Английская Википедия:Iminoborane

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Файл:TBuN=B-tBu (CETSUP).png

Structure of tBuN=B-tBu (tBu = tert-butyl). The B-N bond distance is 126 pm.

Iminoboranes comprise a group of organoboron compounds with the formula RB=NR'. They are electronically related to acetylenes but are usually more reactive due to the polarity.

Structure and bonding

The parent iminoborane, HB=NH, is produced by the photolysis of H₃BNH₃. Bonding in iminoboranes can be described by two resonance structures:

<chem>R-\overset{\ominus}{B}{\equiv}\overset{\oplus}{N}-R <-> R-B=\ddot{N}-R</chem>

The stability is dramatically affected by bulky substituents. One isolable iminoborane is Шаблон:Chem2.

Comparison of bond lengths in simple boron-nitrogen hydrides

Molecule	Ammonia borane	Aminoborane	Iminoborane
Formula	BNH6	BNH4	BNH2
Class	amine-borane	aminoborane	iminoborane
Analogous hydrocarbon	ethane	ethylene	acetylene
Analogous hydrocarbon class	alkane	alkene	alkyne
Structure	Файл:Ammonia-borane-dimensions-MW-1983-2D.png	Файл:Aminoborane-dimensions-MW-1987-2D.png	Файл:Iminoborane-dimensions-IR-1987-2D.png
Ball-and-stick model	Файл:Ammonia-borane-from-xtal-3D-balls.png	Файл:Aminoborane-from-MW-1987-double-3D-balls.png	Файл:Iminoborane-from-IR-1987-triple-3D-balls.png
Hybridisation of boron and nitrogen	sp3	sp2	sp
B-N bond length	1.658 Å	1.391 Å	1.238 Å
Proportion of B-N single bond	100%	84%	75%
B-H bond length	1.216 Å	1.195 Å
N-H bond length	1.014 Å	1.004 Å
Structure determination method	microwave spectroscopy	microwave spectroscopy	infrared spectroscopy

Synthesis

Elimination of fluoro- or chlorosilanes provides a well-tested route. Bulky substituents such as (Me₃Si)₃Si stabilize the iminoborane with respect to oligomerization:

(Me₃Si)₃SiB(F)-N(SiMe₃)₂ → (Me₃Si)₃Si-B=N-SiMe₃ + F-SiMe₃

Thermal decomposition of azidoboranes induces migration of R from boron to the nascent nitrene gives iminoboranes:

R₂B-N₃ → RB=NR + N₂

Reactivity

Oligomerization

Iminoboranes tend to oligomerize, often forming cyclic derivatives. Preventing this reaction is the purpose of bulky substituents. Five types of oligomerization product are produced: cyclodimers (Шаблон:Chem names, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Шаблон:Prime), cyclotetramers (Шаблон:Chem names, Te), and polymers (polyiminoboranes, Po); which are shown below. Which product is dominant depends on the structures of reactants and the reaction conditions. Some of the products can be interconverted.

Файл:Figure product of iminboboranes oligomerization-1.png

Addition reactions

The addition of protic agents is fast and quantitive. Boration reaction of iminoboranes is the addition of B-X single bond to B≡N, where -X can be -Cl (chloro-boration), -N₃ (azido-boration), -SR (thio-boration), -NR2 (amino-boration) and R (alkyl-boration). One of these reactions are illustrated here.

Файл:Chloro-boration of iminoborane.png

Some electron-rich iminoboranes form adducts with Lewis acids.

Файл:Lewis acid addition of iminoboranes.png

Cycloaddition

The typical [2+3]-cycloaddition is the addition of B≡N and RN₃ to give a BN₄ ring. One of the widely investigated [2+2]-cycloadditions is the reaction of aldehydes and ketones.

Coordination to transition metals

Like alkynes, iminoboranes bind transition metals.

Файл:Coordination of iminoboranes to metals.png