Allyloxysilanes

Radical-chain cyclization of alkenyloxysilanes using thiol catalysts give five-membered ring products (via a 5-endo cyclization) in the case of allyloxysilanes... 

Allyloxysilanes (14) undergo radical chain cyclization in the presence of di-tert-butyl hyponitrite as radical initiator and thiol as a catalyst at ca 60 °C (Reaction 6.3) [5]. The thiol promotes the overall abstraction from the Si—H moiety as shown in Scheme 6.4 and the silyl radical undergoes a rapid 5-endo-trig cyclization. Indeed, EPR studies on the reaction of t-BuO radical with silanes 14 detected only spectra from the cyclized radicals even at — 100°C, which implies that the rate constants for cyclization are > 10 s at this temperature. 

Intramolecular hydrosilylation of the higher homologues 15 and 20 under similar conditions gave also excellent yields of cyclized products [5]. The homo-allyloxysilanes 15 afforded a mixture of six- and five-membered ring products in a ratio of 4.5 1 for R = Me and 2.5 1 for R = Ph in favour of the larger ring (Reaction 6.4). The EPR spectra obtained by the reaction of t-BuO radical... 

The [1,4]-rearrangement of the allyloxysilane system is a potentially useful method for allylsilane synthesis (equation 106). However, in general, a [1,2]-rearrangement may occur predominantly in this system (see Section n.F.l). To this end, the author s group recently found that the rearrangement of 171 in the presence of excess amount of HMPA provides the [l,4]-retro-Brook rearrangement product 172 in excellent yield (equation 107). ... 

It is well known that thermal decomposition of allyl-substituted silanes proceeds by retro-ene reaction with formation of transient species having a Si=C bond, such as silaben-zene, silatoluene and dimethylsilaethylene4b e. The kinetic data on the gas-phase pyrolysis of a similar allyloxysilane derivative, (l,l-dimethylallyloxy)dimethylsilane (16), and the results on thermolysis of allyloxydimethylsilane (17) in a flow system both indicate the participation of an intermediate silanone, (CH3)2Si=0 (10), as shown in Scheme 523. 

In 1989, Nefedov and coworkers have reinvestigated the thermolysis of the above-mentioned allyloxysilane derivatives 16-18 and of 2,2,6-trimethyl-2-silapyrane (21) using vacuum pyrolysis and matrix isolation techniques23. IR spectroscopic studies on the products isolated in the matrices enabled them to probe directly the intermediacy of 10 in these reactions and to discuss its thermal stability. Only in the case of allyloxydimethylsilane (17) did they find direct spectroscopic evidence for the formation of 10 by observation of its most intense band at 798 cm-1 in the matrix IR spectrum of the pyrolysis products. In all other cases silanone 10 was not detected and it was assumed that it is thermally unstable, undergoing fragmentation into SiO and CH3 radicals as shown in Schemes 7, 8 and 9 (the species actually observed in the matrix are indicated). In this paper, Nefedov and coworkers have reaffirmed the thermal and kinetic stability of dimethylsilanone 10 in the gas phase, which they had previously described19. 

A 1 1 mixture of (E)- and (Z)-allyloxysilanes 231 was prepared by the reactions of ( )-trimethylsilylvinyl sulfone 229 with aryllithiums followed by the addition of aliphatic aldehydes and cyclohexanone (equation 147). An anionic 1,4-silyl migration from C to O in the lithium alkoxide 230 was proposed to be involved. On the other hand, lithium alkoxide 232 lacking an aryl group was stable and gave no rearranged product even at room temperature (equation 148)360. 

AcylsUanes. A new route to acylsilanes from allyloxysilanes is shown in equation (I). A key step is the palladium-promoted isomerization of the double bond under neutral conditions. ... 

The asymmetric retro-[l,4] Brook rearrangement of allyloxysilane using 582 (R = C00C6H4-/)-N02) as electrophile and its stereochemical course at silicon to give 592 have been evaluated <2006AGE2235>. 

Under carefully optimized conditions, including addition of HMPA to the reaction mixture, deprotonation of allyloxysilanes favored the retro-1,4-Brook rearrangement. The method was applied to systems having an asymmetric center at silicon (e.g., 160), and it was shown that the rearrangement occurred with retention of configuration at silicon. " ... 

Benzyloxysilanes and allyloxysilanes were also utilized for Hf-catalyzed Friedel-Crafts reaction of benzene rings (Equations 24 and 25) [30]. The similar reaction system can be applied for a three-component coupling of benzaldehyde, allylsi-lane, and substituted benzenes to afford diarylpropene (65) in excellent yields (Equation 26) [31]. This method provides a short synthetic route for tamoxifen (68) (Equation 27) [32]. 

A mechanistic study of the retro-[1,4]-Brook rearrangement of 3-silyl allyloxysilanes is reported to explain why acidic hydrolysis gives the enol, whereas basic hydrolysis gives the aldehyde (Scheme 62)P... 

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