Olefin 6-electron ligands

This favoured stereochemistry, together with the 18-electron configuration of the molybdenum atom, accoimt for the preference of the cycloheptatriene to act as a 6-electron ligand in this compound. On the other hand iron often favours a diene ligand rather than a triene this is probably due 

Transition motal complexes containing n-bondod heterocyclic ligands 

Heterocyclic compounds such as pyrrole or thiophene can also act as 5- or 6-electron ligands to transition metals. Only a few of their complexes are known to date, but there seems no reason why a wide variety of unsaturated heterocyclics should not form stable, 7r-bonded metal complexes. Some examples, with preparative routes, are given below  

Of particular interest are various complexes formed by organoboron compounds with transition elements. Treatment of the N-lithio-derivative of the S membered boron-nitrogen heterocycle, 7.28, with anhydrous ferrous chloride yields a dark-brown diamagnetic product, for which the sandwich ferrocene-like structure 7.29, has been proposed. 

A hexamethylborazine chromium tricarbonyl complex is also known. It seems reasonable that it will have an eclipsed configuration with the donor nitrogen in the position trans to the ir-add carbonyls. The ring-chromium bonding would be expected to be more localized than in benzeneCr(CO)3, but less so than in a complex L Ct CO) where L is (for example) pyridine. 

A number of olefins which may act as 6-electron ligands are shown in Table 17. Cycloheptatriene may act as both a 4-electron (see p. 138) and 6-electron ligand. With the Group VI metal heracarbonyls and clohepta-triene the complexes CtHsM(CO)3 are formed [93a]. The structure of ( cloheptatriene molybdenum tiicarbonyl, S.IO, has been determined by X-ray analysis [93]. 

The six sp carbons form a plane and the molybdenum lies below that plane almost equidistant from the six-carbons ( 2 S3 A). It is interesting that the C-C distances of these carbons clearly show alternate bond lengths, as found in the free ligand. The methylene group of the CtHs ligand lies above the C6-plane which suggests some sp character for the carbon atoms to which it is attached. The structure also shows that the three carbon monoxide groups are, essentially, diametrically opposed to the positions of the carbon-carbon double bonds and the metal atom has an approximately octahedral environment. 

Treatment of chromium hexacarbonyl with cyclo-octa-l,3,5-triene gives the deep red complex CsHioCr(CO)3 [95]. X-Ray analysis shows the structure, 5.11 [96] in which six of the carbons are within 2-24-2-28 A of the chromium atom and may be assumed to bond to it, the remaining two carbon atoms are further away (/ 3-l A). The bond lengths of the six-bonded carbons show distinct alternate single and double bond character. 

Ccylo-octatetraene forms the complex CsH8Mo(CO)3 which has a similar structure to that of the cycIo-octa-l,3,5-triene complex, 5.13 (see p. 199). 

Bicyclo-[4,3,0]-nonatriene, 5.12, forms brick-red complexes with tungsten and molybdenum, C9HioM(CO)3 [97]. They undergo catalytic reduction, adsorbing one mole of hydrogen per mole of the complex. The structure, 5.13, is proposed, where the ligand is the monocyclic tetra-ene tautomer of the parent olefin. 

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The C2-symmetric 2,6-bis(2-oxazolin-2-yl)pyridine (pybox) ligand was originally applied with Rh for enantioselective hydrosilylation of ketones [79], but Nishiyama, Itoh, and co-workers have used the chiral pybox ligands with Ru(II) as an effective cyclopropanation catalyst 31 [80]. The advantages in the use of this catalyst are the high enantiocontrol in product formation (>95 % ee) and the exceptional diastereocontrol for production of the trans-cyclopropane isomer (>92 8) in reactions of diazoacetates with monosubstituted olefins. Electronic influences from 4-substituents of pyridine in 31 affect relative reactivity (p = +1.53) and enantioselectivity, but not diastereoselectivity [81]. The disadvantage in the use of these catalysts, at least for synthetic purposes, is their sluggish reactivity. In fact, the stability of the intermediate metal carbene has allowed their isolation in two cases [82]. 

