Lewis acid mechanism titanium complexes

Allyltrialkylsilanes add to aldehydes in the presence of a Lewis acid." The mechanism of this reaction has been examined." " When chiral titanium complexes are used in the reaction, allylic alcohols are produced with good asymmetric... 

Alkenes can be transformed into epoxides by hydroperoxides and a catalyst, which often is a high-valent titanium or molybdenum complex acting as a Lewis acid. The mechanism is not clear in great detail in Figure 2.34 a suggested mechanism is given. Coordination of the alkene to the metal prior to attack of the electrophilic oxygen is not considered as a necessary step. 

Cationic mechanisms are much more characteristic of the polymerization of oxygen heterocycles, both ethers and acetals. A wide variety of catalysts has been used, including protonic acids, such Lewis acids as boron trifluoride, phosphorus pentafluoride, stannic chloride, antimony pentachloride, titanium tetrachloride, zinc chloride, and ferric chloride, and salts of carbocations or tri-alkyloxonium ions having anions derived from Lewis acids. Some complex, coordination catalysts that appear to operate by a mechanism... 

The chemistry of Lewis acids is quite varied, and equilibria such as those shown in Eqs. (28) and (29) should often be supplemented with additional possibilities. Some Lewis acids form dimers that have very different reactivities than those of the monomeric acids. For example, the dimer of titanium chloride is much more reactive than monomeric TiCL (cf., Chapter 2). Alkyl aluminum halides also dimerize in solution, whereas boron and tin halides are monomeric. Tin tetrachloride can complex up to two chloride ligands to form SnCL2-. Therefore, SnCl5 can also act as a Lewis acid, although it is weaker than SnCl4 [148]. Transition metal halides based on tungsten, vanadium, iron, and titanium may coordinate alkenes, and therefore initiate polymerization by either a coordinative or cationic mechanism. Other Lewis acids add to alkenes this may be slow as in haloboration and iodine addition, or faster as with antimony penta-chloride. 

The same conversion is successfully catalyzed by using in-situ generated complexes of Ti(OPr )4 and tridentate Schiff bases (Stmcture 54), which are derived from substituted salicylaldehydes with chiral aminoalcohols [85]. Another similar chiral reagent is derived from reaction of titanium tetraisopropoxide and the Schiff base of 3,5-di-tert-butylsalicylaldehyde and (5)-valinol. The mechanism and stereoselectivity of these chiral Lewis acids are discussed by Corey and co-workers. Other chiral Ti Schiff base complexes have been employed in asymmetric TMSCN addition to benzaldehyde [85]. 

The influence of Lewis acids on the diastereoselectivity of the cycloaddition of /f-alkoxyalde-hydes has also been studied35. Magnesium bromide, highly effective for a-alkoxyaldehydes, fails in the case of the cycloaddition of aldehyde 10 to diene 2 and the reaction does not exhibit any selectivity, probably due to a change of mechanism to Mukaiyama s aldol type. One reason may be the change of solvent from tetrahydrofuran to a mixture of benzene and diethyl ether. The additions of aldehyde 10 to other dienes are more selective but diastereoselectivity is still much lower than for the a-alkoxy aldehydes. Boron trifluoride-diethyl etherate complex also leads to a mixture of four possible products. Excellent selectivity is achieved for the titanium(IV) chloride catalyzed addition of aldehyde 10a to diene 2b, 11c is obtained as the only product. 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

The generally accepted mechanism of the classic Danheiser annulation involves three basic steps the Lewis acid-catalyzed electrophilic combination of the a, 0-unsaturated ketone with the silylallene, a 1,2-sp-silyl migration, and a final cyclization step. This mechanism was first proposed by Danheiser in the original publication of the annulation and has been generally accepted but has never been formally investigated. A more detailed account of the reaction pathway is shown below. Treatment of the a,y5-unsaturated ketone 1 with TiCU produces a titanium complex existing as two resonance-stablized cations 26 and 27. Attack of the 2,3-7c-bond of the... 

The structure of titanium complexes is helpfid in understanding the interaction mechanism between Lewis acidic titanium ion and substrate [4]. Some crystal structures of chelate complexes have been reported [5]. It has been shown that most of this kind of titanium complexes take in-plane coordination marmer rather than out-of-plane [6] (Scheme 14.1). The attacking nucleophile is either a ligand bonded totitanimn complex or an external reagent The stereoselectivity results solely from the bias of the chiral titanium center. 

In the inverse electron-demand (lED) HDA reaction systems, a,p-unsaturated ketones, thiones, nitroalkenes, and related compounds often serve as heterodiene units and the electron-rich olefins are usually used as dienophiles for the reaction [158]. A concerted mechanism has been suggested for the HDA reaction between an a,p-unsaturated ketone with a vinyl ether mediated by a titanium complex (Scheme 14.66), although the possibility of a stepwise, cationic pathway, particularly in the presence of a Lewis acid, cannot be completely excluded [160]. 

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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