Four-atom concerted transition state

Four-atom concerted transition state (results in syn addition)... 

Hydroboration occurs by a concerted process and takes place through a four-membered cyclic transition state, formed by addition of a polarized B—H bond (boron is the more positive) to the alkene double bond (5.2). This is supported by the fact that the reaction is stereospecific, with syn addition of the boron and hydrogen atoms. The reaction can also be stereoselective, with hydroboration taking place preferentially on the less hindered side of the double bond. Stereospecific addition of borane to a 1-alkylcycloalkene such as 1-methylcyclohexene, gives, after oxidation of the organoborane product (see Scheme 5.21), almost exclusively the trans alcohol product (5.3). 

Four-atom concerted Syn addition of H and B transition state... 

The characteristic features of hydroboration are consistent with a concerted four-centre transition state carrying charges on the participating atoms (Figure Bl.l) and this model adequately rationalizes the majority of hydroboration results. 

We know that this is not the whole story because of the stereochemistry. Hydroboration is a syn addition across the alkene. As the addition of the empty p orbital to the less substituted end of the alkene gets under way, a hydrogen atom from the boron adds, with its pair of electrons, to the carbon atom, which is becoming positively charged. The two steps shown above are concerted, but formation of the C-B bond goes ahead of formation of the C-H bond so that boron and carbon are partially charged in the four-centred transition state. 

Because syn addition to the double bond occurs and no carbocation rearrangements are observed, carbocations are not formed during hydroboration, as shown in Mechanism 10.5. The proposed mechanism involves a concerted addition of H and BH2 from the same side of the planar double bond the it bond and H-BH2 bond are broken as two new o bonds are formed. Because four atoms are involved, the transition state is said to be four-centered. 

The -elimination with two groups lost from adjacent atoms is another common reaction in pyrolysis, usually taking place with an Ei mechanism and not involving free radicals. An a-atom is the atom bound to a specific group or bond, and any atom adjacent to it is indicated as a p-atom. p-Eliminations or 1,2-eliminations involve, for example, the elimination of a group from a-atom and a hydrogen from the p-atom. For polymers where the pyrolysis takes place in condensed phase, E2 and Ei mechanisms are not excluded. The Ej mechanism involves a cyclic transition state, which may be four-, five- or six-membered [4]. No discrete intermediate is known in this mechanism (concerted mechanism). Two examples of reactions with E mechanism involving different sizes of cyclic transition state are shown below [3] ... 

Boron has only three electrons in the valence shell, and therefore its compounds are electron deficient and there is a vacant p-orbital on the boron atom. Borane (BH3) exists as a mixture of BaHe/BHs, as dimerization partially alleviates the electron deficiency of the boron. This equilibrium is fast, and most reactions occur with BH3. The addition of borane to a double bond is a concerted process going through a four-centered transition state. The formation of the C-B bond precedes the formation of the C-H bond so that the boron and the carbon atoms are partially charged in the four-centered transition state. 

There are three general mechanisms for insertions concerted, free radical, and heterolytic addition. In the 1,2-insertion, the concerted mechanism proceeds via interaction of the 7t system of the unsaturated compound directly with the intact E-H bond, with each end of the n system directed at either the E or the H atom (Scheme 1). This interaction may or may not be preceded by precoordination of the unsaturated molecule to the element. The transition state for this reaction is considered to be four-centered, and yields products that are cis-substituted on the reduced unsaturated substrate. 

The latest proposal by Vedejs (3d) is that the Wittig reaction proceeds via a concerted but asynchronous puckered 4-center cycloaddition pathway in which the stereoselectivity is determined by multiple steric effects and varying degrees of rehybridization at the phosphorus atom in the transition state. At present, there is not definitive evidence to prove that the reaction must proceed in this manner. Recent MNDO-PM3 computations by us (7,8) and somewhat related MNDO computations by Yamataka et al, (9) do not support the puckered 4-center cycloaddition hypothesis for the reactions of unstabilized ylides with aldehydes (3d), Instead, the MNDO-PM3 computations indicate that such Wittig reactions proceed through an essentially planar transition state (TS) with respect to the four central atoms, P-C-C-O. This process is... 

An objection that may be raised to a concerted four-center mechanism is the possibility of violation of orbital symmetry rules (68, pp. 65-78). Formally, the cyclization and cleavage may be viewed as forbidden + J2,] and [Jl, + J2,] cycloadditions (see transition state structure 41). The polarity of the bonds involved may alleviate the situation somewhat (21, 41). However, if the magnesium atom utilizes an additional orbital as in 42 (formally occupied partially by binding a solvent... 

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