1,2-Azaborolyls

Dihydro-lH-l,2-azaboroles derive from cyclopentadiene by the isoelectronic replacement of a C=C group by a BN moiety ". The neutral rings react in xs boiling Fe(CO)j without any other solvent to form red-brown iron-dihydro-1,2-azaborolyldi-carbonyl dimers as their cis and transisomers in 56% yield, or with CojfCOg in petroleum ether at 60-80°C to the half-sandwich complex (dihydro-1,2-azaborolyl)Co(CO)2 (40%). The CO group can easily be substituted by olefins ... 

Azaborolyl sandwich complexes can be prepared by metal-vapor synthesis ( 6.5.3.1) ... 

Enantioselective nucleophilic addition to imines has been carried out with a planar-chiral Lewis acid based on a 1,2-azaborolyl framework. 64 ... 

Azaborolyl Zirconium(iv) Complexes as Alkene Polymerization Catalysts... 

A hybrid DFT method has been used to calculate the basicities of 1,2-azaborolyl 9 and 1,2-thiaborolyl 22, which were found to be more basic than cyclopentadienyl. The catalytic performance of early transition metal polymerization catalysts with these heterocyclic ligands has been evaluated by this MO approach <2003MI417-02>. 

In the past decade, X-ray crystallography has become a more routine method of characterization. Consequently, there are many structures of boron-containing heterocycles. The heterocycles relevant to this chapter are separated into structures which do not contain transition metals and the transition metal complexes of the Cp-like 1,2-azaborolyl, 1,2-thiaborolyl, and 1,2-oxaborolyl rings. 

Figure 4 The formulas of new 1,2-azaborolyl metal complexes which have been structurally characterized by X-ray crystallographic studies.

The extensive pioneering work of Schmid and co-workers on transition metal complexes of 1,2-azaborolyl ligands was thoroughly covered in GHEC-II(1996) <1996GHEG-II(3)755>. More recent work has reported additional complexes... 

Fu and coworkers have extended their work on B-functionalized 1,2-azaborolyls which was outlined in Section 4.17.8.1. In particular they have prepared enantiopure 1,2-azaborolyl iron complexes (—)-151 <2006T11343> and (+)-218 <2005JA15352> which have been used on asymmetric syntheses. 

Azaborolyl complex (- -)-218 has been used in a stereoselective Mukaiyama aldol reaction as illustrated in Scheme 32 <2005JA15352>. Complex (- -)-218 reacts with electron rich aromatic aldehydes and silyl ketene acetals to generate adduct 220. X-ray structures indicate the stereochemistry is as illustrated. This stereochemistry is... 

The reaction of (—)-151 with imines such as 2-methyl-l-pyrrolidine illustrated in Scheme 33 affords complex 222. An X-ray structure of 222 shows that the coordinated imine is oriented perpendicular to the 1,2-azaborolyl ring. This orientation which contrasts with that assumed for 219 must be due to the greater steric bulk of the imine. Reaction of 222 with allylmagnesium bromide gives 223 with excellent stereoselectivity. Hydrolysis affords the free amine 224. The reactions illustrated in Schemes 32 and 33 demonstrate that 1,2-azaborolyl iron complexes can efficiently transfer chirality to B-bound organic substrates. The development of catalytic versions of these stoichiometric reactions would be a highly desirable extension of this work. 

Theoretical attempts to describe structure and bonding in five-membered rings containing one boron and one additional heteroatom are known in only one case. MNDO Calculations have been performed for the l/f-2,5-dihydro-l,2-azaborole and the anionic 1,2-azaborolyl ring system... 

Table 1 Experimental and MNDO-calculated structural parameters of l//-2,5-dihydro-1,2-azaborole and the 1,2-azaborolyl anion.

One of the numerous 1/f-1,2-azaborolyl anions was analyzed structurally as its lithium salt (Figure 5) <88CB1873>. The C—C bond lengths are more or less completely averaged and the BN unit is also involved in the delocalization of n electrons. 

This heteroaromatic character of 1,2-azaborolyl rings qualifies them to form numerous transition metal complexes. In contrast to the cyclopentadienyl ligand there is still a remarkable disturbance caused by the boron and nitrogen atoms. This becomes visible if those transition metals are used for complexation that cannot reach the 18-electron configuration in normal sandwich complexes. As a consequence, electron-rich metals tend to increase the distance to the electron-rich nitrogen atoms, whereas electron-poor metals tend to increase the distance to the boron atom. Three examples illustrate these structural features of 1,2-azaborolyl rings. 

An example showing the usefulness of a H NMR spectrum is given by l-/-butyl-2,3-dihydro-2-methyl-1 H-1,2-azaborole and the corresponding 1,2-azaborolyl derivative (1 //-1,2-azaboratol) (26). 

The transition of a 17/-2,3-dihydro-l,2-azaborole (30) to the deprotonated l//-l,2-azaboratole (1,2-azaborolyl) (31) again shows the sensitivity of this tool <88CB1873>. 

The delocalization of the n electrons in the anionic 1,2-azaborolyl system increases the electron density at the boron atom, manifested in a high-field shift. Oxygen is a less effective n donor for a Lewis acidic boron atom compared with nitrogen. 

Among the five-membered rings with boron and a second heteroatom, aromaticity is only relevant for anionic systems which are isoelectronic with C5H5. This is the case for only one type of compound, the 1,2-azaborolyl ring (1,2-azaboratole) (42). This anion derives from the neutral l//-l,2-azaborole by abstraction of a proton, as when cyclopentadiene is transformed into the cyclopentadienide. 

Type (i) If 1,2-azaborolyl salts react with various element halides, substitution in the 3-position usually occurs. 

Combination with a second C3BN ring leads to diastereoisomers which can be separated (46a,b). In the diastereoisomer (46a) the metal M uses the same ring sites, whereas in (46b) different sites are coordinated. Many examples have been described by Schmid where both isomers are formed and could be separated and characterized by single x-ray structure analysis <85MI 317-01 >. This chiral property of 1,2-azaborolyl rings in combination with metal atoms has some interesting consequences with respect to stereoselective reactions. 

Both systems are anionic and so only exist in combination with various cations. In the simplest case this can be Li, or, as frequently described for the 1,2-azaborolyl rings, other main-group or transition metals are used to synthesize half-sandwich and sandwich complexes. Some of the 1,2-azaborolyl compounds are selected for discussion here. 

The reactivity of the 1,2-azaborolyl rings is mainly influenced by the heteroatoms boron and nitrogen. Thus, if 1,2-azaborolyllithium salts are reacted with element chlorides, the substitution... 

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