Novel catalysts for the overman rearrangement

Article abstract of DOI:10.1055/s-2005-918952

PtCl₂, PtCl₄, AuCl, and AuCl₃ were found to be efficient catalysts for the Overman rearrangement of O- allyltrichloroacetimidates to N-allyltrichloroacetamides. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-918952

2
984
LETTER
Novel Catalysts for the Overman Rearrangement
N
I
ovel Ca
e
talysts for
v
the Overman
a
R
earrangement Jaunzeme, Aigars Jirgensons*b
a
a
Faculty of Material Science and Applied Chemistry, Riga Technical University, Riga 1048, Latvia
b
Latvian Institute of Organic Synthesis, Riga 1006, Latvia
Fax +371(7)541408; E-mail: aigars@osi.lv
Received 17 August 2005
(
Scheme 1). No reaction was observed with 10 mol% of
Abstract: PtCl , PtCl , AuCl, and AuCl were found to be efficient
catalysts for the Overman rearrangement of O-allyltrichloroacetim-
idates to N-allyltrichloroacetamides.
2
4
3
Ti(OEt) , Ti(i-PrO) , Al(i-PrO) , CeCl , FeCl , MgBr ,
Cu(OAc) , Rh(OAc) , RuCl , or NiCl after a reaction
4
4
3
3
3
2
2
2
3
2
time of 70 h. Traces of the desired rearrangement product
a together with unreacted imidate 1a, [1,3]-rearrange-
Key words: amines, amides, rearrangement, catalysis, Lewis acids
2
ment product 3a, and trichloroacetamide (4) were ob-
tained in the presence of 10 mol% of (CuOTf) ·C H ,
2
6
6
The aza-Claisen rearrangement of O-allyltrichloroacetim-
idates to N-allyltrichloroacetamides was first demonstrat-
ed by Overman and since then it has found widespread
Cu(OTf) , CuCl, CuCl , and ZnCl within 18–70 hours.
2
2
2
The conversion of imidate 1a to amide 2a was slow (ca.
0%, 70 h) with 50 mol% of AgOTf. After attempting
3
1
application as a C–N bond forming reaction. Trichloro-
such a range of catalysts we were pleased to find that 10
mol% of PtCl , PtCl , AuCl, and AuCl effectively pro-
acetimidates are readily available from allylic alcohols
and the trichloroacetyl group in the products can be rela-
tively easily cleaved rendering the Overman rearrange-
ment a method of choice for the synthesis of primary
2
4
3
moted the rearrangement of trichloroacetimidate 1a
Table 1). In the case of PtCl , AuCl, and AuCl , the ap-
(
2
3
propriate solvent was CH Cl , however, PtCl in CH Cl
2
2
4
2
2
1
,2
allylamines. The rearrangement can be performed ther-
mally at elevated temperatures (typically 110–140 °C) or
can be catalyzed by Hg(II) and Pd(II) salts under very
gave a low yield of product 2a due to competitive forma-
tion of the [1,3]-rearrangment product 3a and elimination
of trichloroacetamide (4). The unwanted side reactions in
1
,3
mild reaction conditions. The catalytic approach offers
additional opportunities – chelation controlled diastereo-
selectivity has been reported for the Pd(II)-catalyzed rear-4
rangement providing protected anti-1,2-amino-alcohols
the PtCl -catalyzed rearrangement of imidate 1a were
4
suppressed by changing the solvent to THF and substan-
tially increased the yield of product 2a. The efficiency of
the newly found catalysts was explored with additional
structurally diverse trichloroacetimidates 1b–e (Table 1).
Imidates 1b and 1c gave the expected Overman rearrange-
ment products 2b and 2c in moderate to good yields. The
elimination of trichloroacetamide (4) dominated in the
5
and 1,2-diamines. Moreover, the first chiral cobalt ox-
azoline palladacycle catalyst has been recently developed
for the enantioselective rearrangement of trichloroacetim-
1
d,e
idates.
To the best of our knowledge Hg(II) and Pd(II) salts are case of imidate 1d, with the trisubstituted double bond and
the only efficient catalysts reported to date for Overman the expected amide 2d formed only in trace amounts.7
and related imidate rearrangements,3a,6 although the po- Imidate 1e derived from (Z)-(3-benzyloxy)crotyl alcohol
tential utility of other metals has been mentioned.3b We yielded vinyl-oxazolidine 5. Such a product probably
believed that broadening the scope of catalysts for the forms via a mechanism different to that of the Overman
Overman rearrangement would offer new perspectives for rearrangement, and has been reported also in the
this useful transformation, in particular with respect to PdCl (PhCN) -catalyzed reactions of a similar substrate,
2
2
8
stereochemical aspects.
bis-trichloroacetimidate, derived from cis-butanediol.
The search for new catalysts began with the screening of Noteworthy, AuCl, AuCl , and PtCl were soluble in
3
4
metal salts and alkoxides using trichloroacetimidate 1a solvents selected for the reaction, while PtCl was poorly
2
1
2
(
R = Me, R = H) as a model substrate in CH Cl
soluble in both CH Cl and THF resulting in hetero-
2
2
2 2
Scheme 1
SYNLETT 2005, No. 19, pp 2984–2986
0
1
.1
2
.2
0
0
5
Advanced online publication: 27.10.2005
DOI: 10.1055/s-2005-918952; Art ID: D24105ST
©
Georg Thieme Verlag Stuttgart · New York

