Reductive Silylation Using a Bis-silylated Diaza-2,5-cyclohexadiene

Article abstract of DOI:10.1002/chem.201900879

1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1, was tested as a reagent for the reductive silylation of various unsaturated functionalities, including N-heterocycles, quinones, and other redox-active moieties in addition to deoxygenation of main group oxides. Whereas most reactions tested are thermodynamically favorable, based on DFT calculations, a few do not occur, perhaps giving limited insight on the mechanism of this very attractive reductive process. Of note, reductive silylation reactions show a strong solvent dependence where a polar solvent facilitates conversions.

Full text of DOI:10.1002/chem.201900879

A Journal of
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Title: Reductive Silylation using a bis-silylated diaza-2,5-
cyclohexadiene
Authors: Daniel M. Beagan, I.J. Huerfano, Alexander V. Polezhaev,
and Kenneth G. Caulton
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Reductive Silylation using a bis-silylated diaza-2,5-cyclohexadiene
Daniel M. Beagan, I.J. Huerfano, Alexander V. Polezhaev, and Kenneth G. Caulton*[a]
Department of Chemistry, Indiana University, Bloomington, Indiana
Abstract
1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1, is tested as a reagent for the reductive silylation of
various unsaturated functionalities, including N-heterocycles, quinones, and other redox-active moieties in
addition to deoxygenation of main group oxides. Whereas most reactions tested are thermodynamically
favorable, based on DFT calculations, a few do not occur, perhaps giving limited insight on the mechanism
of this very attractive reductive process. Of note, reductive silylation reactions show a strong solvent
dependence where a polar solvent facilitates conversions.
Introduction
Chemical reduction of unsaturated functionalities, often in the form of hydrogenation, is a highly desired
molecular transformation. In practice, the unassisted bimolecular reaction of H2 with a non-metal substrate
generally suffers from the lack of a low energy mechanism[1], thus proton coupled electron transfer[2]
demonstrates a valued, and evolutionarily selected alternative. The advantage of this process, over H2 itself,
is that it is polar, and thus especially suited for reduction of polar multiple bonds. Another approach is the
use of two H atoms “carried” by some delivery reagent chosen so the H-reagent bonds are both
thermodynamically weak and kinetically labile (e.g. allylic or benzylic). The departure from symmetry,
and thus from non-polarity, in moving from addition of H2 to addition of H-SiR3 is an attractive advance,
and the resulting silyl product can itself have value not only for the reduction of the bond to which it has
added. Silicon is an element which has well developed “heterophilicity,” i.e. forming strong bonds to
heteroatoms such as N, O, and halogens. The next step in reductive reagent development is to enhance the
polarity of the bond delivering the silyl group, for example, to a Si-N bond. Mashima and Tsurugi[3] have
recognized the desirable property of having nitrogen substituents able to stabilize either radical or anionic
products of Si-N bond scission, and this has led them (Scheme 1) beyond the bis-silyl cyclohexadiene A to
Scheme 1. Silyl Transfer Reagents
[a] Department of Chemistry, Indiana University, Bloomington Indiana 47405
Supporting information and the ORCID identification number(s) for the author(s) for this article is available on the
WWW under: http://dx.doi.org/10.1002/chem.2018xxxxx
1
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bis-silylated diaza heterocycles B. These have the potential advantages to deliver two silyl groups, and the
two conjugated ring nitrogens make both radical and charged intermediates more energetically accessible.
Kaim, who pioneered these reagents,[4] synthesized the compounds originally because they are very electron
rich, thus subject to outer sphere electron transfer. The electron richness of the reagent results from two
bis-amino olefin functionalities, and even nonconjugated enamines are established as electron rich. These
reagents have the stoichiometry to transfer two silyl groups in the same way that 1,4-cyclohexadiene or
dihydroanthracene supply two H. However, this transfer is not found in seemingly simpler silyl sources,
R3Si─SiR3 because analogous to H2 these disilanes lacks polarity. The utility of bis-silyl pyrazines
especially 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1, and the tetramethylated analogue, 2
(Scheme 1)[3b, 5] to abstract chloride and oxide from metal complexes have been actively explored, and this
process clearly represents reduction of the metal center. We are interested in broadening the application of
their reductive character, including to main group elements in the form of reductive silylation or
deoxygenation, but also in the mechanism, concerted or stepwise, of these silyl transfer reactions.
