Synthesis conditions favoring structure formation

Table V. Trends in Synthesis Conditions Favoring Structure Formation with Di-n-Propylamine...

Finally, the high-pressure synthesis favors the formation of borides that are difficult to prepare under standard conditions. In this way SmBj with the AlB2-type structure and GdB,2 and ThB,2 with the UB,2-type structure are prepared. ... 

The structural similarity of Al22Br2o 12(THF) 6 to (3-rhombohedral boron with the familiar B84 unit raises the question whether another, more covalent, form of aluminum in addition to the metallic a-form exists, similar to the geometry of the Al atoms in the Al12 icosahedra of 6. The mild conditions applied for the synthesis of Al22Br20 might also favor the formation of such a modification. In order to support this hypothesis, we investigated the structural competition between the fee and ff-B type for Al by means of ab initio full-potential calculations, with the results shown in Fig. 15.52... 

The synthesis conditions observed to favor formation of a particular structure-type are shown in Table V. 

Materials chemistry contains all the elements of modem chemistry. These include synthesis, structure, dynamics, and properties. In synthesis, one employs all possible methods and conditions from high-temperature and high-pressure techniques to mild solution methods (chimie douce or soft chemistry).3 Chemical methods generally tend to be more delicate, often yielding novel, metastable products. Kinetic rather than thermodynamic control of reactions favors the formation of such structures. Supramolecular organization provides new ways of designing materials. 

This precedents prompted several groups to develop reaction protocols that lead exclusively to the reduced products (the reductive PKR), in order to use them in synthesis. Thus, Becker obtained diazabicyclooctanones like 208 from amines (207) as the only reaction product when they used DSAC conditions under an inert atmosphere. The nitrogen atmosphere was essential as in air they obtained mixtures of the cyclopentanones and the cyclopentenones. This group has used this methodology for the synthesis of azaadamantanes like 209 as part of the structure of certain antagonists (Scheme 59) [ 182-184], Addition of TFA to the reaction favors the formation of reduced products. A series of alkynes produced cyclopentanones as the major product when reacted with norbornene, Co2(CO)8 and TFA. The authors think TFA reacts with the cobalt complex 210, prior to the reductive elimination that gives the final product 211 (Scheme 60) [185]. 

Compound I must be activated by the strongly acidic medium. One possible protonation site is the basic pyrrolidinium nitrogen atom (structure X), which would favor the formation of carbenium ion XI (see Scheme 16.3). This intermediate would gain additional stabilization from the polar solvent. Conditions for an SnI substitution would prevail to yield XII, whose fragmentation would resemble, formally speaking, the retro cycloaddition process used in the synthesis of I (i.e., VIII - I). The resulting ketone (XIII) would contain the elements required for the construction of a five-membered carbocycle with the desired incorporation of the functional carbon of one of the nitrile groups, as... 

Some facts make primary formation of alcohols probable also for iron catalysts Alcohols and hydrocarbons produced by the synthesis show similar structure of the branching, and process conditions favorable for recovery of intermediate products (short contact times, high recycle ratio, low temperatures, etc.) usually increase the yields of alcohols. 

D-RNA-containing particles do not interact with ribosomes or with ribosomal subunits under any of the conditions used (Samarina et al., 1966 1967b). However, the isolated D-RNP of polysomes easily combine with ribosomes to reform polysomal complexes capable of protein synthesis (Armentrout and Weisberger, 1968). It may be that the globular structure of nuclear D-RNP does not favor the formation of complexes with ribosomes, whereas the removal of some proteins and resulting configurational changes may make such interaction possible. 

The mechanism for the Niementowski quinoline synthesis is presumably similar to that of the closely related Friedlander reaetion. The mechanism for the Friedlander reaction has been studied extensively" and two possible mechanistic pathways exist, as illustrated below. There is support for both pathways. Most of the evidence, however, tends to favor initial formation of the SchifTs base intermediate (13) followed by cyclization to give quinoline 15 however, the reaction conditions and structures of the reactants may influence the pathway by which the reaetion proeeeds. ... 

Further evidence to support the role of self-assembly in divergent dendrimer synthesis can be found in the construction of poly(amidoamine) PAMAM dendrimers. First, ideality and fidelity of structure is usually a signature of a self-assembly event. This is a universal observation throughout biological systems. In all cases, nearly ideal PAMAM structures are obtained only under mild conditions favoring such organizational events as a prerequisite to bond formation [88]. Attempts to impose more severe reaction conditions (i.e., reaction temperature >40 C) dramatically reduces the ideality of dendrimer structures even in the early generations. Under mild, kinetically favorable reaction conditions however, nearly ideal dendrimer structures are obtained up to the onset... 

---