Silanes elimination reactions

Fluoride-induced /S-elimination reactions of silanes having leaving groups in the position are important processes in synthetic chemistry, as, for, example in the removal of / -trimethylsilylethoxy groups. 

A one-pot reaction between a tryptophan ester, benzotriazole, and 2,5-dimethoxytetrahydrofuran in acetic acid gives the diastereomeric benzotriazolyl tetracycles, 349, in good yield. Substitution of the benzotriazole by reaction with silyl enol ethers and boron trifluoride etherate gives the corresponding ketones 350 and 351, and reaction with allylsilanes gives the corresponding alkenes 352 and 353. If the boron trifluoride etherate is added to the mixture before the silane, elimination of benzotriazole from 349 is also observed (Scheme 83) <1999T3489>. 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Following the determination of the geometry and the thermochemistry of transition states, the rate parameters for the two silane decomposition pathways can be obtained directly by the TST formulation presented earlier. These calculations have led to unimolecular rate constant expressions 10 exp(-91000/RT)s-S and 10 exp(-62000/l r)s" for Si-H bond scission and H2 elimination reactions, respectively. These results clearly... 

Jones and coworkers developed a new method of generating silenes based on the addition-elimination reaction. Addition of t-BuLi to an appropriately substituted chloro(vinyl)silane produces a neopentyl-substituted silene67,68. Among many reactions, it has been shown that the transient silene adds to anthracene to afford stereoisomers, 40a and 40b, as isolable compounds (Scheme 11). Fractional crystallization of the adduct 40 from hexane gave pure 40a, leaving a 69/31 mixture of 40a and 40b. 

Using the metalloradical reactivity of the Rh(II)OEP (OEP = 2,3,7,8,12,13,17,18-octaethylphorphynato) dimer, the preparation of silyl rhodium complexes was achieved by the hydrogen elimination reaction with silanes I R SiH (R = R = Et, Ph R = Me, R = Ph, OEt). The Rh—Si bond length of 2.32(1) A, found when R = Et, is comparable to those in other Rh(III) complexes (Table 11). The crystal packing indicates that all the ethyl groups on the porphyrin periphery are directed toward the silyl group. Consequently, the aromatic part of one complex molecule is in contact with the aromatic part of the next molecule and the aliphatic part is in contact with the aliphatic part of the next molecule204. 

Linearly connected polycyclic silanes are usually made by salt elimination reactions, whenever the cyclosilanyl alkali metal compound involved is stable enough. Otherwise, the photochemical cleavage of bis(cyclosilanyl)mercury compounds may be employed. An... 

Attention has also been focused on reactions of /3-functional silanes. Sommer et al. 140) first reported the -elimination reaction of /3-chloro-alkylsilanes catalyzed with Lewis acids [Eq. (30)]. Transient formation of... 

In this article we concentrate on the first step of the reactions, viz. the initial reaction of silanes with the square-planar Pt(II) complexes (PnN)Pt(X)CH3 (X = CH3, H, Cl, SiH3). Only stereochemical aspects of the initial oxidative addition/reductive elimination reactions (Scheme 1) are considered in this contribution. Energetic aspects and details of the reaction mechanisms were discussed elsewhere [4 - 6], Some of the conversions of the Pt(II) complexes shown in Scheme 1 are endothermic, and therefore only occur if succeeded by another energy-releasing reaction, for example in a catalytic cycle. 

Desilylation of the C-silylaziridine (91) with caesium fluoride produced the 2//-azirine (92) in good yield by an elimination reaction (Equation (41)) <89CC836>. The aziridine (91) was prepared by addition of a nitrene to a vinyl silane (see Section 1.01.9.2). 

Incoming nucleophiles may also cause cleavage of the M-Si bond by inducing a reductive elimination reaction. Thus, MeLi and phosphines displace silanes HSiR3 from Cp(CO)2Mn(H)(SiR3) compounds11 195. 

As mentioned earlier, reductive elimination reactions are commonly observed processes that involve M-Si bond cleavage. Usually the transition-metal reductive elimination product is trapped by an added reagent such as a silane (equation 63)204, a germane (equation 64)205, a phosphine (equation 65)167 or hydrogen (equation 66, dppe = Ph2PCH2CH2PPh2)206. The latter reaction with hydrogen probably proceeds via initial oxidative addition of H2 to form a Pt(IV) intermediate. In the case of chiral complex ds-(SX-)-[(l-Naph)PhMeSi]PtH(PPh3)2, elimination of the silane upon addi-... 

After the metathesis, either ethylene-assisted C-H and Si-C reductive eUm-inations or metatheses of Cp2Zr(H)(CH2CH2SiH3) and Cp2Zr(SiH3)(CH2CH3) with silane take place to afford CH3CH2SiH3, whereas the direct C-H and Si-C reductive elimination reactions do not occur [40]. Also, metathesis with silane more easily occurs with lower activation barrier and larger exothermicity than... 

Water molecules are eliminated during silane bonding reactions, and oxane linkages (M-O-Si) are formed. Since calcium carbonate is not responsive to chemically reactive silanes, stearic acid treatment has become an important technique, widely used in calcium carbonate filled PVC, including rigid PVC products, flexible PVC and plastisols. 

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