Mashima and Tsurugi[3b] have shown that 1 reduces binary W and Ti chlorides to lower oxidation states
by chloride abstraction, a conversion which is perhaps even superior to using elemental Zn or Mg toward
the same end. While it was originally envisioned that elimination of (Me3Si)2O from MOn by 1 might be
a useful approach to MOn-1, a recent application to support-bound WO3 gave instead tetravalent tungsten
silyl esters, which is still a valuable 2 electron reduction, but without full elimination of W/O bonds and
creation of lower metal coordination number.[5c] The method has also been applied to nanoparticle
formation,[5g] silylation of B-F bonds[6] and of α−halo carbonyls,[7] and deoxygenation of nitro
compounds.[8] In summary, the strengths and limitations of this attractive reagent are only now
emerging.[5c, 9]
Finally, the combination of Me3SiCl and KC8 has become a popular way to functionalize reduced N2 in
metal complexes [10] and is speculated to involve Me3Si radicals;[1a, 11] the parasitic production of (SiMe3)2
is a problem with this reagent combination, and this might be improved using the bis-silyl transfer reagents
1 or 2.
Results
N-Heterocycle Silylation: Tetrazines, Triazines, and Pyrazines
The reagent 1 acts as a two-electron reductant, thus upon double silyl transfer produces pyrazine (Pz), itself
a weak oxidant, meaning in order for silyl transfer to occur, the substrate must be a stronger oxidant than
pyrazine. Tetrazines, because of their high nitrogen content, are easily reduced[4b, c, 12], and this raises the
question of both the thermodynamics and kinetics of double silyl transfer from reagent 1 to a tetrazine
(Scheme 2)
Equimolar 1 and diphenyl tetrazine (Ph2Tz) in dichloromethane react in time of mixing with a characteristic
color change from the pink of tetrazines to a deep orange indicative of reduction. The 1H NMR spectrum
of the product solution (Figure S1) is consistent with the bis-silyl tetrazine (Scheme 2) together with a
singlet identical to that of pyrazine showing complete consumption of starting reagents. The silyl product
is C2 symmetric showing three phenyl chemical shifts and a singlet for two equivalent Me3Si.
2
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Scheme 2. Reaction of Diphenyl Tetrazine (Ph2Tz)
with 1
This result confirms, unsurprisingly, that pyrazine is a weaker oxidant than diphenyl tetrazine, so silyl
groups are transferred to the heterocycle better able to stabilize additional reducing equivalents. This is
consistent with the reaction free energy of Scheme 2 determined by DFT which yields a value of -23.9
kcal/mol. The analogous reduction of Ph2Tz does not occur with H2 despite a DFT calculated Go of – 6.7
kcal/mol, confirming a barrier in the unassisted mechanism of reduction with H2, which 1 is able to
circumvent.
After showing the ability of 1 to silylate tetrazines, we explored the reaction of 1 with a bis-tetrazinyl
pyridine (BTZP). Because there are two tetrazine rings in addition to two pyridine-based azabutadiene
moieties, we tested (Scheme 3) if equimolar 1 would selectively reduce one tetrazine ring, reduce each
tetrazine ring by one electron, or even reduce the azabutadiene fragment including silylation at pyridine
(Scheme 3).
Reaction of equimolar BTZP, with 1 at 25oC in dichloromethane gives color change from pink to a deep
orange/red at a slower rate than with diphenyl tetrazine. NMR monitoring over time in CD2Cl2 shows
complete conversion of starting reagents within 2 h. The final 1H NMR spectrum shows a product bearing
Scheme 3. Reaction of BTZP with 1
3
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Scheme 4. Possible products of reductive silylation of 1,3,5-triazine
inequivalent tetrazine rings on the pyridyl linker judged by the two sets of ethyl multiplets, three pyridyl
multiplets, and two SiMe3 singlets, all with intensity consistent with the product in Scheme 3. Unconsumed
1 is absent, and the singlet of pyrazine is present, at appropriate intensity compared to those of the pyridyl
product. Further silylation was tested by addition of 1 to equimolar (TMS)2BTZP in CD3CN, upon which
there was an immediate color change from red to orange. The 1H NMR spectrum after 10 minutes shows
complete consumption of 1 and clean conversion to (TMS)4BTZP (Scheme 3).
