Alkylation molecular orbital calculation

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

An alkyl radical and a nitroxide radical exist in an equlibrium with the corresponding alkoxyamine as their coupling product (Eq. 57). Moad and Rizzardo [213] and Kazmaier et al. [214] independently estimated the effects of the structure of the alkyl group and the nitroxide on the dissociation energy of various alkoxyamines into the radicals by semiempirical molecular orbital calculations. The bond dissociation energies determined are summarized in Table 5 ... 

Several types of molecular orbital calculation have been carried out on the H- and the 2.ff-structures of w-triazole, and on simple alkyl derivatives from these, estimates of several physical properties, in-... 

Dimercapto-4-cyanoisothiazole is alkylated primarily on the 3-mercapto group in agreement with molecular orbital calculations.143 A 5-methylmercapto group may be displaced by other nucleophiles such as hydrazine122 or alkoxide.51 3-Mercapto-5-phenylisothiazole has been oxidized to the 3-sulfonic acid and sulfonamide.66... 

The photoelectron spectra of [Co3(CO)9(/i,3-CR)], and the results of molecular orbital calculations, lead to two conclusions. First, the alkyl-idyne bridge donates a lone pair to the metal triangle, and second, the presence of this bridge, rather than the metal-metal interactions, is crucial to the stability of the complex (184, 185) and, by inference, its radical anion. 

Bonds become weaker as we move down the Periodic Table. Compare C O and C S, or the carbon-halogen bonds C—F, C—Cl, C—Br, C—1. This is a consequence of the first generalization, since bond distances must increase as we go down the periodic table because the number of inner electrons increases. However, it is noted that high-level ab initio molecular-orbital calculations confirm that the effect of alkyl substituents on R—X bond dissociation energies varies according to the nature of X (the stabilizing... 

Cossee and Arlman (13,14) were among the first to propose a comprehensive mechanism for Ziegler-Natta catalysis and supported their proposals with molecular orbital calculations. The Cossee-Arlman proposal involves a "migratory alkyl transfer" (15) and, with some refinements, remains the most widely cited mechanism for Ziegler-Natta catalysis. A summary is presented below. (For more details, see references 5,12,16 and 17.)... 

The reactions of Fischer and Schrock carbyne complexes are of interest because they may act as intermediates in chemical synthesis. Typical reactivity of carbyne complexes with nucleophiles (i.e., alkyl lithium reagents and metal(I) alkoxides) are consistent with the electronics and molecular orbital calculations for these types of complexes [Eq. (9) 22]. The nucleophile adds to the carbyne-carbon, resulting in the formation of a carbene complex. The reaction of a Grignard reagent with a carbyne complex is expected to demonstrate similar reactivity. 

As discussed in 11.3.3.1 a molecular orbital calculation favors cis coplanar geometry for 1,2-insertion of an olefin into the metal-alkyl bond of d metals. Although similar behavior is expected for alkyne insertion reactions, the assumption of a coplanar transition state may not be valid because of the greater availability of alkyne n orbitals . Experimental evidence for the coplanar transition state was obtained in a study of the kinetics of reactions (1) and (m). 

The methylidyne cubane [Cp Ti(/x-CH)]4 (Scheme 202) is obtained as a dark brown crystalline solid by thermolysis of Cp TiMe in toluene with methane elimination. This transformation was monitored by NMR and no intermediates are observed. The signals assignable to the methylidyne groups appear as singlets at 6 17.75 in the NMR and at 6 490.8 in the 13C NMR spectra. In order to analyze the interaction between the /i3-ligand and the titanium centers, extended Huckel molecular orbital calculations have been carried out. In contrast, the thermolysis of the trinuclear oxo alkyls [Cp Ti(/i-0)(CH2R)]3 (R = H, Me) affords the //3-alkylidyne derivatives [Cp Ti(//-0)]3-(/x3-CR).505-507... 

More recently, there have been several investigations of CO insertion using high-level molecular orbital calculations at the DFT level. These have confirmed that CO insertion is indeed alkyl migration, and two intermediates, structures 3 and 4, have been shown to exist along the reaction path.10... 

Marks and co-workers (12) have studied the alkyl substituted compounds 7-16. Assuming that INDO/2 molecular orbital calculations on alkyl radicals can reasonably predict experimental electron-nuclear hyperfine coupling constants, a, they have calculated the a values for each of the alkyl substituents. Taking the ratio of the contact shifts of the ortho positions in 7 and vinylic position in 16 as equal to the ratio of calculated a values and the ratio of the geometry factors as equal to the ratio of pseudocontact shifts, Marks and co-workers could solve for the contact and pseudocontact shifts in 7 and 16. Factoring the... 

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

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