Substituent effect on the formation and reactivity of platinum carbenoids

Article abstract of DOI:10.1002/adsc.200800544

A propargylic ester containing a propargylic alkoxy group has been observed to preferentially undergo [1,2]-acyl shift over [1,3]-shift. In addition, the complementary and contrasting reactivity of vinyl vs. alkynyl platinum carbenoids has been discovered. Vinyl platinum carbenoids are more prone to undergo [1,2]-H shift over addition to pbonds whereas alkynyl platinum carbenoids preferentially add to π-bonds.

Full text of DOI:10.1002/adsc.200800544

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DOI: 10.1002/adsc.200800544
Substituent Effect on the Formation and Reactivity of Platinum
Carbenoids
Eun Jin Choa and Daesung Leeb,*
a
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, USA
b
Fax : (+1)-312-996-0431; phone: (+1)-312-996-5189; e-mail: dsunglee@uic.edu
Received: September 3, 2008; Published online: November 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800544.
Abstract: A propargylic ester containing a propar-
gylic alkoxy group has been observed to preferen-
tially undergo [1,2]-acyl shift over [1,3]-shift. In ad-
dition, the complementary and contrasting reactivi-
ty of vinyl vs. alkynyl platinum carbenoids has been
discovered. Vinyl platinum carbenoids are more
prone to undergo [1,2]-H shift over addition to p-
bonds whereas alkynyl platinum carbenoids prefer-
entially add to p-bonds.
generate allene 3[4] over a [1,2]-shift to form vinyl
metal carbenoid 2, if the alkyne moiety is internal. In
general, only terminal alkynes (R3 =H)[5] and internal
alkynes with electron-withdrawing substituent (R3 =
CO2R)[6] were known to provide the metal carbenoid
2.
Recently, in the study of the metallotropic [1,3]-
shift[7] of electrophilic metal complexes, we have
shown that a 1,3-diyne-containing system (R3 =alkyn-
yl group) provided strong preference for [1,2]-acyl
shift, thereby selectively generating vinyl carbenoid
2.[8] Recognizing the subtlety of the electronic influ-
ence by the R3 substituent for the [1,2]- and [1,3]-acyl
shift, we envisioned that the introduction of an elec-
tron-withdrawing oxygen substituent at the propargyl-
ic carbon of 4 would provide enough driving force for
Keywords: alkynes; carbenoids; cyclopropanes;
platinum; substituent effects
Carbenes and carbenoids are versatile intermediates the formation of carbenoid 6 over 5 via an [1,2]-acyl
in organic chemistry. Usually, diazo compounds serve shift (Scheme 2).[9] Furthermore, the oxygen substitu-
as precursors to these reactive species under photolyt- ent would also allow an alternative trapping mecha-
ic, thermal, or transition metal-catalyzed conditions.[1] nism of metal carbenoid 6 to form 1,3-diene 7 through
And yet, due to the inconvenience of their prepara- a [1,2]-hydride shift[5f,10] due to an electronic effect of
tion and their hazardous nature, an alternative source the oxygen substituent. This would nicely complement
for these species is highly desirable. In this context, the more prevailing trapping manifolds of reactions of
an alkyne such as propargylic ester 1[2] would be an 6 through [1,2]-acyl migration forming 8 or intra-/in-
ideal precursor to generate a metal-carbenoid 2 if the termolecular cyclopropanation generating 9. We
alkyne can be properly activated by electrophilic tran- report herein a prominent role of the propargylic
sition metals[3] under appropriate conditions oxygen substituent to induce [1,2]-acyl migration in
(Scheme 1). However, the potential of this transition the initiation step and a favorable [1,2]-hydride shift
metal-based alkyne activation is significantly reduced at the termination step. This ultimately allowed us to
by the propensity of 1 to undergo a [1,3]-acyl shift to observe the conspicuous difference in trapping prefer-
ence between vinyl and alkynyl carbenoids, where the
former have a higher tendency to undergo [1,2]-hy-
dride shift whereas the latter favour an addition to al-
kenes and alkynes.
First, we examined substrate 10 containing a prop-
argylic methoxy substituent for its [1,2]-acyl shift and
the termination behaviour of the resultant carbenoid.
When 10 was subjected to typical reaction conditions
Scheme 1.
