Dehydrogenative silylation

Hydrogenation of the product 147 removes the benzyl protecting groups and at the same time reduces the triazine to its dihydro derivative 148. A roundabout scheme is required for dehydrogenation due to the sensitivity of the intermediates. The product is thus converted to its silyl ether 149 exposure to air results in oxidation and desilylation. There is thus obtained the antineoplastic agent fazarabine (150), also known as ara-A C. 

The N-bis-silylated o-phenylenediamine 1511 reacts with DMF at 120°C to give benzimidazole, in 97% yield, and dimethylamine and hexamethyldisiloxane 7, whereas reaction of benzaldehyde with 1511 gives only 29% 2-phenylbenzimida-zole 1513, because the intermediate benzimidazoline 1512 is only rather slowly dehydrogenated to 1513 [52]. Heating of N,N -bis(trimethylsilyl)ethylenediamine 1514 with DMF affords imidazoline 1515 and dimethylamine and HMDSO 7 ]52] (Scheme 9.32). The lactam 1516 cycHzes analogously with SiCU 57/triethylamine in 63% yield to give 1517 ]53]. 

We hoped to define conditions that would allow us to use toluene as solvent for the reaction. The toluene solution of 8 could be taken directly into the dehydrogenation following distillation to remove water and alcohol solvents. The boiling point of toluene was suitable for the thermolysis but silylation in toluene was very slow. 

A breakthrough came when trifluoromethanesulfonic acid (triflic acid, TfOH) was found to catalyze the silylation reaction in toluene. Using TfOH in toluene, silylation is complete within minutes at 25 °C. Other strong acids, FS03H and MsOH gave lower overall yields in the dehydrogenation. 

In discussion of the dehydrogenation of silyl enol ethers using DDQ, Jung and Murai had proposed that DDQ abstracts a hydride producing a stabilized cation 70 which loses TMS+ to give the enone 23 (Scheme 3.31) [8]. 

Reinvestigating the reaction of cyclohexanone silyl enol ether 22 with DDQ, adducts were also observed [26]. At 25 °C, both C-C and C-0 adducts (75 and 76) were formed along with the dehydrogenation products 23 and 77. The ratio of... 

The list of Process Chemists, Chemical Engineers, and Analytical Chemists who contributed to the success of this effort is too long to include here. I would, however, like to acknowledge the contribution of Apu Bhattacharya in discovering the silylation-mediated dehydrogenation of the azasteroids and Ulf Dolling whose guidance made our success possible. 

The disilanickela complex 21 was also found to be a good catalyst for the dehydrogenative double silylation of aldehydes. The nickel-catalyzed reactions of 1,2-bis(dimethylsilyl)carborane 11 with aldehydes such as isobutyraldehyde, trimethylacetaldehyde, hexanal, and benzaldehyde afforded 5,6-carboranylene-2-oxa-l,4-disilacyclohexane.32 34 36 The dehydrogenative 1,4-double silylation of methacrolein and tram-4-phenyl-3-buten-2-one in the presence of a catalytic amount ofNi(PEt3)4 also took place under similar conditions. In contrast, the reaction of 11 with a-methyl-tran.s-cinnamaldehyde and irans-cinnamaldehyde under... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

Recently, Kumada et al. (49) have published a report on what they refer to as dehydrogenative, stereoselective cis double silylation of internal acetylenes. This appears to be a variation of Eq. (53), with diethyl bipyridyl nickel(11) as the catalyst, in which hydrogen is liberated instead of being added to an alkene to form a saturated product. 

Keywords Alkane metathesis Borylation C-H bond activation Dehydrogenation Hydroarylation Iridium catalyst Silylation... 

Falck has recently reported dehydrogenative silylation of heteroarenes with triethylsilane (18) [97]. Coupling with the Si-H bond of triethylsilane, rather than the disilane Si-Si bond, in conjunction with the use of norbomene that presumably acts as a hydrogen acceptor, gives good yields with indoles, thiophenes, and furans, under relatively mild condition (80°C). Unlike the reaction shown in Scheme 7, silylation of indole did not require protection of the N-H group. 

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

Hydrosilylation and Dehydrogenative Silylation of Carbon-Carbon Multiple Bonds... 

Unlike Pt catalysts, the triad complexes of iron and cobalt catalyze competihvely both dehydrogenative silylation and hydrosilylation [11]. The reaction can proceed via a complex containing the o-alkyl and o-silylalkyl ligands (Scheme 14.1). 

The concurrent P-H transfer from the two ligands to the metal is a key step for two alternative reactions, namely hydrosilylation and/or dehydrogenative silylation [6, 11]. 

Scheme 14.1 Organometallic intermediate competitive hydrosilylation and dehydrogenative silylation of alkenes.

Ir(I), Ir(II) andlr(V) complexes stabilized by an O-donor ligand (e.g. [Ir(coe)(triso)] and [Ir(C2H4)2(triso)] (triso = tridentatetris(diphenyloxosphosphoranyl)methanides) are effective catalysts for the dehydrogenative silylation and hydrosilylation of ethylene [16-18]. 

Scheme 14.2 Competitive hydrosilylation and dehydrogenative silylation of ethylene and 1-hexene.

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