Titanium tetrachloride methylenation

A solution of 72 g. (0.60 mole) of mesitylene in 375 ml. of dry methylene chloride is placed in a 1-1. three-necked flask equipped with a reflux condenser, a stirrer, and a dropping funnel. The solution is cooled in an ice bath, and 190 g. (110 ml., 1.0 mole) of titanium tetrachloride is added over a period of 3 minutes. While the solution is stirred and cooled, 57.5 g. (0.5 mole) of dichloromethyl methyl ether2 is added dropwise over a 25-... 

Another titanium-based reagent for the methylenation of carbonyl compounds is that prepared from dibromomethane/zinc/titanium tetrachloride and related systems (Scheme 14.25) [48]. These systems transform a wide variety of carboxylic acid derivatives to terminal olefins in the same way as titanocene-methylidene does. 

A ubiquitous co-catalyst is water. This can be effective in extremely small quantities, as was first shown by Evans and Meadows [18] for the polymerisation of isobutene by boron fluoride at low temperatures, although they could give no quantitative estimate of the amount of water required to co-catalyse this reaction. Later [11, 13] it was shown that in methylene dichloride solution at temperatures below about -60° a few micromoles of water are sufficient to polymerise completely some decimoles of isobutene in the presence of millimolar quantities of titanium tetrachloride. With stannic chloride at -78° the maximum reaction rate is obtained with quantities of water equivalent to that of stannic chloride [31]. As far as aluminium chloride is concerned, there is no rigorous proof that it does require a co-catalyst in order to polymerise isobutene. However, the need for a co-catalyst in isomerisations and alkylations catalysed by aluminium bromide (which is more active than the chloride) has been proved [34-37], so that there is little doubt that even the polymerisations carried out by Kennedy and Thomas with aluminium chloride (see Section 5, iii, (a)) under fairly rigorous conditions depended critically on the presence of a co-catalyst - though whether this was water, or hydrogen chloride, or some other substance, cannot be decided at present. 

Reactions in various alkyl halide solvents. A preliminary survey of polymerisations catalysed by titanium tetrachloride in various alkyl halide solvents was undertaken using highly purified materials and a vacuum technique. The most important qualitative result obtained was that in the solvents methylene dichloride, ethyl chloride, ethylene dichloride,... 

Table 6 Polymerisation of isobutene by titanium tetrachloride and water in methylene dichloride [80, 87] ...

The technique and apparatus used in this work have been described in detail [81]. The reaction vessel was made hydrophobic by exposure to the vapour of trimethylchlorosilane and evacuated for several hours. Then isobutene, dried by sodium, and methylene dichloride, stored over calcium hydride, were distilled into it, the temperature adjusted, and the reaction started by the breaking of a phial containing a solution of titanium tetrachloride in methylene dichloride and one containing water. These could be broken in this, or the reverse, order, or simultaneously. The ensuing reaction was registered as a time-temperature curve by an automatic recorder. The range of conditions studied was [C4H8] = 0.05 -0.6 mole/1, [TiClJ = (0.1-5) x 10 3 mole/1, [H20] = (0.05-5) x 10 4 mole/1, T= 18°- -95°. 

Chain-Transfer with anisole. The phenomenon of chain-transfer, especially with aromatic compounds, has been extensively investigated for the polymerisation of styrene, but there is only one such study with isobutene [13]. Isobutene (0.1 mole/l) was polymerised by titanium tetrachloride (3 x 10 3 mole/l) in methylene dichloride with a constant, low, but unknown concentration of water in the presence of anisole (0.02 to 0.15 mole/l) over the temperature range -9° to -90°. The reactions were stopped at 10-20 per cent conversion by the addition of methanol. 

Supposition 1. In the paper in which we first described the self-ionisation of titanium tetrachloride in methylene dichloride or ethyl chloride, we also showed that the conductivity measurements on aluminium bromide solutions available at that time could be explained by a self-ionisation of the type... 

Detailed study [18] of the system isobutene-titanium tetrachloride-water-methylene dichloride showed it to be highly complex, but the kinetics and the temperature-dependence of rate and DP could be explained, at least qualitatively, on the hypotheses that the chain-carriers are ions, that paired and free cations have appreciably different reactivities, and that the degree of dissociation of the ion-pairs increases with decreasing temperature. 

The Polymerisation of Isobutene by Titanium Tetrachloride in Methylene Dichloride, R.H. Biddulph, P.H. Plesch and P.P. Rutherford, International Symposium on Macromolecules, Wiesbaden, 1959, Paper No.III.A.10. 

The Polymerisation of Styrene by Titanium Tetrachloride, Part IV. Kinetics of Polymerisation in Methylene Dichloride, W.R. Longworth, C.J. Panton and P.H. Plesch, Journal of the Chemical Society, 1965, 5579-5590. 

A similar type of acid-catalyzed condensation of aldehydes with 4-methylene-2-oxetanone (diketene), giving 4-oxo-6-methyl-l,3-dioxins, has been patented (73GEP2149650). However, other work has established that <5-hydroxy-/3-keto acids or unsaturated keto acids are formed as the principal products (equation 24) (78CPB3877, 78CL409). The latter reaction probably involves electrophilic attack of the protonated aldehyde on the nucleophilic exocyclic methylene carbon atom of the diketone. A closely related reaction of acetals with diketene, catalyzed by titanium tetrachloride, gives the corresponding <5-alkoxy-/3-keto esters (74CL1189). 

No other aliphatic olefin has been investigated in detail with titanium tetrachloride. 2-methyl-2-butene is known to oligomerise in methylene chloride at —78 but... 

AH the evidence gathered in this section must now be rationalised into a reasonable mechanism. Titanium tetrachloride is monomeric in methylene chloride even at low temperature. Sli t interactions exist between TiCl4 moIecules but these do not amount to any strong association. Given the spectroscopic observations concerning the initial stoicheiometry of initiation and the structure of the products of this reaction, we are led to formulate a revival of the old Hunter-Yohe mechanism Thus, the first step in the direct initiation processes involving TiCl4 and olefins takes the form ... 

Longworth and Plesch examined the conductivity of solutions of titanium tetrachloride in methylene chloride and observed that this was directly proportional to the Lewis acid concentration. This correlation was taken as evidence of a 2 2 ionogenic equilibrium involving the monomeric halide. The presence of a TiCls deposit at the cathode seemed to support the presence of TiCl. This type of work is extremely delicate and demands all possible experimental care (purity, dryness, etc.) to provide meaningful results. The interpretation of such results reflecting ionc enic equilibria of various possible stoicheiometry was recently discussed in a very useful paper by Grattan and Plesch ... 

Titanium tetrachloride and a tertiary amine are a useful catalyst for Knoevenagel condensation [149] as shown in Eq. (45) [150]. Because the reaction can be performed under mild conditions, acid-sensitive functional groups survive the reaction conditions and the optically active center at the enolizable position did not racemize (Eq. 45). More examples of the titanium-catalyzed Knoevenagel condensation are shown in Table 5. Alkylation of an (unsaturated) (iV,0)-acetal with active methylene compounds was performed analogously in the presence of TiCU and NEts (Eq. 46) [154]. Depending on the structure of the active methylene compounds, carbon-carbon bond... 

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