1,2-Addition four-center transition state

In the second step, the n complex becomes the addition product by passing through a four-center transition state in which the boron atom is partially bonded to the less substituted carbon atom of the double bond. 

Addition takes place through the initial formation of a n complex, which changes into a cyclic four-center transition state with the boron atom adding to the less hindered carbon atom. The dashed bonds in the transition state represent bonds... 

A theoretical study of the reaction mechanism for addition of organozincate complexes to aldehydes was recently performed using density functional theory.298 It has been suggested that the addition takes place through formation of a four-centered transition state and, therefore, it can be considered a typical nucleophilic reaction. 

In 1998, Kubota, Nakamura and coworkers stndied the addition of substituted (alkoxy)allyllithium and zinc reagents to cyclopropenone acetaP. They found that the regioselectivity of the (alkoxy)allyllithiation is dependent on the substituent, while the diastereoselectivity remains constantly high (>97%). This is supported by their theoretical study (at the B3LYP/6-31G level. Scheme 3) which reveals that a (hydroxy)allyllithium species of 7r-allylmetal nature can react with cyclopropene via two [2 + 2]-type four-centered transition states of similar energies leading to a- and y-adducts. 

Hydroboration is a sterospecific syn addition. The addition occurs through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C—B and C—H bonds are, therefore, formed from the same side of the double bond. In molecular orbital terms, the addition is viewed as taking place by interaction of the filled alkene n orbital with the empty p orbital on boron, accompanied by concerted... 

Interestingly, the Zr-promoted crotylzincation of 5-decyne occurred without allylic transposition (equation 75), presumably via a four-centered transition state, unlike the uncatalyzed addition of crotylzinc bromide to alkynylsilanes which proceeded with complete S 2 regioselectivity108. 

The characteristic features of hydroboration of alkenes—namely, regioselec-tivity, stereoselectivity, syn addition, and lack of rearrangement—led to the postulation of a concerted [2 + 2] cycloaddition of borane353,354 via four-center transition state 37. Kinetic studies, solvent effects, and molecular-orbital calculations are consistent with this model. As four-center transition states are unfavorable, however, the initial interaction of borane [or mentioned monobridged dimer, Eq. (6.56)] with the alkene probably involves an initial two-electron, three-center interaction355,356(38, 39). 

The addition of B2C14 to multiple bonds was shown to be stereoselective. The initial diborane products isolated after the addition of B2C14 to cis- and trans-2-butene were oxidized to yield meso- and racemic-2,3-butanediols, respectively.478 480 A similar treatment of the addition product of cyclohexene resulted in cis-1,2-cyclohexanediol 480 These observations are consistent with syn addition and the involvement of four-center transition state 42. Similar studies with addition... 

New catalysts have been described,646 and ab initio MO calculations have shown that the transformation takes place through a four-center transition state.647 In addition, the anomalous relative reactivities of substrates, specifically, the higher reactivity of alkynes compared to those of alkenes, can be explained by considering the reaction to essentially be a nucleophilic attack by an alkyl anion, rather than an electrophilic one. 

In either case, one has to proceed through a four-centered complex for which there is no appreciable driving force and for which all evidence indicates an order of 20 to 26 kcal. of strain energy. In addition, the A factor for such a reaction, because it involves a tight transition state with loss of internal rotation, is expected to be low by about a factor of 10. The over-all result is that one expects a four-centered transition state, a relatively high activation energy, and a relatively low A factor. 

In addition, studies indicate that Zn(CH3)2 undergoes rapid exchange with Cd(CH3)2, and various postulates concerning the mechanism of this reaction have been made (57, 87). Recent studies of this system in methyl-cyclohexane solvent have clearly shown that the reaction is first order in each of the components and proceeds with an activation energy of 17 kcal/mole (57). This study indicates that the exchange process proceeds through a four-centered transition state... 

A small kn/ko of 1.3 for addition of (H/D)SiMe3 is consistent with hydride transfer from the silane via a four-center transition state. 

Bulky titanium alkylamido complexes are capable of equilibrating with aliphatic and aromatic hydrocarbons via concerted 1,2-elimination/l,2-addition processes a Ti=NR imido species is formed as a transient intermediate. Kinetic parameters suggest a four-center transition state with an almost linear NHR arrangement 122... 

Nitrosyl halides add to alkenes references are scattered through the litnnture back to 1875 (ref. 194 and references cited therein). The adducts vary enormously in their stability, but when their structures allow they, like nonhalogenated nitroso compounds, isomerize to oximes or dimnize. The orientation of the reaction is consistent with an electrophilic medumism, in which the reagent is polarized as NO Hat. Bicyclic substrates and reaction media of low polarity favor syn addition, suggesting a four-center transition state (Scheme 81). Aziridine synthesis via NOCl/alkene adducts is discussed in Section 3.5.2.1. 

As with C-alkylation, the mechanism of amino group replacement can follow the elimination/addition or the nucleophilic replacement path. Both mechanisms are indeed mentioned in the literature and are cxrcasionally claimed to occur concurrently. The elimination/addition path is suggested in the reaction of p-aminoketones with uracils, and a four-centered transition state is proposed for the. same reaction with indole Mannich bases. ... 

The addition proceeds through a four-center transition state 2 involving a four-membered ring. 

The site-selective addition of nitrosyl chloride to an f-configurated double bond, determined by kinetic studies18 with acyclic and cyclic (E)- and (Z)-alkenes in chloroform or by the reactions with (Z,E, )-1,5,9-cyclododecatriene19 and (Z,E)-, 5-cyclodecadiene"fl in dichloromethane, can be explained in terms of conformational stability in the four-centered transition state or by a syn-type addition however, the configuration of the products of such reactions was not elucidated. 

There is similar controversy over whether the monomer (M-2) or open-dimer (OD-2) transition state governs the mode of addition of organolithium compounds to carbonyl compounds (Fig. 2). Although a four-center transition state M-2 involving the monomer has long been believed to participate in the alkylation [34], some ab initio calculation evidence shows that an open dimer OD-2 or related dimeric structure is more likely to be involved in the reactions, especially when (MeLi)2 participates [35, 36]. McGarrity and co-workers showed that the dimer of n-BuLi is tenfold more reactive than the tetramer toward benzaldehyde in THF. Even at high dilution there is no detectable concentration of the monomer [37],... 

Other Nitrile Complexes. While similar additions of iron (i) and rhodium (11) hydrides to acrylonitrile to form 1-cyanoethylmetal complexes have been reported, 2-cyanoethylmetal complexes also form in certain cases (43, 51). Organotin hydrides may add to acrylonitrile in either direction, depending on the conditions of the reaction (25). Formation of the 2-cyanoalkyltin adduct apparently involves a radical mechanism, whereas a polar mechanism is operative in forming the 1-cyano-alkyl adduct. A four-center transition state was not considered probable in the latter case. 

Stereochemistry. The four-center transition state for hydroboration of alkenes discussed above implies that addition of the B-H bond to a double bond proceeds in a syn-mannev. Moreover, hydroboration of a bicyclic alkene, such as a-pinene, results in the addition of B-H from the less hindered face of the molecule to give IpCjBH. 

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. 

Little work has been reported on insertion of alkynes into Al-H bonds. ( 4119)2 AlH reacts readily with internal alkynes to give uniquely the cis addition product, as expected for a migratory insertion involving concerted addition via a four-center transition state". On the other hand, addition of LiAlH4 to internal alkynes results in trans addition by attack on the triple bond by hydride ion. 

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