Borylenes addition

Cobaltocene is partially oxidized and in part undergoes insertion of a borylene group, RB. The borinato ligands derive from the unknown borabenzene ( 6.5.3.4). Some porphyrinatoindium and thallium complexes ( 6.5.2.2) can also be synthesized via oxidative addition reactions (TPP)InCl is added oxidatively to Co2(CO)g and Mn2(CO) to give (TPP)In—Co(CO)4 and (TPP)ln — Mn(CO)j, respectively, and (oep)InCl is added to CojfCOg to yield (oep)ln— 0(00)4. 

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... 

The addition of borylenes to cyclohexene was reported to give reactive boranes (46) (76TL4765, 77JA3182), but reinvestigation of this reaction proved the formation of boratricyclic derivative (47 R = Me) (83POL1337). 

The reaction was complete after three weeks, and no additional signals of further byproducts were detected by multinuclear NMR monitoring. Compound 42 can be viewed as the first example of a hetero-metdllic x3-borylene complex. According to the results of a crystal structure determination 42 con-... 

Scheme 9 Synthesis of mesityl(halo)boryl complexes via halide addition to a cationic terminal borylene complex...

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. 

Metal borylene complexes are another recent addition to the library of metal boron compounds, and these are now finding applications in organic synthesis, initially via their role as stoichiometric borylene transfer agents in the synthesis of borirenes from alkynes [13]. It is expected that additional applications of newly discovered metal borylene complexes will emerge as new routes to them are developed. 

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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