The Boryl Anion

Unlike carbanions and silyl anions, boryl anions (BR2 ), in the form of boryllithiums, have been unknown until recently. In 2006, Japanese researchers reported the synthesis of a boryllithium via the reduction of a cyclic (diamino)bromoborane with lithium/naphthalene in THF, that is, Li+Np , where Np is the anion radical of naphthalene (Segawa, Y. Yamashita, M. Nozaki, K. Science 2006, 314, 113-115)  

Note the close structural similarity of the boryllithium with an NHC. This is not a coincidence the two species are in fact isoelectronic. 

In spite of its uniqueness, the above boryllithium was found to behave more or less like a regular organolithium. Thus, it undergoes transmetallation with MgBr2-OEt2 to yield a boryl-Grignard reagent. Similarly, with electrophiles, it reacts like a conventional carban- 

Subsequently, borylcopper reagents were also synthesized, as shown below. As softer nucleophiles than boryllithiums, borylcopper reagents exhibit significantly different reactivity, undergoing, for instance, conjugate additions with a, -unsaturated carbonyl compounds  

Suggest a rationale for the apparent inaccessibility and instability of boryl anions (BR2 ). Why, for instance, can you not obtain a boryl anion by deprotonating a dialkylborane with n-butyllithium  

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Very recently, diamino-substituted boryl, -B(R NCH=CHNR ) (R = 2,6- Pr2CgH3), complexes of Li", Mg ", and Zn have been synthesized and structurally characterized [78-82]. As expected, the boryl anion displays its nucleophilicity and undergoes... 

The compounds described herein were prepared by three methods. The first route involves deprotonation/substitution at the N-H sites of 1, the second consists of a cleavage reaction of an Si-N derivative of 1 with PhBCI2, and the third route is a transamination reaction between a bis(dimethylamino)boryl derivative of 1 and an aliphatic diamine. In the first approach, compound 1 is deprotonated by treatment with one equivalent of n-BuLi. Quenching of the resulting anion with various electrophiles produces the monosubstituted products 2-4 (eq 3). 

A number of tethered osmium boryl complexes have been developed recently in which one of the boryl substituents features an additional pendant donor which coordinates to the metal centre as a tether . For systems resulting from reactions with bifunctional donors such as 2-aminopyridine two possibilities exist, depending on which donor atom interacts with the boron centre and which with the metal. Thus, tethered boryl complexes result from coordination of the anionic donor at boron, with the neutral donor tether coordinated at osmium the reverse coordination possibility leads to the formation of intramolecular base-stabilized borylene complexes. 

Free ftom a transition metal, tiie boryl group is highly basic. The first fuUy diaracter-ized alkali metal boryl anion was reported in 2006 by Yamashita, Nozaki and co-workers. Previous studies on the generation of boryl anions have not been reproduced. Theoretical studies imply that much of the charge on a free boryl anion would actually reside on the substituents, and that the boron would bear positive charge. ... 

As a side note, it is worth mentioning that NHC-stabilized r-boryl anions (30 and 31) were recently reported [84-86]. The potassium salt of 30 has been synthesized and structurally characterized [84]. Computational and reactivity studies indicate that there is a n-nucleophilic boron atom and 30 cleanly reacts with the electrophile Mel [84]. 31 was suggested as a reactive intermediate, which easily reacts with a diverse range of electrophiles to give different types of NHC boranes [86]. 

He X, Hartwig JF. Boryls bound to iron carbonyl. Stmcture of a rare bis(boryl) complex, synthesis of the first anionic boryl, and reaction chemistry that includes the synthetic equivalent of horyl anion transfer. Organometallics. 1996 15 400-407. 

The search for boron-centered nucleophiles has been a fundamental challenge owing to the six-valence electron, hypovalent nature of boron which renders boron-based molecules electrophilic. A major breakthrough in this area came in 2006 when Nozaki and Yamashita isolated and structurally characterized a boryllithium reagent that was isoelectronic to an NHC and reacted as a boron-centered nucleophile. Since, there have been additional reports of boron-centered nucleophiles where the nucleophilic boron atom was a ligand to a transition metal [i.e. borylene). In the past five years, however, carbenes have emerged as excellent ligands to stabilize sp nucleophilic boryl anions. 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

NH-Phosphinous amides can be metallated at nitrogen, via their corresponding anions [78,112,113] as well as borylated [114] and silylated [22,115, 116]. In this last case, the isomeric P-silylphosphazenes are also occasionally obtained. 

Ort/io-phenylene diboranes constitute another important class of polydentate Lewis acids which have been considered for the complexation of anions.In this context, most efforts have centered on the study of the ligative behavior of 1,2-bis(bis(pentaliuorophenyl)boryl)tetraliuorobenzene (17). Similar to 22, compound 17 forms chelate fluoride ([17- r2-F] ) and hydroxide ([17- r2-OH] ) complexes when treated with KF/18-C-6 and KOH/18-C-6, respectively (18-C-6 = 18-crown-6) (Scheme 35). The crystal structure of these anionic complexes has not been... 

The A -unsubstituted 5-dimethylamino-l,2,4,3-triazaphosphole is converted to its anion by sodium, potassium, or their hydrides (80ZN(B)1222>. Boryl (86PS(28)7I>, silyl (80ZN(B)1222>, stannyl, phosphino, and (thio)phosphinyl <80ZN(B)1222> derivatives have been prepared mostly by reaction of the triazaphosphole with the corresponding chloride the substituents become attached to N-2. 

Addition of the anion of the a-bromoester to the alkylborane initiates the reaction. Transfer of one alkyl group then takes place and the resulting a-boryl ester tautomerizes to an alkenyloxyborane. Finally hydrolysis releases the alkylated ester (Figure B3.6). 

Unsymmetrically en -substituted 1,8-diborylnaphthalenes have also been prepared and investigated as bidentate Lewis acids. The synthesis of such derivatives is non-trivial because it necessitates the sequential introduction of boryl moieties at the / en-positions of the naphthalene backbone. Thus far, this strategy has only been applied successfully on few occasions. The reaction of 1,8-dilithionaphthalene-tmeda with one equivalent of dimesitylboronfluoride results in the formation of dimesityl-1,8-naphthalenediylborate 26 as a monoborylated naphthalene product (Scheme ll).32 This derivative is the only example of an anionic 1,8-boron-bridged naphthalene derivative. However, it is important to note that Siebert has reported the synthesis and structure of a neutral 1,8-boron-bridged naphthalene derivative which features a (di-Ao-propylamino)boron moiety bridging the two naphthalene peri-carbon atoms.33 A single-crystal analysis carried out on 26-Li(py)4 confirmed... 

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