Alkoxides tertiary

Michael condensations are catalyzed by alkaU alkoxides, tertiary amines, and quaternary bases and salts. Active methylene compounds and aUphatic nitro compounds add to form P-substituted propionates. These addition reactions are frequendy reversible at high temperatures. Exceptions are the tertiary nitro adducts which are converted to olefins at elevated temperatures (24). 

Mixed anhydrides (234 equation 126) are formed in the reaction of silver benzoate with imidoyl chlorides. Imidoyl chlorides react with alcohols or phenols in the presence of base (alkoxide, tertiary amine) to give imidates, e.g. (235 equation This method is especisdly useful for the... 

Amidinium salts (obtainable by various methods, compare Section 2.1.2.5) which are not peralkylated at nitrogen are converted to amidines by treatment with strong bases, e.g. aqueous alkali metal hydroxides, alcoholic alkoxides, tertiary amines etc. As already mentioned it is not difficult to alkylate amidines (see Section 2.7.2.5.4), thus by subsequent alkylation and deprotonation reactions amidines can be synthesized carrying the desired substituents at nitrogen, e.g. (308 Scheme 47). A series of trisubstituted formamidines have been prepared by reaction of anilines with the azavinylogous form-amidinium salt (309). ... 

Secondary and tertiary alkyl halides are not suitable because they react with alkox ide bases by E2 elimination rather than by 8 2 substitution Whether the alkoxide base IS primary secondary or tertiary is much less important than the nature of the alkyl halide Thus benzyl isopropyl ether is prepared m high yield from benzyl chloride a pri mary chloride that is incapable of undergoing elimination and sodium isopropoxide... 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Other Rea.ctions, The photolysis of ketenes results in carbenes. The polymeriza tion of ketenes has been reviewed (49). It can lead to polyesters and polyketones (50). The polymerization of higher ketenes results in polyacetals depending on catalysts and conditions. Catalysts such as sodium alkoxides (polyesters), aluminum tribromide (polyketones), and tertiary amines (polyacetals) are used. Polymers from R2C—C—O may be represented as foUows. 

Alkaline earth metal alkoxides decompose to carbonates, olefins, hydrogen, and methane calcium alkoxides give ketones (65). For aluminum alkoxides, thermal stability decreases as follows primary > secondary > tertiary the respective decomposition temperatures are ca 320°C, 250°C, and 140°C. Decomposition products are ethers, alcohols, and olefins. 

Higher alkoxides, such as tetra(2-ethylhexyl) titanate, TYZOR TOT [1070-10-6], can be prepared by alcohol interchange (transestenfication) in a solvent, such as benzene or cyclohexane, to form a volatile a2eotrope with the displaced alcohol, or by a solvent-free process involving vacuum removal of the more volatile displaced alcohol. The affinity of an alcohol for titanium decreases in the order primary > secondary > tertiary, and... 

In the case of tertiary and some of the more complex alcohols, the use of alkaU hydroxides is not feasible, and it is necessary to use reagents such as sodium hydride, sodium amide, or the alkaU metal to form the alkoxide ... 

Many aromatic steroids submitted to the Birch reduction contain hydroxyl groups which are deprotonated to the corresponding alkoxides during the reduction, particularly if a tertiary alcohol is used as the proton donoi. The steroidal alkoxides and the one derived from the proton donor often precipitate and cause foaming of the reaction mixture, as was noted by Wilds and Nelson. These alkoxides can be kept in solution by adding an excess of the proton donor alcohol to the mixture the alcohol also assists in dissolving the starting hydroxylic steroid. A particularly useful reaction medium for hydroxylic steroids contains ammonia, tetrahydrofuran and -butyl alcohol in the volume ratio of 2 1 (Procedure 2, section V). This mixture... 

The activation energy of substitution of an unactivated aromatic halide (e.g., fiuorobenzene and 2-chloronaphthalene ) is over 30 kcal while that of activated compounds is 5-20 kcal. For the tabulated reactions (Tables II-VIII) with alkoxide and with primary, secondary, or tertiary amines, resonance activation (cf. 278 and 279) by ortho or para nitrogens is found to be greater than inductive activation (cf. 251). This relation is qualitatively demonstrated in... 

Together with a shift of the proton from the a-carbon to the alkoxide oxygen, the tertiary amine is eliminated from the addition product to yield the unsaturated product 3. Early examples of the Baylis-Hillman reaction posed the problem of low conversions and slow reaction kinetics, which could not be improved with the use of simple tertiary amines. The search for catalytically active substances led to more properly adjusted, often highly specific compounds, with shorter reaction times." Suitable catalysts are, for example, the nucleophilic, sterically less hindered bases diazabicyclo[2.2.2]octane (DABCO) 6, quinuclidin-3-one 7 and quinuclidin-3-ol (3-QDL) 8. The latter compound can stabilize the zwitterionic intermediate through hydrogen bonding. ... 

Tertiary magnesium alkoxides 20, bearing a /3-hydrogen, may undergo a dehydration reaction upon acidic workup, and thus yield an alkene 21 ... 

A variant of the Williamson ether synthesis uses thallium alkoxides. The higher reactivity of these can be of advantage in the synthesis of ethers from diols, triols and hydroxy carboxylic acids, as well as from secondary and tertiary alcohols on the other hand however thallium compounds are highly toxic. 

The ether-forming step is an S -like reaction of the alkoxide ion on the silicon atom, with concurrent loss of the leaving chloride anion. Unlike most Sn2 reactions, though, this reaction takes place at a tertiary center—a trialJkyl-substituted silicon atom. The reaction occurs because silicon, a third-row atom, is larger than carbon and forms longer bonds. The three methyl substituents attached to silicon thus offer less steric hindrance to reaction than they do in the analogous ferf-butyl chloride. 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

The details of the mechanism are poorly understood, though the oxygen of the alcohol is certainly attacking the carbon of the isocyanate. Hydrogen bonding complicates the kinetic picture. The addition of ROH to isocyanates can also be catalyzed by metallic compounds, by light, or, for tertiary ROH, by lithium alkoxides ° or n-butyllithium. ° ... 

This is not a new reaction. This is just an Sn2 reaction. We are simply using the alkoxide ion (ethoxide in this case) to function as the attacking nucleophile. But notice the net result of this reaction we have combined an alcohol and an alkyl halide to form an ether. This process has a special name. It is called the Williamson Ether Synthesis. This process relies on an Sn2 reaction as the main step, and therefore, we must be careful to obey the restrictions of Sn2 reactions. It is best to use a primary alkyl halide. Secondary alkyl halides cannot be used because elimination will predominate over substitution (as seen in Sections 10.9), and tertiary alkyl halides certainly cannot be used. 

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