Insertion of Alkenes and Alkynes

Intermolecular insertion of alkenes to a-allyl intermediates is possible with an Ru catalyst. For example, 3,5-dienecarboxamide 274 is formed in high yield by Ru(cod)(cot)-catalysed coupling of 2-butenyl methyl carbonate (273) with acrylamide in the presence of A-methylpiperidine [122], Ni-catalysed transformation of allyl 3-butenoate (275) to heptadienoic acids 276a and 276b proceeds by insertion of the double bond to 7r-allylnickel intermediate [123], 

However, whereas intermolecular insertion of alkene to 7r-allylpalladium is difficult, intramolecular insertion proceeds smoothly. This reaction is known as the metalla-ene (pallada-ene) reaction [124]. Use of AcOH as the solvent is crucial. The Pd-catalysed reaction of 277 under CO atmosphere affords the keto ester 279 by one-pot reaction 

The first step of the Pd-catalyzed reaction of allyl acetate bearing allene moiety 284 is attack of the 7i-allyl group at the central carbon of the allene to form 285, (or insertion of one of the allene double bonds) which is the 7r-allylpalladium 286. Then domino insertions of double bond, CO, double bond, CO and double bond occur to form six C—C bonds, affording 287. Finally, the tetracyclic diketone 288 was obtained by -elimination in 22% total yield [127], 

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Carbamoyl or alkoxycarbonyl complexes 87 are obtained by the attack of amines or alkoxides to metal carbonyls. They are important intermediates of carbonylation reactions and undergo insertion of alkene and alkyne. 

Polycyclic compounds are prepared by domino insertions of alkenes and alkynes to the allenylpalladium intermediates. As examples, 1,6-enyne 28 underwent intramo-... 

Hydride complexes of palladium and platinum are almost invariably stabilized by phosphine ligands and play an important role in catalytic processes such as hydrogenation. Examples are Pt(H)ClL2 and Pt(H)2L2, as well as hydrido alkyls and aryls, trans,-Pt(H)(R)L2. There are cis and trans isomers. A typical reaction is the insertion of alkenes and alkynes into the Pt—H bond 33... 

The insertion of alkenes and alkynes into M-E (E = SiRa, SnR3, BR2) is not substantially different from the insertion into M-H or M-C bonds. The same orbital considerations hold in these cases (see Section 6.2.1) and theoretical methods have been applied to analyze these processes, generally in the context of mechanistic studies of catalytic reactions where this may be an important step (hydrosilation, hydroboration, diboration, silylstannation, etc.). Table 6.11 contains some calculated activation energies for insertion of ethylene and alkynes into M-ER bonds of comparable systems. The balance of bond breaking/bond making is less favorable for M-Si than for M-C or M-H bonds. Moreover, the directionality of the 0(M-Si) bonds is detrimental for insertion as discussed when... 

In contrast to a facile intermolecular insertion of alkenes and alkynes to Pd-aryl and Pd-alkenyl bonds as observed in Heck reaction, curiously intermolecular insertion of alkenes and alkynes to Pd-allyl bond in 7r-allylpalladium is very rare. On the other hand, intramolecular insertion of alkenes and alkynes to r-allylpalladium occurs smoothly to form five- and six-membered rings. Oppolzer and co-workers have developed a useful synthetic method of cyclic compounds by intramolecular insertions of alkenes and alkynes, and they explained the reaction as Pd-ene reaction as shown by 404. The method has been applied to the syntheses of several natural products and complex cyclic compounds [155,156]. 

D. INSERTION OF ALKENES AND ALKYNES VIA ADDITION OF Pd(II) COMPLEXES HYDROPALLADATION, CARBOPALLADATION, HETEROPALLADATION, AND METALLOPALLADATION... 

Few direct comparisons between the rates for insertions of alkenes and alkynes have been made. However, one comparison indicates that the insertions of alkenes are slower than die insertions of alkynes. The insertion of acetylene into the cationic palladium-alkyl complex in Equation 9.7H° is directly analogous to the insertions of ethylene into cationic palladium alkyl complexes. This insertion of acetylene is faster than the insertion of ettiylene into ttie same palladium-methyl complex. The insertion of 1-hexyne ismuch slower than the insertion of acetylene and gives a mixture of the two vinyl complexes that result from 1,2- and 2,1-insertion. 

The insertion of alkenes and alkynes with the alkyne complex generates metallocyclopentenes and met-allocycopentadienes 5.247 (Scheme 5.70). The metal-carbon bonds in these complexes are quite polar and may be cleaved by a variety of electrophiles, including acids, halogens and molecular oxygen. Addition of reagents such as sulfur and phosphorus chlorides yields various unusual and interesting heterocycles. 

