Phenyl silene

Summary l-[2,6-bis(dimethylaminomethyl)phenyl]silenes (2a-d) were prepared by treatment of the (dichloromethyl)oligosilanes R (Me3Si)2Si-CHCl2 la-d (a R = Me b R=tert-Bu c R=Ph d R = Me3Si) with 2,6-bis(dimethylaminomethyl)phenyl-lithium and were characterized by NMR studies and (in part) by X-ray stmctural analyses as well as by their reactions with water to give silanols. Treatment of 2a-d with benzaldehyde produced 2,2-bis(trimethylsilyl)styrene (4) and a silanone polymer as the expected products. For the reaction of 2a and 2c with benzaldehyde, an interesting side reaction was observed leading to the 2-oxa-l-sila-l,2,3,4-tetrahydro-naphthalenes 6a and 6c, respectively. 

Silenes (compounds with an Si=C double bond) are known to be extremely reactive compounds. Till now, only a few silenes, stabilized kinetically by bulky substituents, could be isolated under normal conditions and were characterized by X-ray structural analyses [1]. An effective stabilization of Si=C systems is also achieved by coordination of a base, such as amines, THF, or fluoride ions, to the electrophilic silene silicon atom [2], and recendy we succeeded in synthesizing thermally unexpectedly stable intramolecularly donor-stabilized silenes [3]. In this paper, the synthesis and, in particular, the behavior of the l-[2,6-bis(dimethylaminomethyl)phenyl]silenes 2a-d are described. 

A few other routes to new silenes involving photochemical processes have been reported, most of which have the structure RR Si=CH2110-112 and are listed in Table I. Photolysis of l,l-dimethyl-2-phenyl-l-silacyclo-but-2-ene 22 in a glass at 77 K led to the siladiene 23, which absorbed at 338 nm,113 as shown in Eq. (17) ... 

When phenyl (Ph) groups replaced both Me3Si groups, again a rather unstable 1,2-disilacyclobutane dimer appeared to be formed,90 as shown by NMR data but when f-butyl replaced a Me3Si group, the silene failed to dimerize.87 Thus, it is evident that whether or not head-to-head [2 + 2] cyclodimerization occurs depends on the bulk of the substituents on both sp2-hybridized silicon and carbon. 

Analogous behavior was followed by the phenyl-substituted silene 156. The initially formed silene 157 underwent 1,3-methyl migration to give the silene 158, which then dimerized in a head-to-tail manner to yield three different stereoisomeric dimers 159, two of which were characterized by crystal structures. Again, the exchange of trimethylsilyl and trimethylsi-loxy groups at the ends of the Si=C bond occurred, followed by 1,3-methyl silicon-to-silicon rearrangements. The steps are summarized in Eq. (54). 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

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... 

The products of the thermolysis of 3-phenyl-5-(arylamino)-l,2,4-oxadiazoles and thiazoles have been accounted for by a radical mechanism.266 Flash vacuum pyrolysis of 1,3-dithiolane-1-oxides has led to thiocarbonyl compounds, but the transformation is not general.267 hi an ongoing study of silacyclobutane pyrolysis, CASSF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the 3-21G and 6-31G basis sets have modelled the reaction between silenes and ethylene, suggesting a cyclic transition state from which silacyclobutane or a trcins-biradical are formed.268 An AMI study of the thermolysis of 1,3,3-trinitroazacyclobutane and its derivatives has identified gem-dinitro C—N bond homolysis as the initial reaction.269 Similar AMI analysis has determined the activation energy of die formation of NCh from methyl nitrate.270 Thermal decomposition of nitromethane in a shock tube (1050-1400 K, 0.2-40 atm) was studied spectrophotometrically, allowing determination of rate constants.271... 

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. 

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). 

Ishikawa and coworkers investigated the relative ease of migration of a Me3Si group to vinyl and phenyl groups in precursor compounds containing both groups133. In the case of the irradiation of compound 237 in the presence of methanol, silene 238 was found to be the major reactive intermediate, while with acetone and 2,3-dimethylbuta-l,3-diene the number and nature of different ene products can only be explained by the existence of silatriene 239 (equation 59). 

The regiochemistry of such reactions was further investigated for the dihydropyranyl-substituted phenyldisilane 240134. It was found that two types of silene intermediates, 241 and 242, are formed by 1,3-silyl shifts to the dihydropyranyl and the phenyl group, respectively (equation 60), and that the distribution of trapping products from these silenes depends on the trapping agent used. 

The competitive migration to either a vinyl or phenyl group has been investigated by Ishikawa and coworkers133. While with traps other than methanol there is proof for the intermediacy of both silenes 238 and 239, formed from 237 (of equation 53), just one product, i.e., 400, is found for the reaction with the alcohol (equation 119). 

Photolysis of 1-benzyl-l-methylsilacyclobutane and 1-benzyl-l-phenylsilacyclobutane also leads to formation of isotoluene derivatives as the major primary products (Scheme 5). The primary photolysis in both cases is dominated by reactivity specific to the benzyl chromophore rather than the SCB moiety, unlike the case with other SCBs. Silene formation competes significantly in the case of 1-benzyl-l-phenylsilacyclobutane, when the role of the primary chromophore is shared between the benzyl group and the SCB ring via a second phenyl group attached to silicon. 

THF can also have an accelerating effect on reactivity, in cases where the weakly basic solvent can get directly involved in the reaction via a catalytic pathway and complexation with the free silene is weak. Such an effect has been observed for the reaction of alcohols with the aryldisilane-derived l,3,5-(l-sila)hexatriene derivatives 21a-c, as shown by the second-order rate constants for reaction of the three silenes by MeOH and TFE in isooctane, MeCN and THF solution (Table 11 note that the third-order rate constants for reaction of 21a-c with MeOH have been omitted)48. Table 11 also includes data for the reactions with acetone in the same three solvents, as an example of a reaction which has no catalytic component47. The rate constants for all three reactions decrease in the order isooctane > MeCN > THF for 21a, which complexes relatively strongly with the ether solvent, as demonstrated by the distinctive red shift in its UV absorption spectrum in THF (kmax = 460 nm) compared to isooctane and MeCN (kmax = 425 nm)48. Compound 21b exhibits a ca 10 nm shift of its absorption band in THF solution while none is detectable in the case of 21c, indicating that the equilibrium constant for THF complexation within this series of silenes decreases with increasing phenyl substitution at... 

The effects of substituting a phenyl group for —H or —Me at the silicon end of the Si=C bond has different effects on the rates of silene reactions depending on the substituents present at carbon. For example, the l,3,5-(l-sila)hexatriene derivatives 21a-c exhibit a significant decrease in reactivity toward methanol and acetone with increasing phenyl substitution (Table 11). This too is consistent with the stepwise mechanisms proposed for... 

TABLE 12. Absolute rate constants for reaction of 1-methyl- and 1-phenyl-substituted silenes (RR Si=CH2) with MeOH and acetone in hexane or isooctane solution at 23 - 25 °C... 

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