Silanes promoters

Chelation control has been invoked to explain the stereoselectivity of the reaction between 2-methoxycyclohexanone and trimethyl(2-propenyl)silane promoted by titanium(IV) chloride59, and has been observed for addition of allylsilanes to a-amidoaldehydes60, although the stereoselectivity is very dependent upon the amount of Lewis acid used and the structure of the substrate, particularly when the Lewis acid can bind to more than one site61. 

Another anti-cancer agent in clinical use is podophyllotoxin (3-59) this has an aryl tetrahydronaphthalene lignan lactone skeleton, and demonstrates potent tubulin-binding, anti-mitotic properties (Scheme 3.16) [30]. The Sherburn group [31] prepared this molecule by a tris(trimethylsilyl)silane promoted conversion of thionocarbonate 3-55 into the lactone 3-58, which proceeded with a yield of 38 %. As intermediates, the radicals 3-56 and 3-57 can be assumed. 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/silylformylation of enynes that invokes several of the same intermediates proposed for the rhodium-catalyzed cyclization/hydrosilylation of enynes (Scheme 7). Silylmetallation of the G=G bond of the enyne followed by / -migratory insertion of the pendant G=G bond into the resulting Rh-G bond could form rhodium cyclopentyl complex Illf. a-Migratory insertion of GO into the Rh-G bond of Illf followed by silane-promoted reductive elimination from the resulting rhodium formyl complex rVf could release the silylated cyclopentane carboxaldehyde with regeneration of silylrhodium hydride complex If (Scheme 7). 

The moisture resistance properties of filled molding compounds are enhanced by the treatment of the fillers with silane adhesion promoters prior to compounding. Silane promoters on wollastonite fillers in thermoplastic polyester molding compounds (50 percent filled) will improve the flexural strength after 16 h in 50°C water by as much as 40 percent. Silane-treated silica fillers have been found to significantly increase the moisture resistance of epoxy adhesives used in the electronics industry for chip, surface-mounted, and printed-circuit processes.8... 

Conventional silanes are limited to high-solids and solvent adhesive and sealant applications in which moisture is not encountered until use. However, water-stable epoxy silane promoters have recently been developed to enhance the wet strength of waterborne adhesives and sealants.17... 

Palladium-catalyzed cross couplings of organofluorosilanes with vinyl [37] and aryl [38] halides and triflates are known, where the activation of the Si—C bond by fluoride ion plays an essential role in the coupling. The pentacoordinated fluoroorganic silanes promote the transmetallation to the aryl halide-palladium complexes strongly. At least one fluorine... 

In sulfur-cured EPDM systems, the primary amino- and mercaptofunctional silanes can participate in the cure mechanism to a far greater extent than the vinylsilane as shown by the higher modulus values. In the peroxide-cured EPDM system, all silanes promote significant improvements in modulus but in different degrees depending on their relative reactivity. The methacryloxy functional silane... 

The proposed mechanism based on product observation is depicted in (Scheme 16). Regioselective insertion of the proximal double bond of allene 31 into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, HlRhJSiRs, affords intermediate D-L Subsequent allqme insertion forming the cyclic vinyl-Rh complex D-II followed by silane promoted reductive elimination provides the product 32 and the active Rh-Si complex. When allenynes that did not have substitutions on the alkyne terminus or on the distal allene double bonds were subjected to the optimized conditions, complex product mixtures were obtained. These... 

Indium salts also exhibited good catalytic activity in a reductive aldol reaction. For example, the silane-promoted diastereoselective reductive aldol reaction has been established by using InBra as a catalyst (173). In their report, the syn-selectivity obtained by an indium catalyst is higher than that of any other... 

Scheme 7.40 Cross-coupling reactions of 2-pyridyldimethyl(vinyl)silane promoted by TBAF.

Marek and co-workers found that magnesium-to-zinc transmetalation of the alkoxides derived from addition of acetylenic Grignard reagents to acyl silanes promoted the 1,2-Brook rearrangement. The resulting propargyl zinc intermediates (e.g., 37), in equilibrium with the silyl allenol ethers of type 38, underwent diastereoselective carbocyclization in suitable systems, providing cyclopentanol 39, for example, after acid quench and desilylation. ... 

Therefore, the utility of this cycloaddition in total synthesis was highlighted by Leighton and co-workers during their asymmetric synthesis of (+)-manzacidin C 70 via a chiral silane-promoted diastereoselective and enantiose-lective acyUiydrazone-alkene [3+2] cycloaddition reaction (Scheme 41.14). The manzacidins were isolated from the Okinawan sponge Hymeniacidon, and they possess activities such as a-adrenoceptor blockers, antagonists of serotonergic receptors, and actomyosin adenosine... 

The mechanism of the Leighton silane promoted acylhy-drazone-alkene cycloaddition is assumed by the authors to be stepwise. The approach of the alkene from the re face of the hydrazone-silane should be controlled by the chiral silane. The exact conformation 66 of the hydrazone-silane... 

SCHEME 41.14. Chiral silane promoted [3+2] cycloaddition with acylhydrazones. 

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