The C-B bond has low polarity (the difference in
electronegativity2.55 for carbon and 2.04 for boron) and therefore alkyl boron compounds are in general stable though easily oxidized. Vinylgroups and arylgroups donate electrons and make boron less electrophilic and the C-B bond gains some double bondcharacter. Like the parent borane, diborane, organoboranes are classified in organic chemistryas 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 oxygenare borinic esters R2BOR, boronic esters RB(OR)2 and borates RB(OR)3 such as trimethylborate. In organometallic chemistrycompounds with metal to boron bonds are called boryls (M-BR2) or borylenes (M-B(R)-M).
* Simple organoboranes such as
triethylboraneor tris(pentafluorophenyl)boroncan be prepared from trifluoroborane(as the ethercomplex) and the ethyl Grignard reagent.
* Boranes react rapidly to
alkenes in a process called hydroboration. This concept was discovered by Dr. Herbert Charles Brownat Purdue Universitywith help from Georg Wittig. Although diborane as a pure compound is a dimer, BH3 forms a 1:1 complex with oxygen in for instance THF. In an ordinary electrophilic additionreaction the Markovnikov's ruledetermines regioselectivitybut with boranes the mode of action is the exact opposite. The reason is that boron is less electronegativethan hydrogen. When a positive charge develops in the alkene on "most substituted" carbon atom, that is where the partially negatively charged hydrogen atom adds to, leaving the "least substituted" carbon atom for the boron atom. The so called anti-Markovnikov addition is most pronounced when the boron compound has very bulky substituents. One organoboron reagent that is often employed in synthesis is 9-borabicyclo [3.3.1] nonane or 9-BBNwhich is generated from the reaction of cyclooctadieneand diborane["Advanced Organic Chemistry", F.A. carey, R.J. Sundberg ISBN 0-306-41088-5] . Hydroborations take place stereoselectivein a "syn" mode, that is on the same face of the alkene. In this concerted reactionthe transition stateis represented as a square with the corners occupied by carbon, carbon, hydrogen and boron with maximum overlap between the two olefin p-orbitals and the empty boron orbital.
*Hydroboration-oxidation reaction. In organic synthesis the hydroboration reaction is taken further to generate other
functional groupsin the place of the boron group. The Hydroboration-oxidation reactionoffers a route to alcohols by oxidation of the borane with hydrogen peroxideor to the carbonylgroup with the stronger oxidizing agent chromium oxide.
*A second group of reactions that organoboron compounds are involved in create new carbon carbon bonds.
Carbon monoxideis found to react very easily with a trialkylborane. What follows is a 1,2-rearrangementwhen an alkyl substituent on the anionic boron migrates to the adjacent electrophilic carbon of the carbonyl group. The carbonyl group can then be reduced to an alcohol group Fact|date=February 2007.
*Asymmetric allylboration demonstrates another useful application of organoboranes in carbon-carbon bond formation. In this example from Nicolaou's synthesis of the
author = Nicolaou, K.C.
coauthors = Sarabia, F.; Ninkovic, S.; Finlay, M.R.V.; Boddy, C.N.C.
year = 1998
title = Probing The Ring Size Of Epothilones: Total Synthesis Of 14-, 15-, 17-, And 18 Epothilones A
journal = Angewandte Chemie. International edition in English
volume = 37
issue = 1-2
pages = 81–84
url = http://cat.inist.fr/?aModele=afficheN&cpsidt=10349604
accessdate = 2008-03-02] asymmetric allylboration (using an allylborane derived from chiral alpha-pinene) is used in conjunction with TBS protection and
ozonolysis. Overall, this provides a two-carbon homologation sequence that delivers the required acetogeninsequence.
*Organoboron compounds also lend themselves to
transmetalationreactions with organopalladiumcompounds. This reaction type is exemplified in the Suzuki reaction.
* Borane hydrides such as
9-BBNand L-selectride(lithium tri-sec-butylborohydride) are reducing agents. An example of an asymmetric catalystfor carbonylreductions is the CBS catalyst. This catalyst is also based on boron, the purpose of which is coordination to the carbonyl oxygen atom.
* Trialkyl boranes can be oxidized to the corresponding
borates. One method for the determination of the amount of C-B bonds in a compound is by oxidation of R3B with the nitrosocompound "nitrosomethane" (MeNO) to RB(OR)3 and "trimethyl amine" Me3N which can be titrated.
