Heteroatomic substituents

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive dyes. 

In the case of esters, carboxylate anions, amides, and acid chlorides, the tetrahedral adduct may undergo elimination. The elimination forms a ketone, permitting a second addition step to occur. The rate at which breakdown of the tetrahedral adduct occurs is a function of the reactivity of the heteroatom substituent as a leaving group. The order of stability of the... 

Aziridines bearing heteroatom substituents are best prepared through treatment of the corresponding azirines with heteroatom nucleophiles. Thus, azirine carbox-ylates (in this case prepared by thermal decomposition of the corresponding vinyl... 

In cases where the heteroatom substituent is the medium (M) group, the cyclic and the open-chain model predict the same stereochemistry. In cases where the heteroatom substituent is small (S), the two models predict opposite stereochemical results. This leads to an order of stereospecificity, with the stereospecificity highest when both models predict the correct stereochemistry, with substantially lower specificity when the cyclic model only applies, and with the lowest degree of stereospecificity when only the open-chain model predicts the correct stereochemical result. 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Assuming that a carbonyl compound 1 with a substituent Y (which may be either a heteroatomic substituent or an alkyl group) forms enolate 2 exclusively, and that the aldehyde 3 functions as the only carbonyl-active component12, four stereoisomeric products 4a, 4b, 5a, 5 b may result. 

Transfer to monomer is of particular importance during the polymerization of allyl esters (113, X=()2CR), ethers (113, X=OR), amines (113, X=NR2) and related monomcrs.iw, 8, lb2 The allylic hydrogens of these monomers arc activated towards abstraction by both the double bond and the heteroatom substituent (Scheme 6.31). These groups lend stability to the radical formed (114) and are responsible for this radical adding monomer only slowly. This, in turn, increases the likelihood of side reactions (i.e. degradative chain transfer) and causes the allyl monomers to retard polymerization. 

Meerwein reactions can conveniently be used for syntheses of intermediates which can be cyclized to heterocyclic compounds, if an appropriate heteroatom substituent is present in the 2-position of the aniline derivative used for diazotization. For instance, Raucher and Koolpe (1983) described an elegant method for the synthesis of a variety of substituted indoles via the Meerwein arylation of vinyl acetate, vinyl bromide, or 2-acetoxy-l-alkenes with arenediazonium salts derived from 2-nitroani-line (Scheme 10-46). In the Meerwein reaction one obtains a mixture of the usual arylation/HCl-addition product (10.9) and the carbonyl compound 10.10, i. e., the product of hydrolysis of 10.9. For the subsequent reductive cyclization to the indole (10.11) the mixture of 10.9 and 10.10 can be treated with any of a variety of reducing agents, preferably Fe/HOAc. 

This approach did not seem to be as satisfactory for those sulfamates having heteroatom substituents (hetero-sulfamates). Spillane suggested that the various electronic effects of the hetero-atoms probably introduce an additional variable that is apparently absent, or constant, for the carbosulfamates. Because molecular connectivity correlates structure with molecular volume and electronic effects, Spillane included molecular connectivity, (computed for the entire molecule, RNHSOO to the four variables, x, y, z, and V, and applied the statistical technique of linear-discrimination analysis to 33 heterosulfamates (10 sweet, 23 not sweet). A correlation of >80% was obtained for the x, z, x subset 5 of the 33... 

Heteroatom substituents also introduce polar effects. In the case of a-alkoxy aldehydes the preferred TS appears to be F and G for the E- and Z-enolates, respectively. These differ from the normal Felkin TS for nucleophilic addition. The reactant conformation is believed to be determined by minimization of dipolar repulsion between the alkoxy substituent and the carbonyl group.96 This model predicts higher 3,4-anti ratios for Z-enolates, and this is observed. 

Ono and Kamimura have found a very simple method for the stereo-control of the Michael addition of thiols, selenols, or alcohols. The Michael addition of thiolate anions to nitroalkenes followed by protonation at -78 °C gives anti-(J-nitro sulfides (Eq. 4.8).11 This procedure can be extended to the preparation of a/jti-(3-nitro selenides (Eq. 4.9)12 and a/jti-(3-nitro ethers (Eq. 4.10).13 The addition products of benzyl alcohol are converted into P-amino alcohols with the retention of the configuration, which is a useful method for anri-P-amino alcohols. This is an alternative method of stereoselective nitro-aldol reactions (Section 3.3). The anti selectivity of these reactions is explained on the basis of stereoselective protonation to nitronate anion intermediates. The high stereoselectivity requires heteroatom substituents on the P-position of the nitro group. The computational calculation exhibits that the heteroatom covers one site of the plane of the nitronate anion.14... 

Heteroatomic Substituents. The effect of attaching a heteroatomic substituent to the carbyne carbon, for example, NMe2, is to remove the degeneracy of the 7r-type LUMO s (28). There is extensive mixing (w-donation) from the lone pair on nitrogen into one of the p orbitals on carbon. The ir -orbital for the N - C -interaction is moved to higher energy, and the other p orbital becomes the sole LUMO. 

Table II summarizes the structural studies of Ru, Os, and Ir carbene complexes with the heteroatom substituents N, O, S, or Se. These X-ray data clearly illustrate two features of the bonding in these compounds ...

The /Tamino alcohol structural unit is a key motif in many biologically important molecules. It is difficult to imagine a more efficient means of creating this functionality than by the direct addition of the two heteroatom substituents to an olefin, especially if this transformation could also be in regioselective and/ or enantioselective fashion. Although the osmium-mediated75 or palladium-mediated76 aminohydroxylation of alkenes has been studied for 20 years, several problems still remain to be overcome in order to develop this reaction into a catalytic asymmetric process. 

Reactions of various terminal alkenes, some of them bearing heteroatom substituents, with Me3Al and a catalytic amount of Erker s chiral neomenthylindene-zirconocene dichloride provide, after oxidation with 02, 2-methyl-l-alkanols in high yields with up to 85% ee [76] (Scheme 8.37). 

Cyclopropylcarbinyl-cyclobutyl ring expansions (Eq. 5) are facilitated by the presence of the heteroatom substituent in the order O > S > Se. In this case, the heteroatom stabilized cyclobutyl cation (see Eq. 39) can suffer hydrolysis to give the... 

The most widely used method for the preparation of [l,2,4]triazolo[3,4-A][l,3,4]thiadiazoles 85 employs 4-amino-5-thio-4/7-[l,2,4]triazoles 83 or 4-amino[l,2,4]-triazole-5(47T)-thiones 84 as starting materials. The reaction of the triazoles 83 or 84 with carbonic acid derivatives furnishes [l,2,4]triazolo[3,4-4][l,3,4]thiadiazoles with a heteroatom substituent (N, O, S) at position 6 the O- and S-functions are formulated as 6-hydroxy and 6-thio derivatives 85a or as thiadiazol-(5/7)6-ones and -thiadiazole-(577)6-thiones 85b, respectively reaction with carboxylic acid derivatives provides the 6-substituted-[l,2,4]triazolo[3,4-4][l,3,4]-thiadiazoles 85c (Equation 20 Table 3). 

By beginning with the simplest type of pz macrocycle, the entirely unsubstituted species 3 (Scheme 3), this section will focus on porphyrazines that do not involve heteroatom substituents. 

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