Group alkoxide chemistry

The formation of tetrameric aggregates with a hetero-cubane structure is a structural motif that is observed for several alkylzinc alkoxides in the solid state (Figure 80). In these structures the alkoxy group is -bonded with its oxygen atom to three zinc atoms. The structural elucidation of [MeZnOMe]4 (152) represented the first example of such a structural motif in organozinc alkoxide chemistry . Later, similar structures were observed for alkylzinc alkoxides containing other alkoxy groups (153, = f-... 

In the following periodic table (Table I), the elements for which the homo-metallic and heterometallic alkoxide chemistry have been explored in detail are circled and put within a square, respectively. The metals of later Ad and 5d groups are underlined by a semicircle to indicate that not much is known about their alkoxide chemistry (40, 41) due to their instability, which is brought about by facile decomposition pathways (e.g., /3-hydrogen elimination). The elements whose alkoxides have been utilized as synthons for ceramic and other materials are marked with an asterisk ( ). 

Hexamers (239, 240) and tetramers (34, 238) are common in alkali metal alkoxide chemistry, although with more sterically demanding alkoxo groups, the oligomeric nature becomes lower in the solid state (cf. Section IV.B). Interestingly, [NaO-/-Bu] , crystallizes as two independent molecules in the asymmetric unit as a hexamer (n = 6) and a nonamer (n = 9). 

An interesting as well as unusual structure (Fig. 29) was reported for the compex Nd6(0-/-Pr)17Cl (256) in 1978. This structure attested to the complexity possible in lanthanide alkoxide chemistry (28). The structure of the neodymium complex can be regarded as two triangular Nd3(jt3-OR)(ft2-OR)3(OR)6 (where R = O-i -Pr) units connected by sharing the /x3-Cl site (which becomes a /i6-Cl) and three terminal OR ligands (which become /t2-OR groups). 

As mentioned in the introduction to this study guide chapter, the chemistry of ethers is very limited, showing a tendency toward nucleophilic displacement reactivity only under fairly special conditions. As is the case with alcohols, for any kind of nucleophilic displacement to occur to an ether (S l or Sn2), the leaving group (alkoxide in this case) has to be improved. This improvement is again done most simply by protonation with a strong acid. Then reaction can occur with a good nucleophile. 

The chemistry of ethyl alcohol is largely that of the hydroxyl group, namely, reactions of dehydration, dehydrogenation, oxidation, and esterification. The hydrogen atom of the hydroxyl group can be replaced by an active metal, such as sodium, potassium, and calcium, to form a metal ethoxide (ethylate) with the evolution of hydrogen gas (see Alkoxides, metal). 

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

Diorganoboryloxides, R2BO, bearing bulky organic substituents are quasi-alkoxide ligands for main group and transition metals. Because of the 7r-acceptor properties of boron, these anions are weaker 7r-donors than alkoxides, and this is reflected in their structural chemistry. Diethylzinc reacted with the borinic anhydride (9-BBN)20, Scheme 82, to afford the alkylzincboryloxide [EtZnO(9-BBN)]4 129, which also crystallizes in heterocubic form.187... 

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

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