Insertions Involving Alkenes

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

In any homogeneous catalytic reaction involving an alkene, isomerization of the alkene is always a possibility. The insertion of alkene into the M-H bond can occur in a Markovnikov or anti-Markovnikov manner (see Section 5.2.2). Alkene isomerization involves Markovnikov addition, which is followed by a /3-hydride elimination. A simplified catalytic cycle is shown in Fig. 7.1. 

Insertion reactions involving metal alkoxides are also known. For example, carbon dioxide is known to react with some metal alkoxides as shown in equation (12). The formation of a bidentate ligand is a significant thermodynamic driving force for some of these reactions. The isoelectronic aryl and alkyl isocyanates and carbodiimides can react similarly. Insertion reactions involving alkenes and carbon monoxide are known for platinum alkoxides. 

Phosphalactones result from the addition of CICH2PCI2 with propiolic acid, CHCCOOH F Free radicals are involved in the insertion of alkenes into PX3 (X = C1, Br). The reactions may be initiated by radiolysis or photochemical irradiation. With O2, phosphonate products are obtained, often in conjunction with phosphates . 

The many reactions that involve insertion of alkenes or alkynes into metal-carbon or metal-hydrogen bonds provide further examples of hypercoordination of carbon atoms during reactions. For example, an alkene may coordinate to the coordinatively unsaturated metal atom of a metal hydride complex prior to inserting into the metal-hydrogen bond [Eq. (1.9)] ... 

Studies cited earlier (Section VI.D.l)209,21 212 and the observation of vinylsilane products under conditions of some catalyzed hydrosilylation reactions262,269,270 have implicated another possible mechanism involving alkene insertion into a M-Si bond. This possibility is shown in Scheme 11. Insertion into the M-Si bond would produce a... 

Alkyl migrations are not confined to the formation of acyl groups, and, for example, alkene insertion involves the conversion of a coordinated alkene to a cr-bonded alkyl group. Equation 23.37 shows the migration of an H atom related alkyl migrations occur and result in carbon chain growth. 

As with other diazoalkanes, diazomethane reacts with alkenes to form cyclopropane derivatives (sec. 13.9.C.i).272 Reaction with aromatic derivatives leads to ring expansion to cycloheptatriene derivatives.223 Both of these reactions (addition to an alkene or arene insertion) involve generation of an intermediate carbene and addition to a jt bond they will be discussed below. Many of the reactions of diazomethane tend to be ionic in nature and are, therefore, set aside from the other diazoalkane chemistry in this section. One of the commonest uses of diazomethane itself is esterification of small quantities of acids, especially acids that are precious for one reason or another. The reaction is quantitative and gives good yields of a single product, as in Tadano s conversion of 338 to the methyl ester of 339224 in a synthesis of (-)-verrucarol. 

This reaction is often assumed to be of importance in alkyne polymerization. The first clues to the mode of action came from the work of Thorn and Hoffmann on the insertion of alkenes into Pt—H bonds, which they suggested involved a four-coordinate intermediate in which the alkane replaces a ligand and achieves a coordination site cis to the hydride. 

The 1,2-insertion of alkenes into transition metal-carbon o-bond leads to C-C bond formation under mild conditions, as described in Chapter 6. This reaction is considered to be a crucial step in the coordination polymerization and carbometalation of alkenes catalyzed by transition metal complexes. A common and important carbometalation is the Heck-type arylation or vinylation of alkene catalyzed by Pd complexes [118], The arylation of alkene, most typically, involves the formation of arylpalladium species and insertion of alkene into the Pd-aryl bond as shown in Scheme 5.20. The arylpalladium species is formed by the oxidative addition of aryl halides to Pd(0) complexes or the transmetalation of aryl compounds of main group metals with Pd(II) complexes. Insertion of alkene into the Pd-aryl bond produces 2-arylalkylpalladium species whose y6-hydrogen elimination leads to the arylalkene. Oxidative chlorination of the 2-arylalkylpalladium intermediate forms chloroalkanes as the product. 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

Since most of the important insertion// -elimination processes affect molecules where A=B is a carbon-carbon double or biple bond, the discussion is organized considering brst the different types of M-X bond involved, and the insertion of alkenes or alkynes. Less abundant cases are discussed later. The reader should have in mind that sometimes the word alkene is used in a more general way, meaning an unsaturated carbon-carbon bond not involved in aromaticity. Also the word alkyl can be used in a more general context meaning hydrocarbyl. 

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