Allylsilanes metathesis

In the presence of an imidazolium salt and a base, oxidative cyclization of a Ni(0) species upon the diene and an aldehyde takes place first and forms an oxanickellacycle 25, which equilibrates with a seven-membered oxanickella-cycle 26, naturally possessing a cis double bond. cr-Bond metathesis through 26 with hydrosilane affords (Z)-allylsilane (Z)-23. The role of NHC ligand (AT-heterocyclic carbene, generated by H+ elimination from imidazolium C2H by a base) is not clear at present a Ni(0)-NHC complex is believed to effectively produce 26. 

Allylsilanes Good Cross-/Self-Metathesis Selectivity. 162... 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

As expected, there was no formation of stilbenes or a dinitrile product and, more surprisingly, in all of the reactions reported only 5-7% of the allyltrimeth-ylsilane self-metathesis product was observed. It was proposed that this lack of allylsilane self-metathesis was due to the steric bulk of the TMS group reducing the reactivity of the Me3SiCH2 substituted alkylidene. In a more recent report by Blechert and co-workers it was noted that allyltrimethylsilane and its hydrocarbon equivalent (4,4-dimethylpent-l-ene) had comparable reactivities in the cross-metathesis reaction [28], further suggesting that the selectivity arises from steric rather than electronic effects. 

In a second report by Blechert on the cross-metathesis of polymer-bound alkenes, an immobilised allylsilane 19 was reacted with a variety of highly func-... 

As with the allylsilane cross-metathesis reactions, significant quantities of allyl stannane self-metathesis were not detected in any of the reactions and the trans isomer predominated in the cross-metathesis products. Identical reactions were carried out using allyltributyl stannane, in place of allyltriphenyl stannane, but the yields of the cross-metathesis products were consistently lower and in many cases dropped below 25%. 

A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

An interesting application of the cydization of alkenyl thioacetals is the stereoselective preparation of olefmic diols. Thus, oxidative cleavage of the silicon—carbon bond [32] in the ring-closed metathesis products, i.e. cyclic allylsilanes such as 35 and 36, affords (Z)-alk-2-ene-1,5-diols 37 and 38 (Scheme 14.18) [33],... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

Apart from the tandem metathesis/carbonyl o[efination reaction mediated by the Tebbe reagent (Section 3.2.4.2), few examples of the use of stoichiometric amounts of Schrock-type carbene complexes have been reported. A stoichiometric variant of cross metathesis has been described by Takeda in 1998 [634]. Titanium carbene complexes, generated in situ from dithioacetals, Cp2TiCl2, magnesium, and triethylphosphite (see Experimental Procedures 3.2.2 and 3.2.6), were found to undergo stoichiometric cross-metathesis reactions with allylsilanes [634]. The scope of this reaction remains to be explored. 

Resin-bound (4-acyloxy-2-buten-l-yl)silanes, which can be prepared from resin-bound allylsilanes and allyl esters by cross-metathesis, react with dilute TFA to yield free carboxylic acids (Figure 3.7 [75]). However, the scope of this strategy remains to be explored. Similarly, esters of polystyrene-bound (2-hydroxyethyl)silanes readily undergo acidolysis and have been used as acid-labile linkers (Figure 3.7 [76]). 

Enediynes, silane-initiated cascade cyclization, 11, 400 Ene reactions, allylsilanes, 9, 313 Energetics, key elementary reactions, 1, 617 Enes, niobium complexes, 5, 87 Ene-yne metathesis... 

Cross-metathesis of two different alkenes 11 and 42 usually produces a mixture of products 6 and 15. However, depending on the functional groups R1 and R2, the cross-product 6 is obtained with high selectivity rather than the homoproduct 15 from 11 and 42. Some terminal alkenes, such as allylstannane [16], acrylonitrile [17,18] and allylsilane [19], undergo clean cross-metathesis to give cross-products 6 as the main product, rather than homoproducts 15. Cross-metathesis of the cyclic alkenes 43 with terminal alkenes 42 can be used for the synthesis of dienes 44. 

The Mo-catalysed cross-metathesis of acrylonitrile (59) [17,18] and allylsilane (60) [19] with alkenes 61 and 62 produced cross-products 63 and 64 with high selectivity. Reaction of 1-octene with 2 equivalents of styrene (65) afforded 66 in 89% yield. Only small amounts of stilbene (68) and 67 as the homoproducts were formed [23]. 

Murai et al. showed that the cycloisomerization of enynes catalyzed by PtCl2 has several feasible pathways (1) to 1,3-dienes via a formal metathesis, (2) to a 1,4-diene if the enyne substrates contains an allylsilane or stannane, (3) to a homo-allylic ether if it the reaction is performed in an alcoholic medium, or (4) to bicycle[4.1.0]heptene derivatives (Scheme 4) [26]. Further studies conducted by other groups have indicated the cyclization might proceed via a cationic mechanism triggered by coordination of Pt(II) with the alkyne moiety [27, 28]. Very recently, Oi and coworkers also observed a formal metathesis reaction mediated by a cationic Pt complex [29]. 

In some systems self-metathesis of allylsilanes has been observed to accompany the CM. The CM of allylsilane with unsaturated compounds is a convenient method of introduction of the silyl group into the olefin. Moreover, a chemoselective run of CM has been demonstrated (Eq. 38) [68]. 

A polymer-supported allylsilane was prepared by the cross-metathesis of the simpler immobilized allylsilane and an olefin and was used as a new reagent for allylation of an acetal in the presence of TiCE (Eq. 71) [191]. 

Silyl group transfer. Allylsilanes and vinylsilanes serve as silyl group donors to alkenes such as styrenes and vinyl ethers. It is apparently a metathesis reaction. 

Olefin metathesis. Cross metathesis permits a facile access to allylsilanes and Z)-a,P-unsaturated nitriles. Furthermore, employing an alkene metathesis in tandem with asymmetric allylboration provides functionalized alkenes. ... 

We first synthesized suitably functionalized amides in four steps from the corresponding alkenyl bromides (Scheme 11). The allylsilane function was introduced by cross-metathesis between the terminal alkenes and allyltri-methylsilane. Then, the bromide was substituted by an azide and the latter was reduced to the primary amine by the action of LiAlELj. As before, a pep-tidic coupling with vinyl acetic acid led to the desired starting compound with a diastereomeric ratio /Z=70/30. 

A simultaneous intermolecular/intramolecular enyne metathesis process using diynes (e.g., 267) and allylsilane 268 afforded conjugated triene derivatives (e.g., 270). Gyclopropanation was observed as a side-reaction in the tandem enyne metathesis RCM of diene-yne 271. ... 

Cross-metathesis of allylsilanes with alkenes also occurs via their preliminary isomerization followed by the reaction of l>propenylsilane with exemplary olefin - 1-decene resulting in l-(triethoxysilyl)-l-decene as a product (equation 13b). If crossmetathesis took place, one could expect l-silyl-2-undecene as a product of this reaction (equation 13a). 

In the intermolecular process, cross-metathesis offers a usefid and versatile alternative to alkyne hydroboration for the synthesis of functionalized alkenyl pinacol boronates with a moderate to high -selectivity. Styrenes, allylsilanes, protected alcohols and amines are good partners as 1,1-disubstituted olefins [105-107]. This reaction has been exploited recently as part of a macrocyclization strategy in the epothilone series (Scheme 9.52) [108]. 

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