5494
F. Cadoret, Y. Six / Tetrahedron Letters 48 (2007) 5491–5495
Table 3. Scale of reactivity of some unsaturated compounds with
O
O
i
i
Pr
Pr
O
i
Pr
Pr
O
i
Pr
Pr
N
O
diisopropyloxy(g2-cyclopentene)titanium
(6À30 ꢁC)
1
at low temperature
Ti
Ti
Ti
Ph
O
i
O
i
Ph
Ph
24
25
26
Unsaturated
Relative
Reactivity
compound
rate
Figure 1.
PhCHO
240
MPV Reduction
16.5
Tishchenko reaction,
MPV reduction, 1,2-insertion
and ligand exchange
CHO
ylacetylene (Table 1, entry 3), and about 3 times faster
with acetophenone than with benzyl cyanide (Table 2,
entry 4).
Ph-CN
5.5
3.5
Ligand exchange and 1,2-insertion
1,2-Insertion
Ph CN
In summary, through the means of competition experi-
ments, a scale of reactivity has been established for the
reaction of diisopropyloxy(g2-cyclopentene)titanium 1
with several unsaturated compounds, including nitriles,
carbonyl derivatives, alkynes and alkenes. Among the
molecules tested, aldehydes reacted fastest, following
an unusual mechanistic pathway. With the knowledge
of the results presented herein, novel coupling reactions
mediated by 1 can be envisioned, both in the inter- and
intramolecular modes. We are currently developing such
an application, which will be published in due time.
O
2.5
Ligand exchange
Ligand exchange
Ligand exchange
Ligand exchange
1,2-Insertion
Ph
1
Ph
<0.2
<5 · 10À3
<10À4
Ph
Ph
O
EtO OEt
Finally, it should be underlined that a part of the ob-
served amounts of compounds 4, 7, 9, 15, 17 and 18
may also proceed from the reaction of residual amounts
of cyclopentylmagnesium chloride with the nitriles,
ketone or aldehydes put into play. The total yields of
products resulting from such direct processes should
normally not exceed 10%.10
Acknowledgement
`
´ ´ ´ `
We are grateful to the Ministere delegue a l’Enseigne-
´
`
ment superieur et a la Recherche for the funding granted
to F.C.
From all the results collected, a scale of reactivity of the
unsaturated compounds under study can be proposed.
Relative rates can be roughly estimated on the basis of
the reactivity ratios found in the competition experi-
ments, and are presented in Table 3, with phenylacetyl-
ene being chosen as a reference. Future work in the area
should help refining these values and enrich the table. It
should be pointed out that for the time being, these
results can only be deemed valid in the special case of
complex 1, and differences in reactivity should be ex-
pected with other titanacyclopropanes. Indeed, very dif-
ferent results have been reported for some reactions,
depending on the nature of the complex involved, either
1 or its [4.1.0] homologue diisopropyloxy(g2-cyclohex-
ene)titanium.17 The behaviour of diisopropyloxy(g2-tri-
methylsilylethylene)titanium may also be cited here.
This complex was reported to add to aldehydes via a
1,2-insertion pathway in good yield,18 in sharp contrast
with our results obtained using 1.
Supplementary data
Supplementary data associated with this article can be
References and notes
1. Kulinkovich, O. G.; Sviridov, S. V.; Vasilevskii, D. A.;
Pritytskaya, T. S. Zh. Org. Khim. 1989, 25, 2244–2245;
Kulinkovich, O. G.; Sviridov, S. V.; Vasilevskii, D. A.;
Pritytskaya, T. S. Russ. J. Org. Chem. 1989, 25, 2027–
2028.
2. Kulinkovich, O. G. Chem. Rev. 2003, 103, 2597–2632.
3. Chaplinski, V.; de Meijere, A. Angew. Chem., Int. Ed.
1996, 35, 413–414.
4. Bertus, P.; Szymoniak, J. Chem. Commun. 2001, 1792–
1793.
5. Kulinkovich, O. G.; Savchenko, A. I.; Sviridov, S. V.;
Vasilevski, D. A. Mendeleev Commun. 1993, 230–231.
6. Harada, K.; Urabe, H.; Sato, F. Tetrahedron Lett. 1995,
36, 3203–3206.
7. Eisch, J. J.; Gitua, J. N. Organometallics 2003, 22, 24–26.
8. Cho, S. Y.; Lee, J.; Lammi, R. K.; Cha, J. K. J. Org.
Chem. 1997, 62, 8235–8236.
9. Laroche, C.; Harakat, D.; Bertus, P.; Szymoniak, J. Org .
Biomol. Chem. 2005, 3, 3482–3487.
10. Cadoret, F.; Retailleau, P.; Six, Y. Tetrahedron Lett. 2006,
47, 7749–7753.
Importantly, besides establishing the reactivity scale
mentioned above, the competition experiments also fur-
nished some secondary information on the reactivity of
diisopropyloxytitanocene complexes 24, 25 and 26
resulting from ligand exchange of 1 with benzonitrile,
acetophenone and phenylacetylene (Fig. 1).
Namely, the results suggest that 24 reacts with benzonit-
rile in preference to phenylacetylene (Table 1, entry 1),
and 26 reacts chemoselectively with phenylacetylene
rather than ethyl carbonate (Table 1, entry 5). Similarly,
25 reacts exclusively with acetophenone rather than phen-
11. In a separate experiment carried out at 0 ꢁC, a Kulinko-
vich reaction performed from styrene and ethyl carbonate,
using Ti(OiPr)4 and cC5H9MgCl, also afforded 23 and the