Boratabenzene

Herberich s 1970 report of the synthesis of a cobalt-bound boratabenzene1 and Ashe s 1971 description of the synthesis of lithium 1-phenylboratabenzene2 marked the starting point of the study of borabenzenes, a family of molecules that includes neutral borabenzene-ligand adducts as well as anionic boratabenzenes (Scheme 1). 

Neutral borabenzene-PMe3, generated through the route described in Scheme 2, reacts with a variety of anionic nucleophiles to furnish 5-substituted borataben-zenes (Scheme 7).15 This approach provides efficient access to boratabenzenes that bear a range of boron substituents (H, C, N, O, P) with diverse electronic and steric properties.16 Mechanistic studies establish that this novel aromatic substitution process follows an addition-elimination pathway. 

Reaction of the same neutral borabenzene-ligand adduct, C5H5B-PMe3, with a transition, rather than an alkali, metal alkyl or amide can furnish r 6-boratabenzene complexes in a single step (Scheme 8).17 This efficient transformation presumably proceeds through initial ir-coordination of CsHsB-PMes to the transition metal, followedby an intramolecular substitution reaction. In contrast to other approaches to the synthesis of T 6-boratabenzene complexes, this synthetic route does not have a parallel in if-cyclopentadienyl chemistry. 

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

In contrast to these adducts in which the boratabenzene ring is bound ti to the main-group metal, reaction of [C5H5B-Me]Li with PbCl2 affords a bent-sandwich complex, Pbfi/ -CsI LBMeh.31 This report provided the first structural characterization of an r 6-bonding mode to a p-block metal. Reaction of Pb(Ti6-C5H5BMe)2 with a Lewis base such as bipyridine leads to a complex wherein the bipyridine is bound in the pseudoequatorial plane. 

The first triple-deck complexes that contain a bridging boratabenzene have been prepared through treatment of Cp Ru(C5H5B-Me) withmetallo-electrophiles (Scheme 17).32 The structure of the illustrated adduct has been established by X-ray crystallography. 

It has been shown that boratabenzenes can provide the framework for a new family of stable, 19-electron iron-sandwich complexes. For example, treatment of (C6Me6)FeCp with MeBBr2 leads to insertion to furnish the corresponding r 6-boratabenzene adduct (Scheme 18).33 The structures, EPR spectra, and reactivity of these boratabenzene complexes are very similar to their well-studied (arene)Fe(cyclopentadienyl) precursors. The unpaired electron resides in an antibonding orbital that is largely metal based. 

An interesting example of t 3 coordination of a boratabenzene to a transition metal has been observed for a Zr(IV)-boratanaphthalene complex (Scheme 20).35 Thus, treatment of the illustrated 1-boratanaphthalene with Cp+ZrCl3 furnishes an adduct in which the metal-carbon distance for the 3, 4, and 5 positions of the heterocycle (2.53-2.56 A) is significantly shorter than for the 2 and 6 positions (2.77-2.81 A). It is suggested that this distorted bonding mode is the consequence of the high electron demand of Zr(IV), which prefers coordination to the most electron-rich carbon atoms. ... 

A very different mode of r 3 complexation has been observed for the Sc(III)-(bis)boratabenzene complex illustratedbelow.36 In this instance, a crystallographic study reveals that r 3 coordination to one of the boratabenzene rings occurs through the exo nitrogen, the boron, and the 2-carbon. In solution, the complex is fluxional. 

Among the most exciting frontiers in boratabenzene chemistry is the development of transition metal-boratabenzene complexes as catalysts. As early as 1984, it had been demonstrated that these adducts can accelerate useful reactions— specifically, Bonnemann established that (C5H5B-Ph)Co(cod) serves as a catalyst for pyridine-forming cyclotrimerization reactions of alkynes and nitriles.39... 

A dozen years later, another breakthrough was reported the use of Zr(IV)-boratabenzene complexes as catalysts for olefin polymerization (Scheme 24).40... 

Prior to this 1996 study, there had been no reports of boratabenzene complexes of early transition metals.42 An X-ray crystal structure of the catalyst revealed a C2-symmetric geometry that resembles Cp2Zr-based bent metallocenes. The bond lengths suggest a strong B-N it interaction (rotational barrier measured by NMR 18 kcal/mol) and a very weak Zr B interaction ( t 5 coordination of the boratabenzene ring). 

The OEt-substituted Zr(IV)-boratabenzene complex has been employed in an interesting dual-catalyst approach to the synthesis of branched polyethylene.47 Capitalizing on the ability of this boratabenzene complex to generate 1-alkenes (Scheme 25) and the ability of the titanium complex illustrated in Scheme 27 to copolymerize ethylene and 1-alkenes, with a two-catalyst system one can produce branched polyethlene using ethylene as the only monomer (Scheme 27). The structure and properties of the branched polyethylene can be altered by adjusting the reaction conditions. 

A series of novel, bridged Zr(IV)-boratabenzene complexes have been synthesized and structurally characterized (Scheme 28).48 The geometries of these adducts closely resemble those of classical ansa-metallocenes. In the presence of excess MAO, these boratabenzene complexes catalyze the polymerization of olefins. 

Boratabenzene analogues of commercially significant constrained geometry catalysts have also been investigated.49 For the MAO-activated copolymerization of ethylene/l-octene, the illustrated Ti(IV)-boratabenzene complex is about four times more active than the Zr(IV) complex. The level of 1-octene incorporation is significantly lower than for the corresponding Cp-derived catalysts, due perhaps to the greater steric demand of the amidoboratabenzene framework. 

A crystallographically characterized Ti(III)-boratabenzene complex has been found to be ineffective at catalyzing ethylene polymerization.50... 

Boratabenzene complexes of Group 3 and Group 6 metals serve as effective catalysts for the oligomerization/polymerization of ethylene. For example, [(C5H5B-Ph)2ScPh]2, pretreated with H2, oligomerizes ethylene to furnish 1-alkenes.17a In the case of a Cr(III)-boratabenzene complex, ethylene is polymerized to afford... 

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