Silicon/boron exchange

Silicon/boron exchange. For the sake of increasing reactivity in coupling an alkenylsilane is converted to a boronate by reaction with BCI3 and then with catechol. Such a product can be used in Suzuki coupling. 

Arylsilanes, after a silicon-boron exchange, can eventually be transformed into the corresponding arylzincs (Scheme 4.13) [77]. This method allows, for example, the one-pot preparation of (4-iodophenyl)isopropylzinc 56 from 1,4-bis(trimethylsilyl)benzene (55). 

Dibutyl acetyleneboronate is a moderately active dienophile since the Diels-Alder adduct with cyclopentadiene was obtained in a 25% yield after heating at 130 °C for 15 h [62]. In contrast, alkynyldibromoboranes, generated in situ by silicon/boron exchange, appeared to be highly reactive dienophiles [63]. Unfortunately, their great electrophilidty restricted their synthetic application to unfunctionalized substrates. 

X = S, 0, NR). Lithium-, silicon-, or tin-boron exchange provide facile access to the desired borylated species. On the other hand siloles and stannoles (X = Si, Sn) with organoboron substituents in the 3-position are conveniently prepared via organoboration reactions.Another new... 

In three-dimensional mixed-valence systems, electron transfer can manifest itself as electrical conduction, thermally activated. Most work continues to focus on the better known semiconducting materials such as silicon-boron or silicon nitride " (at low temperature), or organic crystals of the anthracene type (at high temperature),or redox polymer-coated electrodes. In the last-mentioned case, the importance of ion migration as well as electron transfer has recently been emphasized. In the mixed-valent Tl(I)3Tl(III)Cl6, conductivity and isotopic exchange studies have been taken to indicate that cation transfer is the principal charge-carrying mechanism, and not electron transfer as such. " Mossbauer... 

Boron has been shown to be an efficient catalyst. Various aldehydes and silyl enol ethers afforded the corresponding syn-substituted /3-hydroxyketones in high diastereoselectivities (80-94% de) when the reaction was performed in water with 10 mol% Ph2BOH, surfactant (SDS), and a Bronsted acid (Scheme 8.6). A mechanism involving a boron enolate intermediate generated by a silicon/metal exchange was proposed the improvement observed in the presence of benzoic acid could be due to an increase of the rate of the Si/B exchange. ... 

Difluoro-l-silacyclobutane is obtained by the exchange of two chlorine atoms for fluorine in the 1,1-diehloro analog using antimony(III) fluoride in w-xylene yield 46% bp 36 -38 C.64 Antimony(lll) fluoride is supposed to interact, through its lone electron pair, with coordinative-ly unsaturated silicon rather than with chlorine atoms at silicon. Because of this Si —Cl compounds are fluorinated slower than C — Cl derivatives in the presence of boron trifluoride.65... 

Apparently chlorine and trifluoromethanesulfonate groups attached to boron and silicon can be replaced by fluorine in lithium fluoride at lower temperatures than that reported for C-X (X = halogen) exchange. Dichloroborane 5 is converted to the difluoro derivative 6 with lithium fluoride in diethyl ether at 0-22°C.7... 

Most importantly, the scope of the Diels-Alder reaction is very high - not only allowing the synthesis of cyclohexenes and 1,4-cyclohexadienes using 1,3-butadienes and alkenes and alkynes, respectively, but also giving access to a multitude of different heterocycles by exchanging the atoms a-d in the butadiene as well as the atoms e and f in the alkene by hetero atoms such as oxygen, nitrogen and sulfur. However, also dienes and dienophiles with several other atoms as phosphorous, boron, silicone, and selenium have been described. Thus, many different heterodienes and heterodienophiles have been developed over the years (Tables 1-1 and 1-2). 

The exchange of tin for lithium or other metals is probably the most valuable job it does. Reagents such as BuLi attack tin or boron directly rather than removing a proton. Silicon is not usually attacked in this way and proton removal is more common. In the next chapter we shall see how transition metals open up a treasure chest of more exotic reactions for which the reactions in this chapter are a preparation. 

---