Boron bridges

The boron-bridged tetrathiaporphyrinogen 28 can be prepared in good yields (62%) from the dilithiated di-2-thienylborane amine 27 (Fig. 9). 

In 1978, Sugasawa et al., at Shionogi Pharmaceutical Co. reported ortho-selective Friedel-Craft acylation with free anilines with nitrile derivatives [4]. Sugasawa reported that the reaction requires two different Lewis acids (BC13 and A1C13) and does not proceed when N,N-dialkyl anilines are used. He proposed that boron bridging between nitriles and anilines led to exclusive ortho-acylation but a conclusive mechanism was not elucidated. The report did not offer any reason why two different Lewis acids were required and why the reaction did not progress with N,N-dialkyl anilines. Therefore, we initiated mechanistic studies. 

During the time of the Olin reports, the first examples of oligomeric boron-bridged (l-pyrazolyl)borate systems appeared from the laboratory of Trofimenko at DuPont Chemicals 24 He reported the synthesis of poly(l-pyrazolyl)borates (6) (Fig. 5) from the reactions of alkali metal borohydrides with the pyrazole ligand. The (l-pyrazolyl)borate ligand was obtained from two pyrazole units when bridged by a BR2 unit on one side and by a metal or onium ion on the other. Even though reports... 

Figure 25 The ring-opening polymerization of strained boron-bridged [ 1J ferrocenophanes (35a-c) to produce poly(ferrocenylboranes) (36a-c). (Adapted from ref. 57.)...

The electrochemical properties of the imidazo[4,5-,7 [l,3,2]diazaborolidines 56, especially the AgEM constants (10 ), are close to the boron-bridged violenes indeed, the values obtained for 56 are on a par with those of the acceptor part in organic metals derived from TCNQ or -dicyanochinodiimine (DCNQI) (AgEM 10 -10 ) <2004CC1860>. 

Replacement of the hydrogen bonds in oxime complexes with boron bridges leads to macrocyclic complexes such as those from dimethyl glyoxime (equation 59).201,202 This kinetic template technique has been used for the encapsulation of metals inside cage ligands.203 204... 

More recently, imidazole- and pyrazole-type ligands with a phosphorus atom as the bridge have been reported,97,98 as depicted in formulae (46)-(48). Also rings with imidazolines (49)," or bridged by a —C—OH group (SO),100 have been reported. Boron-bridged polypyrazolylborates and related systems are dealt with in Chapter 13.6. 

A close scrutiny of the seminal report on the boranes reveals that the equivalence traditionally presented between the solid geometrical and the topological pictures of these compounds is incomplete. Because of this mathematical disparity, an alternate geometrical description of the bonding in the boron hydrides is introduced, as well as a new interpretation for the boron bridge. 

Unsymmetrically en -substituted 1,8-diborylnaphthalenes have also been prepared and investigated as bidentate Lewis acids. The synthesis of such derivatives is non-trivial because it necessitates the sequential introduction of boryl moieties at the / en-positions of the naphthalene backbone. Thus far, this strategy has only been applied successfully on few occasions. The reaction of 1,8-dilithionaphthalene-tmeda with one equivalent of dimesitylboronfluoride results in the formation of dimesityl-1,8-naphthalenediylborate 26 as a monoborylated naphthalene product (Scheme ll).32 This derivative is the only example of an anionic 1,8-boron-bridged naphthalene derivative. However, it is important to note that Siebert has reported the synthesis and structure of a neutral 1,8-boron-bridged naphthalene derivative which features a (di-Ao-propylamino)boron moiety bridging the two naphthalene peri-carbon atoms.33 A single-crystal analysis carried out on 26-Li(py)4 confirmed... 

Treatment of anionic four-membered rings with electrophiles leads to boron-bridged diboriranes (Equation 26). The specific structure varies with the nature of the electrophile, but three-membered rings are always formed <2003AGE669, 2003AGE671>. 

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