Silyl base-free

The mono-silylated or free acetamides, which are liberated during silylation with 22 a, can, furthermore, interfere with any subsequent reaction, e.g. with electrophiles. Thus in the one-pot/one-step silylation, Friedel-Crafts catalyzed, nucleoside synthesis starting from protected sugar derivatives and pyrimidine or purine bases, the mono- or bis-silylated amides such as 22 a can compete with less reactive silylated heterocycHc bases for the intermediate electrophilic sugar cation to form protected 1-acetylamino sugars in up to 49% yield [42, 47]. On silylation with trimethylsilylated urea 23 a the Hberated free urea is nearly insoluble in most solvents, for example CH2CI2, and thus rapidly precipitated [43]. 

All cationic phosphenium complexes reported to date are base-free species. In contrast, silylene complexes tend to be stabilized by forming base adducts. Most silylene complexes are base-stabilized species54 59 and base-free silylene complexes are still rare.59-63 Typical examples are shown in Scheme 7. A phosphite-phosphenium complex exists in a solid state and even in solution, and does not take an OMe bridging form between the two P atoms,64 whereas a silyl-silylene complex is not detected, but exists as an OMe bridging form between the two Si atoms.65... 

Another germanium-based hnker has been developed for solid-phase synthesis of a pyrazole hbrary [79]. A new iterative synthesis of regioregular oligothiophenes has been developed in which double-couphng after each iteration minimizes deletion sequences. The method exploits the susceptibility of a-silyl- but not a-ger-myl-substituted thiophene derivatives towards nucleophilic ipso protodemetalation and features an unusual base-free Suzuki-type coupling product. The strategy... 

The unusual bis-silyl methyl ligand, (CH(SiMe3) (SiMe20Me) ), forms a cyclo octamer with lithium (5). The bis(trimethylsilyl)dimethylphenylsilylmethyl ligand forms monomeric compounds with lithium and sodium in the presence of additional electron donors (TMEDA, diethyl ether, and THF). In each structure, there are additional interactions between the metal ions and carbons from the phenyl rings. The tris(dimethylphenylsilyl)methyl compounds of sodium, potassium, rubidium, and cesium crystallize in polymeric chains. Each of these compounds also has intra- and inter-molecular phenyl-metal cation interactions. The monomeric [LiC(SiMe2CH2PPh2)3] forms as a base-free compound even in the presence of a large excess of electron donors (such as TMEN) (6). " ... 

Beryllium-silicon bonds are a recent development. They are likewise prepared by the reaction of the metal halide with alkali metal silyls, but unlike the Mg analogues, both of the well-characterized Be examples are base-free. While [(te/T-Bu3Si)BeCl] is oligomeric, the molecular structure of (tert-Bu3Si)2Be features a perfectly linear Si-Be-Si unit. CpBe(SiMe3), prepared from CpBeCl and trimethylsilyllithium in pentane, exhibits an intriguing thermal chemistry. The identity of the primary thermolysis products, Scheme 14, implies the participation of silylene intermediates, a contention supported by DFT/NBO (density functional theory/natural bond order) analysis. ... 

In general, low magnetic shielding of Si is typical for low-coordinated silicon atoms, as in silylenes" (e.g. 1 and 4-7 in Scheme 2), free " (e.g. 8-10" in Scheme 3) or almost free " silyl cations, in contrast with silylene adducts" (Scheme 4), Low Si nuclear magnetic shielding is typical for base-free silylene transition metal complexes " (Scheme 5), and also for numerous compormds, where silicon is involved in multiple bonding with other elements (Scheme 6). 

Hashimoto H, Matsuda A, Tobita H (2005) Nonphotochemical synthesis of a base-free silyl (silylene)iron complex and its reaction with CO another direct evidence for reversible 1,2-and 1,3-group migrations. Chem Lett 34 1374... 

Thus, it appears at this point that the condensation of free sulfonimidates is an easier and more efficient route to oxothiazene polyiners than the condensation based on N-silylsulfonimidates. Although the free sulfonimidate route has yielded polymers with lower molecular weight in most cases, it may be possible to overcome this apparent shortcoming by varying condensation conditions. Some representative molecular weight (GPC) data for polymers from both N-silyl- and free sulfonimidates are provided later (Table I) in the section on polymer characterization and properties. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

Attack of excess methanol, in the presence of bases or acids, on trimethylsilyl-oxy ethers such as 13 or on dimethylsilyloxy compounds such as 50 during trans-silylations (cf the preceding section) probably also proceeds via transition states such as 73 or 74 to afford the free alcohols or phenols 11 and methoxytrimethylsi-lane 13a. 

As might be expected, the add-catalyzed silylation-amination of the free purine-bases such as hypoxanthine 242 (R=H) and guanine 242 (R=NH2) proceeds, via 243, to the corresponding substituted adenines 244 in high yields [64] (Scheme 4.24). 

With trimethylsilyl iodide 17 the 0,N-acetal 457 gives the iminium iodide as reactive intermediate this converts the enol silyl ether 107 a in situ into the Man-nich-base 669, in 81% yield, and hexamethyldisiloxane 7 [195]. On treatment of the 0,N-acetal 473 (or the N-silylated Schiff base 489) with TMSOTf 20 (or Zny, the intermediate iminium triflate adds to the ketene acetal 663 to give mefhoxytri-methylsilane 13 a and silylated / -amino esters such as 670, which are readily transsilylated by methanol to give the free / -aminoester [70, 196] (Scheme 5.61). 

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

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