Journal of the American Chemical Society
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’ REFERENCES
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Figure 1. Molybdenum enolate structures.
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nonbonded interactions. The first lies between the R group of the
cyanoester nucleophile and aryl group of the molybdenum
π-allyl complex. Additionally, the bulky tert-butyl ester also has
an unfavorable interaction with the aryl group of the π-allyl
complex. Alternatively, the enolate in which the nitrile and tert-
butyl ester are syn can be considered as in TS10 and TS20.
However, these two transition states each contain two unfavor-
able interactions as in TS2. The larger the R group of the
cyanoester, the greater the discrimination between the two
possible diastereomeric transition state structures. Indeed this
is what was observed experimentally from cyanoester 19 which
has slightly diminished diastereoselectivity. Furthermore, it has
been previously demonstrated4b,d,8,11 that a more electron-rich
molybdenum center should disfavor reductive elimination and
promote equilibration between the two isomers, moving toward
a CurtinꢀHammett situation. Indeed the utilization of bis-
methoxy ligand L2 rather than L1 supports this conclusion.
Finally, the enantioselectivity observed in the reaction speaks
to the utility of L28,11 as a catalyst for π-allyl molybdenum
catalysis.
In summary, we have developed a catalytic Mo-AAA of cyanoe-
sters that proceeds in high regio-, diastereo-, and enantioselectivity.
The products of the transformation provide a fully functionalized
quaternary stereocenter which contain a vicinal tertiary stereocenter
that is otherwise difficult to access. The substituted cyanoester
products are interesting from the standpoint that they contain a
number of functional handles with orthogonal reactivity that can
allow for further elaboration. Also interesting is the high selectivity
for the branched product in all cases even though in some cases the
cyanoester nucleophile is extremely sterically demanding. The
versatility of this mode of reactivity with molybdenum in the Mo-
AAA reaction is ongoing in our laboratory.
(7) It was found that upon using an equimolar ratio of cyanoester to
allylic carbonate the conversion suffered greatly.
(8) (a) Belda, O.; Moberg, C. Synthesis 2002, 1601. (b) Belda, O.;
Kaiser, N. F.; Bremberg, U.; Larhead, M.; Hallberg, A.; Moberg, C. J. Org.
Chem. 2000, 65, 5868.
(9) Martin, S. F.; Dappen, M. S.; Dupre, B.; Murphy, C. J.; Colapret,
J. A. J. Org. Chem. 1989, 54, 2209.
(10) For mechanistic studies, see: (a) Krska, S. W.; Hughes, D. L.;
Reamer, R. A.; Mathre, D. J.; Palucki, M.; Yasuda, N.; Sun, Y.; Trost, B. M.
Pure Appl. Chem. 2004, 76, 625. (b) Hughes, D. L.; Lloyd-Jones, G. C.;
Krska, S. W.; Gouriou, L.; Bonnet, V. D.; Jack, K.; Sun, Y.; Mathre, D. J.;
Reamer, R. A. Proc. Nat. Acad. Sci. U.S.A. 2004, 101, 5379. (c) Lloyd-Jones,
G. C.; Krska, S. W.; Hughes, D. L.; Gouriou, L.; Bonnet, V. D.; Jack, K.;
Sun, Y.; Reamer, R. A. J. Am. Chem. Soc. 2004, 126, 702. For X-ray
structure of O-bound enolate molybdenum complex see: (d) Morales, D.;
Clemente, M. E. N.; Perez, J.; Riera, L.; Riera, V. Organometallics 2003,
22, 4124.
’ ASSOCIATED CONTENT
(11) Emura, T. Final Report, Stanford University, 1997ꢀ1999;
Dogra, K. Ph.D. Thesis, Stanford University, 2003.
S
Supporting Information. Complete experimental details
b
and characterization of all new compounds. This material is
’ AUTHOR INFORMATION
Corresponding Author
’ ACKNOWLEDGMENT
We thank the National Science Foundation (CHE-0846427)
for their generous support of our programs.
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dx.doi.org/10.1021/ja2029602 |J. Am. Chem. Soc. 2011, 133, 8165–8167