Functionalized silylenes

Furthermore, we found that trapping products with an NH-functionalized silylene, MeSiNH/Pr, can be obtained in reasonable amounts. No hints for thermal isomerization were found in cycloadditions with 2,3-dimethylbutadiene or oxadienes. Such reactions are at least slower than cycloadditions. 

The alkoxy-functionalized silylene-bridged ansa-Qp-Flu zirconocene dichlorides bearing a 6-/[Pg.959]

This method can be used for the synthesis of a variety of silylenes and functionalized silylenes. By varying the substituent on the nitrogen atom or carbon atom, it should be possible to generate many more amidinato-stabilized-fimctionalized silylenes. 

In the past 5 years, much effort was dedicated to the exploration of the chemistry of amidinate-functionalized silylenes (Scheme 12). As the starting silylene already comprises a tricoordinated silicon atom, various oxidative addition reactions furnished silicon(IV) complexes with pentacoordinated Si atoms. Some of them comprise the striking features of three-membered SiNN (60) [196], SiNC (61) [196], SiCC (62) [196], or SiOC (63) [110] heterocycles, which are genuine novelties in silicon coordination chemistry. o-CH-activation of diazobenzene has also been observed (formation of 64) [197]. 

Silylene complexes are not only stable with donor substituents but also with simple alkyl residues at silicon. These alkyl complexes still have a sufficient thermodynamic stability, but otherwise are reactive enough to allow a rich and diverse chemistry. Particularly the chlorocompounds 7 and 11 are valuable starting materials for further functionalization reactions the details of these reactions will be discussed in the forthcoming sections. The data for the known compounds are summarized in Table 1. 

Starting from (OC)5MnSiR2H (R = Me, Ph, Cl), the p-silylene complex 70 is accessible via the oxidative addition of the Si —H bond to Pt(C2H4.)(PPh3)2 and Pt(PPh3)4, respectively. Structure 70 can be functionalized by displacement of the phosphine ligands alcoholysis and hydrolysis of the compound 70 leads to silicon-free complexes [175]. 

The experimental results presented do not interfer with the mechanism of PICVD of silicon via the silylene route. However, the results of the measurements of the silane relaxation time as a function of the silane concentration in the discharge and the comparison of disilane and silylene relaxation times do contradict to the SiH3-route. The latter mechanism has to be abandoned. 

The sulfoxide method has been applied to the concept [319,374] of intramolecular aglycone delivery for the formation of [1-mannosidcs by means of a silylene linker. In the original work, the acceptor and a thioglycoside donor were joined by means of a silylene group before the oxidation to the sulfoxide [141]. However, it was later found that the preformed sulfoxide was tolerated by the chemistry for the introduction of the linker [286,375]. The intramolecular aglycone delivery step was shown to function effectively for the transfer of the donor to the 2-, 3- and 6-position of glucopyr-anosides, as exemplified in Scheme 4.64. 

The product from the dehydrocoupling of PhSiHj, poly(phenyl(H)silylene), can itself be used as a precursor polymer for further functionalization, using the susceptibility of the Si-H functionality to radical attack, as is described in Section 3.11.4.2. 

Functionalization of polysilanes by chemical modification (post-polymerization) was covered in COMC II (1995) (chapter Organopolysilanes, p 101), where the formation of precursor polysilanes with potentially functionalizable side groups such as chloride, type 34 (via HCI/AICI3 chlorodephenylation of PMPS), 6 triflate, type 35 (via triflate replacement of phenyl groups)135,137 or alkyl halide (via chloromethylation of phenyl groups,138,139 type 36, or addition of HC1 or HBr to double bonds140) was discussed. Four other precursor polysilanes, which utilize the reactivity of the Si-Cl or Si-H bond, have been successfully applied in functionalization since COMC (1995) perchloropolysilane, 17 (see Section 3.11.4.2.2.(i) for synthesis),103 poly[methyl(H)silylene-f >-methylphenylsilylene],... 

Related crown ether-pendant polysilanes 45 were recently prepared by hydrosilylation post-polymerization functionalization of poly[methyl(H)silylene- -methylphenylsilylene], 37, although due to the low molecular weight of 37, the product 45 is also of low molecular weight.153... 

Other than this system, metallated polysilanes contain the metal in low-valent oxidation states. Such systems have been reported by two groups. In 1995, an alternative functionalization route starting from poly[methyl(H)silylene] or poly[methyl(H)silylene-fo-methylphenylsilylene], 37, was reported, in which the polysilane Si-H moiety was hydro-silated using 1,3,5-hexatriene, affording the diene-modified polymer 67, which was metal functionalized using triiron dodecacarbonyl to give the iron tricarbonyl-polysilane coordination complex, 68.177... 

