Disilanes, coupling reactions

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

Attempts have been made to prepare polysilanes containing the 8-dimethylaminonaphth-l-yl ligand.863 The coupling reaction of 8-dimethylaminonaphth-l-yl lithium with MeSiClj has given the dichlorosilane 909, whose mild reduction with Mg has surprisingly yielded the disilane 910 rather than the expected polysilane (Scheme 128). The formation of the disilane may be rationalized by the insertion of a transient silylene 904 into an Si-N bond of... 

In recent years Furstner [15, 16] described the very rapidly and completely conversion of chloro-silanes into disilanes by the help of potassium-graphite (CgK). These coupling reactions were characterized by low temperatures (0-25 C), short reaction times (5-60 min), and high yields (> 90 %). So we tried to reduce our silyltriflate derivatives with potassium-graphite and we succeeded in preparing... 

Disilane-coupling products are obtained from the reaction of some vinyl and aryl silyl chlorides with magnesium [Eqs. (72-76) 140,141]. 

Gatalytic G-F activation has been demonstrated in several reactions. Most of the catalytic conversions involving the cleavage of a G-F bond achieve hydrodefluorination reactions (i.e., the replacement of fluorine by hydrogen) or cross-coupling reactions of fluorinated aromatics. Prominent exceptions consist of Murai s rhodium-catalyzed exchange reaction between fluorobenzenes (see Scheme 41) and a disilane as well as the amination of 2-fluoroni-trobenzene. At present, turnover numbers are invariably low and there are no detailed studies of kinetics and mechanism of catalytic G-F activation. 

The Pd-catalyzed cross-coupling reactions of metal nucleophiles with carbon electrophiles are of considerable value for the regio- and stereocontrolled synthesis of functionalized organometalhc compounds, in particular, silanes, stannanes, and boranes, which are important reagents for Pd-catalyzed carbon-carbon cross-coupling as shown in Sects. in.2.2-in.2.4. Symmetrical bimetallic compounds such as disilanes, distannanes, and diborons are usually used as metal nucleophiles. The present metallation is applicable to aryl, benzyl, vinyl, acyl, and aUyl (Sect. V.2.3.3) electrophiles. 

Substituted aroyl- and heteroaroyltrimethylsilanes (acylsilanes) are prepared by the coupling of an aroyl chloride with (Me3Si)2 without decarbonylation, and this chemistry is treated in Section 1.2[629], Under certain conditions, aroyl chlorides react with disilanes after decarbonylation. Thus the reaction of aroyl chlorides with disilane via decarbonylation is a good preparative method for aromatic silicon compounds. As an interesting application, trimel-litic anhydride chloride (764) reacts with dichlorotetramethyidisilane to afford 4-chlorodimethylsilylphthalic anhydride (765), which is converted into 766 and used for polymerization[630]. When the reaction is carried out in a non-polar solvent, biphthalic anhydride (767) is formed[631]. Benzylchlorodimethylsilane (768) is obtained by the coupling of benzyl chloride with dichlorotetramethyl-disilane[632,633]. 

The dehydrogenative coupling of silanes does not stop at the stage of disilanes in the coordination sphere of early transition metals like Zr and Hf, but chain polymers of low molecular weight are formed. As reactive intermediates in this reaction, silylene complexes are also assumed. However, alternative mechanisms have been discussed (sect. 2.5.4). 

Falck has recently reported dehydrogenative silylation of heteroarenes with triethylsilane (18) [97]. Coupling with the Si-H bond of triethylsilane, rather than the disilane Si-Si bond, in conjunction with the use of norbomene that presumably acts as a hydrogen acceptor, gives good yields with indoles, thiophenes, and furans, under relatively mild condition (80°C). Unlike the reaction shown in Scheme 7, silylation of indole did not require protection of the N-H group. 

When a copper anode is used instead of platinum, the resulting chlorosilane is subsequently reduced to a Si—Si coupling product in a one-pot reaction (equation 49)57. Interestingly, when a mixture of hydrosilane and chlorosilane is electrolyzed using a copper anode and a platinum cathode, a Si—Si coupling product is obtained in 64% yield on the basis of the sum of both reagents used. Thus, the paired electrolysis of hydrosilane on the anode and chlorosilane on the cathode proceeds to give disilane (equation 50)57. 

Zirconocene-mediated coupling of bis(methoxyethynyl)disilanes led to zirconacycles, which were converted into 3,4-benzo-l,2-disilacyclobutanes via transmetallation with tin followed by a Diels-Alder reaction (Scheme 25) <2000CL1082>. 

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