Organoborane

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Organoborane or organoboron compounds are chemical compounds that are organic derivatives of BH3, for example trialkyl boranes. Organoboron chemistry or organoborane chemistry is the chemistry of these compounds [1] [2]. Organoboron compounds are important reagents in organic chemistry enabling many chemical transformations, the most important one called hydroboration.

Properties

The C-B bond has low polarity (the difference in electronegativity 2.55 for carbon and 2.04 for boron) and therefore alkyl boron compounds are in general stable though easily oxidized. Vinyl groups and aryl groups donate electrons and make boron less electrophilic and the C-B bond gains some double bond character. Like the parent borane, diborane, organoboranes are classified in organic chemistry as strong electrophiles because boron is unable to gain a full octet of electrons. Unlike diborane however, organoboranes do not form dimers.

Other boranes (of academic interest) are carboranes, cluster compounds of carbon and boron and borabenzene, the boron equivalent of benzene.

Organoboranes with carbon replaced by oxygen are borinic esters R2BOR, boronic esters RB(OR)2 and borates RB(OR)3 such as trimethylborate. In organometallic chemistry compounds with metal to boron bonds are called boryls (M-BR2) or borylenes (M-B(R)-M).

Synthesis


Reactions

Boryllithium

Nucleophilic anionic boryl compounds have long been elusive but a 2006 study described a boryllithium compound which reacts as a nucleophile [4] [5]:

This is remarkable because in other period 2 elements lithium salts are common e.g. lithium fluoride, lithium hydroxide lithium amide and methyllithium. Reaction of base with a borohydride R2BH does not result in deprotonation to the boryl anion R2B- but to formation of the boryl anion R2B-H(base)+ because only this reaction path gives a complete octet [6]. Instead the boryl compound is prepared by reductive heterolysis of a boron-bromide bond by lithium metal. The new boryl lithium compound is very similar to and isoelectronic with N-heterocyclic carbenes. It is designed to benefit from aromatic stabilization (6-electron system counting the nitrogen lone pairs and an empty boron p-orbital, see structure A) and from kinetic stabilization from the bulky 2,6-diisopropylphenyl groups. X-ray diffraction confirms sp2 hybridization at boron and its nucleophilic addition reaction with benzaldehyde gives further proof of the proposed structure.

Other uses

TEB - Triethylborane was used to ignite the JP-7 fuel of the Pratt / Whitney J-58 ramjet engines powering the Lockheed SR-71 Blackbird.

External links

See also

  • Compounds of carbon with other elements in the periodic table:
CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl Ar
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr Kr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI Xe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr Ra Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uup Uuh Uus Uuo
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ac Th Pa CU Np Pu Am Cm Bk Cf Es Fm Md No Lr


Chemical bonds to carbon
Core organic chemistry many uses in chemistry.
Academic research, but no widespread use Bond unknown / not assessed.

References

  1. The Roles of Boron and Silicon, Susan E. Thomas; Oxford Chemistry Primers No.1; 1991: Very good general book covering all the important reactions of boron and organoboranes in organic chemistry.
  2. Organometallics Christoph Elschenbroich 3rd Ed. 2006 ISBN 3-527-29390-6 - Wiley-VCH, Weinheim
  3. Advanced Organic Chemistry, F.A. carey, R.J. Sundberg ISBN 0-306-41088-5
  4. Boryllithium: Isolation, Characterization, and Reactivity as a Boryl Anion Yasutomo Segawa, Makoto Yamashita, Kyoko Nozaki Science 6 October 2006: Vol. 314. no. 5796, pp. 113 - 115 doi:10.1126/science.1131914
  5. Boron Attacks Electropositive element pressed into action as nucleophilic boryllithium Bethany Halford Chemical & Engineering News October 9, 2006 Volume 84, Number 41 p. 11 Link
  6. Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine. Dennis G. Hall ISBN 3-527-30991-8



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