Siloles, formation

Reductive cleavage of the Si-Si bond of bis(siloles) with alkali metals resulted in the formation of silole monoanions (Scheme 2.49). Silole monoanions were found to be aromatic on the basis of NMR spectral data and calculations. 

Scheme 5 Formation of polyfl, 1 -silole), 8, by Li-mediated Wurtz-type coupling.

Scheme 12 Proposed mechanism of silole-incorporated polysilane 22 formation by ROP.

The flash vacuum pyrolysis (FVP) of 1-allyl-l-methyl-l-silacyclopent-3-ene gives rise to elimination of propene and formation of silole 3 identified as its dimer 5, and by its Diels-Alder adducts with maleic anhydride and hexafluorobutyne. However, the silole 3 has been detected as a monomer by the MS-FVP technique6. 

The decomposition of the S-methylxanthate ester of 1,1-dimethyl-1-silacyclopcnt-4-cn-3-ol takes place in refluxing ether, but this ester cannot give the silole 4 which dimerizes in situ (61% yield), together with siloxane formation (Me2SiC4H5)20 (31%) due to a ft C—Si elimination14. 

Other mechanisms have been proposed, and the formation of 2 was verified by high-temperature catalytic hydrogenation17. Although the identification of 2 in these experiments seems questionable5,10, Gaspar and coworkers, in 1986, reported that gas-phase pyrolysis of 1,1,1,3,3,3-hexamethyltrisilane with butadiene gives the silole [2 + 4]... 

The mechanisms proposed by Caspar and coworkers in their approach to the problem have a common point, the formation of the intermediate 9. This silylene then undergoes two prototropic rearrangements initial isomerization to a silene, the 2//-silole (11), followed by a second to give the 1H-silole (2)19 (equation 3). 

Also in 1992, Michl, Nefedov and coworkers20a,b reported that vacuum pyrolysis (800 °C, ] 0 1 - ] 0 4 Torr) of 5-silaspiro[4.4]nona-2,7-diene (12) resulted in the formation of silole (2), that was isolated in an Ar matrix, and characterized by its IR and UV-visible spectra. Matrix photolysis and vacuum pyrolysis of l,l-diazido-l-silacyclopent-3-ene (13) also produced the parent silole. After warming the pyrolysate to RT, the [2 + 4] dimer 10 of silole 2 was identified by GC-MS. 

An extensive theoretical investigation does not exist for the siloles, but PM3 calculations of formation enthalpies of 2 and its tautomers have indicated that the l//-silole is the most thermodynamically stable species200. The activation barrier for 11 — 2 isomerization was calculated to be 96 kJ moC1, comparable to that for cyclopentadiene2d 116. The (1H + 1H) dimer 1019 is isolated rather than the (2H + 1H) dimer as in the case of phosphole. This directly confirms the thermodynamic stability and the Diels-Alder kinetic instability of 2. The marked difference in the stability of the parent silole and phosphole was explained3 by the relative stabilities of the a bonds in silanes and phosphines (Si > P) and of the ji bonds in silenes and phosphenes (P > Si)117. 

Iodine reacts differently with the C-methylated silole ring. Cleavage of the ring and formation of a complex mixture instead of the expected l,4-diiodo-2,3-dimethylbutadiene is observed12. 

Dehydrocoupling of 1 catalyzed by <50 mol% (i.e., M-H/Si-H < 0.5) of Red-Al yielded 2 as light yellow powders. Polymers with molecular weights (Mw) of 4600 and 4100 were isolated in 86 and 78% yields when 15 mol% and 50 mol% of Red-Al were used, respectively. Polymer yields and molecular weights when 15 mol% of Red-Al was used were higher compared to polymers obtained when 50 mol% of Red-Al was used. Products from the reaction of 1 with 15 mol% 25 mol%, and 50 mol% Red-Al were separated by preparative GPC and were characterized by NMR spectroscopy. Shorter oligomers such as silole dimer or trimer were not found in products. However, when 100 mol% of Red-Al (i.e., M-H/Si-H = 1) was used, the formation of silole dianion 333 was observed without forming 2 [Eq. (6)]. 

Thermolysis of the five-membered silole 148 quantitavely resulted in the formation of the 2,4-disila-l-germatri-cyclo[2.1.0.0z,s]pentane 149 (Scheme 58) <2002JA9962>. 

The reaction of the mixed cyclotrimetallenes 22 and 23 with phenylacetylene also proceeds through the initial [2 + 2] cycloaddition of the first molecule of phenylacetylene across the E=Si (E = Si, Ge) double bond with the formation of the three- and four-membered bicyclic compounds 30 and 3122. But the reaction does not stop at this stage valence isomerization of the bicyclic compound takes place to form the silole type structures 32 and 33 with one Si=C and one Ge=C double bond, which in turn... 

Silole rings have been constructed by treating 3,3 -dibromo-2,2 -bithienyl with BuLi and reacting the dilithio derivative so obtained with a dialkyldichlorosilane <2006JA9034>. The aryl-aryl bond formation by electron-transfer oxidation of Lipshutz cuprates has been utilized for the construction of macrocycles involving thiophene rings <2006JOC6110>. 

If the silole bears hydrogen atoms in 2,5-positions (e.g., 48), it can undergo cycloaddition with the very weak dienophile 47 to give 7-sila-norbornadiene 49 or with itself during its formation to give 7-sila-norbornenes such as SO <2002ZNB741>. 

One of the most versatile routes to siloles, germoles, stannoles, and eventually to plumboles of type 194, allowing for a great variety of substituents in all positions, is provided by 1,1-organoboration of the respective alkyn-l-ylmetal compounds <1995CCR125, 2006HAC188>. The sequence of reactions involved does not fit into the concept of bond formations (Sections 3.17.8.1-3.17.8.3) outlined so far and therefore it is dealt with separately. 

Digermacyclohexadienes 244 give germoles 245 as sole products by irradiation in benzene <19980M1782>. When silagermacyclohexadiene 246 is treated in the same way, the formation of two metalloles is possible, but germole 247 is formed as a major product (84%) compared with silole 248 (9%). 

Alkoxy-substituted highly coordinate Si-heterocycles 89YGK831. Siloles, germoles, stannoles, plumboles, formation of ionic species and coordination compounds 90CRV265. 

Silole dianions, which are of current interest as aromatic species [1 - 3], behave as nucleophiles in simple displacement reactions, generally yielding simple trapping products [4 - 6]. However, the reaction of l,l-dilithio-2,3,4,5-tetraphenylsilole (1) with l,l-dichloro-2,3-diphenylcyclopropene (2) takes a most unexpected course, leading to ring-opening of the cyclopropene ring and ultimate formation of a tetracyclic disilapentalene and a stable bis-silole diradical [7]. 

The Pd-catalyzed reaction of silacyclopropanes wifh terminal or electron-deficient alkynes gives siloles along wifh silacyclopentenes (Scheme 10.255) [685]. The mechanism for fhe formation of fhese products might involve oxidative addition of fhe carbon-silicon bond to a Pd(0) complex generated in situ. 

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