Silyl-substituted acetylenes

The reactions of complex 2a with ketones and aldehydes show a strong dependence on the substituents. With benzophenone, substitution of the silyl-substituted acetylene leads to the r]2-complex 58, which is additionally stabilized by a THF ligand. This complex can serve as an interesting starting material for other reactions. With benzaldehyde and acetophenone, the typical zirconadihydrofuran 59, akin to 2c, is obtained from a coupling reaction. This complex is unstable in the case of benzaldehyde and dimerizes, after elimination of bis(trimethylsilyl)acetylene, to yield 60. In this respect, it is similar to the above discussed complex 2c, since both of them show a tendency to eliminate the bis(trimethyl-silyl)acetylene. The reaction of methacrolein with complex 2a depends strongly on the solvent used [40]. 

The reactions of complex (25) with carbonyl compounds show a strong dependence on the substituents leading to the substitution or elimination of the silyl-substituted acetylene or the insertion into the Zr-C bond. Moreover, the reaction of methacrolein with (25) depends strongly of the solvent used in hexane elimination of bis(trimethylsilyl)acetylene leads to the formation of dimer (33) while in THF insertion with the formation of zirconacycle (34) is observed. 

Silyl-substituted acetylenes. The hyperconjugative interaction between the C=C moiety and SiH3 as well as CH3 groups of local C3v symmetry has been discussed extensively as one of the best examples of how to parametrize LCBO-MO models by PE spectroscopic M e state energies (Section IV.E). The ionization patterns of the following mono- and disilyl acetylene derivatives are also reported in the literature153 (cf. Table 62). 

Computational studies of the thermal reactions of silene (Me3Si)2Si C(OSiMe3)(t-Bu) with silyl-substituted acetylenes, bis(trimethylsilyl)butadiyne, t-butyldimethylsilylacet-ylene, and bis(trimethylsilyl)acetylene report substituent effects in the reactivity of the silene. A regioselective migration of a sulfonyl group for the synthesis of functionalized pyrroles can be controlled with high selectivity for the formation of both a- and / -(arylsulfonyl)methyl pyrroles. 

The structural comparison (Fig. 3) with both the twofold tris(trimethylsilyl)methyl substituted acetylene and 1,4-benzene derivatives (Fig. 1) as well as with the literature data [6a] for hexa-kis(rm.butyl)disilane [6b] containing a SiSi bond elongated to 270 pm ( ), for the linear ( ) hexa-kis(rm.butyl)disiloxane [6c] or for di(tris(trimethylsilyl)silyl)zinc [6d] is based advantageously on a model in which the two substituent half-shells are separated along their central C3 axes by spacers of different lengths. 

Sakurai et al. (1984) prepared a series of silyl substituted potentially trishomoaromatic acetylenes [120]-[123], However, no evidence supporting homoaromaticity was reported. 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

Trimethylsilyldiazomethane, 327 Silyl substituted arenes Bis(trimethylsilyl)acetylene, 97 Chromium carbene complexes, 82 Titanium(IV) chloride-Diethylalu-minum chloride, 309 Other organosilanes Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Tributyltin chloride, 315 Di- x-carbonylhexacarbonyldicobalt, 99 Trimethylsilyl trifluoromethanesul-fonate, 329... 

The reactions of silylated synthons to form heterocycles are well appreciated methods in the heterocyclic chemistry. A great variety of nitrogen containing products can be obtained in a classical approach via a 1,3-dipolarophilic addition of substituted acetylenes plus diazo-compounds204 (Scheme 43). It could be shown205 that these additions can be considered as evidence for a directive influence of the TMS moiety because one of the two possible reaction products is usually favoured (Scheme 43) and e.g. 320 is the only reaction product, whereas the addition of TMS-acetyl-ace-tylene (317) plus diazomethane (318) leads to two isomers featuring 325 as the main product (Scheme 43). 

