Borabenzene adducts

Borabenzene
Names
Preferred IUPAC name
Borinine
Identifiers
3D model (JSmol)
ChemSpider
  • InChI=1S/C5H5B/c1-2-4-6-5-3-1/h1-5H
    Key: HXNZTJULPKRNPR-UHFFFAOYSA-N
  • B1=CC=CC=C1
Properties
C5H5B
Molar mass 75.91 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Borabenzene is a hypothetical organoboron compound with the formula C5H5B. Relative to the related but highly stable benzene molecule, borabenzene has two fewer electrons; by Hückel's rule, it would be anti-aromatic.

Adducts of borabenzene with Lewis bases are aromatic and indirectly isolable. A particularly large family are derived from the boratabenzene anion, [C5H5BH]−.

Synthesis

Since borabenzene is unavailable, 4‑silyl-1‑methoxyboracyclohexadiene or similar boracyclohexadienes (derived from dibutylstannacyclohexadiene, itself from double hydrostannylation of 1,4-pentadiyne) are used as a borabenzene equivalent, e.g.:: 217, 221 : 102–104 : 8–9 

C
5H
5N + MeOBC
5H
5SiMe
3 → C
5H
5N-BC
5H
5 + MeOSiMe3

Possible Lewis bases for the reaction (besides pyridine) include tertiary amines and phosphines, phosphole anions, isonitriles and cyanide anion, bis(IMes)platinum, and (under heat) certain metal carbonyls.: 102 : 5–11, 19–20 

Alternatively, piperyleneboramides and -borate esters cyclize in base (e.g., lithium diethylamide).: 106–107 

Boratabenzene ligands form through (reversible) ring-expansion-oxidation of certain 19-electron cyclopentadienide complexes, and can then be released from the metal through either ligand exchange with cyanide or alkali metal reduction.: 216–218, 225 

In rare cases, dihydroborepins rearrange and deprotonate to borabenzenes;: 222  likewise hydride reduction of a pentagonal pyramidal boracation also rearranges to a borabenzene.: 12–13 

As of 2019, there were no known syntheses of uncomplexed borabenzene. Boracyclohexadiene flash vacuum pyrolysis only proceeded under a nitrogen atmosphere to give a borabenzene-dinitrogen complex.

Neutral adducts

Adducts of borabenzene with pyridine and triphenylphosphine.

Adducts of the form BC5H6−L react with many nucleophiles, exchanging out the L moiety in a nucleophilic aromatic substitution addition-elimination.: 105  These reactions are generally catalyzed by minute quantities of Lewis acid;: 21  thus for example the pyridine adduct is tightly bound in free pyridine: no exchange is observed even at elevated temperatures.

The pyridine adduct C
5H
5N-BC
5H
5 is structurally related to biphenyl. It is a yellow whereas biphenyl is colorless, indicating distinct electronic structures.

Adducts with amines generally have a B-N π bond; adducts with phosphines do not.: 109  Nevertheless, both behave like a diene in Diels-Alder reactions:: 24–25 

Borabarrelene synthesis via a Diels-Alder reaction of borabenzene-pyridine adduct

Boratabenzenes

Boratabenzene
Names
IUPAC name
Boranuidabenzene
Identifiers
3D model (JSmol)
  • InChI=1S/C5H6B/c1-2-4-6-5-3-1/h1-6H/q-1
    Key: JFCOHHLERXSTLY-UHFFFAOYSA-N
  • [BH-]1=CC=CC=C1
Properties
C5H6B−
Molar mass 76.91 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Boratabenzene, sometimes called borinate, is the heteroaromatic anion with the formula [C5H5BH]−. Derivatives of boratabenzene often replace the adduced hydride with a (formal) organyl or alkoxide anion. They have some use as spectator ligands in olefin polymerization catalysts.: 15 

Structure of Rh(C8H12)(η6-C5H5BH).

