Siloxy-metal derivatives

Since group 4 derived species are of particular interest as catalysts for olefin polymerization and epoxidation reactions, the thermal stability of surface metal-alkyl species, as weU as their reactivity towards water, alcohols and water, deserve some attention. On the other hand, mono(siloxy) metaUiydrocarbyl species can be converted into bis- or tris(siloxy)metal hydrides by reaction with hydrogen [16, 41, 46-48]. Such species are less susceptible to leaching and can be used as pre-catalysts for the hydrogenolysis of C-C bonds, alkane metathesis and, eventually, for epoxidation and other reactions. 

Coupling of alkenylcarbene complexes and siloxy-substituted 1,3-dienes affords vinylcyclopentene derivatives through a formal [3C+2S] cycloaddition process. This unusual reaction is explained by an initial [4C+2S] cycloaddition of the electron-poor chromadiene system as the 471 component and the terminal double bond of the siloxydiene as the dienophile. The chromacyclohexene intermediate evolves by a reductive elimination of the metal fragment to generate the [3C+2S] cyclopentene derivatives [73] (Scheme 26). 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

Apparently, the by-products of the metallic aluminum are different from the trimethylsilyl case again siloxy compounds can be collected from the walls of the reaction vessel and can be characterized by spectroscopic and X-ray structural means as [(tBu)Me2SiO]2 A1H 2 and HAl2[OSiMe2(fBu)]5 [34]. It is remarkable that in this case no Al[OSiMe2(tBu)]3 seems to form, and that, in contrast to the corresponding trimethylsilyl derivative, [(tBu)Me2SiO]2 A1H 2 is stable. 

This very versatile method for the synthesis of organoimido functions makes use of the great thermodynamic strength of Si—O and Si—F bonds. With metal oxides two synthetic strategies can be used leading to either complete removal of the oxygen or its conversion to a siloxy derivative (equations 45 and 46). 

The catalytic desymmetrization shown in Scheme 5 involves a meso-tetraene substrate optically pure unsaturated siloxane 23 is obtained in >99% ee and 76% yield [16]. The unreacted siloxy ether moiety is removed to afford optically pure 24. Mo-alkylidenes derived from both enantiotopic terminal alkenes in 22 are likely involved. Since the initial metal-alkylidene generation is rapidly reversible, the major product arises from the rapid RCM of the matched segment of the tetraene. If any of the mismatched RCM takes place, a subsequent and more facile matched RCM leads to the formation of meso-bicycle. Such a byproduct is absent from the unpurified mixture containing 23, indicating the exceptionally high degree of stereodifferentiation induced by the chiral Mo com-... 

The second harmful effect of tetrahydrofuran is that it may undergo ring opening and insert into the silicon-transition-metal bond, again with the formation of a siloxy derivative (252, 336, 337) e.g ... 

The sodium condensation reaction of a,co-bis(chlorosilyl)-substituted compounds and the coupling reaction of dilithio derivatives of compounds bearing 7t-electron systems with dichlorosilanes offer a convenient route to various silicon containing polymers. However, the polymers prepared by these methods always contain a small proportion of siloxy units in the polymer backbone, which would interrapt the electron delocalisation. Therefore, new synthetic routes to organosilicon polymers have been developed in which no alkali metal halide condensations are involved [6, 7]. We report syntheses of organosilicon... 

In the last time we also investigated the reductive coupling of the prepared a,co-bis[(trifluoro-methyl)sulfonyloxy]-substituted organosilicon derivatives with alkali metals. We found, that these coupling reactions did not occur completely. The isolated polymers always contained siloxy units after hydrolysis of the reaction mixture. Therefore we looked for a better reducing agent. 

Similarly, reference is made to another more recent review (41) of the author s contributions to the chemistry of rhodium, iridium, rhenium, osmium and ruthenium alkoxo, and hydroxo complexes, following a similar earlier publication (40) on the alkoxo derivatives of platinum metals. Reference may also be made to the first X-ray structural study (265, 266) of a siloxy derivative, [(cod)Rh(/i-OSiMe)3]2. 

