N tert.-Alkyl

N-Tert-Alkyl-Anilides as Bulky Ancillary Ligands 123... 

An extensive screening of structure-activity relations revealed ( 1 ) the outstanding properties of N,N -disubstituted PD. It is generally accepted that the presence of N-sec.alkyls accounts for better antiozonant protection than that of N-prim. and N-tert.alkyls or N-aryls (21). This may be one of the clues to decipher the chemical pathways of the antiozonant mechanism. The final effect is moreover fully dependent on the composition of the vulcanizate. The structure of commercially used antiozonants is an optimum compromise of efficiency, physical properties and toxicity. N, N -Disec.alkyl-1-4-PD are used in the U.S.A., N-sec.alkyl-N -aryl-l,4-PD are preferred in Europe. N,N -Diaryl derivaties are not applied as antiozonants in NR, BR, IR, or SBR. One of the reasons may be their low solubility in rubber vulcanizates (22). It does not allow them to reach a concentration level in the rubber bulk which is able to act as a long-term operative store of a stabilizer ready to supply the rubber surface slowly but continuously with active compounds by migration and to maintain the protective effect without inefficient quick blooming of an incompatible PD. A chemical reason accounting for the minority antiozonant role of N,N -diaryl PD is discussed later. 

Carboxylic acid esters, N-tert. alkyl acyl amines and saturated aldehydes are also described as starting materials in the Koch carboxylic acid syntheses. Koch and Haaf [591] carbonylated tert. hexylformiate with formic acid and obtained a mixture of carboxylic acids as follows 15% Cg- and C -acids, 28 % of 2,2-dimethyl valeric acid, 14 % Cg- and Cg-acids and 43 % higher acids, mainly Cja-acids. The carbonylation of tert. hexyl acetate yielded 11 % Cg- and Cg-acids, 33% 2,2-dimethyl valeric acid, 10% Cg- and Cg-acids and 16% higher acids, mainly Cig-acids. 

Bi(0Tf)3 xH20 has been used for the catalysis of the Ritter reaction (Equation 53) [127]. Thus, a variety of N-tert-alkyl and aryl amides were obtained in good to excellent yield by reaction of various nitriles with tertiary alcohols. 

A number of alternative multi-step procedures for the synthesis of a-tert-alkyl ketones are known, none of which possess wide generality. A previous synthesis of 2-tert-penty1cyclopentanone involved reaction of N-1-cyclopentenylpyrrol 1 dine with 3-chloro-3-methy1-l-butyne and reduction of the resulting acetylene (overall yield 46 ). However, all other enamines tested afford much lower yields. Cuprate addition to unsaturated ketones may be useful in certain cases. Other indirect methods have been briefly reviewed. ... 

In an alternative approach, the isomeric unsaturated pyrrolidine or piperidine aldoximes 245 a and 245b were prepared and subjected to lOOC reaction affording 246a and 246b, respectively (Eq. 28). Esterification of 240 followed by N-tert-BOC protection and DIBALH reduction provided aldehyde 244 (X = 0) which was subjected to Wittig olefination. Introduction of a two carbon aldoxime chain on N in 244 (X = CH2) was carried out by alkylation with Et a-bromoacetate after deprotection of the N atom in 244. Reduction and oxima-tion led to 245. 

Die Reduktion von Amidinen mit Natrium-boranat fuhrt normalerweise zu N,N-Acetalen. Im Fall von N l-AlkenylEN-alkyl-N -tert.-butyl-amidinen [II z.B. aus N -tert.-Butyl-N-methyl-N-(trimethylsilyl-methyl)-formamidinen(I) und Aldehyden oder Ketonen] lassen sich die entsprechenden N,N-Acetale(III) ohne vorhcrige Isolierung in reiner Form durch Behandeln mit 10%iger Salzsaure zu sekundaren Aminen hydrolysieren3. 