Following our study with tetrazines, we proceeded to test whether 1 can generally silylate and reduce other
N-heterocycles. In general, to avoid diradical products, the bis silylation reaction can only add to ring
nitrogens at the 1,4 or 1,2 sites, and steric factors might preclude 1,2 disilylation. Still more demanding is
1,3,5 triazine, since the only N, N’ disilylation product creates diradicals (Scheme 4), and the other two
demand Si─C bond formation, expected to be less favorable than the initial Si─N bond of the starting
reagent. In fact, after combining 1 and 1,3,5 triazine in a 1.5:1 mole ratio, no reaction is observed over 12
o
hours at 70 C in C6D6 or even at 110oC for 6 hours in toluene-d8; only retention of starting materials is
seen. We take this as a thermodynamic limitation, in that sacrificing two Si-N bonds for one Si-N and one
Si-C is not favorable. DFT shows (Scheme 4) that both reactions are nearly thermoneutral, with the ortho
isomer slightly less stable. As polar solvents are known to facilitate electron transfer, we repeated this
reaction in CD3CN to probe solvent effects and observed no change for this reaction even when heated at
80 oC for 24 hours.
Further, we tested possible silyl transfer to more electron poor pyrazines, using diethyl pyrazine-2,5-
dicarboxylate and 2,5 dicyano pyrazine (Scheme 5). We envisioned that a pyrazine bearing two electron
withdrawing groups might be more stabilized as the corresponding bis(silyl)diazacyclohexadiene due to the
lowering of the LUMO π* orbitals imparted by ring substitution. This stabilization should favor silyl
transfer to the more oxidizing pyrazine moiety vs. the starting reagent. The reaction of equimolar diethyl
pyrazine-2,5-dicarboxylate with 1 in C6D6 shows no silyl transfer to the more electron poor pyrazine even
upon heating at 80 oC for 12 hours. In contrast to the results seen with 1,3,5-triazine. this reaction can be
facilitated using the polar solvent CD3CN which shows slow silyl transfer (roughly 50% conversion over 2
o
days at room temperature) which can be driven to completion by heating at 80 C for 12 hours. If the
4
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Scheme 5. Reaction of Substituted Pyrazines with 1
reaction is heated initially, conversion is observed within 2 hours. In contrast, combining 2,5
dicyanopyrazine with equimolar 1 in C6D6, gives a fast color change to a pale green. Despite this drastic
color change, 1H NMR shows no significant reaction within 30 minutes. We suggest that this color indicates
formation of a small mole percent (undetectable by NMR) of a charge transfer complex between the
differently polarized reagents.[13] After an additional 12 h at room temperature, there is further color change
to a deep orange accompanied by some formation of a product with a new SiMe3 signal as well as a new
vinylic ring hydrogen signal in appropriate intensity to be product bis-silylated pyrazine. After an
additional 3 h at 80 oC, the conversion is increased, but not yet complete, and after 18 h at 80 oC, complete
conversion to a single product is observed (Figure S15). Consistent with the solvent effects observed in the
silyl transfer to the diester pyrazine, when done in CD3CN at room temperature, the dicyano pyrazine can
be silylated in time of mixing. This silyl transfer is thus not thermoneutral, but favors silyl groups on the
ring with strong electron withdrawing substituents, consistent with computated Go values of -14.4
Kcal/mol for silylation of dicyano pyrazine and -12.3 kcal/mol for silylation of the diester pyrazine. The
better silyl transfer in acetonitrile compared to arene solvents despite much lower solubility of 1,
demonstrates the significant solvent-effect on the mechanism of this electron-transfer process.
Figure 1. Experimental solid-state structures of planar 1 (left) and boat-shaped 2 (right)
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Scheme 6. Reaction of pyrazine with 2
To further test silyl transfer between related functionalities, we attempted the reductive silylation of
pyrazine using the potentially more potent tetramethyl substituted reducing agent, 2. DFT calculations
(Scheme 6) were used to compare the relative reductive silylating ability of the two reagents. Though the
methyl-substituted silyl pyrazine reagent has a more positive reduction potential,[3a] possibly due to loss of
antiaromatic character through distortion from planarity (Figure 1), DFT predicts based on Gibbs free
energy that the silylation of pyrazine, C4N2H4, by 2 is favored by 17.3 kcal/mol. The boat conformation[14]
of 2 is also predicted to aid mechanistically as both silyl groups are on the same side of the molecule in
reasonably close proximity to one another. However, experimentally this reaction does not occur at room
temperature, 80 oC in deutero-benzene, or even neat with excess pyrazine at 100 oC within 48 hours. NMR
monitoring of the reaction shows no silyl transfer from 2 to pyrazine, with only retention of starting
materials. As demonstrated with the other pyrazine substrates, the reaction can be promoted using
acetonitrile confirming the thermodynamic favorability of silyl transfer to the more electron poor pyrazine
moiety. With a 9:1 ratio of pyrazine:2 the reaction does occur but is quite slow, showing only 20%
conversion after 18 hours at room temperature and only an additional 20% after another 18 hours at 55 oC.
