes upon reaction with 1, the reaction with terminal alkynes having a keto group at the 5- or y-position induces an intramolecular cyclization, apparently via the above titanium-acetylene complex to afford the four- and five-membered cycloalkanols, as shown in Eq. 9.6 [28]. 

It is also possible to carry out a substrate-controlled reaction with aldehydes in an asymmetric way by starting with an acetylene bearing an optically active ester group, as shown in Eq. 9.8 [22]. The titanium—acetylene complexes derived from silyl propiolates having a camphor-derived auxiliary react with aldehydes with excellent diastereoselectivity. The reaction thus offers a convenient entry to optically active Baylis—Hillman-type allyl alcohols bearing a substituent (3 to the acrylate group, which have hitherto proved difficult to prepare by the Baylis—Hillman reaction itself. 

Titanium—acetylene complexes react with allylic or propargylic halides or acetates through regioselective titanacycle formation and subsequent P-elimination [36,37]. The ... 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

A titanium-acetylene complex reacts with allyl derivatives through regioselec-tive titanacycle formation and subsequent /3-elimination to lead to functionalized 1,4-dienyl derivatives (Scheme 12.65) [84]. 

Generation of an Acetylene-Titanium Alkoxide Complex, Preparation of (Z)-1,2-Dideuterio-1 -(trimethylsilyl)-l -hexene. 

Unlike zirconium, the group IV metal titanium does not form the hydrometalation product but rather a (r -C5Q)-complex. The first titanium-fullerene complex 1 was prepared by reaction of the bis(trimethylsilyl)-acetylene complex of titanocene with equimolar amounts of Cjq (Scheme 7.1). 

Sato and coworkers have reported an asymmetric synthesis of Baylis-Hillman-type allylic alcohols 48, 49 via a chiral acetylenic ester titanium alkoxide complex (Scheme 9) [41]. These reactions rely on the use of the novel acetylenic ester titanium alkoxide complex 44 with a camphor-derived chiral auxiliary. Optically active, stereodefined hydroxy acrylates 46, 47 were obtained in high yields and with excellent regio- and diastereoselectivities. The chiral auxiliary was subsequently cleaved off by alcoholysis. 

Scheme 9. Asymmetric synthesis of Baylis-Hillman-type adducts via a chiral acetylenic ester titanium alkoxide complex (Sato et al.).

GENERATION OF AN ACETYLENE-TITANIUM ALKOXIDE COMPLEX PREPARATION OF (Z)-l,2-DIDEUTERIO-l-(TRIMETHYLSILYL)-l-HEXENE... 

Reduction of the amount of the ethyl ether (initially 400 mL) increases the formation of the 2 1 acetylene-titanium alkoxide complex, i.e., a titanacyclopentadiene. However, under these conditions, hydrolysis of the reaction mixture reveals that the formation of 2,3-dibutyl-l,4-bis(trimethylsilyl)-1,3-butadiene arising from the titanacyclopentadiene is less than 3%. In any event, after distillation, this diene does not contaminate the desired product. 

Treatment of the titanium- ate complex, ( 75-C5H5)Ti p-rf r/2-C2(SiMe i)2 2Mg(r/ -C5H5) 192,112 with excess acetylene furnishes the divalent titanium cyclohexadienyl complex, (if-l, 2,4,5,6-pentakis(trimethylsilyl)-cyclohexadienyl)(77S-cyclopentadienyl)titanium 193 (Equation (2)).113 A series of related compounds prepared from /-butylethyne, cyclohexylethyne, 1-hexyne, and phenylethyne have also been synthesized. 

Treatment of the acetylene complexes Cp2Ti( 72-C2R2) (R = SiMe3 or Ph) with C02 at room temperature resulted in the formation of the dimeric cr-alkenylcarboxylate complexes 131 and 132 (Equation (35)).102 The magnetic moment of both complexes was 1.6 /xB per titanium atom indicating that they contained trivalent Ti centers. The structure of 132 was elucidated by X-ray diffraction, and the presence of two fused metallacyclic rings was confirmed. 

Titanium-alkyne complexes Ti(Me3SiC=CC6Hi3)(OR)2, as well as the chiral complex derived from chloro-tris[(—)-menthoxo]titanium/2MgClPr1 and alkynes, react with carbonyl compounds to afford optically active allylic alcohols in up to 38% ee (Scheme 127).184 Introduction of two different electrophiles at each of the acetylenic terminal carbon atoms was possible in a regio- and stereoselective manner.45 Similarly, the titanacyclopentene compounds react with imines, metalloimines, or hydrazones under mild conditions to afford allylic amines or their derivatives in good to excellent yields (Scheme 128).258... 

Titanium imido complexes [Ti(NBu-f)(L)(py)] (I MeC(2-C5H4N) (CH2N(3,5-C6H3Mc2))2) wdth aryl acetylenes afford [2+2] cycloaddition products 76 (Ar = Ph, p-Tol) (08OM2518). Reaction with a second equivalent of alkyne affords azatitanacyclohexadienes 77 (07OM5522). 

Sato and co-workers have developed an elegant titanium alkoxide promoted MBH reaction between enantiopure acetylenic esters 262-264 (Figure 2.16) and aldehydes. A new chiral dimetallic species, the acetylenic ester titanium alkoxide complex with a camphor-derived auxiliary, enabled the preparation of p-trimethylsilylated MBH adducts 265 with high diastereoselectivity (Scheme 2.150). ... 

Very related to the titanium chemistry described above are experiments with resembling molybdenocene complexes. If the molybdenum-diphenyl-acetylene complex Cp2Mo(PhC=CPh) is used as starting material, no C-C linkage reaction takes place with carbon dioxide. Instead, a crystalline solid was isolated whose x-ray proved the formation of a molybdenum-carbon dioxide complex [85] (Equation 15). 

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