Gold Chemistry of Mostly Alkynes

1
Gold Chemistry of (Mostly) Alkynes
Taken from 4 mini-reviews Hashmi, A. S. K. Gold
Bull. 2003, 35, 3. Hoffmann-Roder, A. Krause, N.
Org. Biomol. Chem. 2005, 3, 387. Hashmi, A. S. K.
Angew. Chem. Int. Ed. Engl. 2005, 44, 6990. Ma,
S. Yu, S. Gu, Z. Angew. Chem. Int. Ed. Engl.
2006, 45, 200.
General Points 1) Most common oxidation states
Au(I), and Au(III) -both are used, often do
analogous things -are slightly different, and
are probably not interconverting 2) Gold is too
expensive, right? AuCl3 17,500/mol Ph3PAuCl
35,600/mol Pd(PPh3)4 20,300/mol Pd2(dba)3
29,700/mol
2
Why Gold?
Au(I) and Au(III) are soft, carbophilic Lewis
acids -particularly with alkynes -also
allenes, and sometimes alkenes
Original report
Hutchings, G. H. J. Catal. 1985, 96, 292.
3
Early Work - Heteroatom Nucleophilic Attack
Pt(II) also works OK, but not as well and at
higher temperatures Fukuda, Y. Utimoto, K.
Synthesis, 1991, 975.
4
Nitrogen Nucleophiles, Contd.
and Au(I)? yes, at least one example
5
Oxygen Nucleophiles
Many trivial cases, such as hydration of alkynes,
but.
6
Functions Other Than Alkynes
Alkynes are not the only CC function whose
reactions may be induced by Au(I) / Au(III), but
there are the most commonly employed Allenes
will also do much analogous chemistry, and in
some cases alkenes will react too It is not
clear that allenes are less reactive than alkynes
(or more reacive, for that matter), but it is
pretty apparent that alkenes are less
reactive -exceptions.some activated alkenes
7
Allenes
8
Alkenes
9
Carbon-Carbon Bond Formation
In C-C bond forming reactions, Au(I) is more
commonly encountered, perhaps even more
than Au(III)
The easy ones first intramolecular reactions,
presumably by the enol
10
More Intramolecular Reactions
Intermolecular Cases
11
Multistep Processes
C-5 of the furan must be substituted, or else you
a mixture of get two regioisomeric phenols
12
Enynes - Multistep Processes
In most (but not all) cases, enynes react to
undergo a non-obvious rearrangement of the
carbon skeleton A gold carbene has been proposed
as a central reactive intermediate
Echavarren, A. M., et al Angew. Chem. Int. Ed.
Engl. 2004, 43, 2402.
13
Other Enynes
Compare previous to
Why these do exo attack, as opposed to the
previous endo attack, is not clear
14
Vinyl/Arene Substituted Enynes
Typical Au(I) catalysts (Ph3PAuCl) work, but are
somewhat slower
Nieto-Oberhuber, C. Lopez, S. Echavarren, A. M.
J. Am. Chem. Soc. 2005, 127, 6178.
15
More Conventional Multistep Cyclizations