Isocyanates metal alkoxides

Zirconium and hafnium tetraalkoxides are highly reactive compounds. They react with water, alcohols, silanols, hydrogen halides, acetyl halides, certain Lewis bases, aryl isocyanates and other metal alkoxides. With chelating hydroxylic compounds HL, such as j8-diketones, carboxylic acids and Schiff bases, they give complexes of the type ML (OR)4 these reactions are discussed in the sections dealing with the chelating ligand. 

Insertion reactions involving metal alkoxides are also known. For example, carbon dioxide is known to react with some metal alkoxides as shown in equation (12). The formation of a bidentate ligand is a significant thermodynamic driving force for some of these reactions. The isoelectronic aryl and alkyl isocyanates and carbodiimides can react similarly. Insertion reactions involving alkenes and carbon monoxide are known for platinum alkoxides. 

Metal alkoxides undergo insertion reactions across the M—O bonds (6) when treated with unsaturated substrates such as isocyanates, isothiocyanates, aldehydes, ketones, ketenes as well as with carbon dioxide, carbon disulfide, and sulfur dioxide. 

In addition to the general procedure iUusIrated by Eq. (2.182), another route has been employed for the synthesis of metal alkoxides (and particularly analogous siloxides) by the reaction of metal alkoxides with esters (and silyl esters) (Section 4.3). Further, the reactions of metal alkoxides with acyl halides (or hydrogen halides) and unsaturated substrates such as CO, CO2, CS2, organic-isocyanates and isothiocyanates, chloral, etc. have been utilized for the preparation of haUde-alkoxides (Section 4.11) and interesting insertion products (Section 4.12), respectively. 

Metal alkoxides and metal amides undergo this reaction readily and some substrates act as catalysts for the trimerization of isocyanates. Also a stepwise conversion of different isocyanates leads to the selective formation of trimers with different substituents. A review on this reaction appeared in 1987 ". A summary of the earlier literature appeared in my previous book " . 

The details of the mechanism are poorly understood, though the oxygen of the alcohol is certainly attacking the carbon of the isocyanate. Hydrogen bonding complicates the kinetic picture. The addition of ROH to isocyanates can also be catalyzed by metallic compounds, by light, or, for tertiary ROH, by lithium alkoxides ° or n-butyllithium. ° ... 

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

The volatile metal-containing precursors which satisfy the ALD criteria fall into four main categories (i) halides, (ii) y0-diketonate complexes, (iii) alkoxides, and (iv) true organometallics, viz. metal alkyls and cyclopentadienyl-type compounds (Fig. 3). Also amido complexes have recently gained attention as possible ALD precursors. Occasionally other compounds have been used as ALD precursors for thin films, for instance, metal nitrates, carboxy-lates and isocyanates [17,18]. 

Metal-Oxygen Compounds. Trialkyltin alkoxides are remarkable for the variety of addition reactions they undergo with carbonyl and thiocarbonyl compounds. Bloodworth and Davies have reported reactions of tri-w-butyltin alkoxides with isocyanates, carbon dioxide, sulfur dioxide, isothiocyanates, carbon bisulfide, chloral, and ketene. The reactions observed were as follows ... 

In conclusion it is necessary to note the considerable change in chemical activity occurring on transformation from the alkoxides into oxocomplexes. An example is the synthesis of a bimetallic Bi-Ti complex. The complex formation of 2 isopropoxides occurs only in the presence of water (h = 0.2-0.7), which leads to the formation of Bi-oxoisopropoxide, which then reacts with Ti(OPr )4 already at room temperature providing BiTi20(0Pr% [447] (see also Chapter 8). Teyssie et al. [760] have proposed a large group of alkoxides of 3d-transition metals, and also those of Zn, Al, and Mo as highly effective selective catalysts for polymerization of lactones, isocyanates, and so on. 

Other catalysts effective in the conversion of isocyanates into carbodiimides include the naphthenates of Mn, Fe, Co, Cu and Pb, derivatives of metallic acetylacetonates, the alkoxides of titanium, zirconium and niobium and vanadium oxides or chlorides. Sterically hindered isocyanates are readily converted into carbodiimides upon heating in the presence of a catalytic amount of a strong base. For example, heating of... 

Table 1. The Reaction of Metal Oxides and Alkoxides with Organic Isocyanates ...

Reactions of isocyanates with alcohols are catalyzed by a variety of compounds, including bases (tertiary amines, alkoxides, and carboxylates), metal salts and chelates, organometallic compounds, acids, and urethanes. The most widely used catalysts in coatings are organotin (IV) compounds, most commonly dibutyltin dilaurate (DBTDL) (dibutylbis[(l-oxododecyl)oxy]stannate) [77-58-7] and tertiary amines, commonly diazabicyclo[2.2.2]octane (DABCO). Combinations of DABCO and DBTDL often act synergistically. 

Insertion of phenyl isocyanate into the metal-oxygen bonds of dimolybdenum or ditungsten hexa-alkoxides " " affords dinnclear derivatives containing two bridging carbamate ligands, as shown in Eq. (2.294) ... 

In all of the above R is the same as defined above and X includes but is not limited to OH, Cl, Br, I, alkoxide (OR), acetate (OOCR), peroxide (OOR), amine (NR ) isocyanate (NCO), and R. The symbol M refers to metallic elements within the composition that include high and low Z metals and... 

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