Extended Open Chains

The charge redistributions that accompany formation of a H-bond, and the consequences for a string of water molecules, were investigated with two rather small basis sets. By extrapolating the results up to the pentamer level, Scheiner and Nagle found the component of the dipole moment, in the direction of the chain, is increased by 40-60% as compared to the moment in the water monomer. Shortly thereafter, Karpfen and Schuster used a different formalism wherein infinite chains could be considered explicitly to examine the same question . Their means of analysis yielded a reduced enhancement of the moment of only 16% they attributed the discrepancy to edge eflects. 

A later work continued the investigation of extended chains of water molecules, incorporating the effects of electron correlation. As in the oligomers of HF, the length of the H-bond contracts as the chain enlarges. The small nonlinearity present in the dimer vanishes as well. Crystal orbital techniques were employed to consider infinitely extended chains. Some of the more interesting features of the infinite chain are listed in Table 5.11 

The highest nonadditivity, defined by these authors as the difference between the total interaction energy and that computed by summing pairwise interactions, is observed for the full heptamer, where it amounts to 16% of the total binding energy. Fig. 5.16 illustrates 

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Several types of diisocyanates (aromatic, aliphatic, cyclo aliphatic) and many different glycol-chain extenders (open-chain aliphatic, cyclo aliphatic, aromatic aliphatic) can be used to produce TPU-elastomer hard segments. In the more conventional and practical formulations only a single diisocyanate component is used to make a TPU, so the diisocyanate is common to both the hard and soft segments. The polymer chemist makes his diisocyanate and glycol-chain-extender component selections based on such considerations as desired TPU mechanical properties, upper service temperature, environmental resistance, solubility characteristics, and economics. 

If the number of B03 groups in the ring is increased indefinitely we reach in the limit the infinitely extended open chain of composition [BO 71- shown in fig. 11. oie. Such an arrangement differs fundamentally from all the other complex anions so far described in that it is no longer possible to point to any discrete radical. The repeat period in the crystal may extend over several units of the chain, at different points of which different atoms may be attached, so that in chemical terminology such a... 

Reaction of hydrazine 165 with a number of monosaccharides give hydrazones 166 (92BCJ546). Their acetylations gave peracetyl derivatives 167. The same reactions were extended to the /V-methyl and the 7-methyl derivatives of 165 (93 JPROO). That compounds 166 exist as open-chain structures was based on a 2D NMR study (93MI1) (Scheme 35). 

Compared to oxo-squaraines or other ring-substituted squaraines, amino-squaraines 39 [45, 52, 112] have ionic character, similar to open-chain cyanine dyes, and due to the positive net charge, these dyes are to some extend water-soluble. Amino-squaraines absorb and emit at longer wavelength than the corresponding oxo-squaraines the absorption maxima are between 650-710 nm (eM = 85,000-300,000 M-1cm-1) [45, 112], The increase in solvent polarity is accompanied by a hypsochromic shift of the absorption. Amino-squaraine dyes are potentially used as fluorescent probes but because their photostability is inferior to those of oxo-squaraines and other ring substituted squaraines of similar structure, their applications are rather limited. 

The high barriers in compounds 31 and 32 may be attributed to the fact that in these compounds the two carbonyl groups are coplanar with the enamine moiety, whereas such a coplanar structure is impossible for the open-chain compounds. Based on this point of view, Kdlle and associates extended the work further and were able to isolate a series of compounds (33,34) in one crystalline atropisomeric form (73). 

The AgBp4 method was extended to cyclizations onto the alkoxyl side chain. Novel heterocycles A-acyl-3,4-dihydro-lP -2,l-benzoxazines (25) and A-acyl-4,5-dihydro-l//,3//-2,1-benzoxazepines (26) were synthesized by the treatment of open-chain A-chloro-A-(2-phenylethyloxy)- and A-chloro-A-(3-phenylpropyloxy)amides (24, n = 2, 3) with silver tetrafluoroborate in benzene (Scheme 2). Optimal yields of 25 were ultimately obtained with AgBp4 in ether or TPIP. ... 

Amidation of W-BOC-tetrahydro-l,2-oxazine-6-carboxylic acid 47 with free oxanipecotic acid afforded amide 48 <2003TL3447>. The 3-methyl-substituted 1,2-oxazine Woxide 280 can be selectively transformed into 2-silyloxy-1,2-oxazines 281, upon treatment with silylating reagents (ClSiMe3). Now, the synthetic utility of 2-silyloxy-l,2-oxazine 281 is extended and it can be rearranged into 3-silyloxymethyl-l,2-oxazine 282 and can further react with morpholine to produce 3-morpholinomethyl-l,2-oxazine 283 which exists in a tautomeric equilibrium with the corresponding open-chain oxime <2003JOC9477>. 

Enolates may be derived from a,/l-unsaturated ketones 16 by base-catalyzed proton abstraction. Under kinetic control the a -proton is abstracted and a cross-conjugated metal dienolate is formed, whereas under thermodynamic conditions the extended dienolate is the major product3,, l. Successful alkylations of dienolates derived from cyclic a,/l-unsaturated ketones have been performed (see Section 1.1.1.3.1.1.2.1.). The related a,/ -unsaturated ester systems have also been investigated22-24. Open-chain structures 16 pose a rather complicated... 

The reactive open-chain substrate 29 with the natural D-threo configuration was prepared along a chemoenzymatic route by making use of the common constitutional and stereochemical relationship which substrates of transaldolase share with those of transketolase. Thus, the R-configured 2-hydroxyaldehyde 28 was chain-extended under transketolase catalysis in the presence of 20 as ketol donor to yield the desired aldol. By this approach, several transaldolases could indeed be shown to display different levels of kinetic stereoselectivity. 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

Cyanobacteria and red algae employ phycobilins such as phycoerythrobilin and phycocyanobilin (Fig. 19-40b) as their light-harvesting pigments. These open-chain tetrapyrroles have the extended polyene system found in chlorophylls, but not their cyclic structure or... 

Introduction of ring-opening metathesis as a versatile polymerisation technique (ROMP) by Chauvin and Herisson Nobel Prize Chemistry to Paul J. Flory for his fundamental achievements, both theoretical and experimental, in the physical chemistry of the macromolecules Fully aromatic polyamides developed Aramids, being lyotropic liquid crystalline polymers of high strength, due to extended molecular chains (Morgan and Kwolek)... 

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