After an additional day of heating the reaction only goes to 57% completion. This result indicates 2 does
indeed serve as a thermodynamically more potent reductive silylating reagent than 1.
Quinone Silylation
Given its propensity to reductively silylate various unsaturated N-heterocycles, we sought to silylate other
redox-active organic moieties. Quinones serve this purpose as they contain adjacent ketone oxygens in a
heterobutadiene fragment while also benefitting from aromatization upon reduction. We further tested the
limits of steric bulk near the targeted heteroatom (Scheme 7). A green solution of 3,5-di-tert-butyl-1,2-
benzoquinone in C6D6 is immediately decolorized when treated with equimolar 1 at room
temperature. Proton NMR spectra taken within 20 minutes showed no peaks corresponding to the reagent
quinone and the appearance of stoichiometric amounts (based on integration) of pyrazine and bis-silyl ortho
catechol (Figures S13 and S14). The tert-butyl group ortho to the targeted oxygen does not prevent
silylation. Success here depends on the thermodynamic benefit of aromatization in the resulting silyl ether.
Scheme 7. Reaction of 3,5-di-tert-butyl-1,2-benzoquinone with 1
6
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For comparison, 3,5-di-tertbutyl-orthoquinone and hexamethyldisilane (1:29 mole ratio) shows no reaction
heating at 80 oC for 24 h in C6D6; both reagents are recovered unchanged. Hexamethyldisilane serves as a
silyl analogue of H2, thus its failure to reductively silylate the orthoquinone demonstrates the importance
of the polarized N-Si bond to silyl transfer.
Testing Heteroatom Selectivity of Multifunctional Compounds
Scheme 8. Possible Products of the reaction between EtTAAQ with 1
We next probed silylation targets which offer multiple heteroatoms in their oxidized state. The
tetraazaanthraquinone (EtTAAQ, Scheme 8) offers the potential products of silylation of only the para
Et
quinone, of only the pyrazine or even of a mix of these two. Finally, since TAAQ is designed to be
reducible by more than two electrons, this reagent offers the possibility to test adding four silyl groups. In
practice reaction of 1 with EtTAAQ in a 2:1 mole ratio in benzene is complete within minutes at 25oC, as
judged by color change from gold-brown to orange and occurs with precipitation of a yellow solid (Figures
S11 and S12). Assay of the mother liquor from this reaction shows production of pyrazine and new ethyl
and Me3Si signals of a highly symmetric product, consistent with 1:1 mole ratio reaction at the two oxygens
only. The yellow precipitate is purely this same product, whose mass spectrum shows an m/z value of
471.3, corresponding to the mass of singly protonated EtTAAQ with two additional SiMe3 groups. Its 13C
NMR spectrum also shows the symmetry consistent with the bis silylated quinone with only six unique
carbons. The persistence of unreacted 1 in the reaction mixture of this 2:1 reaction establishes that fourfold
silylation (Scheme 8) does not occur under the conditions employed.
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Scheme 9. Potential Silylated PDO Products on reaction with 1
1,10-phenanthroline-5,6-dione (PDO) offers another target for evaluating silyl transfer rate and N/O
selectivity, providing multiple sites for reduction via silylation (Scheme 9). Reaction of 1 with PDO in a
1:1 mole ratio in CD2Cl2 is complete in less than 20 minutes and selectively forms a single product,
evidenced by 1H NMR spectroscopy (Figure S3). The product maintains C2 symmetry, which eliminates a
trans N/O silylation product, and equimolar pyrazine is also observed. The 1H NMR spectrum of the product
has one downfield chemical shift, as does PDO, consistent with the ortho proton in an aromatic (pyridine)
heterocyle. Because N silylation on PDO should have a greater effect on the chemical shift of the ortho
protons, the 1H NMR suggests O silylation. The 13C NMR spectrum (Figure S5) is also consistent with a
bis-silyl catechol, containing six aromatic signals and one TMS signal. The IR spectrum of the product
lacks the C=O stretch present in PDO at 1690 cm-1 and shows the appearance of an Si-O band at 929 cm-
1
1, also consistent with O silylation. When PDO is reacted with 1 in a 1:2 mole ratio, the final H NMR
spectrum (Figure S7) shows this same disilylated product together with unconsumed 1, indicating that
further silylation does not occur under these conditions.
