Substituted silabenzenes

As mentioned above, silatoluene (85) is the only substituted silabenzene that has been characterized experimentally241. Schleyer and coworkers245 pointed out that the relative stabilities of structures 86-90 are influenced also by the weakness of Si-H bonds relative to C-H bonds. By this argument, substitution at silicon may therefore alter the relative energies of the various silabenzene isomers. Hanamura and coworkers147 have indeed found that methyl substitution at silicon favors multiply bonded silicon structures including 86, by nearly 20 kcal mol 1 relative to the silylene isomers, in comparison to the energy difference in silabenzene itself. 

FIGURE 40. Density difference plots (substituted aromatic-parent aromatic), taken in a plane 0.7 bohr above the plane of the ring of phenol (a, b) and of 1-hydroxysilabenzene (c, d) in planar (a, c) and perpendicular (b,d) conformations (3-21G//STO-2G). Reprinted with permission from Organo-metallics, 7, 144 (1988). Copyright (1988) American Chemical Society247. 

A general feature of all the density difference plots is the lack of any negative charge buildup on silicon, whether or not the particular substituent is ortho, para or meta directing. For all molecules considered, silicon is the least electronegative atom and therefore prefers to remain positive. This feature appears to determine also the relative stabilities of the ortho-, meta- and para-substituted isomers (see below). 

The second important conclusion is that, of the four possible isomers, substitution at silicon gives the most stable isomer (except for COOH), as shown in Table 29. This is to be expected since all substituents (except silyl) are more electronegative than silicon, so that substitution at the 1-position will produce a stabilizing positive charge at the silicon. Similar conclusions were reached by Apeloig and Kami in their study of substituted silenes106 (see Section V.A.l.b.ii). 

TABLE 29. Relative energies (3-21G, inkcalmol-1) of the four isomers of substituted silabenzenes  

Of the four possible isomers, substitution at silicon produces the most stable isomer (except for COOH), as shown in Table 1. The authors concluded that the degree of stability 

Substituent Relative energies position of substitution Directing effect  

The effect of ipso-substitution on the stability of the silabenzene was evaluated by equation 10,41b which compares the effects of the substituent X in the aromatic system with that in IT3S iX (we think that a comparison with H2C=SiHX would have been a better choice). The results presented in Table 241b show that F, Cl, SH, NO2 and CN 

TABLE 2. Calculated substituent effects on the thermodynamic stability, AE (in kcalmol-1), of (pso-substituted silabenzenes and of substituted benzenes (HF/3-21G)  

TABLE 1. Relative energies (HF/3-21G//HF/STO-2G, kcalmof ) of the four isomers of several substituted silabenzenes (see 17) and the directing effects of the substituents  

TABLE 2. Calculated substituent effects on the thermodyn nic stability, 

---

Closely related studies by Markl using a 2-diazo-l-silacyclohexadiene such as 20 led in some cases to highly substituted silabenzenes,108 109 one of which, 21, was stable at -100°C, based on its 29Si NMR spectrum (Eq. 16). 

The measured IEs for all the germabenzenes were compared and found to be in good agreement with those of substituted silabenzene derivatives, found in the literature81. 

The first unambiguous evidence for the existence of a substituted silabenzene was reported by Barton and coworkers in 197 831,32. The pyrolysis of precursor 19 in the... 

In 1988, Markl and Schlosser51 reported the synthesis of the substituted silabenzene 40 by irradiation of the diazo compound 39, which was found to be stable in solution up to 170 K (equation 12). 

TABLE 30. Calculated substituent effects on the thermodynamic stabilities of substituted benzenes and of ipso-substituted silabenzenes (3-21G AE in kcalmol - lf... 

Along similar lines, l,4-di-tm-butyl-l-chloro-l-silacyclohexa-2,4-diene (80) reacted with lithium diisopropylamide in n-pentane and the formal dimer of a presumably initially formed transient 1,4-di-tm-butylsilabenzene was isolated (equation 153)312. The transient formation of silabenzene was supported by trapping various 1,3-dienes. An even more highly substituted silabenzene was presumably formed as a transient when the diazirine 60 was heated in solution (equation 76)128,249. The products obtained with 2,3-dimethylbutadiene and benzophenone upon thermal decomposition of the diazirine suggest strongly the intermediate formation of a 1-methyl-2,3,4,5-tetraphenylsilabenzene (81) as well as the isomeric fulvene 63. 

Markel and Schlosser reported the synthesis of the substituted silabenzene 3 which was stable in solution up to 170 K . Kinetic stabilization by bulky substituents led to the isolation in an argon matrix of 9, R = MesSi or -PrMe2Si by Jutzi, Maier and coworkers 9, R = -PrMe2Si was stable up to 90 K even without an argon cage . In 1991 Maia-and coworkers isolated and characterized spectroscopically in the gas phase and in an argon matrix at 12 K the 9-silaanthracenes, 10, R = H, Ph . Most recently, Okazaki and coworkers reported the synthesis and isolation of 2-silanaphthalene (1), the first silaben-zenoid compound which is a stable crystalline material even at room temperature. ... 

---