Nucleophilic attack hydrocarbon complexes

Nitration of benzofuroxans (Section VII, A) and decomposition of polynitrophenyl azides, provide generally satisfactory routes to nitrobenzofuroxans. The nitro groups render the ring susceptible to nucleophilic attack (see Section VII,B). 4,6-Dinitrobenzofuroxan, 5,6-dinitrobenzofuroxan, and nitrobenzodifuroxan (34) act as acceptors in change-transfer complex formation with aromatic hydrocarbons. Nitrobenzofuroxans have not been reduced to the... 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

Electron-rich, unsaturated hydrocarbons, which are normally resistant to nucleophilic attack, become generally reactive towards nucleophiles upon complexation to an electrophilic transition metal such as palladium(II), platinum(II) or iron(II). Complexation also directs the regio- and stereo-chemistry of the nucleophilic attack, the result of which is a new organometallic complex, which can often be used to promote additional functionalization of the original substrate. Synthetically useful examples of such processes are presented in the following sections. 

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

Palladium complexes are generally superior catalysts for oxidation reactions, whereas other noble metals are more active for other reactions, e.g., rhodium for hydroformylation. All of these reactions seemingly involve activation of the olefin substrate by rr-complex formation with the noble metal catalyst.513 The oxidation reactions discussed in the following generally depend on nucleophilic attack on the coordinated olefins (or other hydrocarbons) to effect oxidation of the substrate. 

As is the case for monometallic complexes, ligands, such as CO and r-bound hydrocarbons, are activated toward nucleophilic attack. These reactions, which do not require coordination of the nucleophile to the metal, can be extremely rapid. 

Carbon-Carbon Bond-forming Reactions via Organometal1ic Electrophilies. - Functionalization of unsaturated hydrocarbons by nucleophilic attack on their metal complexes continues to receive attention. Sequential double nucleophilic addition to a cationic cobalt complex of butadiene... 

Davies et al. have developed a succinct set of guidelines to predict the most favorable position of nucleophilic attack on electronically saturated cationic metal complexes beating more than one unsaturated hydrocarbon ligand. In essence, the so-called DGM rules state that (i) polyenes (L -type ligands) are more reactive than polyenyls (L, -type ligands see also Scheme 11) (ii) open or acyclic ligands react before closed or cyclic ligands and (iii) addition to... 

Among the earliest reports of alkane activation by a transition metal complex were the articles by Shilov in which Pt(n) served as a catalyst for methane oxidation and Pt(iv) served as a stoichiometric oxidant." The mechanism of C-H activation was termed electrophilic, as the cationic metal was postulated to interact with the electrons of the C-H bond which then lost a proton, forming a metal-carbon bond without a change in oxidation state. Oxidation of the complex by two electrons was then followed by nucleophilic attack at carbon, giving a functionalized hydrocarbon (Scheme 1). 

Factor (b) is a well-known phenomenon (80). Recently Skulski and Kan-abus (81) presented experimental results hinting at the formation of such complexes between benzene and p-nitroanisole, one of the indicators used to establish the tt scale. Since the work of Winstein and Feldman (82) has established the existance of C.T. complexes between aromatic hydrocarbons and tropylium cation, the possibility of weak nucleophilic attack by aromatics on incipient carbonium ions seems reasonable. 

Nucleophilic attack on coordinated unsaturated hydrocarbons is one of the fundamental and particularly well studied reactions in Organometallic Chemistry. The addition of carbonylme-tallates instead of common nucleophiles provides a directed synthesis of hydrocarbon bridged complexes. Carbonylmetallates (particularly Re(CO)5", Os(CO)4 ") add to 7C-bonded olefin, acetylene, allyl, diene, trimethylenemethane, dienyl, benzene, triene and cycloheptatrienyl ligands in cationic complexes and give hydrocarbon bridged bi- and trimetallic, homo- or he-teronuclear complexes [1]. 

This difficulty does not arise with tertiary amines since the C—H bonds adjacent to nitrogen are less susceptible to nucleophilic attack than those next to oxygen in ethers. TMEDA (p. 38) complexes strongly with alkyllithiums. The n-butyllithium chelate is monomeric and very soluble in hydrocarbons. It is a very strong metallating agent. It reacts with toluene at room temperature and more slowly, with benzene. Butyllithium in the absence of TMEDA does not normally attack benzene or toluene and can even be prepared in these solvents. 

The mechanism by which a zinc perthiomercaptide complex reacts with rubber is not known with certainty. It is possible that reaction involves nucleophilic attack of a terminal perthiomercaptide sulphur atom on an a-methylenic or a-methylic carbon atom in the rubber hydrocarbon (RH) ... 

The nucleophilic attack on a polyhapto ligand occurs with a decrease of the hapticity by one unit in the reaction product. In addition, three rules established by Davies, Mingos and Green appear, when the reaction is under charge control, to determine the regioselectivity of the attack, if several hydrocarbon ligands are present in the same complex or if several possible sites are available for the nucleophilic attack ... 

An attempt has been made to predict the sites of nucleophilic attack on [M(CO)3(fl--hydrocarbon)] complexes using the perturbation theory of reactivity. For the model allyl substrate [Co(CO)3( j -C3H5)] the site preference CO > M > C3H5 was predicted for reaction with hard nucleophiles in polar solvents. On the other hand, with soft nucleophiles initial attack at the ir-allyl ligand was favored. Mechanistic studies have suggested only a small energy difference between attack by alkoxide ions on the allyl ligand and the metal in related ( Tr-allyl) palladium(II) complexes. ... 

Unactivated, unsaturated hydrocarbons are not routinely subject to attack by nucleophiles. Complexation of alkenes, alkynes, or arenes to a positively charged metal center may lead to activation of the hydrocarbyl group towards nucleophilic attack and, in particular, to addition of hydride ion. The effect of the [LnM] fragment on the reactivity of the unsaturated ligand is largely due to the transfer of electron density from the unsaturated hydrocarbyl to the metal center upon coordination, a process that is most effective when the metal center is positively charged. Figure 10.5 ... 

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