Boronic acids ROH2 react with potassium hydrogen fluoride to a trifluoroborate salt RBF3M ["Conversion of Arylboronic Acids into Potassium Aryltrifluoroborates: Convenient Precursors of Arylboron Difluoride Lewis Acids " E. Vedejs, R. W. Chapman, S. C. Fields, S. Lin, M. R. Schrimpf J. Org. Chem.1995; 60(10); 3020-3027. DOI|10.1021/jo00115a016] which are precursors to nucleophilic alkyl and aryl boron difluorides (ArBF2). The salts are more stable than the boronic acids themselves and used for instance in alkylation of certain aldehydes [Organoboron compounds as mild nucleophiles in Lewis acid- and transition metal-catalyzedC–C bond-forming reactions Robert A. Batey, Tan D. Quach, Ming Shen, Avinash N. Thadani, David V. Smil, Sze-Wan Li, and D. Bruce MacKay Pure Appl. Chem., Vol. 74, No. 1, pp. 43–55, 2002. http://www.iupac.org/publications/pac/2002/pdf/7401x0043.pdf] [Displayed is a reaction sequence starting with reaction of allyl magnesium bromidewith trimethylborate, followed by hydrolysisof the boronic ester to the boronic acid with hydrochloric acid. The aldehyde is p-nitrobenzaldehyde] : :
Nucleophilic anionic boryl compounds have long been elusive but a 2006 study described a boryllithium compound which reacts as a
nucleophile["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] ["Boron Attacks Electropositive element pressed into action as nucleophilic boryllithium" Bethany Halford Chemical & Engineering NewsOctober 9, 2006 Volume 84, Number 41 p. 11 [http://pubs.acs.org/cen/news/84/i41/8441notw8.html Link] ] :
This is remarkable because in other
period 2 elements lithium salts are common e.g. lithium fluoride, lithium hydroxide lithium amideand methyllithium. Reaction of base with a borohydride R2BH does not result in deprotonationto the boryl anion R2B- but to formation of the boryl anion R2B-H(base)+ because only this reaction path gives a complete octet ["Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine". Dennis G. Hall ISBN 3-527-30991-8] . Instead the boryl compound is prepared by reductive heterolysisof a boron-bromide bond by lithiummetal. The new boryl lithium compound is very similar to and isoelectronicwith 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 diffractionconfirms sp2 hybridizationat boron and its nucleophilic additionreaction with benzaldehydegives further proof of the proposed structure.
Chemical compounds with boron to boron
double bonds are rare. In 2007 the first neutral diborene (RHB=BHB) was presented ["A Stable Neutral Diborene Containing a BdB Double Bond" Yuzhong Wang, Brandon Quillian, Pingrong Wei, Chaitanya S. Wannere, Yaoming Xie, R. Bruce King, Henry F. Schaefer, III, Paul v. R. Schleyer, and Gregory H. Robinson DOI|10.1021/ja075932i] ["Neutral Diborene Is A First" Ron Dagani Chemical & Engineering NewsOctober 1, 2007 Volume 85, Number 40 p. 10 [http://pubs.acs.org/cen/news/85/i40/8540notw7.html] ] [The boron precursor is Boron tribromideand the reducing agentis KC8which abstracts the required protons from diethyl ethersolvent. ] . Each boron atom has a proton attached to it and each boron atom is coordinated to a so-called NHC carbene.
Triethylboranewas used to ignite the JP-7 fuel of the Pratt / Whitney J-58 ramjet engines powering the Lockheed SR-71 Blackbird.
* [http://www.npi.gov.au/database/substance-info/profiles/15.html National Pollutant Inventory - Boron and compounds]
* [http://www.chem.wisc.edu/areas/reich/orgmet/boron.htm Boron in organic synthesis]
* Compounds of carbon with other elements in the periodic table:
Wikimedia Foundation. 2010.
organoborane — noun any organic compound containing a carbon to boron bond … Wiktionary
Organometallic chemistry — n Butyllithium, an organometallic compound. Four lithium atoms are shown in purple in a tetrahedron, and each lithium atom is bound to a butyl group (carbon is black, hydrogen is white). Organometallic chemistry is the study of chemical compounds … Wikipedia
Triethylborane — Chembox new Name = Triethylborane ImageFile = Triethylborane.png ImageName = Triethylborane ImageFile1 = Triethylborane 3D balls.png ImageName1 = Ball and stick model of triethylborane IUPACName = Triethylborane OtherNames = triethylborine,… … Wikipedia
Brown, Herbert Charles — ▪ American chemist Introduction original name Herbert Brovarnik born May 22, 1912, London, Eng. died Dec. 19, 2004, Lafayette, Ind., U.S. one of the leading American chemists of the 20th century. His seminal work on customized reducing… … Universalium
Organoboron chemistry — Organoboron 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. … Wikipedia
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9-Borabicyclononane — chembox new ImageFileL1 = BBN structure.png ImageSizeL1 = 120px ImageFileR1 = 9 BBN 3D balls.png ImageSizeR1 = 120px ImageFile2 = 9 BBN dimer xtal 3D sticks skeletal.png IUPACName = 9 Borabicyclo [3.3.1] nonane OtherNames = Borabicyclononane… … Wikipedia
Catecholborane — Chembox new Name = Catecholborane ImageFile = Catecholborane structure.png IUPACName = Catecholborane OtherNames = 1,3,2 benzodioxaborole 7,9 dioxa 8λ2 borabicyclo [4.3.0] nona 1,3,5 triene Section1 = Chembox Identifiers SMILES = [B]… … Wikipedia