Classical double bond rule

The chemistry of unsaturated silicon compounds, i.e. silylenes and molecules having (p-p)ic-sili-con element multiple bonds >Si=E (E = C, Si, Ge, Sn, N, P, As, O, S), is an interesting field of research for the theoretician as well as for the preparative chemist because of the unexpected and fascinating results which can be obtained. Yet 30 years ago, such compounds were considered "non existent" because of the classical "double bond rule", established by Pitzer and Mulliken in the early fifties. Since then, the chemistry of unsaturated silicon compounds proceeded from the investigation of small" species in the gas phase to the synthesis and isolation of stable species with bulky substituents at the > Si =E moiety, and to the determination of their structural features. 

Following the classical double-bond rule, multiple bonds between silicon and sulfur, both elements of the third period, should be very difficult to obtain. Except for SiS119,120,... 

As mentioned before, carbon and silicon mostly differ in their ability to from multiple p,-p, bonds E=Y with suitable partners (E = C, Si Y = element of group 14 to 16 ). While the p orbital overlap in compounds >C=Y is sufficient to yield stable multiple bonded species, this overlap is strongly reduced in the case of silicon (classical double bond rule of Pitzer and Mulliken). Consequently, under comparable conditions the equivalents of many unsaturated monomeric compounds of carbon, such as H2C=CH2, R2C=0 or CO2 are silicon single bonded polymeric products, e g. polysilanes (-H2Si-SiH2-)n, silicones (-R2Si-0-) and silicon dioxide (Si02)n... 

Following the classical double bond rule , compounds such as H2Si=SiH2, H2Si=CH2, and HSi SiH (disilyne) with multiple bonding to silicon are too... 

The hydration of an alkene double bond under strongly acidic conditions is again a classical reaction that involves a carbocation intermediate, which often leads to various competing reaction products.27 The regiochemistry of the water addition follows the Markovnikov rule.28... 

Benzene is [6]annulene, cyclic, with a continuous ring of overlapping p orbitals. There are six pi electrons in benzene (three double bonds in the classical structure), so it is a (41V+2) system, with N = 1. Hiickel s rule predicts benzene to be aromatic. 

Cyclooctatetraene is [8]annulene, with eight pi electrons (four double bonds) in the classical structure. It is a (41V) system, with N = 2. If Htickel s rule were applied to cyclooctatetraene, it would predict antiaromaticity. However, cyclooctatetraene is a stable hydrocarbon with a boiling point of 153 °C. It does not show the high reactivity associated with antiaromaticity, yet it is not aromatic either. Its reactions are typical of alkenes. 

During the introduction of this review, reference was made to the classical rule of the double bond. The theory, which in its original predictions supports the exceptional position for the elements boron, carbon, nitrogen, and oxygen, has lost its validity and needs modifications, as can be seen from the erratic increase in the numbers of compounds that contradict the rule that have been discovered within the last 12 years. These compounds are not found only as low-valent phosphorus-carbon species, but also increasingly as heteronuclear and even homonuclear molecules built up by heavier elements of the fourth to sixth main groups, such as Si, Ge, As, Sb, S, and Se. 

By classical" addition mechanisms, we include the large number of mechanisms advanced to date to rationtdize various aspects of the bromination of double and triple bonds. The major reaction deduced thus far, conversion of the triple bond to a dibromo double bond, cotild be a very localized process. Yet the triple and double bonds of the backbone are in an extended conjugated system and do not have the energetics of the isolated systems. It is not possible to rule out such mechanisms at present. 

The annulenes are that series of monocyclic polyolefins (C H ) containing a complete system of contiguous double bonds. While benzene (the best known member of this class of compounds) has been in evidence for some time it is only of late that interest in the higher members has become apparent. This interest has its origins in the LCAO-MO theory of re-elec-tron systems as formulated by E. Hiickel (in particular the "Hiickel rule relating aromatic stability to structure). Although the non-classical chemistry of the benzenoid hydrocarbons had previously been the subject of some conjecture, Httckel s theoretical studies provided the first satisfactory explanation of the peculiar stability of this class of compounds and, incidently, the elusiveness of cyclobutadiene. 

The results of many investigations [2-11] demonstrate that at small substrate conversions Z-isomers prevail when the reactant is Z structure and vice versa. If the catalyst has bulky ligands, the stereo-content can be up to 100 % Z or E, when starting Z- or E-2-pentenes are used correspondingly [12]. When using starting terminal olefins R-CH=CH2 and classical catalysts, initial E-content of resulting symmetrical olefins at 25 is close as a rule to thermodynamic equilibrium (83-86 %) [2,3, 13-18]. When a branch point at double bond arises ( for example with R = Pr(Me)CH ), the initial E-content decreases to 46 % [16]. In many cases metathesis of a-olefins in the presence of well- defined Mo-imido... 

Regio-selective functionalization of the terminal double bond in the 1, A-dienes with help of hydroboration-oxidation process and some other combinations allows a great variety of pheromone components by 2-3 steps to be obtained. At the same time, classical routes require as rule 7-9 steps. At the approach to cometathesis of cycloolefin stereo-content of... 

The cationic reaction pathway also allows for predictable effects on differential substrate substitution. In the rate-limiting step, the distribution of charge (marked by ) changes from C(l), C(3) and C(5), in pentadienylic cation 18, to C(2) and C(4), in cyclopentenylic cation 19. Therefore, substituents that stabilize positive charge (electron-donating groups), accelerate the reaction in the a-position (R ), or decelerate the reaction in the p-position (R ). Classically, under rather vigorous conditions, the reactions are under thermodynamic control and result in the formation of product 20, where the double bond occupies the most substituted position (Saytzeff s Rule). 

The rules for aromaticity in the previous section do need qualification in one respect. We have throughout this chapter assumed implicitly that it is possible to write at least one classical structure, i.e., a structure obeying the rules of valence and stereochemistry, for each molecule. Now this is not necessarily the case. Consider, for example, triangulene (28). However one struggles, one cannot write a structure in which all the carbon atoms are linked in pairs by double bonds. There are always two nonadjacent atoms left over. The reason for this can be seen at once if we star the molecule (29). There are two more starred atoms (total 12) than unstarred ones (total 10). Since each double bond in a classical structure must by definition link a starred atom to an unstarred one, it is clearly impossible to pair up the atoms into doubly bonded pairs unless the numbers of starred and unstarred atoms are the same. 

Rule 1. If only one classical structure can be written for a molecule, all the bonds in it are essential single or essential double bonds. Such bonds are localized. This has already been shown to hold (see Sections 3.6 and 3.12). 

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