Calcium amides

Higher molecular weight materials are typically prepared using coordination catalysts, such as the calcium amide-alkoxide system developed by Union Carbide,932 and double metal cyanide initiators, e.g., [Zn(Fe(CN)6)].933 The nature of the active sites in these catalysts is not well understood and only a few studies have been published outside the patent literature.810,934,935... 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

Even a cursory reading of Chapter 3 of the previous volume/ dealing with alkaline earth diorgano amides, shows that detailed information on their preparation, structure and reactivity was quite sparse. For example, there were no data available for strontium or barium amide derivatives and there was just one structure of a complex related to a calcium amide - that of [(thf)3Ca( AIH j(. - in which calcium was incorporated as part of an... 

The synthesis of substituted calcium amides is generally achieved either via salt elimination between a halide (often Cal2) and an alkali metal amide or via transamination using bis(trimethylsilylamido)calcium derivatives, most commonly [Ca N(Si-Me3)2 2(thf)2]. The latter was also conveniently prepared from Ca(OSO2CF3)2 and 2Na- N(SiMe3)2 in thf. ... 

Most monomeric calcium amides involve the use of neutral donor molecules to prevent association. The only example of a three-coordinate mononuclear Ca amide, [Ca N(SiMe3)-(SiBu Ph2) 2(thf)], was prepared from CaF and KN(SiMe3)(SiBu Ph2) similar reaction... 

Scheme 3.11 Formation of calcium-carbon bonds by reaction of a calcium amide with an aluminium trialkyl (R— SiMe,) ... 

The existence of alkyl- or aryl calcium amides has not as yet been unambiguously demonstrated. One reason for this seems to be because calcium— carbon bonds are much less stable than calcium—nitrogen bonds, as is illustrated by the following reaction [Eq. (2)] (12). 

Lithium amide. Lithium nitride. Calcium amide Mercuric nitride. Bismuth nitride. ... 

Ca(NH2)2 (c). Guntz and Benoit1 measured the heat of solution of calcium amide in aqueous HC1. 

The Ca-N-H system possesses even more complicated properties. Hino et al. [34] reported the thermal desorption properties of the mixture of calcium amide (Ca(NH2)2) and calcium hydride (CaH2) with two different ratios (1 1 and 1 3) and the mixture of CaNH and CaH2 as shown in Figure 6.5. From this result, Ca(NH2)2 and CaH2 with the 1 3 ratio desorbs H2 gas by the following two-step reaction (3.5 and 2.1mass%, respectively) ... 

As in the 1960s the demand for (3-pinene far exceeded its availability owing to its versatile applications, a large part of the more commonly occurring a-pinene was isomerized to (3-pinene using, for example, basic catalysts such as calcium amide and alkaline earth metal oxides. A major amount of (3-pincnc is converted to resins with adhesive properties and by pyrolysis to the acyclic monoterpene myrcene. 

More recently, Hino et al. explored the Ca-N-H system starting from the amide, Ca(NH2)2 and focusing on the desorption process [79]. First synthesising calcium amide by ball milling CaH2 under ammonia gas, the decomposition product upon heating the amide at 350 °C was found to be calcium imide according to Eq. (16.16) ... 

The ammonia is boiled off and excess Ca precipitated with CO2 [62]. Small amounts of Ca(N3)2 have also been made with the Wislicenus process [85,120] the intermediate calcium amide forms at 20-60°C, and the azide at 90-100°C. 

The coordination catalysts for these reactions are diverse. They can be compounds of alkaline earth metals, like calcium amide, or calcium amide-alkoxide. They can also be Ziegler-Natta-type catalysts. These can be alkoxides of aluminum, magnesium, or zinc combined with ferric chloride. Others are reaction products of dialkylzinc with water or alcohol. They can also be bimetallic //-oxoalkoxides, such as [(RO)2A102]Zn. Other catalysts are aluminum or zinc metalloporphyrin derivatives (see Fig. 4.1). 

In contrast, the p-diketiminato-stabilized calcium amide [ (ArNCMe)2CH Ca N(SiMe3)2 (THF)] and a tris(pyrazolylborate)-stabilized analogue were reported by Chisholm and coworkers as precatalysts for the ring-opening polymerization of rac-lactide to form heterotactic polylactide [25, 26], while substituted fluorenyl ligands have been reported by Harder and coworkers for the stereocontrolled polymerization of styrene to yield highly syndiotactic polystryene [23]. 

Asymmetric catalysis Initial attempts to develop non-racemic catalysts for the enantioselective hydrosilylation of alkenes have not been successful with C2-symmetric (5)-Ph-pybox and p-diketiminate calcium amide complexes catalyzing the addition of phenylsilane to styrene in the absence of solvent at 50 °C in, at best, 9 % e.e [69]. As with intramolecular hydroaminatiOTi catalysis, these results have been explained in terms of the loss of ligand control due to facile ligand redistribution reactions under the catalytic reaction conditions. 

While calcium isopropoxide, Ca(0 Pr)2 has been employed successfully in the preparation of chiral catalysts, it was also found that calcium amide (e.g., Ca (HMDS)2) worked well in these reactions as a catalyst. Calcium amides have a stronger Brpnsted basicity and a higher solubility in many solvents compared with calcium alkoxides. Interestingly, it was reported that chiral calcium amide catalysts prepared from Ca(HMDS)2 could be employed successfully in [3+2] cycloaddition reactions (Scheme 4) [48]. 

Scheme 4 Catalytic [3-1-2] cycloaddition reaction using a Box-caldum COTiplex prepared from calcium amide and Box...

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