LETTER
Novel Catalysts for the Overman Rearrangement
2985
Table 1 PtCl , PtCl , AuCl, and AuCl -Catalyzed Rearrangement of O-Allyltrichloroacetimidates 1a–e
2
4
3
Entry
Imidate
Product
Catalysta
PtCl2
Solvent
CH Cl
Yieldb
1
2
3
4
5
6
89%
83%
30%
75%
82%
80%
2
2
THF
PtCl4
CH Cl
2
2
THF
AuCl
AuCl3
CH Cl
2
2
2
a
1a
CH Cl
2
2
7
8
9
PtCl2
PtCl4
AuCl
AuCl3
CH Cl
THF
81%
56%
74%
71%
2
2
CH Cl
2
2
1
9
CH Cl
2
2
2
2
b
c
1
1
b
c
1
1
1
1
1
2
3
4
PtCl2
PtCl4
AuCl
AuCl3
CH Cl
THF
61%
28%
52%
34%
2
2
CH Cl
2
2
CH Cl
2
2
1
1
1
1
5
6
7
8
PtCl2
PtCl4
AuCl
AuCl3
CH Cl
THF
traces
traces
traces
traces
2
2
CH Cl
2
2
CH Cl
2
2
1
1
d
e
2
5
d
1
2
2
9
0
1
PtCl2
AuCl
AuCl3
CH Cl
83%
74%
55%
2
2
2
2
CH Cl
2
CH Cl
2
a
Rearrangement of imidate 1a: 10 mol%; imidates 1b–e: 5 mol%.
Yield of isolated product.
b
geneous reaction mixtures. The conversion of imidate 1b In summary we have found that PtCl , PtCl , AuCl, and
2
4
was considerably slower using soluble cis-PtCl (PhCN)2 AuCl catalyze the rearrangement of O-allyl-trichloro-
2
3
(
Acros) in THF (ca. 30% conversion, 18 h). This was un- acetimidates complementing the known Hg(II) and Pd(II)
expected since PdCl (PhCN) is the most often used form salts. Further exploration of the utility of these newly
2
2
of Pd(II) for Overman and related rearrangements.
found catalysts as well as their incorporation into chiral
complexes for the asymmetric Overman rearrangement is
currently underway in our group.
A cyclization induced rearrangement (CIR) mechanism
has been proposed for the catalytic Overman rearrange-
ment where the metal coordination with the double bond
induces a nitrogen addition–deoxymetalation sequence
3
b
(
Scheme 2). Considering the soft Lewis acid nature of Pt Typical Procedure
9
The reaction vessel was charged with the catalyst (5–10 mol%) un-
and Au ions, which determines their ability to complex a
double bond, it is likely that catalysis by these metal salts
also proceeds according to a CIR mechanism. Lowering
the oxidation state increases the softness of the metal ion
1
b
der an inert atmosphere and to this a solution of imidate 1a–e (1
mmol) in CH Cl or THF (2.2 mL) was added in one portion. The
2
2
resulting mixture was stirred for 18 h and filtered through a Forisil
Plug. The solvent was removed and the product was purified by
flash chromatography on silica gel (light petroleum ether–EtOAc).
and may account for the higher efficiency of PtCl and
2
1
a,8a
AuCl compared to PtCl and AuCl for the rearrangement The rearrangement products 2a–c and 5 are known compounds.
4
3
of imidates 1a–c and 1e.
Acknowledgment
The work was supported by a grant from the Latvian Council of
Science and the European Social Fund within the National Pro-
gramme ‘Support for Carrying Out Doctoral Study Programs and
Post-Doctoral Research’.
Scheme 2
Synlett 2005, No. 19, 2984–2986 © Thieme Stuttgart · New York

