Unsuccessful attempts to add alcohols to transient 2-amino-2-siloxy-silenes - leading to a new benign route for base-free alcohol protection

Abstract

Thermolytic formation of transient 1,1-bis(trimethylsilyl)-2-dimethylamino-2-trimethylsiloxysilene (2) from N,N-dimethyl(tris(trimethylsilyl)silyl)methaneamide (1) in presence of a series of alcohols was investigated. The products are, however, not the expected alcohol-silene addition adducts but silylethers formed in nearly quantitative yields. Thermolysis of 1 in the presence of both alcohols (MeOH or iPrOH) and 1,3-dienes (1,3-butadiene or 2,3-dimethyl-1,3-butadiene) gives alkyl-tris(trimethylsilyl)silylethers and the [4+2] cycloadducts between the silene and diene, which confirms the presence of 2 and that it is unreactive towards alcohols. The observed silylethers are substitution adducts where the amide group of the silylamide is replaced by an alkoxy group, and the reaction time is reflected in the steric bulk of the alcohol. Indeed, the formation of silylethers from the reaction of alcohols with silylamide represents a new base-free method for protection of alcohols. The protection reactions using 1 progresses at elevated temperatures, or alternatively, under acid catalysis at ambient temperature, and similar protections can be carried out with N-cyclohexyl(triphenylsilyl)methaneamide and N,N-dimethyl(trimethylsilyl)methaneamide. The latter silylamide can be used under neutral conditions at room temperature. The only by-products are formamides (N,N-dimethylformamide (DMF) or N-cyclohexylformamide), and the reactions can be performed without solvent. In addition to alcohols we also examined the method for protection of diols, thiols and carboxylic acids, and also these reactions proceeded in high yields and with good selectivities.

Photoinduced Brook-Type Rearrangement of Acylcyclopolysilanes

“…Additional stability is gained in silenes influenced by reversed (Si δ− =C δ+ ) bond polarization effected by π-donor substituents on the carbon atom. 15 Reverse-polarized silenes such as the Brook-type silenes 1 , the silatriafulvene 8 , or the transient 2-amino-2-siloxysilenes recently investigated by Ottosson et al 16 are less reactive toward moisture and alcohols than naturally polarized silenes (Si δ+ =C δ− ), and addition of alcohol often proceeds by C–O instead of Si–O bond formation. In contrast to the naturally polarized silenes, which give [2 + 2] and ene adducts as well, the reverse-polarized silenes react selectively with dienes to yield only [4 + 2] adducts.…”

Reactivities of Adamantyl-Substituted Metallenes with a C═E (E = C, Si, Ge, Sn, and Pb) Double Bond. A Theoretical Study

“…(b) Similarly to previous studies of the process of methanol addition to silenes, , the initial step in methanol addition reactions is predicted to be the formation of a weakly bonded precursor complex. In path I (the monomeric MeOH attack), our computed structures are quite similar to those reported by Leigh, Bendikov, Apeloig, and co-workers and are consistent with a weak hydrogen-bonding interaction between the methanol molecule and the CE double bond of the Rea-CE species.…”

“…It is well understood that silenes undergo nucleophilic addition of alcohols and that this could be one of the reactions that complicates their isolation . This reaction with alcohols has been more extensively studied, from a mechanistic standpoint, during the last three decades.…”

Silaphenolates and Silaphenylthiolates: Two Unexplored Unsaturated Silicon Compound Classes Influenced by Aromaticity

“…Influence of reverse Si=C bond polarization can be induced by π-electron donating substituents at the C terminus and/or by σ-electron donating substituents at the Si terminus ( Scheme 1 ) [ 13 , 15 ]. It has also been found that reverse polarization leads to a lower reaction rate for the addition of water and alcohols to silenes [ 7 , 16 , 17 , 18 ]. With regard to substituted silabenzenes we earlier found through quantum chemical calculations that reverse Si=C bond polarization could reduce their tendency for dimerization [ 19 ].…”