Sandwich or half-sandwich type complexes with many transition metals have been reported; indeed, the first known instance of a borabenzene adduct was Cyclopentadienylcobalt 1-phenylboratabenzene, derived from cobaltocene and phenylboron dichloride.: 200–201 

Facially coordinated, these species are 6-electron ligands, weaker donors and stronger acceptors than cyclopentadiene.: 209–210  The metal is typically shifted away from boron in the ring, and it appears that any exocyclic π bond to B delocalizes into a metal-B antibond.: 205  Indeed, in ZrCl2(C5H5B−NiPr2)2, the Zr is approximately η5-coordinated (the -OEt analogue returns to η6).: 114–115 

The boron-coordinated hydrogen in the parent ligand ((CH)5BH−) is rather hydridic, and the B−H bond can perform hydroboration.: 28–29 

When coordinated to a metal center, boratabenzenes can exchange boron substituents through nucleophilic substitution.: 228–230  In electrophilic aromatic substitution reactions, carbons 2 and 6 (ortho to boron) react first, with carbon 4 (resp. para) reacting later.: 230 

The 1-methyl and 1-phenyl borinates can form some of the few organo-thallium(I) compounds.

See also

References

  1. 1 2 3 4 5 6 7 8 9 Herberich, Gerhard F.; Ohst, Holger (1986). "Borabenzene metal complexes". Advances in Organometallic Chemistry. 25. Academic Press. doi:10.1016/S0065-3055(08)60575-4.
  2. 1 2 3 4 5 6 Fu, Gregory C. (2001). "The chemistry of borabenzenes (1986–2000)". Advances in Organometallic Chemistry. 47. Academic Press. doi:10.1016/S0065-3055(01)47010-9.
  3. 1 2 3 4 5 6 7 8 Krummenacher, Ivo; Schuster, Julia K.; Braunschweig, Holger (2019). "Synthesis and reactivity of bora- and borata-benzenes". Patai's Chemistry of Functional Groups. Wiley. doi:10.1002/9780470682531.pat0969.
  4. 1 2 Boese, Roland; Finke, Norbert; Henkelmann, Jochem; Maier, Günther; Paetzold, Peter; Reisenauer, Hans Peter; Schmid, Günter (1985). "Synthese und Strukturuntersuchung von Pyridin-Borabenzol und Pyridin-2-Boranaphthalin". Chemische Berichte. 118 (4): 1644–1654. doi:10.1002/cber.19851180431.
  5. ↑ Wood, Thomas K.; Piers, Warren E.; Keay, Brian A.; Parvez, Masood (2006). "1-Borabarrelene Derivatives via Diels−Alder Additions to Borabenzenes". Organic Letters. 8 (13): 2875–2878. doi:10.1021/ol061201w. hdl:1880/44581. PMID 16774279.
  6. 1 2 Ashe, Arthur J., III; Al-Ahmad, Saleem; Fang, Xianggao (1999) [5 Oct 1998]. "Boratabenzenes: from chemical curiosities to promising catalysts". Journal of Organometallic Chemistry. 587. Elsevier: 93.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ↑ Yuan, Yuanyuan; Wang, Xiufang; Li, Yuxue; Fan, Liyan; Xu, Xin; Chen, Yaofeng; Li, Guangyu (2011). "Rapid Entry to Functionalized Boratabenzene Complexes through Metal-Induced Hydroboration at the Anionic 1-H-Boratabenzene Ligand". Organometallics. 30 (16): 4330–4341. doi:10.1021/om200396k.
  8. ↑ Janiak, Christoph (2006). "Metallocene and Related Catalysts for Olefin, Alkyne and Silane Dimerization and Oligomerization". Coordination Chemistry Reviews. 250 (1–2): 66–94. doi:10.1016/j.ccr.2005.02.016.
  9. ↑ Cui, Peng; Chen, Yaofeng (2016). "Boratabenzene Rare-earth Metal Complexes". Coordination Chemistry Reviews. 314: 2–13. doi:10.1016/j.ccr.2015.07.014.
  10. ↑ Janiac (1997). "(Organo) thallium (I) and (II) chemistry: syntheses, structures, properties and applications of subvalent thallium complexes with alkyl, cyclopentadienyl, arene or hydrotris (pyrazolyl) borate ligands" (PDF). Coordination Chemistry Reviews. 163: 124. doi:10.1016/s0010-8545(97)00011-8.