2
986
I. Jaunzeme, A. Jirgensons
LETTER
References
(5) (a) Doherty, A. M.; Kornberg, B. E.; Reily, M. D. J. Org.
Chem. 1993, 58, 795. (b) Gonda, J.; Helland, A.-C.; Ernst,
B.; Bellu, D. Synthesis 1993, 729.
(1) (a) Overman, L. E. J. Am. Chem. Soc. 1974, 96, 597.
(b) Overman, L. E. J. Am. Chem. Soc. 1976, 98, 290.
(c) Overman, L. E. Acc. Chem. Res. 1980, 13, 218.
(d) Anderson, C. E.; Overman, L. E. J. Am. Chem. Soc. 2003,
(6) (a) Schenck, T. G.; Bosnich, B. J. Am. Chem. Soc. 1985, 107,
2058. (b) Overman, L. E.; Zipp, G. G. J. Org. Chem. 1997,
62, 2288.
125, 12412..(e) Kirsch, S.; Overman, E. L.; Watson, M. P. J.
(
7) Introduction of an amino group at a tertiary carbon has been
Org. Chem. 2004, 69, 8101.
1a
achieved using Hg(II)-catalyzed Overman rearrangement.
(
(
2) Johannsen, M.; Jorgensen, K. A. Chem. Rev. 1998, 98, 1689.
3) (a) Metz, P.; Shoop, A. Tetrahedron 1992, 48, 1071.
In our hands, imidate 1d with PdCl (PhCN) also gave a very
2
2
low yield of amide 2d due to competitive elimination of
trichloroacetamide (4).
(
b) Overman, L. E. Angew. Chem., Int. Ed. Engl. 1984, 23,
79. (c) Singh, O. V.; Han, H. Org. Lett. 2004, 6, 3067.
4) (a) Jamieson, A. G.; Sutherland, A. Org. Biomol. Chem.
005, 3, 737. (b) Ovaa, H.; Codee, J. D. C.; Lastdrager, B.;
5
(
(
8) (a) Sabat, M.; Johnson, C. R. Org. Lett. 2000, 2, 1089. (b)
For related reactions see: Overman, L. E.; Remarchuk, T. P.
J. Am. Chem. Soc. 2002, 124, 12.
(
2
Overkleeft, H. S.; van der Marel, G. A.; van Boom, J. H.
Tetrahedron Lett. 1999, 40, 5063.
9) Duffus, J. H. Pure Appl. Chem. 2002, 74, 793.
Synlett 2005, No. 19, 2984–2986 © Thieme Stuttgart · New York