Intramolecular [3-I-2]-cycloaddition reactions have also been performed using alkyne moieties as reactants. An early example of this methodology is the high-yield formation of 27 from alkyne 26. As in the case of the (Z)-3-benzyloxybut-2-enoate derivatives (vide supra), the presence of the 3-siloxy group is essential for the reaction to proceed. A diastereomeric mixture of 26 leads to the formation of the two diastereomeric bicyclic adducts 27 in a combined yield of 88%. ° Interestingly, albeit with only 60 /o conversion after 100 hours, the same products are also formed at room temperature under the influence of ultrasound and 6 mol /o of tetrakis(triphenylphosphane)palladium(0) as catalyst. In a purely thermal reaction in the absence of a metal catalyst, a totally different reaction, presumably involving a Diels —Alder type addition of the alkyne to a vinylbenzene moiety, takes place. ... 

Other types of heterofunctionalized cyclopropane derivatives used as starting materials in transition metal mediated conversions are acylcyclopropanes and similar systems. These systems, being electrophilic cyclopropanes, are opened by carbanion nucleophiles (e.g. cuprates, see Section l.B.2.1.3.) or nucleophilic transition-metal centers. In combination with siloxy groups, push-pull (capto-dative) substituted cyclopropanes are available as homoenolate precursors via Lewis acid mediated ring opening (see Section 1. B.2.1.4.).. This latter procedure has been used in tetrahydrofuran synthesis. 

Thus (XX) reacts with phenol in pyridine to form diphenoxysilicon phthalocyanine (XXII), with benzyl alcohol to form (XXIII), and with triphenylsilanol to form (XXIV) (168,170, 200). These complexes sublime readily without decomposition (cf. corresponding aluminum derivatives). Bis(diphenylmethylsiloxy)silicon phthalocyanine, which melts before subliming, is one of the very few metal phthalocyanines which actually melt (873). The siloxy complex (XXIV) may also be prepared in benzyl alcohol, thus implying that the Si—O—Si(Pc)—0—Si backbone is more stable than C—O—Si(Pc)—O—C. The dibenzyloxy derivative (XXIII) reacts with diphenylsilanediol to form bis(benzyloxydiphenylsiloxy)silicon phthalocyanine (XXV). 

In 2014 several reports described the use of inverse-electron-demand Diels-Alder reactions to furnish naphthalene-based compounds from phthalazine-containing ring systems. Employing nonprecious metal complexes, copper(l) and nickel(O) complexes, instead of silver salts to catalyze formal [4+ 2] cycloadditions of 1,2-diazines and siloxyalkynes, Rawal and his group developed an environmentally friendlier and more economical method for preparing siloxy derivatives of naphthalene, anthracene, and phenanthrene (Scheme 16) (140L3236). In addition, the copper catalyst could also be employed for the synthesis of the corresponding quinolines and isoquinolines. This method provided the respective products in... 

The structures of ether complexes alkali metal silanolates are associated, HMPA ones tetrameric while [PhMe2SiOK(CgHg)]4 shows CgH to K, KBa3(OSiHi3)5(DME)2 occurs as a bridged metal triangle, (Ph3SiO)3Bi(THF)3 is cis and (Ph3 iO)3Bi a polymer . Siloxy derivatives have been made for Ti and Zr, V, Nb and Ta, Mo and W, Fe and Co, Ni, Cu and Zn, and Y and the rare earths . ... 

Einaga, H., Nojima, M., and Abe, M., Photooxygenation (electron transfer) of sdyl enol ethers derived from 1-indanone silyl group and medium effects on the competitive formation of 3-siloxy-1,2-dioxetane and a-peroxy ketone. Main Group Metal Chem., 22, 539, 1999. 

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