Silylation of Olefins

The Pt(CH2 = CH2)(PPh3)2-catalyzed dehydrogenative double silylation of olefins and dienes with o-bis(dimethylsilyl)benzene was also examined by Tanaka and co-workers.61 The major product of the reaction with dienes, such as isoprene and penta-1,2-diene, is a result of 1,2-addition to the less substituted double bond. The reaction pathway for simple alkenes, shown in Eq. (19), appears to be dependent on the alkene substrate and, in some cases, on reaction temperature. Products resulting from 1,2-addition, 1, and 1,1-addition, 2, are detected for various substrates. In addition, hydrosilylation may occur to give the simple hydrosilylated product, 3, or a by-product, 4, derived from 1,4-migration of a methyl group in 3. 

The dehydrogenative silylation of olefins, closely related to hydrosiiyiation, is promoted by a ruthenium carbonyl complex, RujCCOlij. The product vinylsilane is always the frans-isomer ... 

The dehydrogenative silylation of olefins, which is closely related to hydrosilylation, is effectively promoted by a ruthenium carbonyl cluster complex, Ru3(CO)12. The produced vinylsilane was the trans-isomer in every case examined76 (equation 26). 

The bis-silylation of olefins and acetylenes is a potentially desirable reaction on account of the fact that two C-Si bonds are generated in one step. Intramolecular versions would also be attractive. The reaction is analogous to the tethered hydrosilylation described above but has received less attention. One of the major problems which had to be over-... 

The bis-silylation reaction has found application in the synthesis of the antifungal metabolite (-)-avenaciolide [139] and in the enantioselective preparation of ( )-allylsi-lanes [140]. The use of chiral isocyanides prepared from the terpenoid (-H)-ketopinic acid has also been investigated in an enantioselective bis-silylation of olefins, although the levels of enantioselectivity are, in most cases, modest [141]. 

Basic methods for their production involve the hydrosilylation of alkylacetylenes catalyzed by platinum complexes [8] and the dehydrogenative silylation of olefins, e.g. styrene [9], 1-hexene [10,11], are catalyzed by rhodium [10], ruthenium [9,12, 13] and iridium [11] complexes and photocatalyzed by iron and cobalt [14,15] carbonyls. 

The catalytic inactivity of metallacarbene species e.g. Schrock catalyst [32] and Grubbs complex RuCl2(PPh3)(CHPh) in metathesis of vinyl-trisubstituted silanes and siloxanes also supports such a mechanism. This reaction is also called homo(hetero)coupling or trans-silylation of olefins with vinylsilanes. 

The reaction occurs via the formation of a potential complex containing a-alkyl and <7-silylalkyl ligands. The j8-H-transfer from the two ligands to the metal proceeding concurrently is a decisive step for two alternative reactions, that is, hydrosilylation and/or dehydrogenative silylation of olefins that have been recently reviewed in the catalytic and synthetic aspects (8,13,44). 

A new dicationic phosphonium salt, characterised by a high Lewis acidity, was described by Stephan and co-workers (Scheme 4). Studies on the reactivity of this compound showed its potential as Lewis acid catalyst for the hydro-defluorination of fluoroalkanes and the hydro-silylation of olefins. 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

As inert as the C-25 lactone carbonyl has been during the course of this synthesis, it can serve the role of electrophile in a reaction with a nucleophile. For example, addition of benzyloxymethyl-lithium29 to a cold (-78 °C) solution of 41 in THF, followed by treatment of the intermediate hemiketal with methyl orthoformate under acidic conditions, provides intermediate 42 in 80% overall yield. Reduction of the carbon-bromine bond in 42 with concomitant -elimination of the C-9 ether oxygen is achieved with Zn-Cu couple and sodium iodide at 60 °C in DMF. Under these reaction conditions, it is conceivable that the bromine substituent in 42 is replaced by iodine, after which event reductive elimination occurs. Silylation of the newly formed tertiary hydroxyl group at C-12 with triethylsilyl perchlorate, followed by oxidative cleavage of the olefin with ozone, results in the formation of key intermediate 3 in 85 % yield from 42. 

