Organoborane 1895519 223242520 2008-07-03T05:49:47Z MiPe 2036281 [[Image:OrganoboronLogo.png|right|150px|Organoboron]]'''Organoborane''' or '''organoboron''' compounds are [[chemical compounds]] that are organic derivatives of BH<sub>3</sub>, for example trialkyl boranes. '''Organoboron chemistry''' or '''organoborane chemistry''' is the chemistry of these compounds <ref>''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.</ref> <ref>''Organometallics'' Christoph Elschenbroich 3rd Ed. '''2006''' ISBN 3-527-29390-6 - Wiley-VCH, Weinheim</ref>. 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 [[electrophile]]s because boron is unable to gain a full [[octet rule|octet]] of electrons. Unlike diborane however, organoboranes do not form [[dimer]]s. Other boranes (of academic interest) are [[carborane]]s, cluster compounds of carbon and boron and [[borabenzene]], the boron equivalent of benzene. Organoboranes with carbon replaced by [[oxygen]] are [[borinic ester]]s R<sub>2</sub>BOR, [[boronic ester]]s R<sub></sub>B(OR)<sub>2</sub> and [[borate]]s R<sub></sub>B(OR)<sub>3</sub> such as [[trimethylborate]]. In [[organometallic chemistry]] compounds with metal to boron bonds are called '''boryls''' (M-BR<sub>2</sub>) or '''borylenes''' (M-B(R)-M). ==Synthesis== * Simple organoboranes such as [[triethylborane]] or [[tris(pentafluorophenyl)boron]] can be prepared from [[trifluoroborane]] (as the [[ether]] complex) and the ethyl [[Grignard reagent]]. * Boranes react rapidly to [[alkene]]s in a process called [[hydroboration]]. This concept was discovered by Dr. [[Herbert Charles Brown]] at [[Purdue University]] with help from [[Georg Wittig]]. Although diborane as a pure compound is a dimer, BH<sub>3</sub> forms a 1:1 complex with oxygen in for instance [[THF]]. In an ordinary [[electrophilic addition]] reaction the [[Markovnikov's rule]] determines [[regioselectivity]] but with boranes the mode of action is the exact opposite. The reason is that boron is less [[electronegative]] than 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-BBN]] which is generated from the reaction of [[cyclooctadiene]] and [[diborane]] <ref>''Advanced Organic Chemistry'', F.A. carey, R.J. Sundberg ISBN 0-306-41088-5</ref>. Hydroborations take place [[stereoselective]] in a ''syn'' mode, that is on the same face of the alkene. In this [[concerted reaction]] the [[transition state]] is represented as a square with the corners occupied by carbon, carbon, hydrogen and boron with maximum overlap between the two [[olefin]] [[p-orbital]]s and the empty boron orbital. ==Reactions== *Hydroboration-oxidation reaction. In organic synthesis the hydroboration reaction is taken further to generate other [[functional groups]] in the place of the boron group. The [[Hydroboration-oxidation reaction]] offers a route to [[alcohol]]s by oxidation of the borane with [[hydrogen peroxide]] or to the [[carbonyl]] group with the stronger [[oxidizing agent]] [[chromium oxide]]. *A second group of reactions that organoboron compounds are involved in create new carbon carbon bonds. [[Carbon monoxide]] is found to react very easily with a trialkylborane. What follows is a [[1,2-rearrangement]] when 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 [[epothilone]]s,<ref name=Nicolaou1998>{{cite journal | 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 }}</ref> asymmetric allylboration (using an allylborane derived from chiral [[alpha-Pinene|alpha-pinene]]) is used in conjunction with [[Silyl_ether|TBS protection]] and [[ozonolysis]]. Overall, this provides a two-carbon homologation sequence that delivers the required [[acetogenin]] sequence. :[[Image:AllylborationThenOzonolysis.png|center|600px]] *Organoboron compounds also lend themselves to [[transmetalation]] reactions with [[organopalladium]] compounds. This reaction type is exemplified in the [[Suzuki reaction]]. * Borane hydrides such as [[9-BBN]] and [[L-selectride]] (lithium tri-sec-butylborohydride) are [[reducing agent]]s. An example of an [[asymmetric catalyst]] for [[carbonyl]] reductions 