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Silenes carbenes

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. [Pg.47]

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... [Pg.33]

We theoretically studied the reactions of stable West silylenes 32 and 73 with phosphorus ylide H2C=PMe3.74 Similarly to the simplest analogs of carbenes, these compounds can form betaines in which the negative charge is localized on the silicon atom and the positive charge is localized on the phosphorus atom. These betaines can thermally decompose to form silenes (direction A, Scheme 39) or be isomerized to ylides via direction B. [Pg.87]

Scheme 14.18). The silylene-silene rearrangement 27 28 is nearly thermoneutral, with the silene being slightly more stable. The photolysis of a-diazo compounds (30) is the only frequently used reaction path to silenes (31) via a carbene-silene... [Pg.665]

When (trimethylsilyl)carbene (3a) is generated by photolysis of the diazo precursor 19 in an alcohol solution at room temperature, the rearrangement to the silene is so fast that only the addition products of the latter, but not the 0,H insertion product of the carbene, can be isolated (equation 4)34. The same holds true for photolysis in diethylamine38. [Pg.715]

When phenyl(trimethylsilyl)diazomethane (20) is pyrolyzed in the gas phase, typical reactions of carbene 21 can be observed (see Section III.E.4). However, copyrolysis with alcohols or carbonyl compounds generates again products which are derived from silene 2239,40 (equation 6). Thus, alkoxysilanes 23 are obtained in the presence of alcohols and alkenes 24 in the presence of an aldehyde or a ketone. 2,3-Dimethylbuta-l,3-diene traps both the carbene (see Section ni.E.4) and the silene. [Pg.716]

Bis(trimethylsilyl)diazomethane (25) represents an excellent source for silene 2641. It appears that carbene 3c, which is expected from the photochemical or thermal decomposition of 25, escapes most trapping efforts due to rapid isomerization to silene 26 (equation 7). Photolysis of 25 in benzene solution yields 27 and 28 in a combined yield of 64% and disilazane 29 (10%) all these products are likely to be derived from 26. Similarly, photolysis in the presence of methanol or I)20 traps the silene quantitatively (to give 31 and 32). [Pg.716]

Remarkably enough, epoxide 30 was identified among the products of the photolysis of 25 in benzene/benzaldehyde. This seems to be the only reported case where carbene 3c has been intercepted. Pyrolysis of 25 in a nitrogen flow at 400 °C or vacuum pyrolysis at 500 °C led to the same product pattern as the photolysis. Copyrolysis with benzaldehyde or butadiene gave only trapping products of the silene intermediate41. [Pg.716]

When two carbene functions are separated by one or more silicon atoms, one can expect them to enter independently the usual inter- or intramolecular reactions. Among the intramolecular reactions, extensions of those which have been discussed in Sections III.B and IILC are particularly appealing, namely silylcarbene-to-silene rearrangement at one or both carbene centers and intramolecular carbene dimerization to form a C,C double bond and thus an unsaturated silaheterocycle. [Pg.732]

The dramatic influence of a methyl group on the reaction pathway is exemplified by carbene 163 which rearranges to a vinylsilane. In the absence of this methyl group, however, the silylcarbene-to-silene rearrangement (equation 16) occurs48,49. [Pg.744]

Phenyl(triphenylsilyl)carbene has also been trapped without the interference of a silylcarbene-to-silene rearrangement84. It undergoes 0,H insertion with alcohols and is oxidized to the ketone by DMSO the latter reaction is likely to include an S-oxide ylide (equation 56). [Pg.750]

As already discussed in Section III.B, carbenes 43 can rearrange to a silene by migration of R1 to the carbene center and to a ketene by migration of OR2. The situation is complicated further by a subsequent 1,3(C—>-Si) shift of the OR2 group in the silene to form a doubly rearranged ketene (see Scheme 5). [Pg.753]

An analogous mechanistic scheme (equation 87) has been proposed for the flash vacuum pyrolysis of dimethylsilyl(trimethylsilyl)thioketene148 (256). The pyrolysis of bis(trimethylsilyl)thioketene (257) leads to a more complicated product mixture (equation 88). With 47% conversion, a mixture of trimethylsilylacetylene, 1-trimethylsilyl-1-propyne, bis(trimethylsilyl)acetylene, (trimethylsilyl)thioketene, 2,2,4,4-tetramethyl-2,4-disila-l-thietane and 2,2,4,4-tetramethyl-2,4-disila-l,3-dithietane was obtained. All products can be rationalized, however, by the assumption that carbene 258 undergoes not only a silylcarbene-to-silene rearrangement (as in the preceding two cases) but also isomerization to 2-thiirene and insertion into a methyl-C, H bond. [Pg.770]

Steinmetz and coworkers carried out mechanistic studies on the far-UV photochemical ring opening of l-silacyclobut-2-ene 80. The intermediates were trapped by alcohols to give 84-87 and by methoxytrimethylsilane to give 88 and 8955. The main reaction is the formation of 1-silabuta-1,3-diene 81, while the formation of silene 2, probably via the carbene 90, is a minor reaction (equation 19). The mechanism suggested was supported by deuterium labelling studies and ab initio calculations. [Pg.872]

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic generation from a trisilane 280153, thermolytic or photolytic decomposition of cyclic silanes 28114,154,155 and degradation of diazidosilanes 282153,156. The photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies157. The photolysis of w-diazo compounds 283 is the only frequently used reaction path to silenes 284 via a carbene-silene rearrangement8. [Pg.901]

Silenes are formed by rearrangement of silylcarbenes. If polysilylated diazomethanes 298-300 are employed, a selective migration of a silyl group to the carbene centre occurs and silenes 301, 92 and 302 are formed (equations 74-76)164. The outcome of trapping reactions is independent of the mode of silene generation photochemical and pyrolytic methods give the same results. [Pg.904]

The photolysis of dimethylsilyldiazomethane 251 in an argon matrix at 10 K (Scheme 45) was shown by Sander and coworkers to lead to the diazirine 252 and, via the carbene 253 and a 1,2-H shift, to dimethylsilene 254, used to study the thermal reaction of the silene with formaldehyde132. [Pg.1277]

II. SILYL CARBENE-TO-SILENE REARRANGEMENT AND RING FORMATION A. Decomposition of Monosilanyl Diazo Compounds... [Pg.2403]


See other pages where Silenes carbenes is mentioned: [Pg.12]    [Pg.143]    [Pg.143]    [Pg.146]    [Pg.712]    [Pg.718]    [Pg.718]    [Pg.722]    [Pg.722]    [Pg.737]    [Pg.741]    [Pg.743]    [Pg.750]    [Pg.750]    [Pg.753]    [Pg.754]    [Pg.774]    [Pg.857]    [Pg.900]    [Pg.906]    [Pg.1276]    [Pg.1276]    [Pg.1280]    [Pg.2401]    [Pg.2402]    [Pg.2402]    [Pg.2404]    [Pg.2404]   
See also in sourсe #XX -- [ Pg.900 , Pg.901 , Pg.902 , Pg.903 , Pg.904 , Pg.905 , Pg.906 , Pg.907 , Pg.908 ]

See also in sourсe #XX -- [ Pg.900 , Pg.901 , Pg.902 , Pg.903 , Pg.904 , Pg.905 , Pg.906 , Pg.907 , Pg.908 ]




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