1.1- boriozirconocene

During the last ten years, the chemistry of 1,1-boriozirconocene complexes has been studied. Both hydrozirconation and hydroboration reactions are well established, and are widely applicable to a wide variety of vinyl and acetylene derivatives [1], Alkenylboranes and alkenylzirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

However, these compounds proved to be unstable and difficult to characterize. The authors reasoned that the source of the instability was likely to be the trialkylboron moiety. Boronates are more stable than trialkylboranes since the lone-pairs of electrons on an oxygen atom can donate to the empty orbital of a boron atom. The corresponding gem-boriozirconocenes should also be more stable. Thus, hydrozirconation of the alkenyl-... 

The use of N-bromosuccinimide resulted in very complex mixtures and not the expected a-haloboranes. Apparently, the succinimide moiety may have acted as a base, causing various side reactions [81—85]. However, a-haloboronic esters are much more stable than a-halotrialkylboranes. Halogenation of boriozirconocene bimetallics 18 based on boronic esters proceeded very smoothly, affording the expected a-bromoboronic esters 35 (Scheme 7.11) [52],... 

As representatives of this class of compounds, one hindered and one non-hindered gem-boriozirconocene alkene were prepared [54]. Hydrozirconation of a 1-alkynyldioxaborolane with Cp2Zr(H)Cl in either 1,4-dioxane or THF proceeded readily by a syn addition to give the essentially pure ( )-l,l-bis-metallic boriozirconocene 45 [12]. This bis-metallic complex 45 was also reacted with various electrophiles, thereby generating the alkenylboron derivatives 53—57 as shown in Scheme 7.17. 

Stereospecific syntheses of temarotene (a retinoid) [130] and of chokols A and G (fun-gitoxic sesquiterpenes) [131—133] have been developed using 1,1-boriozirconocene complexes 45. 

The stereoselective synthesis of rac-chokols A (61) and G (62) from the same precursor gem-boriozirconocene 59 involves a conjugate addition to 2-methylcyclopentenone to give the common intermediate 60. The latter is then transformed through a series of reactions to provide the chokols in overall yields of 16% and 17%, respectively (Scheme 7.19) [131],... 

The formation of cyclic boriozirconocenes 63 has been observed by treating l-alkynyl-6-alkenyl diboronates 64 with Negishi s reagent (Scheme 7.20) [133]. Hydrolysis of 63 with HC1 in Et20 gave the 1-alkenyl-l-alkyl diboronate 65 in 80% yield. 

Bimetallic Boriozirconocene Complexes with Planar Tetracoordinate Carbon... 

Preparation of Boriozirconocene Complex 16 (Scheme 7.5) [33] To a stirred ice-cooled suspension of the Schwartz reagent (0.26 g, 1 mmol) in dry CH2C12 (1 mL) was added a solution of B-hexenyl-9-BBN 15 (0.20 g, 1 mmol) in dry CH2C12 (1 mL). The resulting cloudy mixture became a dear yellow solution in 1 h at 0 °C (or 10 min at ambient temperature), indicating the formation of 16. 

Gem boriozirconocene alkanes can be prepared by either hydrozirconation of alkenylboranes or hydroboration of... 

Other classical reactions such as halogenation or amination with zirconium and boron compounds respectively are possible starting from 1,1-boriozirconocene complexes. 

In boriozirconocene aUcenes, the chemistry of Csp -Zr and Csp -B bonds differ considerably and this feature allows a sequential route to substituted aUcenes. Furthermore, cleavage of the two types of bonds generally occurs with retention of geometry. On the other hand, heating of aUcynezirconocene complexes with 9-BBN, to 80-90 °C, affords a bimetallic complex (95) showing a planar-tetracoordinate carbon and an wo-BBN structure. 

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