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 [[borate]]s. One method for the determination of the amount of C-B bonds in a compound is by oxidation of R<sub>3</sub>B with the [[nitroso]] compound ''nitrosomethane'' (MeNO) to R<sub></sub>B(OR)<sub>3</sub> and ''trimethyl amine'' Me<sub>3</sub>N which can be [[titration|titrated]]. * [[Boronic acid]]s ROH<sub>2</sub> react with [[bifluoride|potassium hydrogen fluoride]] to a '''trifluoroborate salt''' RBF<sub>3</sub>M <ref>''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}}</ref> which are precursors to nucleophilic alkyl and aryl boron difluorides (ArBF<sub>2</sub>). The salts are more stable than the boronic acids themselves and used for instance in alkylation of certain [[aldehyde]]s <ref>Organoboron compounds as mild nucleophiles in Lewis acid- and transition metal-catalyzed C–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</ref> <ref>Displayed is a reaction sequence starting with reaction of [[allyl magnesium bromide]] with [[trimethylborate]], followed by [[hydrolysis]] of the boronic ester to the boronic acid with [[hydrochloric acid]]. The aldehyde is [[p-nitrobenzaldehyde]] </ref>: :[[Image:AlkyltrifluoroboratesBatey2002.svg|500px|Alkyl trifluoroborates Batey 2002]] ==Boryllithium== Nucleophilic anionic boryl compounds have long been elusive but a 2006 study described a '''boryllithium''' compound which reacts as a [[nucleophile]] <ref>''Boryllithium: Isolation, Characterization, and Reactivity as a Boryl Anion'' Yasutomo Segawa, Makoto Yamashita, Kyoko Nozaki [[Science (journal)|Science]] 6 October 2006: Vol. 314. no. 5796, pp. 113 - 115 {{DOI|10.1126/science.1131914}}</ref> <ref>''Boron Attacks Electropositive element pressed into action as nucleophilic boryllithium'' Bethany Halford [[Chemical & Engineering News]] October 9, 2006 Volume 84, Number 41 p. 11 [http://pubs.acs.org/cen/news/84/i41/8441notw8.html Link]</ref>: [[Image:Boryllithium.png|center|400px|Boryllithium]] This is remarkable because in other [[period 2 element]]s lithium salts are common e.g. [[lithium fluoride]], [[lithium hydroxide]] [[lithium amide]] and [[organolithium|methyllithium]]. Reaction of [[base (chemistry)|base]] with a borohydride R<sub>2</sub>BH does not result in [[deprotonation]] to the boryl anion R<sub>2</sub>B<sup>-</sup> but to formation of the boryl anion R<sub>2</sub>B<sup>-</sup>H(base)<sup>+</sup> because only this reaction path gives a complete [[octet rule|octet]] <ref>''Boronic Acids: Preparation, Applications in Organic Synthesis and Medicine''. Dennis G. Hall ISBN 3-527-30991-8</ref>. 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 carbene]]s. It is designed to benefit from [[aromaticity|aromatic stabilization]] (6-electron system counting the nitrogen lone pairs and an empty boron [[p-orbital]], see structure A) and from [[kinetic reaction control|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. ==Diborenes== Chemical compounds with boron to boron [[double bond]]s are rare. In 2007 the first neutral diborene (RHB=BHB) was presented <ref>''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}}</ref> <ref>''Neutral Diborene Is A First'' Ron Dagani [[Chemical & Engineering News]] October 1, 2007 Volume 85, Number 40 p. 10 [http://pubs.acs.org/cen/news/85/i40/8540notw7.html]</ref> <ref>The boron precursor is [[Boron tribromide]] and the [[reducing agent]] is [[KC8]] which abstracts the required protons from [[diethyl ether]] solvent. </ref>. Each boron atom has a proton attached to it and each boron atom is coordinated to a so-called [[NHC carbene]]. [[Image:DiboreneSynthesis.png|400px|center|Diborene synthesis Wang 2007]] ==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== * [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] ==See also== * Compounds of carbon with other elements in the periodic table: {{ChemicalBondsToCarbon}} == References == <div class="references-small"><references/></small> [[Category:organometallic chemistry]] [[Category:boron compounds]] [[Category:Boranes]] [[pl:Związki boroorganiczne]] [[fi:Organoboraani]]