One Other Heteroatom

Isoxazole 4.1, pyrazole 4.2, and isothiazole 4.3 are the parent structures of the 1,2-azole family of heterocycles, having a nitrogen atom plus one other heteroatom in a 1,2-relationship in a five-membered ring. 

The names borazole, boroxole and borthiole, respectively, for the three compounds in Examples 4, 5 and 6 have been abandoned long ago as they imply five-membered rings in the Hantzsch-Widman system. The names borazin(e), boroxin and borthiin indicate six-membered rings with unsaturation and only one boron atom and one other heteroatom (although the order of the element name stems is wrong) and are also not acceptable. 

Fully unsaturated five-membered ring systems with one other heteroatom adjacent to phosphorus have only been known only since 1986 <86CB4io> and therefore no representative is contained in Comprehensive Heterocyclic Chemistry, First Edition (CHEC-I) <84CHEC-i(i)523>. They include now... 

Ring Systems with One Phosphorus, One Oxygen, and One Other Heteroatom... 

One Other Heteroatom.—Structure (112) and larger-ring homologues have been synthesized by acyloin reaction of the diesters (113), and the phosphepin (115) has been reported as the unexpected major product of thermolysis of (114) in an interesting cyclodephosphonation reaction the expected product (116) was formed in 10 % yield. Several interconversions at Si in the system (117) have been examined and the borepanes (118) are suggested intermediates in the conversion outlined in Scheme 1. ... 

As usual, the largest number of publications dealing with heterocyclic organoboron compounds have concerned molecules containing boron and nitrogen and, in many cases, at least one other heteroatom. Phosphiniminoboranes (21), prepared from iV-trimethylsilyl-triphenylphosphinimine and diorganylhalogeno-boranes, have been found to be dimeric in solution but monomeric in the vapour state. ... 

The electronic environment of an a-substituent on pyridine (319) approaches that of a substituent on the corresponding imino compound (320) and is intermediate between those of substituents on benzene and substituents attached to carbonyl groups (321, 322) (cf. discussion in Chapter 2.02). Substituents attached to certain positions in azole rings show similar properties to those of a- and y-substituents on pyridine. However, the azoles also possess one heteroatom which behaves as an electron source and which tends to oppose the effect of other heteroatom(s). 

As with the crowns, the situation becomes more complicated when there are other heteroatoms or substituents in one of the cryptand bridges. The symbol B is used to designate a benzo- or catechol unit in the bridge and subscripts are used to designate heteroatoms when non-oxygen heteroatoms are present. Examples of this are shown in structures 12 and 13 above. 

From the general structure of 1,2-diheterocines 1 with identical heteroatoms other than the scries with two nitrogen atoms (X — NR, which have already been discussed in Section 1.2. as they can be regarded as 1,2-dihydro-l, 2-diazocines), only one other example of such a 1,2-di-hcterocine system exists i.e. dibenzo derivative 4 containing the 1,2-dithiocin system 2.1... 

Tervalent organophosphorus compounds containing one single P-N bond with the valency of each atom saturated by protons or carbons (but no other heteroatoms) have been known since their discovery by MichaeUs more than one century ago [ 1 ] and named indistinctly as aminophosphanes, phosphanamines, phosphazanes, or phosphinous amides. This last chemical nomenclature is the one used by the Chemical Abstracts Service (CAS) for indexing these compounds and is also the one that best delimits the scope of this review those species derived from the parent H2P-NH2 (phosphinous amide in CAS nomenclature) by partial or total substitution of protons by hydrocarbon radicals (Table 1). 

Systems with one extra N atom and one extra other heteroatom, 1 1 (Section 12.04.3). The other heteroatoms are O, S, and Se. No system with heteroatoms different from these has been reported in the reviewed period. 

Bicyclic 6-6 N-Bridgehead Systems with One Extra Nitrogen and One Extra Other Heteroatom 1 1, and Their Benzo-Fused Derivatives... 

This chapter provides an update of Chapter 8.33 in CHEC-II(1996) <1996CHEC-II(8)863>. The work carried out on the bicyclic ring systems with ring junction P, As, Sb, or Bi has focused primarily on phosphorus. Very little work has been done on the other heteroatoms and as such the synthesis and reactivity of these compounds have been reviewed as one section, Section 12.12.7. Most of the compounds in this class contain more than one heteroatom, the additional atoms usually being oxygen and nitrogen. 

Likewise, the initial proton polarization may be transferred to other magnetically active heteronuclei, most attractively to those associated with a low y-value of their nucleus (i.e., to 15N, 29Si), and similarly difficult ones, using heteroatom PHIP at low magnetic fields [8, 45]. 

The picture is less clear when free electron pairs on other heteroatoms (O, S, etc.) are involved. There is only one clear-cut case providing evidence concerning endocyclic sulfoxides The unusually large shieldings of carbon atoms in gauche position relative to an axial oxygen atom in thiane oxides 79, 80, 82, and 83 (p. 252) (200,201) have been partially ascribed to an upfield-shifting effect of the equatorial antiperiplanar lone pair (cf. Section III-C). 

One-electron oxidation of organoselenium and organotellurium compounds results in initial formation of a radical cation (equations (19) and (20)). The eventual fate of the radical cation depends on several variables, but is typically a Se(lV) or Te(lV) compound. The scope of this section will be the one-electron oxidation of selenides and tellurides that are not contained in a heteroaromatic compound, and ones in which the Se and Te are bonded to two carbons, rather than to other heteroatoms. Tellurium- and selenium-containing electron donor molecules have been reviewed. 

There is no simple explanation for the much more pronounced instability to pressure of CO compared to N2. Since the only structural difference arises from the heteroatomic character of CO, one could expect that the molecular dipole moment increases with pressure leading to a higher compressibility of CO. But no evidence for this is obtained from either the ab initio calculation or experimentally. In fact the equation of state of nitrogen and carbon monoxide are practically coincident in the pressure range of interest. One other point of interest is the head-to-tail disorder present in carbon monoxide because it has been observed in several high pressure experiments that defects and disorder can play an important role. 

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