Silylenes cyclic

Direct spectroscopic studies and calculations of cyclic silylene-to-silene and germylene-to-germene interconversions 99IZV2027. 

Surprising thermal and chemical stability of the cyclic silylene (29) may be explained by an enhanced aromaticity. This can be seen by comparison with its percursor (30) the difference in length between the single N—C (1.415 A) and double C=C (1.322 A) bonds in (30) decreases in (29) (N—C 1.400 A, =C 1.347 A) to imply an increased delocalization. Also the Si—N bond length increases in (29) only by 0.056 A (from 1.697 A to 1.753 A) instead of the 0.080-0.100 A typical for transition from tetra- to dicoordinate silicon this may indicate developing partial Si= N double bond character in (29) <94JA269l>. 

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

As mentioned above, the cyclic silylenes 75c and also 76c (or 79a and 79b) are dramatically less reactive than other known silylenes which are all transients. Thus, 75c has such a low Lewis acidity that, unlike other silylenes, it does not react as an electrophile. However, 75c reacts as a Lewis base, i.e. as a nucleophilic reagent980. The known reactions of Arduengo-type silylenes are summarized in References 2e, 98c, 98d and 99b and the references cited therein. The theoretical study of the possible reactions of Arduengo-type silylenes is quite limited and awaits the attention of computational chemists. 

Nefedov, Michl and coworkers reported the successful spectroscopic characterization of the l-silacyclopenta-2,4-diene 287 and its photochemical transformation into the isomeric species 288-290 (equation 71)159. Caspar and coworkers160 had suggested that silole 287 might be formed by a rearrangement of the cyclic silylene 288 via the 1-silacyclopenta-1,3-diene 289. This proposal was confirmed in an elegant UV-visible, IR-matrix isolation... 

Maier and coworkers have found that pulsed flash pyrolysis of an acetylenic disilane 323 gave rise to the acetylenic silylene 324, which subsequently rearranged to the cyclic silylene 1-silacyclopropenylidene 325162. Irradiation of this cyclic silylene resulted in its isomerization to the isomeric acetylenic silylene 324, which itself could be photo-chemically converted to the allenic silylene 326. As well, both 324 and 325 were said to isomerize on photolysis to the unusual silacycloalkyne 327, which was characterized... 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

Other recent applications of the diazide photolysis route include the matrix isolation of some interesting cyclic silylenes (equations 23 and 24)54,55. Solution photolysis of diazides can give other reactive species in addition to silylenes. 

Kalcher and Sax also examined the lowest singlet and triplet states of cyc/o-(H2Si)2Si , a three-membered cyclic silylene. The Si—Si—Si angles at the divalent silicon are constrained to be much smaller than those in the open-chain (H3Si)2Si for both the singlet (56.2° versus 95.9°) and the triplet (62.8° versus 125.4°) states. It is surprising that AZ st is nearly the same in the cyclic (8.8 kcal mol 1) and open-chain (8.3 kcal mol 1) silylenes208. 

The electronic structures of the l,3,2(A2)-diazasiloles 83-85 and 89 have been probed by He(i) and He(n) photoelectron spectroscopy. The experimental ionization potentials are summarized in Table 3. The molecular orbitals were assigned with the help of quantum mechanical computations for model compounds and in each case the HOMO was found to be a Jt-type MO of b, symmetry. In the case of the benzol] and pyiido[/4 fused compounds 85 and 89 also the HOMO-1 is a Jt-type MO. The next band was assigned to a lone pair-type MO of aj symmetry, which is mostly located at the dicoordinated silicon atom. Therefore, the HOMOs of the cyclic silylenes 83 and 84 differ in nature from those of the homologues V-heterocyclic carbenes which consist of an essentially aj lone pair-type MO at... 

Today X-ray structures of pretty stable cyclic silylenes are available in these compounds silicon is bound to two nitrogen neighbours as part of a five membered ring skeleton resulting in the first stable silylene with a pure coordination number two ... 

Similar cyclic and sterically protected silylene has been prepared recently (equation 88). The silylene must be stabilized to some extent by a-n conjngation instead of n-7T conjugation for other stable cyclic silylenes shown in Figure 9. Indeed the silylene undergoes facile rearrangement (eqnation 88). 

Among the cyclic silylenes 4-7, SiC2 (4) [2] and 1-silacyclopropenylidene (5) [3] have already been investigated by matrix spectroscopy. This series is now completed by silylene 6, the formal hydrogenation product of 5. We also report on the isolation of 7, which can be derived from 5 by formal substitution of one CH unit by a nitrogen atom. 

Silylenes 113 are the silicon analogues of carbenes and formally contain a divalent silicon atom. We will use here the term silylene which is commonly used in the literature in preference to the systematic IUPAC term, silanediyl. Although silylenes of type 113 are the major subject of this section, other types of silylenes, such as the unsaturated 114 (1-silavinylidene) or 115 (2-silavinylidene) or cyclic silylenes, are possible reactive intermediates and we have discussed some of them in previous sections. In Section VII. A.4 silylenes other than 113 are mentioned. 

Tang has found direct evidence for formation of the silacyclopenta-diene in its reduction to silacyclopentene (43). The cyclic silylene intermediate is also a possible source of the silacyclopentene, althou the more likely route to this product is addition by silylene, which has been demonstrated to be an efficient process (4,39),... 

The extreme values found so far for 5 Si are for divalent silicon (Scheme 1), with extreme deshielding for the cyclic silylene 1 and extreme shielding for the si-licocene 2. In the solid state of 2, two different isomers are evident from the X-ray analysis and also from the solid-state Si NMR spectrum." The cation 3, [ / -C5Me5)Si] has almost the same chemical shift Si as the neutral species 2. 

During the past three years, we have studied the reactions of thermally generated silicon atoms with low molecular weight reaetants. Reaction products were isolated in argon matrices and identified by means of IR spectroscopy, aided by calculated vibrational spectra. The method turned out to be very versatile and successful [1-4]. When the reactions of silicon atoms with unsaturated substrates are compared, a general sequence of products can be outlined as follows if a heteroatom is participating in the multiple bond, the silicon atom is bound to a lone pair in the primary product. This primary product can be isomerized photochemically to a cyclic silylene, the formal addition product of the Si atom to the Jt bond, which is the first product in case of substrates with C-C multiple bonds. Consecutive photoisomerizations lead to the formal insertion product of the Si atom into a C-H bond and, finally, to the product(s) of (successive) migration of H atoms to the Si atom. 

A similar method for simultaneous derivatization of two functional groups is the formation of cyclic silylene derivatives for the same types of compounds (Fig. 

The fact that the Si atom in the center is the one to be squeezed out to breed a silylene means that a silicon-bridged transition state like the one shown in equation (50) is likely. The same authors have also synthesized 5,6,11-trisilaspiro-4,6-undecane, which undergoes a similar photochemical decomposition to give a cyclic silylene [equation (52)]. 

Silanechalcogenides were synthesized by the reaction of a cyclic silylene with suitable chalcogen precursors. The sulfur atom was transferred by trimethylphos-phine sulfide (Scheme 53). ... 

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