Carbon, tetra-substituted

A phase change takes place when one enantiomer is converted to its optical isomer. As illustrated in Figure 9, when the chiral center is a tetra-substituted carbon atom, the conversion of one enantiomer to the other is equivalent to the exchange of two electron pairs. This transformation is therefore phase inverting. 

Figure 9. The phase-inverting transformation of chiral system with a tetra-substituted carbon atom.

A solution prepared by dissolving 2 g. of biomine in 100 g. of carbon tetra. chloride is satisfactory. Carbon tetrachloride is employed because it is an excellent solvent for bromine as well as for hydrocarbons it possesses the additional advan. tage of low solubility for hydrogen bromide, the evolution of which renders possible the distinction between decolourisation of bromine due to substitution or due to addition. 

Specifically, the reaction of trifluoromethyl radicals with carbon tetra-iodide produces perfluoro-tert-butyl iodide and perfluoroneopentane in the ratio of 3 1. Incomplete substitution is presumably due to steric factors around the crowded, central carbon atom. 

The hindered secondary amines can be highly effective photostabilizers for various polymers (]+.,5.,.6) Various hindered amines have been shown to retard oxidation, but most share the common feature of being secondary or tertiary amines with the a-carbons fully substituted. The most widely exploited representatives of this class are based on 2,2,6,6-tetramethylpiperidine either in the form of relatively simple low molecular weight compounds, or more recently as backbone or pendant groups on quite high molecular weight additives ( i.,5.,6). The more successful commercial hindered amines contain two or more piperidine groups per molecule. Photo-protection by tetra-methylpiperidines (near UV transparent) must result from the interruption of one or more of the reactions 1 to 3. Relatively recent results from our own laboratories, and in the open literature will be outlined in this context. 

Dithiocarbamates have been prepared by the reaction of carbon disulfide with primary or secondary amines. The addition of DMF, cesium carbonate and a sulfonamide to the crude dithiocarbamate, give di-, tri- and tetra-substituted thioureas in 65-76% yields (Scheme 41).121... 

A chlorine atom attached to a tetra-substituted carbon carrying an isocyanato group can also be replaced by fluorine under the action of antimony(III) fluoride.6... 

Some useful resins that have found their way into quite substantial applications are the carbonate resins. For instance, trimethylammonium bicarbonate resin (8) is used to neutralize carboxylic acids or strong mineral acids formed in situ in specific reactions. An interesting example is the quenching/scavenging of acid bromide reported by Staufer and Kat-zenellenbogen74 during the synthesis of tetra-substituted pyrazoles (40) from immobilized starting materials as seen in Fig. 18. 

Tau P, Nyokong T (2007) Electrocatalytic oxidation of nitrite by tetra substituted oxotita-nium(IV) phthalocyanines adsorbed or polymerized on glassy carbon electrode. J Electroanal Chem 611 (1—2) 10—18... 

The 1995 Aldrich catalogue lists 875 monosubstituted benzenes (44 with an alkyl carbon atom and 831 with a carbon other than alkyl or a heteroatom as the direct link) 407 1,2-disubstituted, 243 1,2,3-trisubstituted, and 65 1,2,3,4-tetra-substituted benzenes. We thank Dr. S. J. Branca, Aldrich Chemical Co., for this information. 

Recognizing that the substitutents residing on a tetra-substituted carbon atom are spherically spaced equidistant from one another, if all substituents are connected with lines, a tetrahedron is formed. Furthermore, as shown in Figure 4.1, if all four substituents are unique, they can be arranged in two configurations where the two molecules are mirror images and not superimposable. Because these two molecules are identical in composition but not configuration in three-dimensional space, they are referred to as... 

The halogenation reaction proceeds in darkness and is reasonably considered as ionic. It has been shown that if chlorine gives primarily free radical addition on mono- and di-substituted alkenes, it gives ionic substitution products with tri- and tetra-substituted alkenes [73], A model compound study, together with NMR analysis of commercial chloro and bromobutyl samples, confirmed that the reaction on isoprenyl unit leads predominantly to the exomethylene-substituted structure A, and this is explained by steric hindrance due to the tetra-substituted carbon in f3-position which favors proton elimination rather than the nucleophilic attack of halide counter ion in the second phase of addition (Fig. 11, Table 1) [74,75]. 

The rate-determining step in the ionic hydrogenation reaction of carbon-carbon double bonds involves protonation of the C==C to form a carbocation intermediate, followed by the rapid abstraction of hydride from the hydride source (equation 45). ° There is a very sensitive balance between several factors in order for this reaction to be successful. The proton source must be sufficiently acidic to protonate the C—C to form the intermediate carbocation, yet not so acidic or electrophilic as to react with the hydride source to produce hydrogen. In addition, the carbocation must be sufficiently electrophilic to abstract the hydride from the hydride source, yet not react with any other nucleophile source present, i.e. the conjugate anion of the proton source. This balance is accomplished by the use of trifluoroacetic acid as the proton source, and an alkylsilane as the hydride source. The alkene must be capable of undergoing protonation by trifluoroacetic acid, which effectively limits the reaction to those alkenes capable of forming a tertiary or aryl-substituted carbocation. This essentially limits the application of this reaction to the reduction of tri- and tetra-substituted alkenes, and aryl-substituted alkenes. 

Tertiary alkylzirconium compounds appear to be generally inaccessible even as intermediates. Tetra-substituted alkenes are not hydrozirconated at all also, zirconium is unable to migrate past a tertiary position in a carbon chain. Attempted hydrozirconation of an internal alkene which is blocked at both ends (20 equation 26) gives only isomerized alkene (21). The only exception, which appears to involve a tertiary intermediate, is found in the hydrozirconation of dienol (22) in equation (27). ... 

This carbenoid approach is not without synthetic complications. It has been demonstrated that bicyclobutane will be predominant if the geminal dibromocyclopropane is tetra-substituted at the remaining carbons. Otherwise allenic compounds will result, as has been shown by the work of Doering and La Flamme . Nevertheless, as shown in equation 9, tetra-substitution does not guarantee that bicyclobutane will be the sole product . ... 

In the recent total synthesis of sphingofungin E (90), Overman rearrangement of an allylic trichloroacetimidate derived from diacetone-D-glucose 91 generated tetra-substituted carbon... 

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