8
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DFT geometry optimization/frequency calculations at the B3LYP/6-31g(d) level of theory were performed
on the different product isomers, revealing product A to be the lowest energy isomer, followed by C (22.9
kcal/mol higher) then B (75.4 kcal/mol higher). The high energy of B is likely due to steric factors, as the
inwardly pointing nitrogens on PDO provide little space for two bulky TMS groups. As silylation of PDO
to form A is formally a two-electron reduction, the frontier orbital of PDO could provide insight into the
observed selectivity for O silylation. As seen in Figure 2, the LUMO of PDO contains mainly -diketone
 character, kinetically favoring O silylation. In particular, there is no nitrogen character to this orbital.
Figure 2. LUMO of PDO (isovalue 0.05)
Applications to Other Potentially Redox-Active Moieties
The above evidence might be generalized to the conclusion that it is thermodynamically favorable to trade
Si-N for Si-O bonds. We next ask whether forming one Si-O bond can enable formation of one companion
Si-N bond. A 1:1 mole ratio of 2-acetyl pyridine and 1 show no change by 1H NMR after 12 h at 80 oC in
benzene or CD3CN. This failure is noteworthy because the DFT reaction energy for this conversion is –
13.6 kcal/mol. This points to a kinetic limit to the reductive silylation of acetyl pyridine possibly stemming
from a steric mismatch of the two compounds.
Is it possible to silylate a Si-C bond? Can formation of a new Si-O bond enable formation of an Si-C bond?
After heating at 80 oC for 12
h in benzene, 1 and benzophenone (Scheme 10) are
recovered unchanged, showing limits of reductive silylation, even with potential of formation of one Si-O
bond.
N,N'-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-butadiene (Scheme 10, MesDiim) was also
tested because this functionality is an important class of redox-active ligands in transition metal chemistry
and is often used by Mashima and Tsurugi to trap low-valent metal complexes resulting from their reductive
o
dichlorination reactions.[15] Even upon heating for 18 hours at 80 C in C6D6 or CD3CN, no reaction is
observed, only signals corresponding to unchanged MesDiim and 1 remain.
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Figure 3. Targeted moieties for reductive silylation
Reductive Deoxygenation
Scheme 10. Attempted deoxygenation of phosphine oxides
With our goal of oxyanion reduction, we also looked toward the use of 1 as a reductant for the
deoxygenation of main group elements due to the high oxophilicity of silicon. Phosphine oxides are
products in Wittig reactions and other widely practiced pharmaceutical conversions so it would be valuable
to be able to deoxygenate these functionalities effectively recycling them to their reduced phosphine form.
Reaction of triphenylphosphine oxide (PPh3O) with equimolar 1 in CD3CN at 80 ºC shows no reaction by
1H or 31P NMR within 12 hours. This clearly shows some limitations of the silylation reagent, but are these
kinetic or thermodynamic in origin? The reaction of PPh3O with 1 to yield triphenylphosphine, Pz, and
hexamethyldisiloxane [(Me3Si)2O] is calculated to have Go of -46.6 kcal/mol suggesting a high kinetic
barrier. In hopes of lowering this kinetic barrier, we next attempted the analogous reaction using
tributylphosphine oxide. We hypothesized that although a more electron-rich phosphorus atom with three
electron-donating alkyl groups may be more difficult to reduce thermodynamically, there could be greater
contribution by the resonance form containing a P─O single bond (Scheme 11), thus labilizing that bond.
o
Despite our expectations this was not the case and no reaction was observed in CD3CN heated at 80 C.
Therefore, we conclude that this negative experimental result indicates a high barrier for full deoxygenation
of phosphine oxides. A mechanism with concerted double silyl transfer is ineffective in this case.
