Titanium diynes

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. 

Rothwell and colleagues352 studied the titanium mediated [2 + 2 + 2] cycloaddition of alkenes with monoynes and diynes. Among the reactions studied, the reaction between styrene (29) and diyne 609 in the presence of titanium catalyst 610 proved cleanest (equation 175). The reaction yielded 614 via a [2 + 2 + 2] cycloaddition followed by a titanium mediated suprafacial [1,5] H-shift involving 611-613. The cis relationship between the trimethylsilyl group and the phenyl group indicated that the initially formed titananorbornene 611 had an endo stereochemistry. 

Metallacyclic complexes containing two molecules of diyne per MCp2 group have also been isolated, that with titanium containing 2,4-alkynyl substituents (225) while with zirconium, the unusual seven-membered metallacumulene structure 226 is adopted, which has only one alkynyl substituent. The bi- and tricyclic 2/2 complexes 227 and 228 have so far been obtained only from reactions of TiCp2 with PhC=CC=CPh.30 ... 

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. 

Titanium Mediated Cycl izat ion of a Diyne 1,2- E, E -bis(Ethylidene)cyclohexane... 

Titanium-catalyzed hydroamination of the diyne 470 has been demonstrated to proceed via a 5-r r/o- /g-cyclization affording the pyrrole 471 (Equation 130). Pyrroles have also been isolated as products from similar reactions involving related 1,5-diynes, which resulted from S-exo-dig-annuhitiom <2004OL2957>. 

E,E-Exocyclic dienes J A titanium reagent (1) consisting of (CsHsljTiCI, CHjPCCaH,), and sodium amalgam in the ratio 1 1 2 effects cyclization of diynes (2) in which n = 3, 4, or 5 to (E,E)-exocyclic dienes (3) (equation 1). Highest yields (80%) are obtained when... 

The synthesis of an ene-diyne -containing [28]porphyrin-(6.0.6.0) has also been described." This macrocycle, tetra-iV-methyl[28]octadehydroporphyrin-(6.0.6.0) 4.152 was prepared by the low-valent titanium-mediated dimerization of... 

Many recent investigations in this field of chemistry have been directed toward the synthesis and study of diyne titanium complexes. The TiCp2 precursor compound Cp2Ti(Me3SiC2SiMe3) reacts with 1,4-disubstituted 1,3-butadiynes to give five-membered titanacyclocumulenes, the structures and stability of which depend strongly on the diyne substituents. Mononuclear or binuclear homobimetallic derivatives can be formed. For the mononuclear complexes, an equilibrium between the cyclocumulene and an alkyne structure is possible (Scheme 561). Binuclear complexes may exhibit different structural types (Scheme 562). 

These titanium systems mediate the metathesis of G-G bonds and recombination processes with disubstituted butadyines.1461 Intramolecular cyclization of terminal disubstituted ct,a -diynes with successive unusual Gp cleavage and new intramolecular G-G coupling processes have been observed in the reaction with the terminal disubstituted diynes RC=C-(CH2) -C=CR (Scheme 567). The stability of the resulting products is determined by the spacer length (n = 2, 4, 5, 6). An increase in spacer length (n > 4) provides undefined secondary and decomposition products. The X-ray structures of the some of the resulting products have been reported.1469... 

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