Electron sextets

Preparation from Nitrene Intermediates. A convenient, small-scale method for the conversion of carboxyhc acid derivatives into isocyanates involves electron sextet rearrangements, such as the ones described by Hofmann and Curtius (12). For example, treatment of ben2amide [55-21-0] with halogens leads to an A/-haloamide (2) which, in the presence of base, forms a nitrene intermediate (3). The nitrene intermediate undergoes rapid rearrangement to yield an isocyanate. Ureas can also be formed in the process if water is present (18,19). 

Migration to the developing electron sextet at nitrogen is not restricted to hydrogen. In (79) there is methyl migration with formation of methylamine and acetone in the acid-catalyzed decomposition of (80), phenyl migration leads to aniline and acetaldehyde. 

A final statement on the mechanism of the diaziridine formation cannot yet be made. The obvious formulation [Eq. (36) ] as a reaction of the CN double bond with the imen 39 (with an electron sextet) is almost certainly excluded. The formation of 39 as an intermediate has been jiroposed for the Raschig hydrazine synthesis, but has been disjmted. The following facts are against a diaziridine formation corresjionding to Eq. (36) ... 

The ketocarbene 4 that is generated by loss of Na from the a-diazo ketone, and that has an electron-sextet, rearranges to the more stable ketene 2 by a nucleophilic 1,2-shift of substituent R. The ketene thus formed corresponds to the isocyanate product of the related Curtius reaction. The ketene can further react with nucleophilic agents, that add to the C=0-double bond. For example by reaction with water a carboxylic acid 3 is formed, while from reaction with an alcohol R -OH an ester 5 is obtained directly. The reaction with ammonia or an amine R -NHa leads to formation of a carboxylic amide 6 or 7 ... 

The various types of Lewis acids are protons, simple cations, electron-deficient molecules, compounds in which the central atom can expand its octet, and elements with an electron sextet. 

An electron-gain centre of similar geometry and electronic structure is generated (43) by radiolysis of the nitrosocarbonyl Mn(C0)4N0. Spectra associated with the electron-loss centres Mn(C0)nX+ (n=4 or 5) are less well-defined and pose analytical difficulties (41). However, there is little doubt that these are high-spin radicals, probably electronic sextets. 

Combination of 1 with electron-sextet fragments X might offer an approach to kinetically stabilized p7i-p7i systems of the type (Me5C5)2Si = X, provided that two a-bonded pentamethylcyclopentadienyl substituents at... 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

Table 1. Numbers R x,y,z) of Basic Monocyclic Aromatic Rings XxYyZz Having a -Electron Sextet,62 with 4 < (x + y+z) = m<8...

Solutions for a jr-electron sextet n = 1) are presented in Table l.62 Analogous tables were obtained for a doublet of jr-electrons (n = 0) and for a decet (n = 2) but are not displayed here. 

In addition to the above imidazole-analogue car-benoid systems, compounds with the electron sextet at N, P, or As between the two nitrogens have also been obtained (Scheme 56) 174 175 2-chloro-l,3,2-dia-... 

Borepin is the heterocyclic analogue of the tropyl-ium cation with a jr-electron sextet. Few borepin derivatives have been reported a monocyclic 1-meth-ylborepin 202,245 benzo derivatives 204,246 and a dithienoborepin 205247 (Scheme 77). Heptaphenyl-... 

In addition to the energetics of 162, 179-Za and 180, Scheme 6.42 contains those of the boraisobenzene 178, which represents the last possible system of that kind with one heteroatom of the second row of the periodic table. The allene structure 178 was indeed calculated to be the ground state, but AG298 of the zwitterion 178-Z2 is only 2 kcal mol-1 greater. The polarization of that state is opposite to that of 179-Za. To allow an aromatic jt-electron sextet to form, the cr-orbital at the central carbon atom of the allene system must remain empty [120], Experiments directed towards the observation of 178 or a derivative thereof seem to be unknown. 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

Hence, the possibility to acquire aromaticity (conferred by the presence of six n electrons in the five-carbon-membered ring) considerably increases the electron affinity of this ring. As a result, one of the two n electrons of the P=C bond remains on the phosphorus atom, and the other combines with the excess electron to create the cyclopentadienyl n-electron sextet. The situation is analogous to that in the diphenylfulvene anion-radical as analyzed in Chapter 3 (see Section 3.2.2). 

Fig. 1. Geometries of the chlorine oxyfluoride molecules and their ions compared to those of the corresponding chlorine fluorides. Since the C1+ cation would possess only an electron sextet, it is stabilized by a GIF molecule to form the C12F+...

In the ease of pyrrole this should require the destruction of the stable -electron sextet of the ring, Therefore its N—H frequency does not... 

In principle, recently published19 rules for writing structural formulas have been adhered to in the present review with the one exception that a full circle in the cycle is used to denote not only a 7r-electron sextet but any number of 7r-electrons consistent with the Hiickel rule in general. 

If, in these formulations, the aluminum, with its electron sextet and its tendency to form an electron octet, is decisive for the reaction course, the question naturally arises whether such a formulation also applies to the other metals which are catalytically effective. The answer is positive, returning to the starting point of the considerations, that is, to the existence of the complex Na(Al(OH)4). It becomes evident that similar addition compounds actually exist with other metals which are catalytically effective. Literature (70, 77, 18) on the subject describes among others, HZn(OH)3, H2Pb(OH)6, nd H2Sn(OH)6. 

The five-membered aromatic heterocycles pyrrole (5), furan (6) and thiophene (7) are formally derived from cyclopentadienyl anion by replacement of one CH group with NH, O or S, each of which can contribute two p-electrons to the aromatic ir-electron sextet. Heteroatoms of this type have in classical structures only single bonds and are called pyrrole-like . Other five-membered aromatic heterocycles are derived from compounds (5), (6) and (7) by further replacement of CH groups with N, 0+ or S+. 

Substituent increments are obtained as usual by subtracting the reference shifts of naphthalene (C-1,4,5,8 127.7 C-2,3,6,7 125.6 C-9,10 133.3 ppm) from the individual data of C-l to C-10 in the substituted derivatives given in Table 4.55. It turns out that comparable Zx, Zmthn, Zmrta, and Zpara increments in naphthalene and benzene differ substantially in magnitude, as exemplified in Table 4.56. In 1-substituted naphthalenes, C-9 increments are attenuated in favor of C-2 relative to comparable ortho effects known from benzene C-3 and C-1 in 2-substituted naphthalenes behave correspondingly (Table 4.56). This can be explained by the cannonical formulae c, which do not contribute so much to the actual molecular state due to disrupted % electron sextets. Full inter-... 

Preparation from Silrene Intermediates. A convenient, small-scale method for the conversion of carboxylic acid derivatives into isocyanates involtcs electron sextet rearrangements, such as the ones described by Hofmann and Cuitius. 

Another index of delocalization, devised and widely applied by Schleyer et al.,163 is the nuclear-independent chemical shielding (NICS) value. When calculated above the ring, this value corresponds to the -contribution which is a significantly negative quantity for situations with aromatic delocalization . Scheme 21 shows the A and NICS values computed by Schleyer et al.141 for benzene, hexasilabenzene, and hexazine. The three species are seen to possess equally aromatic electron sextets, as indeed anticipated from the appreciable vertical resonance ener-... 

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