Another class of heterocyclic compounds are silyl-substituted triazoles which can be obtained by the addition of TMS-azide 14) and manifold substituted acetylenes 1, 35, 350 (Scheme 50)33,209 2ll Those in the first step resulting l-TMS-4-phenyl (3Ji)-, l-TMS-4,5-bis(methoxycarbonyl)- 352) and l-TMS-4-alkyl-l, 2,3-triazole 353) are then hydrolyzed to form 4-phenyl- 354)-, 4,5-bis(methoxycarbonyl)- 355), 4-alkyl-l,2,3-triazole 356). 

On the basis of crossover experiments, Woerpel and Clark attributed the formation of alkynes to an intramolecular rearrangement mechanism (Scheme 7.33).101 For silver-promoted alkyne formation, insertion of the acetylene into the Ag-Si bond produced 116. Then (3 elimination of bromide occurred more rapidly than silacy-clopropene formation to generate silyl-substituted alkyne 117. Substitution of L with bromide then afforded the observed product (113a). 

In one group of reductive-elimination reactions, an HSiR3 molecule can be displaced from a silyl-substituted transition-metal hydride ML (H)(SiR3) by more efficient 7r-bonding ligands such as CO, PR3, C2H4, acetylenes or N2 which favor a lower oxidation state of M ... 

The stability of the -C -Si< bond has been known for a long time But on the other hand they are reactive compounds which undergo either - as precursors to vinylsilanes - various types of addition reactions or - as only silyl-protected acetylenes - an electrophilic substitution under Friedel-Crafts conditions in presence of catalytic amounts of Lewis acids The — SiR3 moiety has a highly useful protecting and/or activating function. 

Quite important are silylated acetylenes for the synthesis of poly-ynes and a large number of variously substituted acetylenes -52)... 

Another class of heterocyclic compounds are silyl-substituted triazoles which can be obtained by the addition of TMS-azide (14) and manifold substituted acetylenes 1, 35, 350 (Scheme 50) yj ose in the first step resulting l-TMS-4-phenyl... 

With diethyl(bipyridyl)nickel(II) and chlorohydrosilanes, the hydrosilyiation of di-substituted acetylenes undergoes dehydrogenative, stereoselective c -double silylation to give 1,2-bissilylalkenes, accompanied by normal hydrosilylated products ... 

Fuchs has examined a number of additional alkynes. One in particular, silyl-substituted triflone 104, may prove most useful, as it provides silylated alkyne products upon reaction with suitable substrates [62c]. In general, attempts to functionalize triflones with other groups at the alkyne carbon or at propargylic positions were unsuccessful. Triflones with more remote functionality, including bis-acetylenes, gave useful reagents. Fuchs triflone methodology can also be extended to vinylation and allylation reactions (Scheme 23) [63, 64). 

The Diels-Alder reaction of oxazoles with alkynes has become a preferred method for the synthesis of substituted furans with diverse applications. A large number of early examples of this reaction have been tabulated. Activated dienophiles such as acetylenic ketones and esters can be used, although unactivated alkyl, aryl, and silyl alkynes have been used as well. In particular, the reaction of 4-phenyloxazole with substituted acetylenes is frequently used for preparing 3,4-disubstimted furans. Cycloadducts derived from 4-phenyloxazole typically decompose under milder conditions than 4-alkyloxazoles, allowing the synthesis of a wider range of functionalized furans. Several examples are shown below. 

The widespread and useful subdivision of larger molecules into parent systems and substituents and their comparison based on first- and second-order perturbation arguments [1, 14] have proved tremendously valuable for organosilicon compounds as well [1]. Especially important is the extreme electron donor effect of (H3C)3SiCH2 substituents, discovered over 30 years ago [17-19] by way of the low first ionization energies of organosilicon molecules containing 3-silyl-substituted lone pairs [20] (Fig. 7) or prototype 7t-systems such as benzene [17], ethene [18], or acetylene [19] (Fig. 8). 

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