The reaction mechanism is proposed after the isolation and structural confirmation of the activated alkene insertion species nickelacyclopentanes 6a from the reaction of nickel-carboryne with 2-vinylpyridine (Scheme 7.5). The sequential insertion of alkene and alkyne with excellent regioselectivity control by electronic effect results in the formation of 5. Alkyne inserts regioselectively into the Ni-Caikyi bond of the nickelacyclopentane, whereas the Ni-Cgage bond remains intact. Reductive elimination yields the final products 5. In fact, treatment of 6a with 3-hexyne affords the expected dihydrobenzocarborane... 

MIGRATORY INSERTION OF ALKENES AND ALKYNES INTO M-H BONDS... 

Insertion of alkenes and alkynes in the Ar-H bond directed by an ortho substituent... 

Several examples of the insertion of alkenes and alkynes into metal-carbon and metal-hydrogen bonds have been mentioned incidentally in the previous chapter, on homogeneous catalysis. In this section we concentrate on reactions in which the mechanistic interest centres on the insertion process. 

Scheme 8 Different orientations of the heteroatom groups in the initial complexes for insertion of alkenes and alkynes into the M-Z bond (coordinated multiple bond is omitted) other isomers are also possible depending on the ligand environment...

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

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. 

The addition proceeds through (a) oxidative addition of the B-X bond to a low-va-lent metal (M=Pd, Pt) giving a ds-B-M-X complex (92), (b) migratory insertion of alkene or alkyne into the B-M bond (93 94), and finally (c) reductive elimination... 

The only other alkenyl carbenoid with a proton trans to the halide that can readily be generated by deprotonation is the parent 1-lithio-l-chloroethene 57 [43] (Scheme 3.13). Insertion into organozirconocenes arising from hydrozirconation of alkenes and alkynes, followed by protonation, affords terminal alkenes and ( )-dienes 59, respectively [38]. The latter provides a useful complement to the synthesis of 54 in Scheme 3.12 since the stereocontrol is >99%. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Imido selenium compounds Se(NR)2, where R = Bu or Ts, were first noted to give allylic amination of alkenes and alkynes.232 Formally the NR function is inserted into the allylic C—H bond yielding the C—NHR moiety. Related reactivity was also found for the sulfur imides, S(NR)2.233 Reactions between 1,3-dienes and Se(NTs)2 give [4 + 2] adducts which, in the presence of TsNH2, react to generate 1,2-disulfonamides.234... 

Zefirov and coworkers have developed procedures for chlorosulfamation of alkenes and alkynes using reagents of the type R2NSO2OCI formed by insertion of sulfur trioxide into the nitrogen-chlorine bond in yV-chloroamines (R2NCI).106... 

Reactions of alkenes and alkynes that generate a carbon-metal bond by nucleophilic addition to a metal ir-complex and subsequently undergo carbon monoxide insertion to yield a carbonyl product are... 

The insertion of alkene to metal hydride (hydrometallation of alkene) affords the alkylmetal complex 34, and insertion of alkyne to an M—R (R = alkyl) bond forms the vinyl metal complex 35. The reaction can be understood as the cis carbometallation of alkenes and alkynes. 

Metalametallations of alkenes and alkynes are useful methods for the construction of 1,2-dimetala-alkanes and 1,2-dimetala-l-alkenes, which react subsequently with suitable electrophiles to form substituted alkanes and alkenes. Metalametallation is carried out usually with bimetallic reagents of the type R Si-M R, or R Sn-M R in which M = B, Al, Mg, Cu, Zn, Si or Sn. Some metalametallations proceed without catalysts Cu, Ag and Pd compounds are good catalysts. The metalametallation with bimetallic compounds, such as Si-B, Si-Mg, Si-Al, Si-Zn, Si-Sn, Si-Si, Sn-Al or Sn—Sn bonds, catalysed by transition metal complexes, is explained by the oxidative addition of the bimetallic compounds to form 478, and insertion of alkene generates 479. Finally 1,2-dimetallic compounds 480 are formed by reductive elimination. Dimetallation of alkynes proceeds similarly to give 481. Dimetallation is syn addition. 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

Acylmetal hydride is formed by the oxidative addition of aldehyde, and hydroacylation occurs by insertion of alkene or alkyne. The Ni-catalysed hydroacylation of internal alkyne 600 with aldehyde gave rise to the v./l-unsaturated ketone 601 [230]. The Ru-catalysed hydroacylation of cyclohexene with aldehyde 602 under CO pressure at high temperature gives the ketone 603 [231]. 

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

Depending on the organic framework, palladium complexes can initiate a series of additions and insertions with alkenes and alkynes, leading to polycyclic structures from linear or monocyclic starting materials. A simple catalytic cycle for the cyclization of 1,6 and 1,7 enynes is given in Scheme 39. 

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