Carbon substitution

Diaziridines also show slow nitrogen inversion, and carbon-substituted compounds can be resolved into enantiomers, which typically racemize slowly at room temperature (when Af-substituted with alkyl and/or hydrogen). For example, l-methyl-3-benzyl-3-methyl-diaziridine in tetrachloroethylene showed a half-life at 70 °C of 431 min (69AG(E)212). Preparative resolution has been done both by classical methods, using chiral partners in salts (77DOK(232)108l), and by chromatography on triacetyl cellulose (Section 5.08.2.3.1). 

The general mechanism for electrophilic substitution suggests that groups other than hydrogen could be displaced, provided the electrophile attacked at the substituted carbon. Substitution at a site already having a substituent is called ipso substitution and has been observed in a number of circumstances. The ease of removal of a substituent depends on its ability to accommodate a positive charge. This fector determines whether the newly attached electrophile or the substituent is eliminated from the [Pg.588]

Heterocyclic enamines often undergo two-step 1,3 cycloaddition with methyl vinyl ketone. This involves electrophilic attacks by an olefinic carbon and by a carbonyl carbon (24,25). For example, 1,2-dimethyl-Zl -pyrroline (14), when treated with methyl vinyl ketone, produces 1,6-dimethyl-2,3,4,5-tetrahydroindole (15) (24). The requirement which must be met so that this type of cyclization reaction can take place is that the a position of the heterocyclic enamine be carbon substituted. This provides... 

If the a position of an enamine is carbon substituted, providing the possibility of an isomeric enamine, and if the amine group and other substituent groups are sufficiently removed from the sites of electrophilic attack as to not cause any steric interference, then simple alkylation of an enamine by an acrylate ester can be followed with a second electrophilic... 

Among metal borides of the formula MjM B or (Mj, M/r)2B, the competing structural units are (a) the antiprism and (b) the trigonal metal prism. In many cases the CUAI2 structure with BMg-antiprismatic B coordination is adopted in close resemblance to transition-metal silicides, but no boron-carbon substitution is ob-served - " . 

Nelson, D.G.A., Featherstone, J.D.B., Duncan, J.E and Cuttress, T.W. 1982 Paracrystalline disorder of biologieal and synthetic carbonate-substituted apatites. Journal of Denial Research 61 1274-1281. 

The thiophene ring can be elaborated using standard electrophilic, nucleophilic, and organometallic chemistry. A variety of methods have been developed to exploit the tendency for the thiophene ring (analogous to that of furan and pyrrole) to favor electrophilic substitution and metallation at its a-carbons. Substitution at the p-carbons is more challenging, but this problem can also be solved by utilizing relative reactivity differences. 

The active form of fohc acid (pteroyl glutamate) is tetrahydrofolate (Figure 45-15). The folates in foods may have up to seven additional glutamate residues linked by y-peptide bonds. In addition, all of the one-carbon substituted folates in Figure 45-15 may also be present in foods. 

Figure 45-15. Tetrahydrofolic acid and the one-carbon substituted folates.

Figure 45-16. Sources and utilization of one-carbon substituted folates.

Generally, only simple silenes having small groups (H, Me, CH2=CH) are obtained as transient species from the thermolysis of silacyclobutanes. In part this is due to the high temperatures (usually above 450°C) required for the ring cleavage. Substitution on the carbon atom adjacent to silicon in the ring can lead to carbon-substituted silenes. 1,3-Disilacyclobutanes do not readily revert to silenes under thermal conditions, but examples... 

Table 3.15. Effects of y-Carbon Substitutents on Type II Cleavage(89>...

The enantioselective hydrogenation of C=C bonds is included in Chapter 3, where the general nature of heterogeneous enantioselective hydrogenation is fully explored, so here we examine only the narrow subject. Enantioselective hydrogenation of C=C bonds occurs only on prochiral double bonds (at least one of the carbons substituted with two different groups other than hydrogen). A chiral environment is required. 

Figure 1. Comparison between silicon and carbon substituted amides...

The structure has an adamantane skeleton consisting of all six bridge carbons substituted by sulfur atoms. Considering its highly symmetrical structure, it can be assumed to be a quite stable crystalline compound. The history of the preparation of this skeleton is briefly documented in the introduction.4-6... 

Tetra- and penta-coordinate carbon-substituted heavy ketones stabilized by the coordination of nitrogen atoms... 

A carbon-substituted five-membered ring, 24, was also successfully ring-opened via a silyl carbanion intermediate after initiator attack on a silicon adjacent to carbon to give a sequence-ordered C/Si... 

Tsuji and co-workers carbonylated a-carbonate-substituted allenes 113 with carbon monoxide and methanol, which provided 114 in excellent yields (Scheme 14.25) [54], They found that allenylic carbonates are more reactive than simple allylic carbonates and that the reaction proceeded rapidly even at ambient temperature under atmospheric pressure of carbon monoxide. Unfortunately, the poor E/Z selectivity diminishes the synthetic value of this very efficient carbonylation reaction. 

By far most of the reports on addition reactions of hetero-nucleophiles to activated dienes deal with sulfur-nucleophiles17,48,80,120-137, in particular in the synthesis of 7/3-sulfur-substituted steroids which, like their carbon-substituted counterparts (Section n.A), are of interest because of their ability to inhibit the biosynthesis of estrogens80,129-137. Early investigations17,120-122 concentrated on simple acyclic Michael acceptors like methyl sorbate and 2,4-pentadienenitrile. Bravo and coworkers120 observed the formation of a 3 1 mixture of the 1,6- and 1,4-adduct in the reaction of methyl sorbate with methanethiol in basic medium (equation 39). In contrast to this, 2,4-pentadienenitrile adds various thiols regioselectively at C-5, i.e. in a 1,6-fashion (equation 40)17,121,122, and the same is true for reactions of this substrate with hydrogen sulfide (equation 41), sodium bisulfite and ethyl thioglycolate17. 

Both categories of sp-hybridized neutral carbon ligands, namely the all-carbon-substituted and heteroatom-conjugated allenylidene and cumulenylidene ligands, are presented in great detail by V. Cadiemo and S. E. Garcia-Garrido in the sixth and seventh chapters of this volume, respectively. 

All-Carbon-Substituted Allenylidene and Related Cumulenylidene Ligands... 

Abstract Allenylidene complexes have gained considerable significance in the context of transition-metal carbene chemistry due to their potential applications in organic synthesis. The aim of this chapter is to draw together a general presentation of the most efficient synthetic routes, the main structural features and reactivity patterns, as well as current applications in homogeneous catalysis, of aU-carbon-substituted allenylidenes and related cumulenylidene complexes containing an odd number of carbon atoms. 

---