Silicon heterocyclic synthesis

Si-Heterocycles possessing chiral silicon centers, synthesis of 84MI18. Silylenes as precursors of Si-heterocycles 90UK918. 

Silicon heterocycles with different ring sizes have been synthesized by variations of the organometallic Wurtz coupling reaction. The method requires synthesis of intermediate organosilicon derivatives that can involve several steps. 

Padwa, A. Heterocyclic synthesis using the Pummerer reaction. Phosphorus, Sulfur Silicon Relat. Elem. 1999, 153-154, 23-40. 

Complex vinyl silanes may also be formed from simpler vinyl silanes by metathesis reactions. A ring closing metathesis (Section 8.3.3) to form a silicon heterocycle 2351, with a necessarily cis alkene was employed in a synthesis of brasilenyne 2.354 (Scheme 2.110). The RCM was followed by an intramolecular Hiyama... 

Reactions of silylenes with unsaturated organic substrates are valuable for the synthesis of novel silicon compounds (1,2). Over the past three decades, the reactions of silylenes with carbonyl compounds have been extensively investigated (3—5). It has been shown that these reactions produce diverse types of products via siloxirane or carbonyl silaylide intermediates while the carbonyl C—O a bond is generally maintained (3—5). Complete cleavage of the C—O bond has only been observed in several cases (4). Furthermore, the synthesis and physical properties of siloles and related unsaturated carbon—silicon heterocycles are of great interest as heteroaromatic molecular precursors and potential molecular optical materials (6). In this experiment, reaction of a NHC-stabilized silacyclopentadienylidene (silole silylene) with aldehydes, leading to the formation of the novel NHC-stabilized a,(3-unsaturated silanone, was described. 

The donor-stahilized hydrosilanimine is potentially useful as a hydrosilylation reagent. In addition, the existence of the Si=N double bond allows the cycloaddition reactions with unsaturated organic molecules for the synthesis of silicon heterocycles. 

Intramolecular reactions of temporarily silicon-tethered molecules in synthesis of 0,Si-heterocycles 97S813. 

Silicon protection is also commonly used to direct lithiation chemistry in five-membered heterocycles. For example, oxazoles , thiazoles and Ai-alkylimidazoles ° ° lithiate preferentially at C-2, where the inductive effect of the heteroatoms is greatest. If C-2 is blocked, lithiation occurs at C-5, where there is no adjacent lone pair to destabilize the organolithium. Functionalization of these heterocycles at C-5 can therefore be achieved by first silylating C-2, reacting at C-5 and then removing the silyl group. The synthesis of 666 illustrates this sort of sequence (Scheme 258) °. ... 

The present volume comprises 17 chapters, written by 27 authors from 11 countries, and deals with theoretical aspects and structural chemistry of peroxy compounds, with their thermochemistry, O NMR spectra and analysis, extensively with synthesis of cyclic peroxides and with the uses of peroxides in synthesis, and with peroxides in biological systems. Heterocyclic peroxides, containing silicon, germanium, sulfur and phosphorus, as well as transition metal peroxides are treated in several chapters. Special chapters deal with allylic peroxides, advances in the chemistry of dioxiranes and dioxetanes, and chemiluminescence of peroxide and with polar effects of their decomposition. A chapter on anti-malarial and anti-tumor peroxides, a hot topic in recent research of peroxides, closes the book. 

The traditional synthesis of miinchnones involves the cyclodehydration of N-acylamino acids usually with acetic anhydride or another acid anhydride. Potts and Yao (3) were apparently the first to employ dicyclohexylcarbodiimide (DCC) to generate mesoionic heterocycles, including miinchnones. Subsequently, Anderson and Heider (4) discovered that miinchnones can be formed by the cyclodehydration of N-acylamino acids using Ai-ethyl-Ai -dimethylaminopropylcarbodiimide (EDC) or silicon tetrachloride. The advantage of EDC over DCC is that the urea byproduct is water soluble and easily removed, in contrast to dicyclohexylurea formed from DCC. Although the authors conclude that the traditional Huisgen method of acetic anhydride is still the method of choice, these two newer methods are important alternatives. Some examples from the work of Anderson and Heider are shown. The in situ generated miinchnones (not shown) were trapped either with dimethyl acetylenedicarboxylate (DMAD) or ethyl propiolate. 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

The synthesis of heterocycles containing both silicon and sulfur in the ring from linear thiols have recently been described by Kirpichenko and coworkers190 (Scheme 66). 

Among the reactions applied in the synthesis of fullerene derivatives cycloaddition reactions such as [2 + 1]-, [2 + 2]-, [3 + 2] and [4 + 2] cycloadditions play a dominant role. In these reactions ring-fused fullerene derivatives are obtained, at least with incorporation of heteroatoms such as oxygen, nitrogen, or silicon. In this section photochemical reactions leading to cycloalkyl ring-fused fullerene adducts will be presented. Photocycloaddition reactions leading to C6o-fused heterocycles will be discussed later. 

As mentioned previously, photocycloaddition reactions are a useful method for obtaining fullerene derivatives fused directly to heterocycles. Such compounds have attracted much interest because they might have interesting properties, e.g., amino acid fullerene derivatives as biologically active compounds or pyrrolidino-fullerenes as key precursors for a great number of fullerenes donor-acceptor bridged dyads and triads. Fullerenes functionalized with silicon compounds are of great interest in materials science. The synthesis of these compounds will be described in this section. 

There have been no reports in the literature between 1996 and 2006 on the synthesis of five-membered heterocycles containing three or more heteroatoms and at least one silicon atom that use this particular synthetic methodology. 

---