Olefins n-bond

Reaction of an alkene with a nitronium ion involves donation of the n -electron pair of the alkene into an orbital of the nitronium ion. Donation of a bonded electron pair necessarily means that the bond from which it comes is broken. Likewise population of an antibonding level by electron donation generally results in breaking of the bond to which the antibonding orbital corresponds. In this case electron donation of the olefinic r-electron pair results in the rupture of file olefinic n bond and acceptance into file N-O n orbital results in breakage... 

In the first step, a cis 1,2-insertion of cyclopentene takes place the intermediate derived from this insertion is sterically too demanding to permit insertion of another monomer molecule and is isomerised before another cyclopentene molecule is inserted. Since only /1-hydrogen atoms are available for the stabilisation of the active species (jS-agostic interaction), /1-hydride transfer, followed by monomer rotation or migration, and then reinsertion occur, leading to 1,3-linked polymer chains. The rotation around the Zr olefin n bond results in the formation of cis-1,3-poly(cyclopentylene) [scheme (3a)] [20] while the n face migration of the olefin across the nodal plane of the double bond results in the formation of trans-1,3-poly(cyclopentylene) [scheme (3b)] [19]. 

G. J. Kubas, Metal-Dihydrogen and cr-Bond Coordination The Consummate Extension of the Dewar-Chatt-Duncanson Model for Metal-Olefin n Bonding, J. Organomet. Chem. 635, 37-68 (2001). 

When an olefin coordinates to a transition metal, the olefin n bond donates electrons to an empty metal orbital (donor bond) and the olefin n orbital accepts metal valence electrons from a filled metal atomic orbital (back-bond). Two molecular orbitals can describe the conventional representation of the metal-olefin bond as originally proposed by Dewar and modified by Chatt 7). 

Note that the activation energy for 4-hydroxy-4-methyl-2-pentanone (No. 9) is 8 kcal.mole lower than that of the /5-y-unsaturated alcohol (No. 5). This is just about half the difference between the carbonyl and olefin n-bond energies (75 and 60 kcal.mole , respectively), and therefore is in agreement with the rather crude method suggested for obtaining activation energy estimates for reverse ene type elimination reactions (see appendix). 

Infrared measurements show that (a) the olefin retains its double bond in the complex, and (b) the olefin is symmetrically bonded to the metal. Further, the olefin is found to be trans-directing. Chatt and Duncanson (1953) suggest that the metal is bound by both o and n bonds, probably a a-type bond between a Pt (dsp hybrid) and the olefin n bond, together with a r-type bond between a Pt (dp hybrid) and the antibonding n orbital of the olefin (Fig. 269). 

In an equally unambiguous paper. Grant and Lambert showed that it was atomically held oxygen which was responsible for the epoxidation of ethylene and that the molecularly held species, though present, was merely a spectator [24]. The intermediate responsible for isomerisation was, therefore, CH2-CH2-O-Ag, and Grant and Lambert stated as much in their paper [24]. They produced the reaction mechanism in their paper, in which they proposed that selective oxidation results from an electrophilic attack by an O(a) on the olefinic n-bond. ... 

Electron flow from the filled to the empty orbital as shown accomplishes the following the metal-ligand bond is broken, the metal-olefin n bond is... 

The authors concluded that the linear correlations between C for a- and 7c-bonded carbons indicated that metal d to olefin n bonding was of little importance and that <7p was the dominant term in the determination of the chemical shifts of coordinated olefinic carbons, [cf., Clark et al. (38)]... 

The dissociation energy of alkyllithium is very large. In the case of MeLi (dimer, trimer and tetramer) they are —42, —82 and — 128kcaI/mole, respectively [42]. Organolithium compounds are non-transition metal compounds but they can form t-bond structures. The elements of non-transition metal compounds which can form the 7t-bond, are Na, Be, Mg, Ca, B, Al, Ga, In, Tl, Ge, Sn, Pb, P, As, Sb, S, Se, Te, etc., besides Li [44]. The olefinic rr-bond with transition metals is well-known the coordination of the Tt-bond is such that the electrons of the olefinic n bond are donated to the vacant d orbitals, and the backdonation of the rr-bond is such that the electrons of the metal d-orbitals are donated to the antibonding n orbital of the olefin. However, as non-transition metals have no vacant d orbitals, the r-electrons of olefins only partially move to the s- or p-orbital of the metal. Then, the electrons largely remain in a non-bonding orbital, and the backdonation is therefore almost none [44]. 

Similar fragmentations to produce S-cyclodecen-l-ones and 1,6-cyclodecadienes have employed l-tosyloxy-4a-decalols and 5-mesyloxy-l-decalyl boranes as educts. The ringfusing carbon-carbon bond was smoothly cleaved and new n-bonds were thereby formed in the macrocycle (P.S. Wharton, 1965 J.A. Marshall, 1966). The mechanism of these reactions is probably E2, and the positions of the leaving groups determine the stereochemistry of the olefinic product. 

