Topics in Current Chemistry (2020) 378:1
1 3
the intramolecular reactions led to a carbocyclic structure derived from the reaction between the enamine and the parallelly formed electrophilic gold triple bond
complex. It is worth mentioning that normally, in these types of reactions, the
Hayashi–Jørgensen-type catalysts preferred, despite the fact that a Michael reaction requires the presence of a quite strong electrophile (represented by the iminium
intermediate). The above mentioned catalyst favors the entire process due to the
fact that the second step of the reaction (the enamine attack) requires mandatorily
an enhanced nucleophilic behavior. Despite the proof of concept, no enantiomeric
excesses were observed for the reaction. An enantioselective variant of this carbocyclization reaction was later reported by Jørgensen [103]. Gold is not only able to
activate π-electron density of triple bond of alkynes, but it is also able to activate
other suitable substrates like allylic alcohols. In fact, allylc alcohols are activated by
gold catalysis to realize a catalytic enantioselective intramolecular α-allylic alkylation of aldehydes through the combination of gold and aminocatalysis (Scheme 17a)
[107].
A range of chiral secondary amines (Hayashi–Jørgensen catalyst, MacMillan I
and II type of catalysts) and different binuclear chiral phosphine gold(I) complexes
were tested in the model ring-closing reaction. Remarkably, decent levels of stereoselection were observed when the organocatalysts were merged with a cationic gold
complex 32, although the combined use of chiral gold complexes with achiral amino
catalysts was proved largely unsuccessful. Apparently, the gold complex is able to
activate the allylic alcohols through the formation of an electrophilic allylic cation.
Furthermore, careful mechanistic evidences for an S N 2′-type mechanism were collected. In other words, although this activation is often described to occur through
the formation of an allylic gold complex, the gold(I)-coordination with the olefin
activates the alkene through the nucleophilic attack of the enamine. Deauration
of the resulting organogold species, with concomitant elimination of water, leads
to the formation of the observed double bond. Gonzales reported an intermolecular reaction of allenamides with aldehydes [108] using a diaryl prolinol silyl ether
1c as the active catalyst in the presence of the ortho-fluorobenzoic acid as additive
(Scheme 17b). The same reaction was reported by Mascareñas and López [109].
6.2 Use of Copper Salts Such as π‑Lewis Acids in Synergistic Enamine Catalysis
The ability of copper to form stable π-complexes with double or triple bonds is well
known in the literature [110]. The Lewis acid organocatalyzed enantioselective
preparation of different five-membered carbo- and heterocyclic structures through
aminocatalysis and copper(I) activation of alkynyl α-disubstituted aldehydes was
reported by Ratovelomanana-Vidal and Michelet (Scheme 18) [111]. The authors
found that the use of various Cu(I) salts, such as copper(I)-thiophene-2-carboxylate,
tetrakis(acetonitrile) copper(I) tetrafluoroborate, the copper(I) triflate·benzene complex, and copper(I)chloride/silver triflate couple, gave lower enantiomeric excesses.
The active Cu(I) catalytic system was generated in situ through the reduction of
copper(II) triflate with (R)-3,5-di-tert-butyl-4-methoxyphenyl-MeOBIPHEP (38)
[MeOBIPHEP = (6,6′-dimethoxybiphenyl-2,2′-diyl)-bis(diphenylphosphine)],
with
Reprinted from the journal
50
1 3
the intramolecular reactions led to a carbocyclic structure derived from the reaction between the enamine and the parallelly formed electrophilic gold triple bond
complex. It is worth mentioning that normally, in these types of reactions, the
Hayashi–Jørgensen-type catalysts preferred, despite the fact that a Michael reaction requires the presence of a quite strong electrophile (represented by the iminium
intermediate). The above mentioned catalyst favors the entire process due to the
fact that the second step of the reaction (the enamine attack) requires mandatorily
an enhanced nucleophilic behavior. Despite the proof of concept, no enantiomeric
excesses were observed for the reaction. An enantioselective variant of this carbocyclization reaction was later reported by Jørgensen [103]. Gold is not only able to
activate π-electron density of triple bond of alkynes, but it is also able to activate
other suitable substrates like allylic alcohols. In fact, allylc alcohols are activated by
gold catalysis to realize a catalytic enantioselective intramolecular α-allylic alkylation of aldehydes through the combination of gold and aminocatalysis (Scheme 17a)
[107].
A range of chiral secondary amines (Hayashi–Jørgensen catalyst, MacMillan I
and II type of catalysts) and different binuclear chiral phosphine gold(I) complexes
were tested in the model ring-closing reaction. Remarkably, decent levels of stereoselection were observed when the organocatalysts were merged with a cationic gold
complex 32, although the combined use of chiral gold complexes with achiral amino
catalysts was proved largely unsuccessful. Apparently, the gold complex is able to
activate the allylic alcohols through the formation of an electrophilic allylic cation.
Furthermore, careful mechanistic evidences for an S N 2′-type mechanism were collected. In other words, although this activation is often described to occur through
the formation of an allylic gold complex, the gold(I)-coordination with the olefin
activates the alkene through the nucleophilic attack of the enamine. Deauration
of the resulting organogold species, with concomitant elimination of water, leads
to the formation of the observed double bond. Gonzales reported an intermolecular reaction of allenamides with aldehydes [108] using a diaryl prolinol silyl ether
1c as the active catalyst in the presence of the ortho-fluorobenzoic acid as additive
(Scheme 17b). The same reaction was reported by Mascareñas and López [109].
6.2 Use of Copper Salts Such as π‑Lewis Acids in Synergistic Enamine Catalysis
The ability of copper to form stable π-complexes with double or triple bonds is well
known in the literature [110]. The Lewis acid organocatalyzed enantioselective
preparation of different five-membered carbo- and heterocyclic structures through
aminocatalysis and copper(I) activation of alkynyl α-disubstituted aldehydes was
reported by Ratovelomanana-Vidal and Michelet (Scheme 18) [111]. The authors
found that the use of various Cu(I) salts, such as copper(I)-thiophene-2-carboxylate,
tetrakis(acetonitrile) copper(I) tetrafluoroborate, the copper(I) triflate·benzene complex, and copper(I)chloride/silver triflate couple, gave lower enantiomeric excesses.
The active Cu(I) catalytic system was generated in situ through the reduction of
copper(II) triflate with (R)-3,5-di-tert-butyl-4-methoxyphenyl-MeOBIPHEP (38)
[MeOBIPHEP = (6,6′-dimethoxybiphenyl-2,2′-diyl)-bis(diphenylphosphine)],
with
Reprinted from the journal
50