Skeletal bonds

The retrosynthetic analysis is performed in two steps in a first step, SYNGEN dissects the skeleton to find all fully convergent bondsets which utili2e starting material skeletons found in two successive levels of cuts. A bondset is a set of skeletal bonds that is cut during the retrosynthetic analysis or formed in any given synthesis. 

The reduction of stereochemical complexity can frequently be effected by stereoselective transforms which are not disconnective of skeletal bonds. Whenever such, transforms also result in the replacement of functional groups by hydrogen they are even more simplifying. Transforms which remove FG s in the retrosynthetic direction without removal of stereocenters constitute another structurally simplifying group. Chart 3 presents a sampling of FG- and/or stereocenter-removing transforms most of which are not disconnective of skeleton. 

Skeletal bonds directly to remote stereocenters or to stereocenters removed from functional groups by several atoms are preserved. Those between non-stereocenters or double bonds which lie on a path between stereocenters are strategic for disconnection, especially if that path has more than two members. 

Macromolecules differ from small molecules in a number of critical properties. First, the linear chain structure confers elasticity, toughness, and strength on the solid state system. This is a consequence of the reorientational freedom of the skeletal bonds and of their ability to absorb impact or undergo elastic deformation by means of conformational changes rather than bond cleavage. 

However, the nature of the skeletal bonds and the elements involved can have a powerful influence on the torsional barrier for individual skeletal bonds. 

In a typical elastomer, the number of skeletal bonds in a network chain range from about 100 to 700 [25]. Networks with chains shorter than 100 bonds have... 

The vector r joining the two ends of the chain takes different values resulting from rotations about the individual bonds. For chains with more than about 50 skeletal bonds, the probability W(r)dxdydz that one end of r is at the origin and the other end is in an infinitesimal volume dV — dxdydz is satisfactorily represented by the Gaussian function [31,32]... 

Figure 1 Distributions for the end-to-end distance of a PDMS chain having n = 20 skeletal bonds of length / = 1.64 A. The Fixman-Alben distribution (dotted curve) and that from a Monte Carlo simulation (solid curve) are compared with the Gaussian approximation (dashed curve).

The end-to-end vector r of any non-crystallized chain portion comprising n skeletal bonds with a length l will be taken as Gaussian-distributed, i.e.,... 

Figure 9. The end-to-end distance per skeletal bond n for regular conformations of polydimethylsiloxane and polyethylene network chains (12). Maximum extensibility rm of this chain molecule occurs at rm/n = 1.34 A.

Figure 10. Radial distributions (in A 1) for PDMS chains having n = 40 skeletal bonds (each of length l = 1.64 A) (53).

The values of 0(ASD) /2.3O3 R listed in Table 5 are the entropic components of log EM. These are the log EM- alues for ideal strainless cyclisation reactions, i.e. reactions where 0AH° = 0. It is of interest to note that, as far as the entropic component is concerned, symmetry corrected effective molarities on the order of 102 106M are found. This observation leads to the important conclusion that cyclisation reactions of chains up to about 7 skeletal bonds are entropically favoured over reactions between non-connected 1 M end-groups. The intercept of 33 e.u. corresponds to an effective molarity of exp(33/R) or 107 2M, which may be taken as a representative value for the maximum advantage due to proximity of end-groups in intramolecular equilibrium reactions. It compares well with the maximum EM of about 108M estimated by Page and Jencks (1971). 

The above considerations have provided a useful guideline for drawing the curved part of the full line in Fig. 23. The portion in the range 15-30 rotors was assumed to coincide with the corresponding portion in Fig. 24, and the remaining short gap was filled by means of a smooth curve starting from the point related to the 7-rotor chain, which still lies on the straight line. Combination of the full lines drawn in Fig. 23 and Fig. 24 offers a spectacular panoramic view of the entropy effect on cyclisation reactions of chain compounds with up to nearly 100 skeletal bonds. 

Figure 3.119 Leading donor-acceptor interactions in AI2H6 (left) and Ga2H6 (right), showing overlap contours in the plane of three-center bridge-bonding (the pi plane, top row) and two-center skeletal bonding (the sigma plane, bottom two rows), with associated second-order stabilization energies in parentheses.

Equation (4.40b) shows that n(h) corresponds to an sd4 00 hybrid (80% d character). If the bond hybrids hi and h2 are directed toward ligands along the x and z axes, then n(h) is oriented in they direction, perpendicular to the skeletal bonding plane. 

However, the important new feature of metal alkylidenes (4.51) is metal-carbon pi-bonding. As discussed in Section 2.8, pi bonds between transition metals and main-group elements are of d -p type, much stronger than corresponding p —pn bonds between heavier main-group elements. Compared with simple metal hydrides and alkyls, metal-carbon pi-bonding in metal alkylidenes affects the selection of metal d orbitals available for hybridization and skeletal bond formation, somewhat altering molecular shapes. 

In p-block elements, a central-atom lone pair is visualized as the limit of bonding to a ligand of lowest possible electronegativity, and thus acquires high s character. In d-block elements, a central-atom lone pair is of essentially pure d character (except for the special n(h) hybrid of ML2 coordination) and s character is essentially reserved for the covalent sigma skeletal bonds. 

An even stronger case of intramolecular RAHB coupling is provided by the maleate ion (HOOCCH=CHCOO-), whose H-bonded and open conformers are shown in Fig. 5.22. Skeletal bond lengths and bond orders of these conformers are compared in Table 5.18. As shown in Fig. 5.22, the H-bonded conformer is favored in this case by more than 26 kcalmol-1, which is indicative of a powerful intramolecular no aoH interaction (estimated second-order stabilization 104 kcalmol-1) that is sufficient to overcome the severe steric repulsion of the extremely short H- -O nonbonded distance ( 1.3 A).56... 

It can be seen that application of the 18-electron rule to clusters necessitates this arbitrary assignment of the number of orbitals of a particular predominant character. The number of orbitals per metal used for cluster skeletal bonding is a consequence of N and E. It varies from two for the M(CO)4 unit in Os3(CO)12 to three for an M(CO)3 unit in Ir4(CO)12. Since these two metal moieties differ, this variation seems reasonable. However, the distribution for an M(CO)3 unit varies with cluster shape as in Ir4(CO)12 and [Os6(CO)18]". ... 

Wade s electron-counting procedures (22) start from a closed polyhedron and require that neither removing a vertex from this polyhedron nor capping a face will alter the number of skeletal bonding orbitals. This capping principle has been demonstrated to be general (31). For... 

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