Communications
The resulting solution was transferred to a flask, and the solvent was
removed under vacuum to afford the crude product polypeptide.
cannot, for steric reasons, be accessed by other means,
including the most frequently used ring-opening polymeri-
zation of amino acid N-carboxyanhydrides.[22–25] These poly-
peptides, unlike the N-unsubstituted ones, are soluble in
common organic solvents, such as THF and chloroform. One
of the unique properties of these polymers is that they can be
facilely degraded by trifluoroacetic acid (TFA).[16] Although
TFA cleavage of small peptides has been reported
recently,[26,27] polypeptide degradation by TFA has never
been observed. The ultimate products were found to be a
mixture of amino acid 5 and dipeptide 6, for the case of the
tert-butyl polypeptide 3e, with regiospecific deuteration on
the a-carbons when the degradation was performed in
[D1]TFA. This result is consistent with the mechanism
involving cleavage of peptide bonds by adjacent carbonyl
Received: February 12, 2007
Revised: April 16, 2007
Published online: July 3, 2007
Keywords: carbon monoxide · copolymerization ·
.
homogeneous catalysis · imines · polypeptides
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oxygen atoms to form Münchnone intermediates (see
[28]
Scheme S3in the Supporting Information).
Such unique
degradation properties would be useful for applications of
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[16] See the Supporting Information.
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Related Materials, Springer, New York, 1987.
We have shown that the highly desirable copolymerization
of imines and CO has been realized through proper choice of
a simple cobalt catalyst, demonstrating once again the ability
of metal catalysis to manipulate organic transformations.[29]
Of particular note are the ready availability and low cost of
the starting materials as well as the atom-economic feature of
the reaction process (see Scheme S1 in the Supporting
Information), which render the reaction well-suited for
large-scale production of the polypeptide materials. In fact,
this method is the shortest possible route for chemical (or
abiotic) synthesis of polypeptides. Whether such a copoly-
merization strategy might have been adopted by nature for
the prebiotic origin of polypeptides might deserve further
investigation.[30] Although the present study has been con-
fined to the use of stable imines, their successful copolymer-
ization with CO has indicated that this transformation can be
both thermodynamically and kinetically viable. Further
expandsion of the scope of this reaction and full elucidation
of the reaction mechanism are the subject of current research.
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[27] M. Goodman, C. J. Creighton, US patent 6,417,397, 2002.
[28] M. Teixido, F. Albericio, E. Giralt, J. Pept. Res. 2005, 65, 153–
166.
Experimental Section
Imine (about 1 mL) was added to a Par pressure reactor containing a
suitable amount of catalyst 1 in dioxane (50 mL), which had been pre-
pressurized under 800 psi of CO overnight. After the addition, the
pressure of CO was returned to 800 psi, and the reactor was heated in
an oil bath at 508C while the reaction mixture was magnetically
stirred for the period of time specified in Table 1. After cooling to
room temperature, the pressure was released and the reactor opened.
[29] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18 – 29.
[30] L. Leman, L. Orgel, M. R. Ghadiri, Science 2004, 306, 283– 286.
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 6068 –6072