Methyl levulinate 679 condenses with silylated y9-alanine 680 in the presence of catalytic amounts of TsOH-H20 to give hexamethyldisiloxane 7 and the Schiff-base 681, whose O-trimethylsilyl groups are saponified by water (derived from TsOH H2O) to give, via 682, the intermediate enamine 683. Subsequent condensation of 683 with the Schiff base 681 affords, via 684, and subsequent saponification, a 4 1 mixture of olefins 685 and 686 [201, 202] (Scheme 5.64). 

Silylation of the 2-qfclohexanone phenylsulfoxide 1213 with the O-silylketene-acetal 1214 in the presence of Znl2 gives 75% of the Sila-Pummerer product 1215, whereas the 2-cyclooctanone phenylsulfoxide 1216 affords a ca. 1 1 mixture of the Sila-Pummerer products 1217 and the olefin 1218 [31] (Scheme 8.12). 

The Michael addition of allyl alcohols to nitroalkenes followed by intramolecular silyl nitronate olefin cycloaddition (Section 8.2) leads to functionalized tetrahydrofurans (Eq. 4.15).20... 

Another useful method for the asymmetric oxidation of enol derivatives is osmium-mediated dihydroxylation using cinchona alkaloid as the chiral auxiliary. The oxidation of enol ethers and enol silyl ethers proceeds with enantioselectivity as high as that of the corresponding dihydroxylation of olefins (vide infra) (Scheme 30).139 It is noteworthy that the oxidation of E- and Z-enol ethers gives the same product, and the E/Z ratio of the substrates does not strongly affect the... 

B. Tetranitromethane. Tetranitromethane forms colored charge-transfer (CT) complexes with a variety of organic donors such as substituted benzenes, naphthalenes, anthracenes, enol silyl ethers, olefins, etc. For example, an orange solution is instantaneously obtained upon exposure of a colorless solution of methoxytoluene (MT) to tetranitromethane (TNM),237 i.e.,... 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

Nucleophilic Reactions.—Attack on Saturated Carbon. Selected examples of the Arbusov reaction include phosphorylation of the chloroacetophenones (1) to give phosphonates, which cyclized to (2) in the presence of acid chlorides,1 formation of the azodiphosphonate (3) from 2,2 -dichloro-2,2 -azopropane,2 3 and the reaction of 2-chloro-3,4-dihydro-3-oxo-2//-l,4-benzothiazine (4) with triethyl phosphite to give the 2-phosphonate (5), which is used as an olefin synthon.8 Bis(trimethylsilyl) trimethylsiloxymethylphosphonite (6) has been synthesized by silylation of hydroxy-methylphosphonous acid, and, as expected, undergoes a normal Arbusov reaction with alkyl halides to give the phosphonates (7).4 This series of reactions, followed by... 

Electroreductive coupling of ketones with silyl-substituted olefins promotes interesting reactions that are useful for organic synthesis. For example, coupKng of ketones with trimethylvinylsilanes affords /I-trimethylsilyl alcohols, which are easily transformed to the corresponding olefins (Scheme 40). This reaction is interesting from the synthetic point of view since vinylsilane behaves as the equivalent to a /I-trimethylsilyl group-substituted anion [77, 83]. 

The iodoetherification strategy was applied to the synthesis of the smaller fragment coupling component 109 as well (Scheme 16). Silylation of alcohol 104 [30] (76% de) allowed the separation of the pure desired diastereomer, which in turn was subjected to hydroboration/oxidation, sulfide formation with thiol 105, and oxidation to give sulfone 106. The requisite y-triethylsilyloxy alkene functionality in 107 was constructed as a diastereomeric E) Z)=l.2 l mixture by another sulfone-based olefination of aldehyde 90 with 106. Treatment of 106 with... 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

During the past two decades, within the series of our studies, we have developed a silylative coupling reaction of olefins with vinylsubstituted siHcon compounds which takes place in the presence of transition-metal complexes (e.g. mthenium and rhodium) that initially contain or generate M—H and M—Si bonds (for reviews, see Refs [5] and [6]). The reaction involves activation of the =C—H bond of olefins and cleavage of the =C—Si bond of vinylsilane. The reaction, which is catalyzed by complexes of the type [ M( x-OSiMe3)(cod) 2] (where M = Rh, Ir) occurs according to Equation 14.12 [71, 72). 

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