Carbenes, generation silyl

Cycloalkyl(silyl)carbenes with an a-C—H bond have not yet been investigated systematically. When cyclohexyl(trimethylsilyl)carbene was generated by thermal a-elimination from cyclohexyl-bis(trimethylsilyl)methanol at 500 °C, only the (1,2) hydride shift took place, whereas cyclopentyl(trimethylsilyl)carbene, generated analogously, gave both the endocyclic alkene and the 1,3-C,H insertion product85,86 (equation 47). 

In the last 15 years we have developed two new catalytic reactions between the same parent substances, i.e. silylative coupling (SC) (also called tmns-silylation or silyl groiq> transfer) and cross-metathesis (CM) of alkenes, which have provided an universal route for the synthesis of well-defined molecular compounds with vinylsilicon functionality. While the cross-metathesis is catalyzed by well-defined Ru and Mo carbenes, the silylative coupling is catalyzed by complexes initialing or generating M-H or M-Si bonds (where M = Ru, Rh, Ir). For recent reviews see Refs. [4-6],... 

Aryl(trimethylsiloxy)carbenes. Acylsilanes (153) undergo a photoinduced C —> O silyl shift leading to aryl(trimethylsiloxy)carbenes (154).73,74 The carbenes 154 can be captured by alcohols to form acetals (157) 73 or by pyridine to give transient ylides (Scheme 29).75 LFP of 153 in TFE produced transient absorptions of the carbocations 155 which were characterized by their reactions with nucleophiles.76 The cations 155 are more reactive than ArPhCH+, but only by factors < 10. Comparison of 154 and 155 with Ar(RO)C and Ar(RO)CH+, respectively, would be of interest. Although LFP was applied to generate methoxy(phenyl)carbene and to monitor its reaction with alcohols,77 no attempt was made to detect the analogous carbocation. 

Enol ether or silyl enol ether as an alkene in RCM can be used by the second-generation mthenium carbene complex Ig. It affords the cyclic enol ether or silyl enol ether, which gives a cyclic ketone [Eqs. (6.17) and (6.18)] °... 

Dixneuf used [RuCl2(/>-cymene)]2 as a catalyst for the reaction of enyne 72a in the presence of imidazolium salt and CS2CO3 and obtained the enyne metathesis product 73a in a high yield. The enyne silyl ether 72b is converted under similar reaction conditions into r/i7ra-compound 73b which after the Tamao oxidation gives diol 74 (Scheme 28). In this reaction, V-heterocyclic carbene should be generated to coordinate to the ruthenium metal, but the actual species for this reaction is not well documented. 

In this context, it is worth mentioning that there is only one other, clear-cut example for the simultaneous occurrence of the acyl(silyl)carbene-to-acylsilene and the acylcarbene-to-silylketene rearrangement of an acylcarbene bearing a Si—Si substituent. Carbene 57, generated by photolysis of diazoketone 56 in benzene, isomerized to both 58 and 59 in about equal amounts44. While the acylsilene cyclized to 1,2-silaoxetene 60, the ketene was isolated and structurally characterized by X-ray diffraction analysis of the derived... 

For alkyl(silyl)carbenes where the alkyl contains an a-C—H bond, 1,2(C C) hydride shift leading to a vinylsilane is the common reaction pathway. Vinylsilane formation has been observed for free (photochemically or thermally generated) carbenes (equations 4384, 4448.49.50 and 45 85,86) but also in carbenoid reactions. In the latter case, the configuration of the alkene could be controlled to a large extent by the choice of the catalyst The -alkene was formed nearly exclusively with copper(I) chloride as catalyst87, whereas rhodium(II) pivalate88 gave mainly the Z-alkene (equation 46). 

Silenes are formed by rearrangement of silylcarbenes. If polysilylated diazomethanes 298-300 are employed, a selective migration of a silyl group to the carbene centre occurs and silenes 301, 92 and 302 are formed (equations 74-76)164. The outcome of trapping reactions is independent of the mode of silene generation photochemical and pyrolytic methods give the same results. 

Methylhydroxycarbene, MeC(OH), has been generated in one of the three reaction pathways of the collision-induced dissociation of protonated butane-2,3-dione.13 Its enthalpy of formation was found to be 16 4 kcalmol-1. Fluorophenoxycarbene (PhOCF) has been generated inside a hydrophobic hemicarcerand (1) by irradiation of the corresponding confined diazirine.14 Its reactivity (especially dimerization and reaction with water) was significantly lowered by the incarceration, allowing its persistence for days at room temperature. New (amino)(silyl)carbenes (2) have been generated and their structure-activity relationship studied. They showed behaviour similar to those of previously reported (amino)(alkyl)carbene.15... 

Even Lewis acid adducts of the acyl metalate 11 (Section 8.3.1.1, Scheme 8) proved effective in generating benzannulation products. The reaction of a trimethylsilyloxy chromium carbene complex with 3-hexyne to give the respective semi-silylated hydroquinone tricarbo-nylchromium complex in 35 % yield [30a] and the use of titanoxycarbene complexes [30b] constitute such examples. Thiocarbene complexes form hydrothioquinones in a Lewis acid supported benzannulation reaction [32]. 

The ruthenium carbene complex (Grubbs catalyst) which has shown high efficiency in alkene methathesis and related processes, since it displays tolerance toward a wide variety of common functional groups, has also appeared of synthetic utility in the hydrosilylation of ketones to yield silyl ethers-one of the most widely used classes of protecting groups in synthetic chemistry (Eq. 97) [ 151 ]. The reaction requires temperatures above 50 °C, which generate a slightly increased amount of silylated by-products. 

Lithium phosphites also can catalyze the silyl benzoin reaction of acylsilanes. Its asymmetric version is successfully achieved by a lithium phosphite derived from a homochiral diol.236 Thiazolium salt 32 effectively promotes conjugate acylation of a, 3-unsaturated carbonyls with acylsilanes in the presence of DBU (Equation (61)).237,237a The active catalyst of this sila-Stetter reaction would be a carbene species generated from 32 by deprotonation. 

The opposite sequence is described with alkynyl silyloxy-tethered enynes. A cascade CM-RCM reaction proceeds in the presence of a second generation ruthenium carbene to give cyclic siloxanes. The initial CM is directed to occur on the alkyne by employing sterically hindered substituted alkenes tethered to the alkyne via a silyl ether group [24] (Scheme 11). 

Bi- and tricyclic carbocyclic ring systems can be easily obtained from (1-alkynyl)carbene complexes in two or more reaction steps without isolation of intermediate products. For example, the bicyclic 1,3-dienone 44 has been generated from compound le in a two-step cycloaddition/annulation procedure involving [4+2] cycloaddition of a diene to the C=C bond of a (l-alkynyl)carbene complex and a subsequent Dotz reaction.68 Furthermore, a 1,4-dioxy dihydronaphthalene 45 was obtained from the silyl derivative lg in a similar reaction sequence. In contrast to the methyl derivative 44, the corresponding silyl derivative generated from lg undergoes facile... 

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