Silicon bridges

The B - C migration can also be induced by other types of electrophiles. Trimethylsilyl chloride or trimethylsilyl triflate induces a stereospecific migration to form (3-trimethylsilyl alkenylboranes having cis silicon and boron substituents.34 It has been suggested that this stereospecificity arises from a silicon-bridged intermediate. 

C. Xu, H. Yamada, A. Wakamiya, S. Yamaguchi, and K. Tamao, Ladder bis-silicon-bridged stilbenes as a new building unit for fluorescent Tr-conjugated polymers, Macromolecules, 37 8978-8983, 2004. 

K. Okita, Y. Harima, K. Yamashita, and M. Ishikawa, Synthesis and optical, electrochemical, and electron-transporting properties of silicon-bridged bithiophenes, Organometallics, 18 1453-1459 (1999). 

The silicon-bridged [l]ferrocenophane LXXXV undergoes polymerization at 120-150°C to yield polymers of number-average molecular weights up to 105 with PDI 1.5-2.5... 

Additionally, further derivatives of sila-dimetacrine [compounds with other substituents bound to the silicon bridge atom (729-131), compounds with a seven-membered central ring containing a Si-Si bridge (132-134), compound 135 with a chlorine atom bound to the tricycle] and sila-melitracene (compounds 136 and 137) were synthesized93, 94). 

The triimide is stable at atm. temp. it decomposes above 400°, chiefly with the rupture of the silicon bridge, and the formation of some silicocyanogen, Si2N2. These compounds are very sensitive to oxygen, and particularly to moisture. A. Stock and F. Zeidler obtained silicon dihydrotriimide ... 

The photolysis of silicon-bridged dihydroaromatic compounds has been utilized to prepare reactive organosilicon species such as disilenes. Thus, photolysis of the... 

Braunstein and coworkers have shown (vide supra) that in the bimetallic Fe—Pt complex (CO)3(SiR3)Fe(yU.-PPh2)Pt(PPh3)CO(Fe—Pt), a silyl migration occurred from the Fe to the Pt center (Scheme 17)102. Remarkably, the first example of a well-characterized insertion of a transition-metal fragment into a strained silicon-carbon bond of a silicon-bridged [l]ferrocenophane was recently reported by Sheridan, Lough and Manners. The... 

Silicon-bridged [l,l]metallocenophanes of type 40 in Scheme 14 represent another class of dinuclear compounds with the ligand system (30). The synthesis of the 1,1,12,12-tetramethyl [1,1] silaferrocenophane was reported by two groups only recently. In one procedure, the dilithium salt of dicyclopentadienyldimethylsilane was reacted with ferrous chloride to give a mixture of the metallocenophane with poly(ferrocenyldimethylsilane) (equation 55)124. In the other procedure, this compound was prepared in a multistep synthesis starting from dilithio ferrocene, as shown in equation 56125... 

There are very few examples of silicon-bridged bent-metallocenes of type 42 in Scheme 14 described in the literature. The class of silicon-bridged [l]-ferrocenophanes is most important due to the exceptional structure and reactivity, and representative examples 43-52 are listed in Scheme 15. 

The special interest in silicon-bridged metallocene units stems from the observation that silicon atoms possess electronic transmission characteristics of unsaturated alkene or alkyne fragments158-161. 

Substantial differences in the electrochemical behaviour have been observed for trifer-rocenes bridged by methylene (compound 80) and by dimethylsilyl (compound 81) units (Scheme 21). Cyclic voltammetry experiments reveal that 80 exhibits three reversible redox processes175, whereas for 81 only two redox events are observed176. The three reversible waves in the carbon-bridged ferrocene (80) are in the ratio 1 1 1 and can be assigned to successive redox events at each iron center, as described in Scheme 21. The two reversible waves in the silicon-bridged ferrocene (81) are in the ratio 2 1 and are consistent with oxidation of alternate ferrocene units, with subsequent oxidation of the intervening iron site (see Scheme 21). 

Comparable effects have been described for silicon-bridged species with up to eight ferrocenyl units176 and also for poly(ferrocenylsilanes) of type 68163. The difference in the electrochemical behaviour of carbon and silicon substituted multiferrocenes may be explained by the different electron donating or withdrawing effects exerted by these substituents on a ferrocene unit. The phenomena described show that a proper choice of the bridges allows one to determine whether a molecule with three or more ferrocenyl units is first oxidized in the center or at the periphery151. 

SCHEME 21. Carbon- or silicon-bridged triferrocenes and their redox behaviour... 

Hartree-Fock methods overview, 1, 645 lanthanide hydrocarbonyls, 4, 4 polyhedral carboranes, 3, 50 silicon-bridged ansa-titanocenes, 4, 624 titanacycles, 4, 568 zinc species, 2, 316... 

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