G
(5 mol% PtCl2 under CO,[11] toluene, 808C) the ex-
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Eun Jin Cho and Daesung Lee
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Scheme 2. Controlled [1,2]-acyl shift to form vinyl carbenoid and its trapping.
pected 1,3-diene 12 was obtained in high yield as a tion occurred in the presence of styrene to yield prod-
mixture of Z/E isomers (Scheme 3). Based on the uct 17 in 76% (3.8:1). This result clearly suggests that
known examples of intermolecular trapping of vinyl vinyl carbenoid 11 preferentially undergoes [1,2]-hy-
carbenoid of type 11 with styrene,[12] we expected that dride shift[13] whereas alkynyl carbenoid 15 prefers to
in the presence of an excess amount of styrene, it react with double bond of styrene, thereby indicating
should deliver cyclopropanated product 13. However, significant differences in reactivity between these car-
even in the presence of an excess amount of styrene benoids.[12,14]
the cyclopropanated product 13 was not obtained, in-
This reactivity difference manifested by putative
stead still 12 was produced as the sole product. In vinyl carbenoid 11 and alkynyl carbenoid 15 was fur-
comparison, under identical conditions, 1,3-diyne 14 ther examined by using substrates 18 and 22 that con-
did not lead to [1,2]-hydride shifted product 16 and tain an allyl ether moiety by which the respective car-
most starting material was recovered unchanged. On benoid intermediates would be readily trapped intra-
the other hand, efficient intermolecular cyclopropana- molecularly (Scheme 4). We surmised that the reac-
Scheme 3. The reactivity difference between vinyl and al-
kynyl Pt-carbenoids.
Scheme 4. The reactivity difference between vinyl and al-
kynyl Pt-carbenoids.
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Substituent Effect on the Formation and Reactivity of Platinum Carbenoids
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tions with these substrates would eliminate any uncer- propanated product 25 in 96% yield devoid of [1,2]-
tainty related to the entropy-based kinetic barrier, hydride shifted product 24.[15] Again, this result is be-
thereby showing the inherent reactivity difference lieved to be caused by the reactivity difference be-
more clearly. Under standard conditions (5 mol% tween vinyl carbenoid 19 and alkynyl carbenoid 23,
PtCl2 under CO, toluene, 608C), the reaction of sub- which further corroborates the previous conclusion
strate 18 selectively produced [1,2]-hydride shifted that vinyl Pt-carbenoids prefer [1,2]-hydride shift
product 20 (91%, Z:E=2.1:1) without any indication whereas alkynyl Pt-carbenoids favour cyclopropana-
of formation of cyclopropanated product 21. On the tion. It is worth emphasizing that propargylic acet-
other hand, the related 1,3-diyne 22 gave only cyclo-
ACHTUNGTRENoNGNU xyalkynes 10/18 and 14/22, although internal alkynes
Table 1. Products from [1,2]-H shift of vinyl carbenoids and addition to alkenes and alkynes of alkynyl
carbenoids.[a]
[a]
Conditions: 5 mol% PtCl2, under CO, toluene.
Isolated yield.
Z/E ratio of crude products.
Additional 20% of the corresponding hydrolyzed aldehyde was isolated.
Z/E ratio of the methyl enol ether (*).
Mixture of diastereomers in 2.9:1 ratio.
[b]
[c]
[d]
[e]
[f]
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are known to promote allene formation, did not pro- PtCl2. In this study, we also have demonstrated that
vide any allenes via [1,3]-acetoxy shift. They delivered vinyl and alkynyl Pt-carbenenoids have significantly
products only derived from the putative vinyl Pt-car- different reactivity: vinyl Pt-carbenoids have the pro-
benoids, which can be rationalized by the presence of pensity to undergo [1,2]-H shift whereas alkynyl Pt-
electron-withdrawing methoxy and allyloxy substitu- carbenoids preferentially add to p-bonds in both
ents for the monoynes and the alkynyl substituent for intra- and intermolecular reactions. Further study to
the diynes.
elucidate the origin of the difference between these
Having shown the salient reactivity difference be- carbenoids will be reported in due course.
tween vinyl and alkynyl Pt-carbenoids, diverse sub-
strates were employed to confirm the observed reac-
tivity in broader context. Especially, the trapping of Experimental Section
initially formed alkynyl Pt-carbenoids by tethered al-
kynes instead of alkenes in substrates 26f will further General Procedure for PtCl2-Catalyzed Reactions
bolster their preferred reactivity toward p-bonds over
[1,2]-hydride shift (Table 1). Substrate 26a that con-
tains a tethered terminal alkene provided only [1,2]-
PtCl2 (5 mol%) was added to a solution of the alkyne in tol-
uene (0.05M), and CO was bubbled through the solution.
