Boron heterocycles, reviews

Ferrocene, bis(cyclopentadienide)iron, was the first transition metal complex with aromatic ligands, and its discovery induced extensive research on complexes of different transition metals and different aromatic ligands. It is therefore not surprising that borinate ion complexes of this type are known. Some complexes with five-membered heterocycles were mentioned in Section 1.21.7. In this section borinate complexes are considered in greater detail because of their formal relationship to benzene. An extensive review on transition metal complexes with boron heterocycles has recently been published (80MI12100). 

Some boron heterocycles have already been mentioned in the section on terpenes, in particular some derivatives of the pinenes with the 9-borabicyclo[3.3.1] moiety, e.g., Alpine-borane and NB-Enantrane (Section 3.1.2.). In this section, five-membered heterocycles will be discussed. Review articles on such compounds and their applications as allylboronates15, a-haloboronic acids20, and catalysts21 are available. 

The current year has seen the appearance of comprehensive data on known carbaboranes up to the end of 1976. - Other reviews have discussed Group IV carbaboranes, carbaborane crystal structures, wfo-monocarbaboranes, car-baborane nomenclature, metallocarbaboranes, and triple decker complexes with boron heterocycles as ligands. Theoretical studies have considered molecular orbitals utilized in bonding carbaboranes and their relationship to cage counting rules. ... 

For a recent review on heterocyclic boronic acids see Tyrrel, E. Brookes, P. Synthesis,... 

The greatest theoretical interest has been devoted to the six-membered boron-nitrogen heterocycles, apparently due to the unusual stability of these compounds, and to their formal similarity to benzene. Most work in the field has been reviewed (77HC(30)38l), in which some unpublished material is also presented. 

There now exists a vast literature on inorganic heterocycles, and much of this has been reviewed in the two recent volumes edited by Haiduc and Sowerby [19], The most extensively studied of the boron-nitrogen heterocycles, and probably the system... 

Phosphorus can be partially replaced in cyclophosp-hazene rings by some heteroelements, for example, boron, silicon, arsenic, antimony, sulftir and the broad variety of such heterocycles cannot be discussed here in any detail, but their chemistry is well covered in several reviews. Examples include boracyclophosphazenes, numerous metal-containing heterocyclophosphazenes, and sulfur-containing heterocyclophosphazenes, to name only a few (Scheme 61). 

Many research groups have started with PPACK and produced inhibitors of a less peptidic nature (to improve in vivo stabihty), and/or with conformational restriction (to tackle the entropy loss problem), and/or with a variety of serine trap functionalities, for example, aldehydes, boronic acids, a-keto amides and acids, a-keto heterocycles, polyfluorinated ketones, and phosphonates. Structures of many of these are known in complex with thrombin but will not be reviewed here (see e.g., pubhca-tions of C.A. Kettner and coworkers). As well as lack of specificity, these potential drugs suffer from slow on-rates and have not progressed to the market. 

No special review of the chemistry of boron-containing heterocycles with ring junction boron atoms has been made however, compounds (l)-(4) <77T233l> and 1-boraadamantane (5) <80Mi 834-01,83PAC1439) were reviewed separately as parts of more general surveys. 

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