The most conspicuous property of aliphatic amines, apart from their fishy smell, is their high basicity, which usually precludes N-alkylations under acidic reaction conditions (last reaction, Scheme 6.3). Hence, alkylation of amines with tertiary alkyl groups is not usually possible without the use of highly stabilized carbocations which can be formed under basic reaction conditions. Rare exceptions are N-alkyla-tions of amines via radicals (Scheme 4.2), copper-catalyzed propargylations (Scheme 6.3), and the addition of amines to some Michael acceptors and allyl palladium or iridium complexes. Better strategies for the preparation of tert-alkylamines include the addition of Grignard reagents to ketone-derived imines [13] or the reduction of tert-alkyl nitro compounds. 

For satisfactory diemo- and stereoselectivity, most catalytic, direct cross-aldol methods are limited to the use of non enolizable (aromatic, a-tert-alkyl) or kineti-cally non enolizable (highly branched, ,/funsaturated) aldehydes as acceptor carbonyls. With aromatic aldehydes, however, enantioselectivity is sometimes moderate, and the dehydration side-product may be important. With regard to the donor counterpart, the best suited pronucleophile substrates for these reactions are symmetric ketones (acetone) and ketones with only one site amenable for enolization (acetophenones). With symmetric cyclic or acyclic ketones superior to acetone, syn/anti mixtures of variable composition are obtained [8b, 11, 19a]. Of particularly broad scope is the reaction of N-propionylthiazolidinethiones with aldehydes, which regularly gives high enantioselectivity of the syn aldol adduct of aromatic, a,fi-unsaturated, branched, and unbranched aldehydes [13]. 

Abb. 7.16. Ritter reaction part I (cf. Fig. 7.17) SN1 reaction between a tertiary alcohol and a nucleophile containing C=N, with hydrogen cyanide acting as the nucleophile. Under reaction conditions the C=N group is tert-alkylated at the N atom and hydrated at the nitrile nitrogen leading to the formation of an W-tert-alkyl-formamide (B). 

The activity of the 4-alkylpyrocatechols (la), 3-alkylpyro-catechols (lb), 3,5-dialkylpyrocatechols (Ic), 3,6-dialkylpyro-catechols (II), 4,5-dialkylpyrocatechols (III), 3,4,6-trialkyl-pyrocatechol (TV), and 4,5- and 4,6-dialkyl-2-alkoxyphenols (V and VI) was studied in isotactic polypropylene at 180 =t 0.1° C. The relative activities Ar were correlated with substituent constants and redox potentials. In the la series the activity of 4-n-alkyl derivatives exceeded that of 4-tert-alkyl derivatives. The length of the main alkyl chain in la and lb and the steric effects in the latter exerted a specific favorable influence. In agreement with these effects, 3-tert-alkyl-5-methyl derivatives are the most active compounds in Series Ic compounds with other combinations of alkyls are weaker antioxidants as are the isomeric substances II and III or compound IV. Etherification of one hydroxyl group exerts an unfavorable effect influences of substitution in 2-alkoxyphenols (V and VI) are generally the same as in the pyrocatechols Z-ZZZ. 

Dirhodium(ll) tetrakis[methyl 2-pyrrolidone-5(R)-oarboxylate], Rh2(5R-MEPV)4, and its enantiomer, Rh2(5S-MEPY)4, which is prepared by the same procedure, are highly enantioselective catalysts for intramolecular cyclopropanation of allylic diazoacetates (65->94% ee) and homoallylic diazoacetates (71-90% ee),7 8 intermolecular carbon-hydrogen insertion reactions of 2-alkoxyethyl diazoacetates (57-91% ee)9 and N-alkyl-N-(tert-butyl)diazoacetamides (58-73% ee),10 Intermolecular cyclopropenation ot alkynes with ethyl diazoacetate (54-69% ee) or menthyl diazoacetates (77-98% diastereomeric excess, de),11 and intermolecular cyclopropanation of alkenes with menthyl diazoacetate (60-91% de for the cis isomer, 47-65% de for the trans isomer).12 Their use in <1.0 mol % in dichloromethane solvent effects complete reaction of the diazo ester and provides the carbenoid product in 43-88% yield. The same general method used for the preparation of Rh2(5R-MEPY)4 was employed for the synthesis of their isopropyl7 and neopentyl9 ester analogs. 

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