The mixture was warmed to the appropriate temperature
hydride shifted product 27a in 70% yield as a mixture
of E/Z isomers (entry 1). Similarly, a trisubstituted
alkene-containing substrate 26b also gave 27b in 92%
yield without any sign of cyclopropanated product
(entry 2). An aromatic substituent[16] in 26c did not
perturb the reactivity of the corresponding vinyl Pt-
carbenoid, producing 1,3-diene product 27c in 77%
yield (entry 3). The reaction of symmetrical monoyne
26d yielded 27d via a consecutive [1,2]-OAc shift fol-
lowed by a [1,2]-hydride shift in 75% yield (Z:E=
1.7:1) at 408C (entry 4). On the other hand, substrate
26e (entry 5), containing mono- and diyne moieties
that can potentially compete, yielded product 27e (Z-
isomer, 67% at 47% conversion), where only the
monoyne was transformed to a 1,3-diene moiety pre-
sumably via a vinyl Pt-carbenoid. This result suggests
that a monoyne is more reactive than the correspond-
ing diyne. In comparison, the reaction of symmetrical
diyne 26f gave symmetrical enediyne 27f in 92%
yield, which is the result of the favourable addition
over a [1,2]-H shift of the initially formed alkynyl car-
benoid onto the other 1,3-diyne moiety across the
tether (entry 6). The differential reactivity of vinyl
and alkynyl Pt-carbenoids was further examined using
a diyne initiator moiety of substrates 26g and 26h.
The reaction of 26g containing a monoyne terminat-
ing group gave 27g in 47% yield (Z:E=1:1.2) via the
penultimate vinyl Pt-carbenoid, which ultimately de-
livered the observed product via a [1,2]-hydride shift
(entry 7). On the other hand, 26h with diyne terminat-
ing group, which eventually generated an alkynyl car-
benoid at the penultimate stage of the catalytic cycle,
did not give any product in the absence of styrene but
yielded cyclopropanated product 27h in 26% yield in
the presence of styrene (entry 8). Decomposition of
products as well as other side reactions such as aro-
matization seem to account for the low yields of the
last two reactions.
(408C, 608C, or 808C) and stirred until the reaction was
complete (30 min–2 h). The mixture was cooled to room
temperature and the solvent was evaporated under vacuum.
Purification by flash chromatography on silica gel (hexane/
ethyl ether) afforded the product.
Acknowledgements
We thank the NIH for financial support of this work as well
as the NSF and NIH for NMR and Mass Spectrometry instru-
mentation.
References
[1] a) G. Mass, Top. Curr. Chem. 1987, 137, 75–253;
b) M. P. Doyle, M. A. McKervey, T. Ye, Modern Cata-
lytic Methods for Organic Synthesis with Diazo Com-
pounds Wiley-Interscience, New York, 1998; c) T. Ye,
M. A. Mckervey, Chem. Rev. 1994, 94, 1091–1160;
d) A. Padwa, M. D. Weinngarten, Chem. Rev. 1996, 96,
223–269.
[2] Reviews on metal-catalyzed reactions of propargylic
esters: a) N. Marion, S. P. Nolan, Angew. Chem. Int.
Ed. 2007, 46, 2750–2752; b) J. Marco-Contelles, E. Sor-
iano, Chem. Eur. J. 2007, 13, 1350–1357.
[3] Recent reviews on gold and platinum-catalyzed reac-
tions: a) A. Fꢁrstner, P. W. Davies, Angew. Chem. Int.
Ed. 2007, 46, 3410–3449; b) D. J. Gorin, F. D. Toste,
Nature 2007, 446, 395–403; c) A. S. K. Hashmi, Chem.
Rev. 2007, 107, 3180–3211; d) E. Jimenez-Nunez, A. M.
Echavarren, Chem. Commun. 2007, 333–346; e) L.
Zhang, J. Sun, S. A. Kozmin, Adv. Synth. Catal. 2006,
348, 2271–2296.
[4] Examples of [1,3]-acyl shift in gold and platinum- cata-
lyzed reactions: a) L. Zhang, J. Am. Chem. Soc. 2005,
127, 16804–16805; b) N. Marion, S. Dꢂez-Gonzꢃlez, P.
de Frꢄmont, A. R. Noble, S. P. Nolan, Angew. Chem.
Int. Ed. 2006, 45, 3647–3650; c) J. Zhao, C. O. Hughes,
F. D. Toste, J. Am. Chem. Soc. 2006, 128, 7436–7437;
d) C. H. Oh, A. Kim, W. Park, D. I. Park, N. Kim, Syn-
lett 2006, 2781–2784; e) L. Zhang, S. Wang, J. Am.
Chem. Soc. 2006, 128, 1442–1443; f) S. Wang, L.
In conclusion, we have observed that propargylic
esters containing a propargylic alkoxy group preferen-
tially undergo [1,2]-acyl shift over [1,3]-shift with
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Zhang, J. Am. Chem. Soc. 2006, 128, 8414–8415; g) S.
Wang, L. Zhang, Org. Lett. 2006, 8, 4585–4587; h) A.
Buzas, F. Istrate, F. Gagosz, Org. Lett. 2006, 8, 1957–
1959; i) A. Buzas, F. Gagosz, J. Am. Chem. Soc. 2006,
128, 12614–12615; j) M. Yu, G. Li, S. Wang, G. Zhang,
Adv. Synth. Catal. 2007, 349, 871–875.
Marion, L. Fensterbank, M. Malacria, S. P. Nolan, L.