Protonated /V-chloroalkyl amines under the influence of heat or uv light rearrange to piperidines or pyrroHdines (Hofmann-Lriffler reaction) (88). The free-radical addition of alkyl and dialkyl-/V-chloramines to olefins and acetylenes yields P-chloroalkji-, P-chloroalkenyl-, and 8-chloroalkenylamines (89). Various N-hiomo- and N-chloropolyfluoroaLkylarnines have been synthesized whose addition products to olefinic double bonds can be photolyzed to fluoroazaalkenes (90). 

A mechanism which is consistent with the various experimental results for olefin formation involves the initial abstraction of the hydrazone proton (103->106) In this case, however, expulsion of the tosylate anion is associated with the abstraction of a second hydrogen from C-16 instead of hydride attack on the C=N bond (compare 97 98 and 106 107). Ex-... 

Olefins with strained, relatively weak n-bonds form cycLobutanes under rather mild conditions [96] (equations 35 and 36). By contrast, cw-l-methylcyclooctene hardly reacts with I,I-dicholo-2,2-difluoroethylene after 15 days at 150 °C, and norbornene gives only a 9% yield of cycloadduct after 3 days at 120 [96]... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

Various fluorinated cyclic compounds containing -N=N- bonding can be decomposed by the elimination of nitrogen The photolysis of phenylfluorodiazi-rine results in the formation of the intermediate phenylfluorocarbene, which can react instantaneously with olefins [7<5] (equation 45)... 

The reaction mechanism clearly involves the oxidative addition of aniline to an unsaturated Ir(I) complex (Scheme 4-4). Interestingly, the azametallacyclobutane intermediate could be characterized by single-crystal X-ray diffraction [141]. This result confirms that insertion of an olefin into the M-H bond is less favorable than insertion into the M-N bond [142]. 

Another approach was developed by Scott in the 1970 s (7.8) which utilises the same mechanochemistry used previously by Watson to initiate the Kharacsh-type addition of substituted alkyl mercaptans and disulphides to olefinic double bonds in unsaturated polymers. More recently, this approach was used to react a variety of additives (both antioxidants and modifiers) other than sulphur-containing compounds with saturated hydrocarbon polymers in the melt. In this method, mechanochemically formed alkyl radicals during the processing operation are utilised to produce polymer-bound functions which can either improve the additive performance and/or modify polymer properties (Al-Malaika, S., Quinn, N., and Scott, 6 Al-Malaika, S., Ibrahim, A., and Scott, 6., Aston University, Birmingham, unpublished work). This has provided a potential solution to the problem of loss of antioxidants by volatilisation or extraction since such antioxidants can only be removed by breaking chemical bonds. It can also provide substantial improvement to polymer properties, for example, in composites, under aggresive environments. 

Treatment of tetrahydroberberine (26) with sodium benzenethiolate (48) or -selenolate (49) in the presence of ruthenium catalyst afforded the C-14—N bond cleavage products 51 or 52 with a phenylthio or phenylseleno group at C-14 (Scheme 12). The latter was converted to the 10-membered amino olefin 53 on treatment with m-chloroperbenzoic acid. 

It is of interest to investigate the usefulness of this theory to the chemical change involving the interaction between the conjugated systems 56,62,145). Such a-n interactions are frequently stereoselective. The addition to olefinic double bonds and the a, -elimination are liable to take place with the fraMS-mode 146h The Diels-Alder reaction occurs with the cis-fashion with respect to both diene and dienophile. 

Resonance hydrids (15) and (16) would suggest that these carbenes should show nucleophilic behavior toward olefins. As predicted, only olefins with electron-poor n bonds have proven to be suitable substrates for these carbenes. 

Diels-Alder reaction of the 1,3,4-oxadiazole with the pendant olefin and loss of N2, the C2-C3 7t bond participates in a subsequent 1,3-dipolar cycloaddition with the carbonyl ylide to generate complex polycycles such as 45 as single diastereomers with up to six new stereocenters. That the cascade reaction is initiated by a Diels-Alder reaction with the alkene rather than with the indole is supported by the lack of reaction even under forcing conditions with substrate 46, in which a Diels-Alder reaction with the indole C2-C3 n bond would be required [26a]. 

The spectra and calculations all led to the conclusion that there is an usually large interaction between both the 7T and lone-pair orbitals in the carbonyl portion of the molecule with the n and a orbitals of the olefin portion. The first ionization potential (9.57 eV) involves ionization of an electron from the oxygen lone pair, whereas the second (11.19 eV) involves ionization of an electron from the olefin rr-bond. The most vertical ionization is from the 7 ax MO (16.11 eV), the second lone-pair orbital on oxygen. 

The organic substrates in Chart 8 can be divided into two main categories in which (i) the oxidation of olefins, sulfides, and selenides involves oxygen atom transfer to yield epoxides, sulfoxides, and selenoxides, respectively, whereas (ii) the oxidation of hydroquinones and quinone dioximes formally involves loss of two electrons and two protons to yield quinones and dinitrosobenzenes, respectively. In order to provide a unifying mechanistic theme for the seemingly disparate transformations in Chart 8, we note that nitrogen dioxide exists in equilibrium with its dimeric forms, namely, the predominant N—N bonded dimer 02N—N02 and the minor N—O bonded isomer ONO—N02 (equation 88). 

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