Complexity of structure

Figure Bl.21.5. Evolution with time of the complexity of structural detennination achievable with LEED.

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of structure. The most useful compounds (not necessarily the simplest structures) are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hydrogenol-ysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and allyl, readily cleaved by Pd-catalyzed isomerization. 

The precise structural role played by the water molecules in these cements is not clear. In the zinc oxychloride cement, water is known to be thermally labile. The 1 1 2 phase will lose half of its constituent water at about 230 °C, and the 4 1 5 phase will lose water at approximately 160 C to yield a mixture of zinc oxide and the 1 1 2 phase. Water clearly occurs in these cements as discrete molecules, which presumably coordinate to the metal ions in the cements in the way described previously. However, the possible complexities of structure for these systems, which may include chlorine atoms in bridging positions between pairs of metal atoms, make it impossible to suggest with any degree of confidence which chemical species or what structural units are likely to be present in such cements. One is left with the rather inadequate chemical descriptions of the phases used in even the relatively recent original literature on these materials, from which no clear information on the role of water can be deduced. 

There has also been some interest in NHC-lanthanide complexes as polymerisation catalysts. Indenyl and fluorenyl functionalised NHC complexes of structures 14 and 15 (Fig. 4.5) were evaluated for isoprene polymerisation following activation... 

Diphenyl cyclopropenone and Ni(CO)4 were reported278) to give a transition-metal complex of structure 453, which was recently re-evaluated in favor of formulation 454 from spectral and chemical evidence199 ... 

The reaction between the nitrone (753b) and trans-[PdCl2(RCN)2] (R = Ph, Me) in the corresponding RCN (or of the nitrone in neat RCN in the presence of PdCl2) proceeds at 45°C with (R = Ph) or reflux with (R = Me). It affords A4-l,2,4-oxadiazoline complexes of structure, similar to (755), but with Pd instead of Pt. The reaction time can be drastically reduced by focused microwave irradiation of the reaction mixture (833). 

Complexes with type II structures are compiled in Table II along with their S—S distances and S—S vibrational frequencies. There are several examples of complexes of structure type Ila (planar trans end-on bridging coordination). Vibrational spectroscopy 174) indicates that this structure occurs in [(CN)5Co(S2)Co(CN)5] (7). Trans arrangement of the metal atoms has been proved by X-ray structure determination for [(NH3)5Ru(S2)Ru(NH3)5]Cl4 2H20 15, 44). 

Group VI Donors. The thio-oxinato-complex Ag(C9HgNS)2 (54 M = Ag) has been prepared, and its N and S co-ordination confirmed from i.r. data. Solution and powder e.s.r. studies of the compound isolated from the reaction between Ag acetate and the NN-di-n-butyldiselenocarbamate anion confirm that it is a Ag" complex of structure (96). A subsequent single-crystal e.s.r. 

Hafnium tetrachloride forms many octahedral complexes of structure HfCl4L2 with neutral donors. For example, with tetrahydrofuran, it forms HfCl4(THF)2. Mononuclear and dinuclear hafnium chloride ions have been reported, produced by reaction with triphenylchloromethane, CPhsCl (Pampaloni, G. O. 1996. J. Organomet. Chem. 518, 189). 

For compilation and arrangement of compounds, earlier reviews and surveys were taken as the basis. In comparison to the previously published reviews, the increasing number and complexity of structures observed is striking. Thus it became quite difficult to list all of these structures in a logical sequence, particularly prenylated derivatives with additional cyclized substituents. Substitution patterns used for grouping of the flavone and flavonol derivatives are as follows. 

Although much information is available on this subject, it is not plentiful enough to draw any conclusions with certainty. The major problem with natural zeolites is that they occur frequently in multiphase assemblages making mineral separation difficult and thus identification and chemical information unsure. Only X-ray diffraction allows a proper mineral identification but this also is not certain due to the complexities of structural variation in zeolites which arise through chemical substitutions. In sum, chemical analyses of so-called single-phase zeolites are likely to be unreliable. 

The entropies of a few substances are given in Table 6-3. Notice how the entropy increases with increasing complexity of structure, with transitions from solid to liquid to gas, and with decreasing hardness of solid substances. 

Direct synthesis of complexes of structure type B from germylenes, stannylenes and plumbylenes is possible in several cases. Many complexes of the very sterically hindered metallenes M[CH-(SiMe3)2]2 (M = Ge, Sn, Pb) have been obtained (Scheme 1).2 27... 

For complexes of structural type A2 the two terminal SH- groups can be in a cis or trans position. The first example of the former was [Cp2Ti(SH)2]22 and some others have been prepared since then, for example [(PPh3)2Pt(SH)2], [(diphos)Ni(SH)2] and [(diphos)Pd(SH)2].23-30 The reaction of HC1 and [(NH4)2 WS4] suspended in Me20 at dry ice temperature is suspected to yield [S2W(SH)2], This material is very unstable, probably due to the high acidity of the two SH protons, and has not been completely characterized. Data for this and related products (which probably also contain terminal SH groups) are compiled in ref. 31. 

For complexes of structural type III the corresponding absorption band is expected to occur at higher energy because both n orbitals of the ligand interact strongly with the metals. 

While some of the Mississippian textiles are of similar structure to the Middle Woodland textiles, others are very complex materials and are lace-like in appearance. Many of the materials from Etowah are preserved by mineralization, and display green-colored deposits on their surfaces. Bast fiber, rabbit hair, and feathers have been identified (2, 11). The textiles from these two sites selected for analysis are representative of the complexity of structure and fineness of yarns seen in the materials they provide evidence of the sophisticated technology of prehistoric people in all phases of fiber collection, processing, yarn spinning, fabric manufacture and, when present, coloration. 

The metal-promoted reactions of 1,2-dihydrazones with formaldehyde (see Scheme 35) can be compared with the purely organic reactions of 1,2-dihydrazones with orthoesters, which lead directly to macrocyclic compounds from which complexes of structural type (85) can be prepared.189190... 

Highly-colored Jackson-Meisenheimer,or a, complexes of structure II or III are formed by addition to the ring system. The controversial history of the study of these compds has been well reviewed (Refs 44,46,51,70 82). They are formed in aq and anhydrous medium, and with a variety of organic and inorganic nucleophiles. Most of these studies have been made with 1,3,5 -trinitrobenzene rather than TNT, since the absence of a reactive methyl group in the former simplifies product isolation and identification. 

Sulfite ion forms a complex of structure III (Refs 44 80), which is a source of yield loss during the purification of TNT if the pH exceeds 8. Cyanide ion also forms a compd of structure III (Ref 44) the kinetics of this reaction has been studied (Ref 76), TNT, as the anion I, can react with itself to form adducts of type II it can similarly react with 13 >5-trinitrobenzene (Ref 57). Primary and secondary aliphatic amines form a-complexes (Refs 46 58), but tertiary amines (in an aprotic solvent), and aromatic amines, form ir-complexes instead. Upon soln in liq ammonia, a complex of type III is initially formed, with no evidence of I (Ref 105). With time, a second NH2 group becomes attached to the ring carbon with the methyl group this compd has cis- and transisomers. Removal of the ammonia gave a red... 

Several tr-allyl palladium chloride complexes have been prepared from steroidal olefins. Reaction of cholest-4-ene (65), for example, with bis(benzonitrile)palladium dichloride produced dimeric complexes of structures (66) and (67) (76). 

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