Reactions of N-Heterocyclic Silylenes with Covalent Azides

Caroline Camic, Nicholas J. Hill Daniel F. Moser, and Robert West

Organosilicon Research Center, University of Wisconsin-Madison, USA

2

• Following the isolation of thermally stable N-heterocyclic

silylenes (NHS) in the mid-1990’s, significant work has

examined their behavior toward a range of organic, inorganic,

and organometallic substrates.

• Amongst the earliest reactions to be studied involved bulky

covalent azide species RN3, a highlight of this work being the

isolation of a base-stabilized silaketimine (NN)Si=NR.

• We have recently sought to isolate a base-free silaketimine

from the reaction of the NHS species 1 and 2 with various

hindered covalent azides - this poster outlines the structural

diversity observed in a range of NHS-azide adducts.

3

• Silicon analog of Arduengo carbene

• Stable at room temperature (under N2!)

• Much less reactive than transient silylenes

• Stability enhanced by aromatic delocalization

N

Si

N

But

ButN

SiN

But

But

Cl

Cl

O

O

N

NH

H

But

But

N- Li+

N- Li+But

But

2 eq t-BuNH2

H2O

2.1 eq Li

THF

1

SiCl4

-78oC

2.1 eq K, THF

65oC

N-Heterocyclic Silylenes

M. Haaf, A. Schmiedl, T.A. Schmedake, D.R. Powell, A. J. Millevolte, M. Denk and R. West, J. Am. Chem. Soc. 1998, 120, 2691

N

Si:

N

1

4M. Haaf, T.A. Schmedake, B. J. Paradise and R. West, Can. J. Chem., 2000, 78, 1526

N

Si:

N

2

NHBut

NHBut

Br

Br

xs.ButNH2

Hexane/H2O

NEt3 / OoC

SiCl4

N

N

But

But

Si

Cl

Cl N

N

But

But

SiNaK2

THF / 10oC

• Saturated analog of silylene 1

• Similar synthesis and reaction profile to 1

• Equilibrium between 2, insertion product and tetramer

• Exists in the solid state as red, diaminodisilyldisilene

2

5

N

Si

N

But

But

N

Si

N

Np

Np

RN

Si

N

Np

Np

N

Si

Me3Si SiMe3

Me3Si SiMe3

N

Si

N

2 3 R = H, CH3 4 5 6

• 1, 2, 3, and 4 are indefinitely stable at room temperature

• 5 and 6 are marginally stable

• 6 is the first stable dialkyl substituted silylene

Lappert 1995

Heinicke 1998

Kira1998

Kira1999

Other Stable Silylenes

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Reactions of NHS 1 with Bulky Azides

N

Si

N

But

But

N3

N(SiMe3)2

Ph3CN3 / THF

N

Si

N

But

But

NR

THF

N

Si

N

tBu

tBu

N

N

N

Nhexane / rtN

Si

N

But

But(R = p-tolyl or Ph3SiN3)

RN3

R

R

Me3SiN3 / THF

M. Denk, R. K. Hayashi and R. West, J. Am. Chem. Soc., 1994, 116, 10813; C. Camic, N. J. Hill, D. F. Moser and R. West,

unpublished work.

Azidosilane: 29Si NMR δ – 46.9 ppm

Silaterazoline: 29Si NMR δ – 53.4 ppm

Silaketimine: 29Si NMR δ – 66.6 ppm

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Reactions of NHS 2 with Bulky Azides

N

Si

N

But

But

N

N

N

Nhexane / rtN

Si

N

But

But

(R = p-tolyl)

RN3

R

R

+

N

Si

N N

Si

N

N

N

But

ButBut

ButR

R

Silatetrazoline Major (ca. 90 %) product

29Si NMR δ – 53.5 ppm

Azidosilane dimerMinor (ca. 10 %)

product29Si NMR δ – 55.6 ppm

C. Camic, N. J. Hill, D. F. Moser and R. West, unpublished work.

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Structure of a Silatetrazoline

Distances (Å) and Angles (o)

Si(1) - N(1-2) 1.690(3) - 1.693(1)

Si(1) - N(3-6) 1.738(3) - 1.744(3)

C(1) - C(2) 1.478(2)

N(4) - N(5) 1.267(2)

N(1) - Si(1) - N(2) 96.22(2)

N(1) - Si(1) - N(6) 117.70(3) N(1) - Si(1) - N(3) 121.20(2)

N(6) - Si(1) - N(3) 85.51(2)

R = 0.053 @ 100 K

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Structure of an Azidosilane Dimer

Distances (Å) and Angles (o)

Si(1) - N(1-2) 1.716(2) - 1.728(2)

Si(1) - N(5-6) 1.741(2) - 1.757(1)

C(1) - C(2) 1.508(1)

N(1) - Si(1) - N(2) 94.74(2)

Si(1) - N(6) - Si(2) 95.95(3)

N(1) - Si(1) - N(6) 130.69(3)

N(3) - Si(2) – N(6) 111.95(3)

R = 0.065 @ 100 K

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A Possible Mechanism ?

N

Si

N

But

But

N

N

N

N

R

R

N

Si

N N

Si

N

N

N

But

ButBut

ButR

R

N

Si

N

But

But

NR

RN3

N

Si

N

But

But

NRNR

N

Si

N

But

But

N

Si

N

But

But

N

R

RN3

RN3 RN3

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Conclusions

• Wide range of structural motifs available from NHS – azide adducts

• Base-free silaketimine not isolated, but appears to react further

• With NHS 1, silaimine reacts with RN3 via [2+3] cycloaddition to give

silatetrazoline

• With NHS 2, azidosilane dimer formed from dimerization of silaimine

or sequential insertion of RN3 into aminosilylsilylene

• Bulkier azide substituents may stabilize a base-free silaketimine

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AcknowledgementsChemistry

DepartmentUW - Madison

Sponsors of the Organosilicon

Research Center

Further Silylene LiteratureM. Haaf, T. A. Schmedake and R. West, Acc. Chem. Res., 2000, 33, 704.

B. Gehrhus and M. F. Lappert, J. Organomet. Chem., 2001, 617, 209.