Asynchronous, 4-centered transition state

While there was initial debate between Pasto5 and Brown6 about the mechanism of the hydroboration reaction, Brown s proposed mechanism6 is now generally accepted as the most likely pathway. Brown suggested that the reaction proceeds via an equilibrium between BH3 THF (5) and free BH3 (6). The free BH3 then rapidly adds B-H across a 71-system of the olefin (7) in an anti-Markovnikov fashion via an asynchronous, 4-centered transition state (8) to afford hydroboration product 9. 

The concerted asynchronous cycloaddition mechanism involving a four-centered transition state was suggested to be operating in these systems. 

Note that the B and the H of the borane are on the same side of the ring in 65, which is a consequence of the four-center transition state and concerted asynchronous delivery of B and H to the C=C unit. This reaction constitutes a cis addition of borane to the alkene, where the B and the H add cis (on the same side). In the case of methylcyclopentene, the cis addition of B and H leads to a trans relationship between the BH2 unit and the methyl group in 65, but the hydroboration reaction is a cis addition. 

The mechanism of the C—H insertion occurs as depicted in Figure 23.1. The insertion probably takes place through a three-centered transition state in which the p orbital of the carbenoid induces an electrophilic attack onto the o-C—H bond orbital of the alkane. The new C—C bond is formed via a concerted and asynchronous manner but occurs with complete retention of configuration at the carbon of the C—H bond to give the desired product. 

The mechanism of the carbo-Diels-Alder reaction has been a subject of controversy with respect to synchronicity or asynchronicity. With acrolein as the dieno-phile complexed to a Lewis acid, one would not expect a synchronous reaction. The C1-C6 and C4—C5 bond lengths in the NC-transition-state structure for the BF3-catalyzed reaction of acrolein with butadiene are calculated to be 2.96 A and 1.932 A, respectively [6]. The asynchronicity of the BF3-catalyzed carbo-Diels-Alder reaction is also apparent from the pyramidalization of the reacting centers C4 and C5 of NC (the short C-C bond) is pyramidalized by 11°, while Cl and C6 (the long C-C bond) are nearly planar. The lowest energy transition-state structure (NC) has the most pronounced asynchronicity, while the highest energy transition-state structure (XT) is more synchronous. 

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... 

Two mechanistic variations can be envisioned to be operative in some Wittig reactions. In some cases (Z-stereoselective reactions) a very asynchronous reaction with a 2-center "anti" pseudo betaine transition state is involved. The other mechanism involves a more synchronous concerted reaction with a 4-center "syn" transition state. These two mechanisms may compete, e.g., in the reactions of semistabilized ylides with aldehydes where mixtures of Z- and E-alkenes are obtained. Further details of these studies will be provided in future publications, as well as additional data about solvent effects. 

Ligation of nitriles to a metal center changes features of the reaction mechanism. In the case of metal-free reactions, the mechanism is concerted and highly synchronous, that is, the reaction occurs in one step via formation of one cyclic flve-membered transition state (TS), and the changes of chemical bonds directly involved in the process take place simultaneously. The estimated degree of asynchronicity of the reactions between nitrones and uncomplexed nitriles is only 5-15% [43, 45-48]. In contrast, the coordination of nitriles to the metal (Pt or Pd) results in a dramatic increase of the reaction asynchronicity to 19-49%, and, in some cases, the TS of the reaction may become acyclic [47]. At the same time, the global mechanism of the DCA usually remains concerted. 

However, the data of direct quantum chemical calculations on the PES of this reaction indicate strong steric repulsion in the case of the supra-antara approach I, which makes this mechanism energetically unfavored [2, 3]. But the route II of the parallel approach with the synchronous formation of two bonds C—C is equally energetically unrealizable. The MINDO/3 calculations with precise localization of the transition state by minimization of the gradient norm lead to the structure III. A three-center Cj—C2—C3 interaction, predictable from the perturbation theory [5], takes place in this structure. The form of the transition vector III reflects the character of the carbon atom shifts which determine the reaction path where the processes of breaking and making of the C—C bonds are sharply asynchronous. The ab initio calculations [3, 6] lead to the same conclusion, they indicate a transition state with the structure IV in which two CC bonds lying in parallel planes are spaced 2.237 A apart. 

The formation of the less favored product XII proceeds in a manner similar to the dimerization of two molecules of ethylene. There are no intermediates on the reaction PES, a type III three-center interaction arises also in the transition state. First a bond is formed between the unsubstituted carbon atoms. Even though the reaction is concerted, the process of the formation of two new carbon-carbon bonds is asynchronous. The calculated activation barrier is 59.7 kcal/mol. 

In this paper, we have shown the theoretical validity of the suggested catalytic cycle for the hydrosilylation of ketones using N-heterocyclic diaminocarbenes Cu(l) hydride catalysts. The activation of the catalyst from a copper fluoride complex, as well as both steps of the catalytic cycle, involves a 4-center a metathesis transition state. The reactants are guided toward these transition states, through the formation of energetically favored van der Waals complexes, formed through favorable electrostatic interactions. Analysis of transition states shows that the a metathesis reactions studied here can be described as asynchronous concerted mechanisms, with the transfer from Cu-linked atoms occurring prior to the transfer toward the metal center. 

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