Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 17, 1998 4241
The transformation of the initial alkyl carbamate nickelacycle
to the amido amidate might occur through a â-H elimination
mediated ring contraction which would involve a hydridonickel
carbamato imine intermediate (eq 3). Similar processes have been
proposed for other metallacyclic ring contractions involving
nickel.8 Abstraction of the hydrogen from the N-H position is
monomer. BpyNi(COD) was reacted with 5 equiv of [13C5]-L-
leucine-N-carboxyanhydride in THF. IR and 13C{1H} NMR
analysis of the crude products verified the presence of
bpyNi(13CO)2 [IR(THF) ν(13CO) ) 1933, 1862 cm-1 13C{1H}
;
NMR (DMF-d7) 198 ppm], 13C-labeled poly-L-leucine [IR (THF)
1613 cm-1 (νAmide I, vs); 1537 cm-1 (νAmide II, vs); 13C{1H}
NMR (DMF-d7) 177 ppm (bpyNiN(H)C(H)R13C(O)N[CH(R)-
13C(O)NH]nCH2R)], and the labeled nickel-amidate endgroup
[13C{1H} NMR (DMF-d7) 174 ppm (bpyNiN(H)C(H)R13C(O)N-
[CH(R)13C(O)NH]nCH2R)]. The reaction with [13C2]-L-leucine-
N-carboxyanhydride gave similar products, except for location
of the 13C label. We identified the presence of bpyNi(12CO)2
[IR(THF) ν(CO) ) 1978, 1904 cm-1],12 poly-L-leucine [IR (THF)
1653 cm-1 (νAmide I, vs); 1546 cm-1 (νAmide II, vs)],13 as well
as the [12C]amidate endgroup [IR(THF) ν(CO) ) 1577 cm-1].
When the reaction was run in DMF-d7, the presence of liberated
13CO2 was also confirmed using 13C{1H} NMR [126 ppm (s,
13CO2)].
All of these experiments were consistent with initial addition
of the NCA to bpyNi(COD) across the C5-O bond, analogous
to the reactions using (PPh3)2Ni(COD). The primary influence
of the ligands manifests itself in the reactivity of the resulting
products. The ligand-free complex from the PPh3 reaction was
inert toward further reactivity with NCAs, while the bpy complex
and complexes formed with other R,R′-diimines and alkyl
phosphines were efficient NCA polymerization initiators. We
were able to directly verify this phenomenon by synthesis of the
reactive metallacycle intermediate formed in the bpyNi(COD)/
NCA reactions. The stable metallacycle (1) was reacted with an
excess of bpy in DMF to form the ligand adduct 2 (Scheme 2).
Reaction of 2 with 100 equiv of Glu-NCA in DMF resulted in
rapid polymer formation. The PBLG formed in this reaction was
identical to that formed using bpyNi(COD) under otherwise
identical conditions (Mn ) 21 600, Mw/Mn ) 1.09). The bpyNi-
(COD)-mediated polymerizations of NCAs are therefore thought
to proceed via amido-amidate nickelacycle active endgroups (eq
4). We are currently exploring other aspects of the chemistry of
these metallacycles.
feasible considering puckering of the eight-membered ring in
addition to precedent for reversible hydride insertion into metal-
imine complexes.9 The successful formation of an amido amidate
with L-tert-butylglycine NCA10 also supports â-H elimination at
the N-H, rather than the C-H, position. The unusual stability
of the final amido species is likely due to the rigidity of the
metallacycle which prevents abstraction of a â-hydrogen.
The structure of these metallacyclic products was further
confirmed by elemental analysis and acidolysis of the complexes.
The product metallacycles contain no phosphine by elemental
analysis and were found to consist of the empirical formula
[NiNHC(H)RC(O)NCH2CHR]x. Osmotic molecular weight mea-
surements in THF (ca. 7 mg/mL) showed that the complexes
aggregate as dimers. Treatment of the metallacyclic complex
derived from L-leucine NCA with HCl in THF gave only a single
organic product. Analysis of this product by 1H NMR spectros-
copy and polarimetry, and comparison of the data with an
authentic sample, showed it to be optically pure L-leucine
isoamylamide hydrochloride (Scheme 2).11
When the donor ligands bound to the nickel(0) precursor were
varied (e.g., alkyl phosphines, R,R′-diimines), the only products
isolable from stoichiometric reactions with Glu-NCA in THF were
some starting nickel(0) compound and poly(γ-benzyl-L-glutamate),
PBLG. When 100 equiv of Glu-NCA was added to bpyNi(COD)5
in DMF, all of the nickel precursor was consumed and PBLG
was isolated in excellent yield (>95%) with narrow molecular
weight distribution (Mn ) 22 100, Mw/Mn ) 1.15). We have
shown that bpyNi(COD) initiates the living polymerization of
NCAs.3 It was suspected that bpyNi(COD) oxidatively adds Glu-
NCA to form the active polymerization initiator in situ which
then rapidly consumes the remainder of the monomer. To identify
this active initiator, a series of experiments were performed where
bpyNi(COD) was reacted with selectively 13C-labeled NCA
monomers.
Acknowledgment. The author thanks Katherine A. Hutchison for the
magnetic susceptibility measurements on the nickel complexes and Dr.
Andrew A. Goodwin for the metal analyses. This work was supported
by the donors of the Petroleum Research Fund administered by the
American Chemical Society, National Science Foundation CAREER
Award (CHE-9701969), and partially supported by the MRSEC program
of the National Science Foundation (DMR-9632716) and the U.S. Army
Research Office Multidisciplinary University Research Initiative (DAAH04-
96-1-0443).
To completely consume all the bpyNi(COD) in reactions with
NCAs it was necessary to use at least a 5-fold excess of NCA
(8) Sano, K.; Yamamoto, T.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1984,
57, 2741-2747. (b) Echavarren, A. M.; Ca´rdenas, D. J.; Castan˜o, A. M.;
Cuerva, J. M.; Mateo, C. Bull. Soc. Chem. Belg., 1994, 103, 549-558.
(9) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163-1188. (b)
Hartwig, J. F.; Richards, S.; Baran˜ano, D.; Paul, F. J. Am. Chem. Soc. 1996,
118, 3626-3633. (c) Wagaw, S.; Rennels, R. A.; Buchwald, S. L. J. Am.
Chem. Soc. 1997, 119, 8451-8458. (d) Fryzuk, M. D.; Piers, W. E.
Organometallics 1990, 9, 986-998.
Supporting Information Available: Details of all reactions and
polymerizations (8 pages, print/PDF). See any current masthead page
for ordering information and Web access instructions.
JA980313I
(12) Nyholm, R. S.; Short, L. N. J. Chem Soc. 1953, 2670.
(13) Bamford, C. H.; Elliot, A.; Hanby, W. E. Synthetic Polypeptides;
Academic Press: New York, 1956.
(10) Amido amidate product: IR (THF) ) 1578 cm-1 (νCdO).
(11) See the Supporting Information.