Like silylenes and carbenes germylenes are isolobal with phosphines and may function as ligands to transition metal complexes. (Adapted from Bazlnet et ah, 2001)... 

Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si—Cl or Si—H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C—H bonds has not been observed. However, the insertion into Si—H bonds has been studied extensively. The occurrence of direct insertion has been indicated by formation of nongeminate homocoupling products. ... 

E = Si) and EN(R)CH=CHNR (e.g. 2, E = Si) (R = H, Bu ) have been reported " " it was concluded that there is significant p -p -delocalisation for the latter compounds. The relationship between stability, acid-base and spin properties, nucleophilicity and electrophilicity in a series of silylenes was studied by conceptual density functional theory." ... 

Finally, atomic and molecular proton affinities (PAs) have also been evaluated for various functionals for ammonia, water, acetylene, silane, phosphine, silylene, hydrochloric acid, and molecular hydrogen. For G2 and G3 theories, the mean unsigned error in PAs is 1.1 and... 

To date, bis(diazomethyl)silanes represent the only precursors to silylene-dicarbenes. It should be noted, however, that the extrusion of N2 from the two diazo functions can occur more or less simultaneously or successively. In the latter case, a diazocarbene is formed in the first place, which is supposed to undergo a fast carbene reaction before the generation of a carbene from the second diazo function occurs. This should be kept in mind for all transformations which are explained mechanistically with the participation of silylene-dicarbenes. There are even examples where only one of the two diazo groups enters carbene chemistry at all (see equations 35 and 36 below). 

Another transformation that should be discussed here briefly is the photochemical conversion of silylenes bearing a hydrogen atom and a substituted ethynyl function at the subvalent silicon center, i.e. 158. If R is methyl (equation 38) or ethynyl (equation 41), irradiation with 254 nm light results in an exchange of the R group and the hydrogen atom. This can be rationalized in several ways (equation 44). Firstly, a series of two 1,3 shifts with either a silylenic (159) or a carbenic (157) intermediate is able to explain the formation of 154. However, silylenes are much more stable than the isomeric carbenes and therefore a 1,3 R shift as the initial step is highly unlikely. A second possible pathway from 158 to 154 involves the cyclic intermediate 156. At this time, it is not possible to differentiate between these possibilities. 

Compounds with two or more silicon atoms directly attached to one another, subdivided into sections based first on the number of silicon atoms and then on the carbon functionality attached to the silicon atoms. Frequently, but not exclusively, the main photochemical behavior involves homolysis of a silicon-silicon bond yielding silyl radicals, but in some cases silylenes result directly from the photochemistry. The resulting compounds are frequently the products of a molecular rearrangement. 

Although the silylformylation of aldehydes is catalyzed by [Rh(COD)Cl]2 or [Rh(CO)2Cl]2, no secondary silylformylation of /i-silylenals (316-318) takes place, probably due to the electronic nature of the aldehyde functionality conjugated to olefin moiety (vide supra). Direct comparison of the reactivity of acetylene and aldehyde functionalities is performed using alkynals328. The reactions of 5-hexyn-l-al, 6-heptyn-l-al and 7-octyn-l-al with different hydrosilanes catalyzed by Rh or Rh—Co complexes at... 

Functionalized cyclotrisilane derivatives bearing oxygen- or nitrogen-containing substituents are also known. The synthesis of the aryl-alkoxy cyclotrisilane trimesityl-tris(2,6-diisopropylphenoxy)cyclotrisilane can be easily accomplished by the photolysis of 2-mesityl-2-(2,6-diisopropylphenoxy)-l,l,l,3,3,3-hexamethyltrisilane via several silylene extrusion and addition steps (equation 7)40. 

In what follows, emphasis will be placed on developments of the past decade, with sufficient (but brief) summaries of previous work and references to earlier reviews and seminal papers to allow this chapter to function as a stand-alone unit. We attempt to place silylene chemistry in a broader context and to interpret as well as describe important discoveries. 

There is a difficulty with the mechanism of Scheme 3 in that the fragmentation of a triplet diradical should conserve spin, yet neither triplet Me2Si nor triplet tetraphenyl-naphthalene have been detected. The diradical pathway for the photofragmentation of silanorbornadienes confirms an earlier proposal by Barton and coworkers52. There is always the possibility that diradical intermediates such as the singlet and triplet 13 S and 13 T could function as silylenoids. Thus, the assumption that products from pyrolysis of 7-silanorbornadienes are formed from free silylenes must be treated with caution. 

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