Bond additivity scheme

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

This approximation corresonds to Allen s bond additivity scheme involving near-neighbor interactions taken two and three... 

The Oppolzer sultam 35-1 (Scheme 35, reaction (101) [84] reacts with even higher stereoselectivies and is easier to remove. The main domains of the Oppolzer sultam are conjugate 1,4-additions or simple double bond additions [Scheme 35, reactions (102) and (103)] [85], which show diastereoselectivities of >95% in most cases. Scheme 36 presents examples of persistent, restorable and selfimmolative auxiliaries which are all based on amino acids or amino alcohols, finders RAMP-SAMP [86] is attached to ketones or aldehydes in form of a hydrazone 36-1 which is used for highly stereoselective electrophilic a-alkyla-tions. After the reaction the auxihary is removed via ozonolysis which generates the nitrosamine 36-2 first. In an ensuing step this is reduced to the original auxiliary. In Schollkopf s bislactim ether alkylations [Scheme 36, reaction (105)]... 

In a bond additivity scheme, a property is related to contributions from each of the bonds in the molecule. For example, the heat of formation of isooctane (2,2,4-trimethyl-pentane), whose structure is shown in Figure 1, would be calculated from its bond units as... 

Table 5 shows that the enthalpies and entropies of these octanes, while not identical, differ by only 4kcalmor and 5.3 eu, respectively, from smallest to largest. In general, bond additivity schemes for thermochemical properties can provide order-of-magnitude... 

Another example of a,j8-imsaturated carbonyl compoimds is provided by p-quinones. Carbonyl yhdes engage p-quinones in a manner reminiscent of their reaction with o ,)S-unsaturated carbonyl compoimds [97,132]. For example, the reaction of 56 with p-benzoquinone yielded to the novel oxa-bridged polycyclic systems 150-152 through stereoselective C=0 and C=C bond additions (Scheme 47). The formation of 150 through tandem cyclization-cycloaddition-Michael addition is quite interesting as four C-O bonds and one C-C bond are formed in a single synthetic step [133]. 

For saturated molecules in which the electrons are locafized in bonds, the moments can be estimated using the vector sum of the polarizabihties of the individual bonds, and the interaction between bonds is neglected (bond additivity scheme). 

Calculated from the bond length given by the authors. f Estimated heats of formation by bond-additivity schemes. 

Two C-C Bond-Forming Events In 2008, Frechet and coworkers described an impressive asymmetric cascade reaction promoted by soluble star polymers with core-confined catalytic entities [10]. The encapsulation of catalysts into soluble star polymers allowed the use of incompatible catalysts and prevented undesired interactions between these catalytic systems. The organocascade corresponded to a nucleophilic addition of Af-methylindole to a,p-unsaturated aldehydes followed by a Michael addition of the adduct to methylvinylketone (MVK) in the presence of H-bonding additive (Scheme 12.5). Each catalyst - imidazolidinone 8 for the nucleophilic addition and diphenylprolinol methyl ether 9 for the Michael addition - or their combination cannot mediate both reaction steps. In particular, p-toluenesulfonic acid (p-TSA) diminished the ability of the chiral pyrrolidine 9 to effect enamine activation. Therefore, p-TSA and 9 were encapsulated in the core of star polymers, which cannot penetrate each other. Imidazolidone 8 was added to the acid star polymer and diffused to the core to form the salt, which allowed the iminium activation and catalyzed the first step. The second step was catalyzed by the pyrrolidine star polymer in presence of the H-bonding additive 10, which... 

One crucial assumption here is that the local field on a segment is given by the Lorentz-Lorenz equation, which implies that the environment of a segment is optically isotropic. (See also imder Local Field). The treatment also involves the assumption of tensor additivity of polarizabilities of noninteracting segments (valence optical scheme or bond additivity scheme). 

Another scheme for estimating thermocheraical data, introduced by Allen [12], accumulated the deviations from simple bond additivity in the carbon skeleton. To achieve this, he introduced, over and beyond a contribution from a C-C and a C-H bond, a contribution G(CCC) every time a consecutive arrangement of three carbon atoms was met, and a contribution D(CCC) whenever three carbon atoms were bonded to a central carbon atom. Table 7-3 shows the substructures, the symbols, and the contributions to the heats of formation and to the heats of atomization. 

The accuracy of an additivity scheme can be increased by going from atomic contributions through bond contributions to group contributions. 

In contrast to the results above, the reaction of a-ketocarboxylic esters follows a different course giving 26, the product of P-H addition of the tautomeric PH phosphazene form to the keto C=0 double bond [126] (Scheme 27). 

Scheldt and Chan have shown that NHC promoted homoenolate formation and addition to azomethine imines 37 generates pyridazinones 41 with high diastereoselectivity, via a proposed highly organised transition state 40 due to a key hydrogen bonding interaction (Scheme 12.6) [12]. 

Based on these observations [18,19,23], a variety of modified catalytic systems have been reported for the diastereoselective reductive carbon-carbon bond formation (Scheme 8). A complex 5 derived from Cp2TiCl2 and MgBr2 is proposed to be an efficient catalyst for the DL-diastereoselective pinacol coupling of aromatic aldehydes [24], Addition of a solution of benzalde-... 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

In addition to its utility in the enantioselective formation of C-0 bonds (cf. Scheme 15), Trost s chiral ligand 102 has been used in the catalytic asymmetric synthesis of C-N bonds. An impressive application of this protocol is in the enantioselective total synthesis of pancrastatin by Trost (Scheme 17) H9i Thus, Pd-catalyzed desymmetrization of 112 leads to the formation of 113 efficiently and in > 95 % ee. The follow-up use of the N3 group to fabricate the requisite cyclic amide via isocyanate 117 demonstrates the impressive versatility of this asymmetric technology. 

---