Borabenzene adducts
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| Names | |
|---|---|
| Preferred IUPAC name
Borinine | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider | |
PubChem CID |
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CompTox Dashboard (EPA) |
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| 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).
Infobox references | |
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.:[1]: 217, 221 [2]: 102–104
- C
5H
5N + MeOBC
5H
5SiMe
3 → C
5H
5N-BC
5H
5 + MeOSiMe3
Possible Lewis bases (besides pyridine) include tertiary amines and phosphines and isonitriles.[2]: 102
Alternatively, piperyleneboramides and -borate esters cyclize in base (e.g., lithium diethylamide).[2]: 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.[1]: 216–218, 225
In rare cases, dihydroborepins rearrange and deprotonate to borabenzenes.[1]: 222
Pyridine adduct
The pyridine adduct C
5H
5N-BC
5H
5 is structurally related to biphenyl.[3] It is a yellow whereas biphenyl is colorless, indicating distinct electronic structures. The pyridine ligand is tightly bound: no exchange is observed with free pyridine, even at elevated temperatures.
The borabenzene-pyridine adduct behaves like a diene, not an analog of biphenyl, and will undergo Diels-Alder reactions:[4]
Trimethylphosphine adduct
BC5H6−PMe3 reacts with many nucleophiles, exchanging out the PMe3 moiety in a nucleophilic aromatic substitution addition-elimination.[2]: 105
Boratabenzenes
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| Names | |
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| IUPAC name
Boranuidabenzene | |
| Identifiers | |
3D model (JSmol) |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| 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).
Infobox references | |
(C5H5BH)_(OZUFAR).png)
Boratabenzene, sometimes called borinate, is the heteroaromatic anion with the formula [C5H5BH]−. Very few salts of the parent boratabenzene anion are known,[1]: 229 the chief one being lithium boratabenzene.[2]: 108 Instead, derivatives of boratabenzene often replace the adduced hydride with a (formal) organyl anion. Methoxyboratabenzene is also common.
Sandwich or half-sandwich type complexes with many transition metals have been reported;[6][7] indeed, the first known instance of a borabenzene adduct was Cyclopentadienylcobalt 1-phenylboratabenzene, derived from cobaltocene and phenylboron dichloride.[1]: 200–201
Facially coordinated, these species are 6-electron ligands, weaker donors and stronger acceptors than cyclopentadiene.[1]: 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.[1]: 205
When coordinated to a metal center, boratabenzenes can exchange boron substituents through nucleophilic substitution.[1]: 228–230 In electrophilic aromatic substitution reactions, carbons 2 and 6 (ortho to boron) react first, with carbon 4 (resp. para) reacting later.[1]: 230
The 1-methyl and 1-phenyl borinates can form some of the few organo-thallium(I) compounds.[8]
See also
- 6-membered aromatic rings with one carbon replaced by another element: silabenzene, germabenzene, stannabenzene, pyridine, phosphorine, arsabenzene, stibabenzene, bismabenzene, pyrylium, thiopyrylium, selenopyrylium, telluropyrylium
- Borazine
- Hexachloroborazine
References
- 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.
- 1 2 3 4 5 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ 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.
- ↑ Cui, Peng; Chen, Yaofeng (2016). "Boratabenzene Rare-earth Metal Complexes". Coordination Chemistry Reviews. 314: 2–13. doi:10.1016/j.ccr.2015.07.014.
- ↑ 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". Coordination Chemistry Reviews. 163: 124. doi:10.1016/s0010-8545(97)00011-8.