Dimethylaluminum chloride, catalyst

Homogeneous catalysts for the ethylene polymerization based on bis(cyclopenta-dienyl)titanium(IV) compounds [4], tetrabenzyltitanium [14], tetraallylzirconium and hafnium are formed with diethylaluminum chloride, dimethylaluminum chloride or triethylaluminum as co-catalysts. Their activities are poor (less than 200 kg PE/mol catalyst per h), so no industrial application resulted. 

Ene reaction of aldehydes. Aliphatic and aromatic aldehydes are not reactive enophiles however, in the presence of dimethylaluminum chloride, which serves as u mild Lewis acid catalyst and proton scavenger, ene reactions occur in reasonable to high yield. Use of C2HSA1C1 results in complex mixtures of products. This ene reaction is a useful route to homoallylic alcohols.2... 

Diels-Alder catalysts Alkylaluminum halides, 5, 173 Boron trifluoride etherate, 43 Diethylaluminum chloride, 173 Dimethylaluminum chloride, 5 Sodium dodecyl sulfate, 281 Titanium(IV) chloride-Diethylaluminum chloride, 309... 

Prins reaction (cf 10, 186-187). Dimethylaluminum chloride is an effective catalyst for the ene addition of formaldehyde (as trioxane or paraformaldehyde) to mono- and 1,2-disubstituted alkenes.5 When 1.5-2.0 equiv. of the Lewis acid is used, homoallylic alcohols are obtained, usually in high yield. y-Chloro alcohols, formed by cis-addition of -Cl and -CH2OH to the double bond, are sometimes also observed when only 1 equiv. of the Lewis acid is present. The advantage of this reaction over the Prins reaction (using HC1) is that m-dioxanes are not formed as by-products, because formaldehyde no longer functions as a nucleophile when complexed to the Lewis acid. 

This reaction is reversible and suitable p-hydroxy alkenes can be cleaved by heat (17-32). There is evidence that the cleavage reaction occurs by a cychc mechanism (p. 1551), and, by the principle of microscopic reversibility, the addition mechanism should be cyclic too. Note that this reaction is an oxygen analog of the ene synthesis (15-23). This reaction can also be done with unactivated aldehydes and ketones " if Lewis-acid catalysts such as dimethylaluminum chloride (Me2AlCl) or ethyMuminum dichloride (EtAlCl2) are used. Lewis acid catalysts... 

Two different alkenes can be combined selectively with metal hydride catalysts. This is termed a codimerization and is selective when one alkene reacts only with the hydride and the other alkene selectively inserts into the metal alkyl formed from the hydride. A good example of a selective reaction is the titanium tetrachloride-dimethylaluminum chloride-catalyzed reaction of ethylene with higher terminal olefins in which the... 

Alkynylation of enones. The nickel catalyst obtained from Ni(acac)2 and DIBAH catalyzes the conjugate addition of organoaluminum acetylides even to transoid enones. The acetylenic aluminum reagent is prepared according to Fried s procedure (4, 144) from a lithium acetylide and dimethylaluminum chloride. In all cases only 1,4-addition is observed. -Examples ... 

Chien [21] reported on the kinetics of ethylene polymerization on a soluble catalyst based on Cp TiCl and dimethylaluminum chloride, (CH3)2AlCl. Chien s work confirmed that the active species was a complex containing both Al and Ti, with the titaniiun in the tetravalent state. The chain initiation process was dependent on the Al/Ti ratio, initiation being slower at higher ratios, suggesting that the active complex may be in equilibrium with one or more inactive species. 

However, almost twenty years later, Breslow [19] reported in 1975 a similar polymerization system with very high ethylene polymerization activity if Cp TiCl was added to toluene in the presence of dimethylalu-minum chloride (DMAC) previously treated with water. The combination of Cp TiCl and only dimethylaluminum chloride (DMAC) in the absence of water had low activity as an ethylene polymerization catalyst. The water/ Al molar ratio of 0.2-0.5 provided a highly active catalyst, while a higher water/Al ratio >0.5 provided a very low activity catalyst. Some of Breslows data is shown in Table 4.2. 

Long and Breslow postulated at the time that the interaction of water with dimethylaluminum chloride provided an intermediate material [Me-Al(Cl)-0] that promoted the catalyst activity by interacting with the Cp TiClj. Breslow proposed a structme shown in Figure 4.1 where an alumoxane dimer imderwent a methyl exchange reaction with Cp TiCl ... 

Thus, carbonyl ene reactions can be used to prepare cyclic alcohols with an exo-cyclic double bond in 3-position (Scheme 3.78). To gain stereocontrol with respect to an on-ring methyl group, the Lewis acid catalyst is of predominant importance [127]. Therefore, the action of relatively small aluminum species such as dimethylaluminum chloride on ene-aldehyde 403 leads to cis-cyclohexanol 406 with dr 9 1. If, however, the bulky MABR is used, the trans-isomer 407 is formed with dr 32 1. This reversal of stereoselection may be attributed to an equilibrium of adducts 408 and 409. For small Lewis acids, the tendency of the methyl substituent to adopt an equatorial position prevails and 406 is generated. For an increasing bulk of the Lewis acid, the repulsion between the methyl group and the Lewis acid enforces conformer 409 and hence formation of the trans-cyclohexanol 407. 

Selective cleavage of THP ethers in the presence of t-butyldimethylsilyl ethers can be accomplished by treatment with mild Lewis acids Dimethylaluminum Chloride (—25 °C to 25 °C, 1 h, 98%) or Methylaluminum Dichloride (-25 °C, 0.5 h, 90%). Selective cleavage of THP ethers of primary, secondary, and tertiary alcohols can be carried out in the presence of t-butyldimethylsilyl, acetyl, mesyl, and methoxymethyl ethers, in the presence of thiostannane catalysts Me2Sn(SMe)2-BF3-Et20 (-20 C or 0 °C, 3-25 h, 80-97%). ... 

---