Borabenzene complexes

Comparable reactions between cyclic divinyl boranes and metal carbonyls lead to borinato (borabenzene) complexes... 

As described ( 6.5.3.1) l-phenylbora-2,5-cyclohexadiene (cyclic divinylborane) reacts with FeCCOj under irradiation to the Fe(CO)3 complex. The dinuclear borabenzene complex [(C5H5BPh)Fe(CO>2]2 is found as a by-product. It is to be compared with the well-known [Tj -CpFe(CO)2]2. CjHjBPh reacts spontaneously with Co2(CO)u to form three products. Above 60°C (C5H5BPh)Co(CO)2 is the only product . At 30°C yellow isomers III and IV having partially hydrogenated borabenzene ligands form above 40°C III isomerizes to IV. 

Borabenzene complexes show XH-NMR spectra of the ABB CC type. Some typical data are collected in Table III. The protons H-2/H-6 always resonate at higher field than H-3/H-5 and are easily recognized by their prominent doublet structure. The proton H-4 is extremely variable in its position which ranges from lowest to highest relative to the other borabenzene protons especially in all true ds complexes (such as, e.g., 15 but not 16) it is the most shielded proton. The coupling constants do not vary significantly. They are J23 = 9.0 and 734 = 6.0 Hz, somewhat smaller than... 

On treatment with T1C1, alkali metal boratabenzenes afford the corresponding T1 compounds (51). The lower reactivity of these can be essential for some syntheses. Some of the rare cationic borabenzene complexes 35-37 could be made using thallium boratabenzenes as reagents. Similarly, (C4Me4)Co(CO)2I yielded the mixed sandwich complexes 38 and 39 in excellent yields (71). 

Ring-member substitution, a very characteristic reaction of some 18-e borabenzene complexes (see Section VII,B), can also occur with 1,4-dibora-2,5-cyclohexadiene complexes. The cobalt complex 53 cleanly reacts with MeCOCl/ A1C13 to give the cation 54 (Scheme 7) (75). The Rh complex (C5Me5)Rh[MeB(CHCH)2BMe] reacts analogously (75). 

As a consequence of the more electron withdrawing character of the boratabenzene ligand, anionic borabenzene complexes are greatly stabilized as compared to their Cp counterparts. The bis (boratabenzene) metal complexes 32 of V (62), Cr (62), Fe (below -10°C) (62), and Co (173) show fully reversible one-electron reductions at easily accessible cathodic potentials. 

By contrast, cationic borabenzene complexes are destabilized as compared to the corresponding Cp compounds. As a consequence only six... 

In most cases oxidation of uncharged borabenzene complexes produces cations which can only be observed electrochemically. The iron compounds 62 and 63 may serve as an example. Oxidation is fully reversible in rigorously dried CH2C12 but irreversible in more basic solvents such as THF and acetonitrile (62). Preparative oxidation with Ce(IV) salts cleanly produces monosubstituted ferricenium cations 64 (Scheme 10) (66). In contrast to the above mentioned boranediyl extrusions, the substituent at boron is retained here in the newly formed cyclopentadienyl ring. 

Borabenzene complexes of cobalt such as Co(C5H5BPh)(COD) (51) and its 5-ethyl analog show the same type of catalysis but improved activity and chemoselectivity (77). Thus, 51 as the catalyst precursor gave the hitherto best results in the catalytic synthesis of the valuable 2-vinylpyridine from C2H2 and CH2=CHCN (120°C, 51 bar, 2 hours, turnover number 2164) (77,101). Furthermore, this catalyst for the first time allowed the synthesis of pyridine from C2H2 and HCN under mild conditions (110°C, 23 bar, 60 minutes, turnover number 103) (77). 

Fio. 29. The nido skeletons of the hexaethylborazine complex (EtsNaBsEta)-Cr(CO)3 and borabenzene complex (C5H5BPh)Mn(CO)3. 

Neutral borabenzene complexes with pyridine undergo interesting addition processes. For example, the reaction of 74 with dimethyl acetylenedicarboxylate leads to the [4+2] adduct 27, a new borabarrelene derivative, via a Diels-Alder reaction in 90% isolated yield (Scheme 2) <2006OL2875>. Similarly, trapping of the borabenzene ring of 74 with in r(/ -generated benzyne affords the novel benzoborabarrelene 75 in 23% yield. The X-ray crystal stmcture of... 

Paramagnetic Co(II) borabenzene complexes (C5HgBR)2Co (R = Br, Me, Ph) have been synthesized by treating (7r-C5H5)2Co with RBBt2. Hydrolysis or methanolysis of the bromo derivative led to complexes (CbH5BOH)2Co and (C5HBBOMe)2Co, respectively (220). 

Behrens, U., Meyer-Friedrichsen, T., Heck, I. Cationic borabenzene complexes as electron accepting groups for molecular material with nonhnear optical properties. Z. Anorg. Allg. Chem. 629, 1421-1430 (2003)... 

A brief review of the cotrimerization of alkynes with nitriles to give pyridines has appeared. Pyridine itself is produced from C2H2 and HCN in benzene at 110 C/60 min under 23 bar pressure. The borabenzene complex (54) as the catalyst gives 103 turnovers. The catalytic formation of substituted thiophenes from alkynes and elemental sulfur in the presence of [CPC0L2] catalysts is also mentioned. Analogous rhodium complexes [Cp RhL2] also catalyze the formation of pyridines. 

Borabenzene Complexes.- The syntheses and properties of (L)2V and (L)V(CO) (L = 1-phenyl-n borabenzene) have been described the latter complex reacts with chpt to give the tropyli im complex (L)V(ti-C7H7)... 

Cobaltocene reacts with oxygen and oxidants, tetrafluoroethylene and halides. The reaction of cobaltocene with dihalogenomethanes gives ring extension. For instance, borabenzene complexes are accessible in this way by reaction with alkyldi-halogenoboranes. 

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