Borylene base-stabilized

Fig. 7. Formation and reactivity of base-stabilized terminal borylene complexes of osmium.

Roper, however, succeeded in converting the osmium boryl complex [Cl2BOsCl(CO)(PPh3)2] (E) into the terminal borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2]147 (26) upon reaction with 8-aminoquino-line according to Fig. 7. Compound 26 is noteworthy as it represents the first example of a base stabilized terminal borylene complex of the type [LXM = B(L)-R] (IVa, Fig. 1). 

As already indicated, the chemistry of terminal borylene complexes is as yet almost unexplored. In addition to the photochemically induced borylene transfer, which was already discussed in Chapter 3.2, studies of the reactivity of terminal borylene complexes are restricted to two recent reports by Roper.147,148 The base-stabilized borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2] (26) undergoes a reaction with ethanol to yield the ethoxy(amino)boryl complex [Os B(OEt)NHCgH6N Cl(CO) (PPh3)2] (35) according to Eq. (13) with a 1,2-shift of the quinoline nitrogen atom from the boron to the osmium center. The alcoholysis of 26 indicates that even the boron atom in base-stabilized borylene complexes displays some electrophilic character—a fact already predicted by a theoretical study.117... 

A detailed comparison of the spectroscopic and structural parameters of the base stabilized borylene complexes 30, 36, 37, and 38 with those of tethered boryl complexes such as F established their close structural relationship and led the authors to the statement that a formal distinction between those species is somewhat artificial. Their reactivity, however,... 

The reaction of 15 with 2-aminopyridine yields a mixture of the amino-(chloro)boryl complex 19a and the base-stabilized borylene complex 20a [40]. 20a reacts readily with ethanol, leading to the corresponding ethoxyborylene complex 20b, which was crystallographically characterized. 19a is stable in boiling ethanol but reacts, in the presence of Ag[SbF6] in undried thf, to the hydroxyboryl complex 19b (Scheme 8). The key structural parameters of 19b and 20b are very similar, which leads to the very important conclusion that there is only little difference in the nature of the Os-B bond in these types of base-stabilized borylene and boryl complexes. 

Scheme 8 Tethered osmium boryl and base-stabilized borylene complexes...

Scheme 9 Synthesis of a cationic base-stabilized borylene complex...

Scheme 11 Synthesis of the base-stabilized bridging borylene complex 27...

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. 

Scheme 23 Interconversion of square planar boryl complex 10.13, T-shaped boryl 10.14 and base-stabilized borylene system 10.15...

In 2011, the previously unknown borylene fragment, H—B , was isolated via stabilization by two CAAC molecules [295]. CAAC A was reacted with BBr3 to give the corresponding Lewis acid-base adduct in high yield. Then, the CAAC-BBr3 compound was reduced using 5 equivalents of KCs while in the presence of... 

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. 

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