A method
for the metal-free reduction of carboxylic amides using
oxalyl chloride as an activating agent and hydrogen as the final reductant
is introduced. The reaction proceeds via the hydrogen splitting by
B(2,6-F2-C6H3)3 in combination
with chloride as the Lewis base. Density functional theory calculations
support the unprecedented role of halides as active Lewis base components
in the frustrated Lewis pair mediated hydrogen activation. The reaction
displays broad substrate scope for tertiary benzoic acid amides and
α-branched carboxamides.
The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.
The metal-free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO-(OCCl 3 ) 2 ) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP-catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP-mediated H 2 -activation.
The vinylation of
various nucleophiles with acetylene at a maximum
pressure of 1.5 bar is achieved by organocatalysis with easily accessible
phosphines like tri-n-butylphosphine. A detailed
mechanistic investigation by quantum-chemical and experimental methods
supports a nucleophilic activation of acetylene by the phosphine catalyst.
At 140 °C and typically 5 mol % catalyst loading, cyclic amides,
oxazolidinones, ureas, unsaturated cyclic amines, and alcohols were
successfully vinylated. Furthermore, the in situ generation of a vinyl
phosphonium species can also be utilized in Wittig-type functionalization
of aldehydes.
The development of the frustrated Lewis pair catalyzed hydrogenation of tertiary and secondary amides is reviewed. Detailed insight into our strategies in order to overcome challenges during the reaction development process is provided. Furthermore, the developed chemistry is extended to the hydrogenation of poly amides and trifluoroacetyl amides for the convenient introduction of trifluoro ethyl groups into organic molecules.
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