A cycle for organic nitrile synthesis via dinitrogen cleavage

Article abstract of DOI:10.1021/ja066090a

In the presence of NaH, the reaction between N₂ and Mo(N[t-Bu]Ar)₃ (Ar = 3,5-C₆H₃Me₂) proceeds at room temperature to afford NMo(N[t-Bu]Ar)₃ (95%). Lewis acidic silyl triflates (Me₃

Full text of DOI:10.1021/ja066090a

Published on Web 10/06/2006
A Cycle for Organic Nitrile Synthesis via Dinitrogen Cleavage
John J. Curley, Emma L. Sceats, and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received August 22, 2006; E-mail: ccummins@mit.edu
Scheme 1. A Synthetic Cycle that Incorporates N2 into Organic
Nitrilesa
Six-electron reductive cleavage of the N2 molecule by soluble
metal complexes has been observed for a handful of early transition-
element systems to provide well-defined terminal or bridged nitri-
dometal products.1-9 We sought to couple dinitrogen cleavage
chemistry with N-atom transfer reactivity,4,10 keeping in mind the
criterion that N-atom transfer should return the metal fragment in
high yield and in a form that is suitable for subsequent, repeated
dinitrogen cleavage. Dinitrogen cleavage by the three-coordinate
molybdenum(III) complex Mo(N[t-Bu]Ar)3 (1) provides 2 equiv
of the terminal nitrido complex NtMo(N[t-Bu]Ar)3 (2) in es-
sentially quantitative yield.1,2,11-13 Others have explored the use of
nitrido 2 as an N-atom source in the synthesis of organonitrogen
compounds upon treatment with TFAA ((F3CCO)2O), a regimen
that did not satisfy the criterion articulated above.14 We have de-
veloped a scheme for N-atom incorporation from 2 into organic
nitriles, a scheme that until now has been entered into only via
independent synthesis.15,16 Herein we describe N-atom transfer from
2 into organic nitriles via a Lewis-acid induced reaction that returns
molybdenum in the form of a chloromolybdenum(IV) complex17
which in turn is reductively recycled18 to dinitrogen-splitting com-
plex 1. Organic nitriles are useful nitrogen-containing building
blocks for synthesis,19,20 and the present methodology may find
application in the 15N-labeling of organic nitriles21 and compounds
derived therefrom.
a (i) (a) Me3SiOTf (b) 1.25 PhC(O)Cl, 0.2 py; (ii) 1.25 t-BuC(O)Cl,
[Me3Si(py)][OTf]; (iii) (i-Pr)3SiOTf, MeC(O)Cl. Yields of RCN were
determined by 1H NMR versus an internal standard. The isolated yields of
7 shown were obtained from reactions using SnCl2.
The terminal nitride complex, 2, is most conveniently prepared
by stirring orange-red solutions of 1 with purified NaH22,23 under
an N2 atmosphere. This method, a modification of published
procedures,11-13,24 requires little purification of the nitride product,
as filtration of the reaction mixture followed by removal of solvent
affords 2 as a golden-yellow powder. Nitrido complex 2 has proven
to be a reluctant nucleophile. This is attributed, at least in part, to
steric crowding of the nitrido functionality by three proximal tert-
butyl residues. Accordingly, 2 is not acylated by acid chlorides in
the absence of additives, even upon mild heating. On the other hand,
nitrido 2 does take part in reactions with strong electrophiles.
Recently described was the reaction of 2 with Me3SiOTf leading
rapidly to the silylimido salt [Me3SiNMo(N[t-Bu]Ar)3][OTf] (3).13
We recognized the latent potential of the Me3Si+ group as a Lewis
acid capable of promoting a reaction between acid chlorides and
2.25-27 Indeed, in the presence of a catalytic amount of pyridine,
mixtures of 3 and PhC(O)Cl are converted to benzoylimido salt,
[PhC(O)NMo(N[t-Bu]Ar)3][OTf] (4-Ph, 75%). This transformation
creates a new N-C bond while introducing a carbonyl functional
group into the molecule that is earmarked for synthetic elaboration.
Reduction of 4-Ph by magnesium anthracene produces the purple
[Mg(THF)2][PhC(O)NMo(N[t-Bu]Ar)3]2 (5-Ph), which can be
isolated from the reaction mixture as a crude material (82%). 5-Ph
participates in reactions with TFAA or PhC(O)OTf to eliminate
PhCN while forming molybdenum(IV) trifluoroacetate, F3CCO2-
Mo(N[t-Bu]Ar)3, or the structurally characterized benzoate, PhCO2-
Mo(N[t-Bu]Ar)3.28 However the molybdenum-containing products
resulting from reactions of 5-Ph could not be isolated as pure
materials, therefore we sought to convert 5-Ph into an easily
manipulated material. Accordingly, treatment of in situ generated
5-Ph with Me3SiOTf cleanly afforded a trimethylsiloxy-substituted
ketimide, Ph(Me3SiO)CNMo(N[t-Bu]Ar)3 (6-Ph, 77%). Crystal-
lographic data (Figure 1) obtained from single crystals of the dark
green 6-Ph show a short Mo-N single bond of 1.828(2) Å, an
NdC bond of 1.280(2) Å, and a nearly linear Mo-N-C angle of
171.0(1)° for the ketimide moiety. The three anilide ligands, which
are equivalent in solution, are found in an “up-down-sideways”
conformation in the solid state.15 Computational studies of 6-Ph
reveal that the HOMO is the back-bond between the d2 metal center
and the ketimide NdC π* orbital. The LUMO is nonbonding and
has the appearance of a dz orbital.29
2
Complexes of the formula Ph(X)CNMo(N[t-Bu]Ar)3 (X ) O2-
CPh, SC6F5) are known to fragment giving (X)Mo(N[t-Bu]Ar)3 and
PhCN.28 We therefore sought out reactions that would release PhCN
from 6-Ph. Both ZnCl2 and SnCl2 were found to react with 6-Ph to
evolve PhCN, with ClMo(N[t-Bu]Ar)3 (7)17,30 as the sole molyb-
denum-containing product. The reaction between SnCl2 and 6-Ph
proceeds over 1 h producing 7 (93%) and PhCN (97%); the tin
byproducts are removed by filtering the reaction mixture. The
reaction between ZnCl2 and 6-Ph requires 3 h to yield PhCN (98%)
and 7 (20%). The isolated yield of 7 is low because the soluble
zinc byproducts must be separated by crystallization. The 15N-
labeled, Ph(Me3SiO)C15NMo(N[t-Bu]Ar)3 (15N NMR: δ ) 404
ppm) was prepared, and its reaction with ZnCl2 was assayed by
15N NMR. In the crude reaction mixture, only one resonance was
9
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J. AM. CHEM. SOC. 2006, 128, 14036-14037
10.1021/ja066090a CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
A key feature of the cycle in Scheme 1 is the set of three
acylation strategies that employ either Lewis acid/Lewis base
combinations or a sterically hindered Lewis acid to promote the
reaction of 2 with acid chlorides. The other essential feature is the
use of SnCl2 and ZnCl2 as both Lewis acids and chloride donors.
Acknowledgment. We thank the National Science Foundation
(Grant CHE-0316823) for financial support and Dr. David S. Laitar
for crystallographic assistance.
Supporting Information Available: Full experimental details. This
material is available free of charge via the Internet at http://pubs.acs.org.
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1
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