Halide metal activation

Many methods have been reported for the addition of allylic groups, including allyltrialkyltin compounds" (in the presence of BF3-etherate), as well as other allylic metal compounds." Allylic alcohols and homo allylic alcohols add to aldehydes in the presence of Sn(OT02 " For allylic halides, both activated... 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

As an additional probe of metal activity, we monitored benzene hydrogenation activity. As seen in Figure 9, Pt-containing rare earth catalysts have lower hydrogenation activity than chlorided alumina catalysts this result reflects inhibition of metal activity on these supports relative to conventional transitional alumina supports. Whereas the acid strength can be adjusted close to that of chlorided and flourided aluminas, metal activity is somewhat inhibited on these catalysts relative to halided aluminas. This inhibition is not due to dispersion, and perhaps indicates a SMSI interaction between Pt and the dispersed Nd203 phase. 

As noted in Section 11.2.2, nucleophilic substitution of aromatic halides lacking activating substituents is generally difficult. It has been known for a long time that the nucleophilic substitution of aromatic halides can be catalyzed by the presence of copper metal or copper salts.137 Synthetic procedures based on this observation are used to prepare aryl nitriles by reaction of aryl bromides with Cu(I)CN. The reactions are usually carried out at elevated temperature in DMF or a similar solvent. 

Metallic zinc is typically covered with a zinc oxide layer that must be removed before the metal can engage in an oxidative addition reaction with organic halides. This activation can be done in one of several ways. 

Restoration of the decayed species to its active valence is thus the key to reactivating the catalyst. It has been known that organic halides with activated C—Cl bonds can add to lower-valent transition metals and convert them to their higher oxidation states by oxidative addition (12-16) ... 

Probably the earliest quantitative experiments on what are now known as cationic polymerisations were made by Gwyn Williams (1938) with styrene and stannic chloride, and by the early 1940s the general belief had become established that in reactions initiated by metal halides the active species is a cation. It appears that Evans and Meadows [3] were the first to state specifically that in hydrocarbon solvents the propagating cations must be paired with the anions and Plesch [4] made the first attempt at calculating the dissociation constant, KD, for an ion-pair comprising a growing cation in a hydrocarbon solvent ... 

The positive modifiers are exemplified by Higashimura s system of monomer + HX (equivalent to HMX) + ZnX2, where the C-X bond of the ester (organic halide) is activated by the metal halide. The transition state (II) for this reaction has been given above. Evidently, the active species PnX.ZnX2 is a reversibly formed donor (PnX) - acceptor (ZnX2) complex. 

Esters, such as alkyl halides, which are too inert to propagate, require a positive modifier (acceptor), e.g., a metal halide, to activate them by forming a D-A complex. 

Arylzinc reagents can be made from aryl halides with activated zinc118 or from Grignard reagents by metal-metal exchange with zinc salts.119... 

Several other Lewis acid metal halides are active (A1C13, SnCl4, FeCl3, AsC13, SbCl3), but none appeared to be more effective than titanium tetrachloride. 

Organometallic compounds which contain a carbon-metal bond are the most reactive carbon nucleophiles. In most cases they are also powerful bases and must be prepared and used under strictly anhydrous and aprotic conditions. A very common way to produce organometallic compounds is to reduce alkyl halides with active metals. Grignard reagents and organolithium compounds are routinely produced in this manner. The transformation is a two-electron reduction of the alkyl halide to a carbanion equivalent the metal is oxidized. 

There are also catalysts that lack any apparent source of metal-carbon bonds. These catalysts include the aforementioned alumina- and silica-sup-ported transition metal oxides (which, in principle, do not demand any activation by organometallic compounds), and also several group 6-8 transition metal chlorides (soluble in hydrocarbons or chlorohydrocarbons), most typically RuC13. Some of these transition metal halides require activation by a cocatalyst of the Lewis acid type (e.g. A1C13, GaBr3, TiCU) [66,67], Noble metal chlorides may be used in alcoholic solvents or in water containing emulsifiers [68]. 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

Fluorinated aUcyl vinyl and aryl cadmium reagents can be prepared in excellent yields just by stirring the fluorinated halides with activated cadmium metal. These reagents can then be used to prepare the corresponding fluorinated stan-nanes, which are versatile synthetic intermediate, by reacting them with tri-n-butyltin chloride at room temperature. Similar methods have been used to synthesize fluorinated copper reagents. ... 

There are fewer reactions with saturated halides than with unsaturated halides. Generally, the ketones and halides range from Cj-C alkyl and the expected alcohols are produced in 50-70% yields [21,35,36]. The same solvents and metal activation procedures used with the unsaturated halides are also applied in these reactions. 

The SjvAr reaction is another attractive method for diaryl ether synthesis, and reactions of o-nitro- and o-cyanofluorobenzenes with phenols were reported . 7r-Complexation of aryl halides with transition metals activates the aromatic nuclei toward S fAr. Segal employed a ruthenium chlorobenzene complex in the poly(aryl ether) synthesis , and the methodology was extensively studied by Pearson, Rich and their coworkers using manganese complex and later iron and ruthenium complexes in natural product synthesis " . The intramolecular substitution of an aromatic chloride with a phenylalanine derivative takes place at room temperature without racemization (equation 27). 

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