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These shortcomings led us to next try deoxygenating the weaker bond of an amine-oxide for its relevance
to nitrogen oxyanions such as nitrate. Reaction of equimolar 1 with trimethylamine oxide at 80 ºC in CD3CN
cleanly leads to the formation of trimethylamine, Pz, and a new silyl product determined to be (Me3Si)2O
as judged by 1H NMR Scheme 12). We next attempted the deoxygenation of dimethyl sulfoxide (DMSO)
to give the corresponding dimethyl sulfide. Addition of TMS2Pz to neat DMSO-d6 shows approximately
1
60% conversion to pyrazine and (Me3Si)2O over 2 days at room temperature based on H NMR. The
reaction can be driven to completion by heating to 80 ºC over an additional 18 hours. As the reaction was
performed neat in deuterated DMSO, no reduced sulfur containing product was observed nor can any
1
information be drawn from changes in integrated intensity of the residual protio-DMSO signal in the H
NMR due to large excess. This same reaction was repeated in CD3CN under the same conditions with 20
1
equivalents of protio-DMSO. H NMR of the final reaction mixture shows identical results to those seen
with neat DMSO, but now with a new singlet signal at 2.06 ppm consistent with dimethyl sulfide.
Scheme 11. Successful deoxygenations
Given these successes. we next attempted the direct deoxygenation of the ions nitrate and nitrite. Both
reactions of AgNO3 with two equivalents and AgNO2 with one equivalent of TMS2Pz give identical results
(Scheme 13). By 1H NMR, we see the clean formation of free pyrazine as well as stoichiometric (Me3Si)2O,
though no reduced nitrogen product is observed. This redox reaction leads to the precipitation of a black
solid predicted to be silver nanoparticles as well as the liberation of nitric oxide (NO) gas. NO gas formation
was confirmed by a trapping experiment with a chromium tris(amide), Cr[N(SiMe3)2]3, to give a known
chromium nitrosyl complex.[16]
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Scheme 12. Deoxygenation of silver nitrate and nitrite
Conclusions
Overall, this work shows that reagents 1 and 2 have broad potential for delivering two formally uncharged
SiMe3 groups, thus effecting two-electron reduction. The reaction favors formation of Si/heteroatom bonds,
and can occur under generally mild conditions. Being homogeneous, it avoids all the surface poisoning
disadvantages of using elemental alkali metals as reducing agents, specifically their often slow rates.
An outstanding feature of this process is two electron reduction character, whereas most reductions are
limited to one-electron processes, and thus involve radical primary products. Since the reactions reported
here are not effected by Me3Si-SiMe3, success here depends on lowered barriers from polar Si-N bonds.
We know nothing yet about whether these reactions involve concerted transfer of two silyl groups, or
whether they are in fact initiated by single electron transfer (reagent 1 is very electron rich), with silyl
migration following from radical cation transient intermediates.
A remarkable related use of 4,4’ bipyridine to catalyze (shuttle) borylpinacolate substituents from B2Pin2
to Pz and its derivatives has been reported, together with assay of substituent and solvent effects, but without
revealing thermodynamics or mechanism.[17] Taken together with the present report, this clearly shows the
broader utility of this entire concept of transfer of both B and Si substituents, termed there “reductive
addition.”
Because many of the reactions here involve transfer of two silyl groups, this leads to thinking of
mechanisms that are concerted, hence one collision effects two silyl migrations. This favors mechanisms
where two rings collide face to face, with expectation that the Si/Si distance in 1 should match that of the
two receiving heteroatoms. However, because it has been shown that bis-silyation of O2 with variants of
this silylating reagent occurs stepwise,[18] it is premature to generalize about mechanisms of reductive
silylation. In the end, different substrates may proceed by different mechanisms, as shown recently.[19]
Broader application can be envisioned. This silylation reagent has potential for stoichiometric use in
derivatization of N2-derived nitride complexes, and in a mechanistically simpler manner than use of KC8
with Me3SiCl.[10] Moreover, because of the preference of Si for O over N, this silylated nitride is certainly
fated to react with OH sources to produce ammonia (i.e. N(SiMe3)3 hydrolysis to NH3). Anion metathesis
could then be used to return Si-O product to Si-Cl, closing the cycle for use of pyrazine as a shuttle for
SiMe3 groups in ammonia from N2.
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Acknowledgments. This work was supported by the Indiana University Office of Vice President for
Research and the National Science Foundation, Chemical Synthesis Program (SYN), by grant CHE-
1362127. We thank Alyssa C. Cabelof for computational contributions.
Conflict of Interest
The authors declare no conflict of interest
Keywords: reductive silylation • silyl transfer • nitrogen heterocycles • DFT • dihydropyrazines
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