Of the various possibilities for the group IB-group IVB bond, only the synthesis of compounds containing a cr or bond between the group-IB metal and the one-electron (e.g., Me, Ph, C=CR) or two-electron (e.g., CO, olefin) carbon ligands is known in detail. The synthesis of -metal IB-carbon-containing compounds is discussed in 5.6.2.3.6. 

Nanoparticles of palladium supported on resins were used in Mizoroki-Heck reactions of electron-poor olefins under ligand-free conditions. For instance, p-bromoanisole proceeded to j)-methoxy butylcinnamate in 94% yield when the power of microwave irradiation was increased to 375 W (Scheme 15.17) [209]. 

Steffanut, P. Osborn, J. A. DeCian, A. Fisher, J., Efficient Homogeneous Hy-drosilylation of Olefins by Use of Complexes of Pt° with Selected Electron-Deficient Olefins as Ligands. Chem. Eur. J. 1998,4, 2008-2017. 

Barrall II, E. M. Flandera, M. A. Logan, J. A., A Thermodynamic Study of the Crosslinking of Methyl Silicone Rubber. Thermochim. Acta 1973,5,415-432. James, P. M. Barrall, E. M. Dawson, B. Logan, J. A., Crosslinking of Methyl Silicone Rubbers. 2. Analysis of Extractables from Samples Crosslinked Under Various Conditions. J. Macromol. Sci. PartA—Chem. 1974, 8(1), 135-155. Steffanut, P. Osborn, J. A. DeCian, A. Fisher, J., Efficient Homogeneous Hy-drosilylation of Olefins by Use of Complexes of Pt° with Selected Electron-Deficient Olefins as Ligands. Chem. Eur. J. 1998,4, 2008-2017. 

The complexation of unsaturated hydrocarbons by transition metals is a powerful activation method that plays a fundamental role in their stoichiometric and catalytic transformation. The bonds formed are governed by jt backbonding. Olefins, dienes and arenes are usually 2-electron L, 4-electron L2, and 6-electron L3 ligands respectively, and alkynes are 2-electron L or 4-electron L2 depending on the needs of the metal. The odd-electron ligands are the 3-electron LX allyl radical and 5-electron L2X dienyl radicals. Cyclopentadienyl and arene sandwich complexes will be examined in Chap. 11. 

The bonding and structures of ir-olefin complexes of the transition elements have already been discussed in some detail in Chapter 5, and their preparation in Chapter 6. We have shown above (p 189) that 2-electron ligands can be formed from and can themselves be converted into 1-electron ligands. The following example shows how one may proceed from a coordinated olefin to an enyl or 3-electron system ... 

Cycloheptatriene complexes, in which the olefin is acting as a 6-electron ligand, may undergo hydride abstraction to give ir-cycloheptatrienyl metal cations, e.g. 

It may be noted that even in the case of conjugated olefins, the number of available electrons on an olefin by no means solely dictates the stoicheio-metry of the olefin-metal product. Thus the reactions on pp. 9, 10 show butadiene acting as a 2- and 4-electron ligand towards iron, and cyclo-octa-l,3,5-triene may act as a 4- or 6-electron ligand [35], e.g.. 

It is worth noting a in that 2-electron ligands such as ethylene form stable complexes with electron-rich transition metals, and hence for those metals with fewer /-electrons, the formation of olefin-metal complexes is favoured when the metal is in a low oxidation state. 

Examples of olefins which act as 4-electron ligands with transition metals are given in Table 7. It can be seen that these ligands form complexes with a wide variety of transition metals. A comparison of Tables 1 and 7 shows... 

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

The ability of Fischer carbene complexes to transfer their carbene ligand to an electron-deficient olefin was discovered by Fischer and Dotz in 1970 [5]. Further studies have demonstrated the generality of this thermal process, which occurs between (alkyl)-, (aryl)-, and (alkenyl)(alkoxy)carbene complexes and different electron-withdrawing substituted alkenes [6] (Scheme 1). For certain substrates, a common side reaction in these processes is the insertion of the carbene ligand into an olefinic C-H bond [6, 7]. In addition, it has been ob-... 

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