Activation of alkenes

Khan, M. M. T. 1974, Homogeneous Catalysis by Metal Complexes, Vol. II, Activation of Alkenes and Alkynes, Academic Press New York - London... 

Compounds (L)AuR have been used as precursor molecules for the in situ preparation of the strong nucleophiles [(L)Au]+ X- by treatment with strong acids HX (X = CF3S03, CF3C02, BF4, PF6, SbF6 etc. L = tertiary phosphine R = alkyl) in polar solvents (Equation (2)). The solutions are used as catalysts for the activation of alkenes and alkynes for addition of water, alcohols, and amines (Sections 4 and 10). 

We have already reviewed the activation of alkenes, alkynes, and carbon monoxide towards nucleophilic attack. The heterolytic splitting of dihydrogen is also an example of this activation it will be discussed in Section 2.10. The reaction of nucleophiles with silanes co-ordinated to an electrophilic metal can be regarded as an example of activation towards nucleophilic attack (Figure 2.28). Complexes of Ir(III) and Pd(II) give t.o.f. for this reaction as high as 300,000 mol.mol. fh"1. 

A development of the last two decades is the use of Wacker activation for intramolecular attack of nucleophiles to alkenes in the synthesis of organic molecules [9], In most examples, the nucleophilic attack is intramolecular, as the rates of intermolecular reactions are very low. The reaction has been applied in a large variety of organic syntheses and is usually referred to as Wacker (type) activation of alkene (or alkynes). If oxygen is the nucleophile, it is called oxypalladation [10], Figure 15.4 shows an example. During these reactions the palladium catalyst is often also a good isomerisation catalyst, which leads to the formation of several isomers. 

Activation of alkenes towards nucleophilic attack by a variety of nucleophiles employing A-phenylselenophthalimide or A-phenylselenosuccinimide as an electrophilic source of the phenylseleno group. 

In addition to the examples outlined above, various funetionalized olefins are also known to undergo CM with high E-selectivity. Functional groups that contribute to high E-olefin formation will be discussed in Section 11.06.4. In these cases, CM provides an orthogonal route to products that are typically generated via either the selective C-H activation of alkenes or allylic oxidation. 

M. M. T. Khan and A. E. Martel, Homogenous Catalysis by Metal Complex Activation of Alkenes and Alkynes" Academic Press, New York, 1974. 

Protection and Activation of Alkenes by the Coordination of Iron Carbonyls... 

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an... 

Chianese, A.R., Lee, S.J., and Gagne F M. R. (2007) Electrophilic activation of alkenes by Platinum(II) so much more than a slow version of Palladium(II). Angewandte Chemie, International Edition, 46, 4042 1059. 

It has already been seen that the reactivity of alkylidene and alkylidyne complexes may be dictated by the nature of both the metal and the carbon substitutents. These principles apply equally to alkene coordination (Figure 6.8). Two metal-ligand fragments serve to illustrate these principles in the activation of alkenes to electrophilic and nucleophilic... 

Alkyne hydrido cluster complexes of the type [Co3(p.-H)2(/i-alkyne)Cp3] can be prepared by 1,2 double C—H activation of alkenes.132 133 For example, the p.-cycloalkyne complexes 36 can be prepared either from the respective cycloalkenes and [Co(C2H4)2Cp] or by using the reductive cleavage of cobaltocene with potassium as a source of CpCo fragments. (See Formula 36.)... 

Activation of alkenes by amides and A-alkylamides has not been used as much as activation by ester groups. The amides are more difficult to reduce than the corresponding esters but have been reported to give the LHDs in 40-80% yields [2,4]. 

Reaction of dihalocarbenes with alkenes, [1 +2] cycloaddition, is the method of choice for the preparation of 1,1-dihalocyclopropanes. The reaction proceeds stereospecifically preserving the configuration of the alkene in the products. These observations allow the conclusion to be made that the dihalocarbene reacts in the singlet state with alkenes. Experimental data (relative activities of alkenes, selectivity indices as well as theoretical calculations indicate that dihalocarbenes are electrophilic species. This means that they react readily with electron-rich (nucleophilic) alkenes. Dihalocarbenes may also react with electron-poor alkenes, but at a much slower rate. In the case of alkenes with a fairly unreactive double bond, dihalocarbene may also attack other sites of the alkene molecule, e.g. insert into a C-H bond. The selectivity (reactivity) of dihalocarbenes depends on the temperature at 20 C typical dihalocarbenes can be arranged in the order given, with respect to their selectivities versus reactivities (Houben-Weyl, Vol. El9b, p 1598). 

Scheme I 71-allylpalladium complexes from Pd-mediated Allyl C-H activation of alkenes...

The activation of hydrogen proceeds according to mechanisms (4.216)-(4,218) therefore, the activation of hydrocarbons probably occurs in a similar way. It was confirmed experimentally that the activation takes place via mechanisms (a), (b), and (c). The activation of alkenes by Pt(II) complexes probably occurs by electrophilic substitution in the first stage ... 

Bimetallic Ru complexes nicely complement the ubiquitous Grubbs alkyli-dene catalysts and related species used as initiators for olefin metathesis. Compared to their monometallic counterparts, they often display a greater propensity at releasing coordinatively unsaturated species essential for the activation of alkene substrates toward metathesis. In the case of homobi-metalUc Ru-arene complexes, the choice of the ancillary ligand was found crucial to achieve high metathetical activities. Thus, complexes 16a and (to a lesser extent) 16b bearing NHC ligands were very efficient catalyst precursors for various types of olefin metathesis reactions, whereas phosphine-based... 

The activation of alkenes by carbophilic Lewis acids is another way to induce cycloisomerization of dienes through a cation cyclization mechanism, provided that undesirable... 

Although there are two distinctive approaches to enan-tioselective sulfa-Michael reactions—nucleophilic activation of thiols and electrophilic activation of alkenes—the recent emergence of bifunctional catalysts has greatly improved our understanding in the wider perspective of origin of asymmetric induction. A comprehensive review of asymmetric sulfa-Michael reactions by Ender and co-work-ers appeared in 2007 that covered the advances in this field from 1970s to 2005. " The reader is advised to refer this review article for a historical overview. The goal of this section, thus, is to focus on a few selected stereoselectivity issues in the asymmetric sulfa-Michael reactions and to document new advances made from 2005 to the present time. 

---