Ferrocenes borylation

Elimination of hydrogen halides from dihalogenoorganylboranes by reaction with ferrocene-1,1 -dithiol resulted in l,3-dithia-2-boryl[3]ferrocenophanes.170 These borylferrocenophanes were air sensitive, but were characterized by NMR, MS, and elemental analysis.170 A series of l,3-dibora-2-X-[3]fer-rocenophanes (X=S, Se, Te) were prepared and characterized and the crystal structure of [Se N Pr2)BC5Fl4 2Fe] has been reported.171... 

Pinacolborane is extensively used in the borylation of aryl halides 114 in the presence of a base (typically pyridine or Et3N or KOAc) and catalytic amount of PdCl2(DPPF) to furnish aryl boronates 115 (DPPF = l,l -bis(diphenyl-phosphino)ferrocene Equation 7) <1997JOC6458, 2000JOC164>. Pinacolborane is compatible with esters, ketones,... 

Highly electron-rich aromatic compounds are known to undergo direct electrophilic borylation reactions with the strong electrophiles BI3, BBrs, BCI3, or HB(C6p5)2 under thermal conditions. As an example, the dibromoborylation of ferrocene in refluxing hexanes is shown in equation (27). Facile intramolecular borylation reactions have also been... 

Metalated ferrocenes also serve as convenient precursors to ferrocenylboranes. Lithiated ferrocenes have been ntilized for the preparation of ferrocenylboronates FcB(OR)2 and l,T-fc(B(OR)2)2 and are especially suitable in the presence of ort/ o-directing donor-substituents. The borylation of disilylated ferrocenes with excess BCI3 on the other hand was reported to yield varying amounts of 1,3-diborylated prodnct (82 M = Fe, X = Cl) in addition to the l,F-diborylated species (83 M = Fe, X = Cl). In contrast, when 1,F-distannylated ferrocenes were treated with equimolar amounts of boron halides, l-stannyl-2-borylferrocenes (86) were formed as the major product rather than the expected... 

Borylated ferrocenes have been exploited by Wagner as building blocks for the assembly of macrocycles and coordination polymers (Scheme 15). Treatment of the diborylated ferrocene l,L-fc(BMe2)2 with the bifunctional aromatic amines pyrazine and 4,4 -bipyridine, respectively, led to spontaneous and reversible formation of (89) and related ferrocenylborane polymers.The polymeric nature of... 

If the heterocycle contains a strongly basic site and a Lewis-acidic boryl group is attached, dimerization or oligomerization through donor-acceptor bonding occurs (see also the formation of ferrocene macrocycles described in Section 5.2). Thus, 2-diethylborylpyridine exists as a dimer (104), whereas 3-diethylpyridine adopts a tetrameric stracture An isomeric species (106), which... 

However, in contrast to benzene, ferrocene is sensitive to oxidation, and the ferrocenium cation, FeCpj, a paramagnetic 17-electron species, is readily formed in the presence of various oxidants. The ferrocenium cation is reluctant to undergo electrophilic substitution, and therefore reactions such as halogenation and nitration, which are important routes to substituted benzene derivatives, cannot be used for the synthesis of substituted ferrocenes. Only electrophilic substitution under nonoxidizing conditions (e.g., Friedel-Crafts acylation, Mannich reaction, borylation, lithiation or mercuration), and radical substitution are available as an entry into the chemistry of substituted ferrocenes. 

Three routes have been preferentially used to prepare ferrocene compounds that contain heteroelements directly attached to the metallocene unit borylation, mercuration, and lithiation. 

The dihaloboryl-substituted ferrocenes can be used to prepare various other boryl derivatives thus, FC-BX2 compounds (X = Cl, Br or I) have been converted into FC-BR2 (R = NMe2 [4, 7], NEt2, OEt, SMe, Me [4, 5]). As an example, the synthesis... 

It has been shown that borylation of ferrocene with BBrj or BIj in hexane solution proceeds in a stepwise manner [9]. Excess BXj leads to mixtures containing essentially l,3,r-tris- and l,3,r,3 -tetrakis(dihalogenoboryl) ferrocene the intermediates are FC-BX2 and fc(BX2)2, although small amounts of l,3-bis(diahalogenoboryl) ferrocene have also been detected in the predominant 1,1 isomer, fc(BX2)2 (X = Br, I). 

Scheme 5-3. Useful intermediates for the synthesis of ferrocene derivatives containing heteroatoms (a) Lithiation (generally with nBuLi) (b) mercuration with Hg(OAc)2, followed by reaction with LiCl or KCl (c) borylation with B(OnBu)3, followed by hydrolysis (d) CUCI2 (e) CuBr2 (f) compounds containing reactive bromo substituents, e.g., BrCX2 -CX2Br (X = F, Cl) or tosyl bromide (g) A-bromo succinimide (h) I2 (i) Br2.

Ferrocene, cymantrene, h -C5H5Mn(CO)j, or methylcymantrene are borylated by BBrj and BI3, MeBIj or PhBl2 in boiling carbon disulfide or cyclohexane to give the metallocenylhaloboranes ... 

Aldridge et al. have demonstrated that a similar boryl-ferrocene 81 can be used as a selective colorimetric sensor for F . When F was added to a GH2CI2 solution of 81, under aerobic conditions, a color change from orange to pale green was observed. This did not occur with any other anion tested. Spectroscopic and electrochemical measurements suggest that complexation of F causes the spontaneous formation of a ferrocenium species, that is, the 150 mV anodic shift in the Fc/Fc redox potential caused by F complexation lowers the redox potential enough for the 81 2F complex to be aerobically oxidized. 

Ferrocene reacts with BX3 (X = Br or I) to give l,r-bis(dihalogenoboryl)-ferrocenes. CpMn(CO)3, on the other hand, readily gives the bis-borylated derivative directly/ ... 

The resonance between 55 and 56 (R = H, alkyl, aryl, ferrocenyl) in ferrocenyl-methyl cations including the diferrocenylmethyl cation has been used by Cais to explain the similar bending of the exocyclic C,C bonds toward the iron atom (Scheme 10.20) [65].Thefieldhas recently been reviewedbyGleiteref a/. [62],who included related isoelectronic boryl substituted systems. Wagner et al. [66] have prepared compounds of this type, and the authors proposed attractive interactions between the iron and boron atoms, because the C-B bonds are bent toward the iron atom. The analogy does not only apply to ferrocenes but also to isoelectronic (cyclobutadiene)(cyclopentadienyl)cobalt complexes and other related compounds [62]. 

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