Titanium alkoxide, mediation

Titanium alkoxide-mediated coupling of functionalized allenes and acetylenes afforded various types of products the first propargyltitanation of acetylenes was achieved by the appropriate choice of allenes.74... 

Bonchio, M., Licini, G Modena, G., Moro, S, Bortolini, O., Traldi, P and Nugent W,A. (1997) Use of electrospray ionization mass spectrometry to characterize chiral reactive intermediates in a titanium alkoxide mediated sulfoxidation reaction. Chem. Commun., 869-870. 

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. 

Priftis et al.used titanium alkoxide catalyst-functionalized MWCNTs for the surface-initiated titanium-mediated coordination polymerizations of L-lactide (LEA), e-caprolactone (e-CL) and n-hexyl isocyanate (HIC) at 120 °C, room temperature and 130 °C in toluene for 24 h, 20 h and 20 h, respectively. The grafted PLEA content could be controlled by the reaction time. In contrast with the other two monomers, HIC had very low conversions. 

In 2003, both Walsh and Yus reported independently on a titanium-mediated phenyl transfer to alkyl-aryl ketones 23 (Scheme 8.8) [22]. The role of the titanium alkoxide was not only to form the active chiral catalytic species, but also to sequester the tertiary alkoxide generated during the catalytic cycle. Yus has discussed the possibility of using either arylboronic acids or Ph3B as precursors for the aryl transfer, and in certain cases ee-values greater than 99% were observed [23]. It is worth highlighting at this point that Walsh also used a similar protocol for the aryl transfer to a,P-unsaturated ketones to produce optically enriched tertiary alcohols [24]. 

ESI-MS was also employed to identify reactive intermediates of some reactions mediated by platinum-alumina, titanium alkoxide, and so on [3-11]. This method has proved to be a good choice for an assay of multiple, competitive, and simultaneously occurring catalytic reactions. 

Protection of Alcohols.—The acylation of alcohols by polymers (48) of 3-acyl-2-oxazolone has been reported using fluoride ion as the catalyst, selective acylation of primary alcohols in the presence of secondary alcohols has been observed. Transesterification, mediated by titanium alkoxides, has been investigated and shown to be a selective, mild, and inexpensive technique for example, for the deacylation of alcohols. ... 

All these transformations are closely related to the respective additions of zinc reagents, which can also be mediated by a catalytic amount of the same chiral ligands and an excess of titanium alkoxides. The mechanism of the latter... 

Aldol reactions. A high degree of syn-diastereoselectivity is exhibited in the aldol reaction mediated by titanium(IV) alkoxides in the presence of a-hydroxy acids. With chiral a-hydroxy acids, asymmetric induction is observed.- ... 

In general, the compounds of the Group 4 metals, such as halides and alkoxides, are well known as Lewis acids to catalyze two-electron electrophilic reactions, and their metallocenes coupled with alkylation and/or reduction agents were effective catalysts for the coordination polymerization of olefins. For the transition metal-catalyzed radical polymerization, their alkoxides, such as Ti(Oi-Pr)4, have also been employed as an additive for a better control of the products. Contrary to the common belief that the Group 4 metals rarely undergo a one-electron redox reaction under mild conditions, there have been some reports on the controlled radical polymerization catalyzed or mediated by titanium complexes, although the conflict in the mechanism between the (reverse) ATRP and OMRP is also the case with the Group 4 metal complexes. 

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