Cavallo, Angew. Chem. Int. Ed. 2008, 47, 718–721.
[10] Examples of [1,2]-hydride shift: a) A. Padwa, D. J.
Austin, Y. Gareau, J. M. Kassir, S. L. Xu, J. Am. Chem.
Soc. 1993, 115, 2637–2647; b) A. Furstner, F. Stelzer,
H. Szillat, J. Am. Chem. Soc. 2001, 123, 11863–11869;
c) C. A. Witham, P. Mauleon, N. D. Shapiro, B. D.
Sherry, F. D. Toste, J. Am. Chem. Soc. 2007, 129, 5838–
5839.
[11] Reactions with Pt-catalyst under CO atmosphere:
a) J. M. Lutton, R. W. Parry, J. Am. Chem. Soc. 1954,
76, 4271–4274; b) A. Fꢁrstner, P. W. Davies, T. Gress,
J. Am. Chem. Soc. 2005, 127, 8244–8245; c) A. Fꢁrst-
ner, C. Aissa, J. Am. Chem. Soc. 2006, 128, 6306–6307.
[12] Examples of intermolecular cyclopropanation of vinyl
Ru-carbenoid, and Au-carbenoids, see refs.[5c,e].
[5] Examples of [1,2]-acyl shift: a) V. Rautenstrauch, J.
Org. Chem. 1984, 49, 950–952; b) E. Mainetti, V. Mo-
uriꢅs, L. Fensterbank, M. Malacria, J. Macro-Contelles,
Angew. Chem. Int. Ed. 2002, 41, 2132–2135; c) K.
Miki, K. Ohe, S. Uemura, J. Org. Chem. 2003, 68,
8505–8513; d) V. Mamane, T. Gress, H. Krause, A.
Fꢁrstner, J. Am. Chem. Soc. 2004, 126, 8654–8655;
e) D. J. Gorin, S. T. Staben, F. D. Toste, J. Am. Chem.
Soc. 2005, 127, 18002–18003; an exception to this gen-
eral trend, see: f) L. Zhang, G. Li, G. Zhang, J. Am.
Chem. Soc. 2008, 130, 3740–3741; application to natu-
ral products, see: g) A. Fꢁrstner, P. Hannen, Chem.
Eur. J. 2006, 12, 3006–3019; h) C. Fehr, J. Galindo,
Angew. Chem. Int. Ed. 2006, 45, 2901–2904.
[13] It is interesting to note that vinyl Rh-carbenoid selec-
À
tively undergo allylic C H insertion over addition to
double bonds. Considering that [1,2]-hydride shift can
À
À
be viewed as a C H insertion to a vicinal C H bond,
the preferred reactivity of vinyl Pt-carbenoid for [1,2]-
[6] B. A. B. Prasad, F. K. Yoshimoto, R. Sarpong, J. Am.
Chem. Soc. 2005, 127, 12468–12469.
À
hydride shift and that of Rh-carbenoid for C H inser-
[7] A recent review on metallotropic shift of alkynyl carbe-
noids: D. Lee, M. Kim, Org. Biomol. Chem. 2007, 5,
3418–3427.
[8] Reactions of diynes: a) E. J. Cho, M. Kim, D. Lee, Eur.
J. Org. Chem. 2006, 3074–3078; b) E. J. Cho, M. Kim,
D. Lee, Org. Lett. 2006, 8, 5413–5416; c) D. J. Gorin, P.
Dubꢄ, F. D. Toste, J. Am. Chem. Soc. 2006, 128, 14480–
14481.
[9] It was observed that the alkyl and aryl substituents at
the propargylic site (R1 and R2 are alkyl or aryl) in-
crease the reactivity of the substrates significantly prob-
ably due to both the electronic and steric reasons (see
ref.[8a]) but their role to affect the selectivity between
[1,2]- vs. [1,3]-acyl shift has not been confirmed. DFT
study for [1,2]- vs. [1,3]-acyl shift, see: A. Correa, N.
tion seems to have close mechanistic similarity. For an
excellent review on donor-acceptor substituted Rh-car-
benoids, see: a) H. M. L. Davies, J, Nikolai, Org.
Biomol. Chem. 2005, 3, 4176–4187. For an example of
À
C H insertion by Pt-carbenoid, see: b) C. H. Oh, J. H.
Lee, S. J. Lee, J. I. Kim, C. S. Hong, Angew. Chem. Int.
Ed. 2008, 47, 7505–7507.
[14] An example of intermolecular cyclopropanation of al-
kynyl Au-carbenoids, see ref.[8c].
[15] Vinyl Pt-carbenoid preferentially reacts with double
bond intramolecularly over [1,2]-hydride shift at the
termination step, see ref.[11b]
[16] The participation of an aryl group at the propargylic
position in the reaction, see refs.[4b,6].
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