Disilyl ether

These reactions presumably proceed by catalytic cycles in which the carbonyl component is silylated. The silyl ether can then act as a nucleophile, and an oxonium ion is generated by elimination of a disilyl ether. The reduction of the oxonium ion regenerates the silyl cation, which can continue the catalytic cycle. 

Disilyl ethers of bicyclo[2.2.0]hexane-l,4-diols were transformed to cyclohexane-1,4-diones by oxidation after hydrolysis in methanol to the diols, which can be isolated.100 Sodium periodate101-103 and molecular oxygen100,101,104 were used as oxidants. 

Anodic oxidation of hydroquinone disilyl ethers also takes place easily at around 1 V vs SCE, as shown in Table 1044. It was proposed that an initial one-electron oxidation generates a cation radical which decomposes by a Si—O bond cleavage to form quinones (equation 40)44. 

For aldohexoses, OH-1, OH-2, and OH-6 seem to be particularly reactive [399,400]. Nevertheless, the initial product of the treatment of D-a/tro-3-heptulose (coriose) with chlorotrimethylsilane in pyridine is claimed to have a free hemiacetal hydroxyl group [401]. 1,6-Anhydro-P-D-glucopyranose yielded 87% of the 2,4-disilyl ether [402],... 

The reagent reacts selectively with the allylic hydroxyl group of 6-deoxyglycals 95 % of 3-0-(/ert-butyldimethylsilyl)-L-rhamnal were isolated after 2 h reaction at room temperature, in addition to 3 % of the 4-silyl and 2 % of 3,4-disilyl ethers [424]. Reinvestigation of this reaction by Thiem and coworkers revealed that temperatures below —20 °C are necessary in order to achieve such a high regioselectivity. At room temperature, a complicated mixture of products was obtained [127a]. Derivatives of N-acetylneuraminic acid such as 59 were also partially silylated with efficiency... 

With the catalytic McMurry reaction a few mole percent of DME-free TiCl3 are sufficient to run the reaction to completion. The trick is that stoichiometrically added zinc continuously provides low-valent titanium and the oxygen that is released is bound by a supersto-ichiometric amount of chlorosilane as a disilyl ether ( disiloxane ). This procedure has been utilized for many indole syntheses based on the McMurry reaction (Side Note 17.6). 

The aldehyde is formed immediately. What happens to the other product illustrates again just how easy nucleophilic substitution at silicon can be. Two of these compounds combine together to give a disilyl ether, called a disiloxane. 

Zincates derived from the ketone ,/(-dianions react with enones, affording unsymmetrical 1,6-diketones (Table 5)29. As shown in the second example of Table 5, quenching with TMSC1 gave the corresponding dienol disilyl ether. A mixed zincate consisting of the dianion and methyllithium in a 2 1 ratio gave comparable results to the symmetrical zincate. 

Oxymethylation of epoxides. " The reaction of a hydrosilane and carbon monoxide with an epoxide catalyzed by CojfCO), results in a regio- and stereoselective ring opening to give disilyl ethers of a 1,3-diol in generally high yield. 

The iodide is liquid at room temperature, the rest are gaseous. Surprisingly, only the bromide is spontaneously inflammable in air. The reaction of the fluoride with water is not recorded the others are hydrolysed immediately to disilyl ether, a colourless gas ... 

In general, silyl compounds present a contrast to their methyl analogues in that they more readily enter into reactions in which the respective group retains its identity. This is exemplified in the instantaneous conversion of the silyl halides by water into disilyl ether, and in their rapid reaction with silver salts. The principal causes of this reactivity are (i) the case with which the co-ordination number of the silicon atom rises from four to six ... 

There is clearly an option for further dihydroxylation. The diol can be protected and how we protect it depends on the dihydroxylation we want to do next. It turns out that the disilyl ether 158 is matched with (DHQ)2PYR whereas dioxolane 159 is matched with (DHQD)2PYR. Hence we may, by judicious protection, maximise subsequent stereoselectivity. Obviously, the disilyl ether of the enantiomer of 157 would be matched with (DHQD)2PYR and the enantiomer of dioxolane 159 with (DHQ)2PYR. 

The structures of trisilylamine, N(SiH3)3, and disilyl ether, (H3Si)20, are shown in Figure 13.11. The NSi3 skeleton in N(SiH3)3 is planar and the N—Si bond distance of 173 pm is shorter than the sum of the covalent radii, (see... 

---