Insertion, alkenes

The insertion of coordinated alkenes into M-H bonds leads to metal alkyls and constitutes a key step in a variety of catalytic reactions (Chapter 9). For example, the commercially important alkene polymerization reaction (Chapter 12) involves repeated alkene insertion into the growing polymer chain. 

In hydrozirconation of alkenes by Cp2ZrHCl, terminal alkenes insert in the anti-Markovnikov direction to give a stable 1° alkyl. Internal alkenes, such as 2-butene, insert to give an unstable 2° alkyl, that -eliminates to give 1- and 2-butene. The 1-butene can now give a stable 1° alkyl that is the final product. This is particularly noteworthy because the free terminal alkene is less stable than the internal alkene. Tlie outcome arises because the 1° alkyl is thermodynamically more stable than a 2° alkyl for steric reasons. The 1° alkyl, R, can subsequently be functionalized in a number of ways to give a variety of RX derivatives. Hydrozirconation is also effective with less reactive substrates, such as nitriles, where addition of Zr-H across the C=N bond is possible.  

Simple a-olefins, where the two ends of the C=C bond are not well differentiated electronically, may give insertion with a mixed regiochemis-try, although steric effects can bias the outcome in suitable cases.  

In the usual syn insertion, the stereochemistry at both carbons is retained. This is best seen for alkynes, where the vinyl product can preserve the syn disposition of M and H. If the initially formed cw-vinyl complex remains 16e, it can rearrange to the sterically less hindered trans isomer, via an 18e t) -vinyl. This can lead to an apparent anti addition of a variety of X-H groups (Eq. 7.21) to alkynes.  

Strain, or the presence of electronegative substituents on the alkene, or moving to an alkyne are some of the other factors that can bias both 

4-center hydride migration with moderate buildup of positive charge on the carbon atom/  

The mechanism of 1,4-addition of [HMn(CO)5] to 1,3-dienes (1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene) as shown in equation (5) has been investigated. Rates of hydrogenation are first order in [HMn(CO)5] and in [diene] and 

The kinetics of CO2 insertion with cw-[WMe(CO)4L] , L = CO or PR3, yielding ci5-[W(0C(0)Me)(C0)4L] , were determined. When L = PMc3 the rate is 250 times faster than when L = CO (Table 10.10) with the reaction being first order in both [complex] and [CO2]. The structure of [Na(18-crown-6)(THF2)]-[WMe(CO)5] provides support for the proposal that the enhanced rate of insertion is related to neutralization of the negative charge buildup on the acetate ligand by Na coordination. 

Enthalpies of activation are 7-lOkcalmor and the negative entropies (-30 to -40 cal cal ) are consistent with the associative kinetics. The linear relationship between log k and either the pK of RCO2H or Hammett cr constants for XC6H4CO2H suggest that the mechanism involves heterolytic 0—0 cleavage and electrophilic insertion of oxygen into the Pd—C bond, in what is proposed to be a concerted process/  

Ab initio calculations were performed on the insertion of methylene into the Ru—H bond. The model complex for [CpRu(PPh3)R(CH2)] or [(C6Me6)Ru(PPh3)-(Me)(CH2)] was ClRuH(=CH2). Methylene insertion is calculated to have a low (11.5 kcal mor ) activation barrier. The insertion reaction is exothermic by T.lkcal mol .  

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Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

Unlike the intermolecular reaction, the intramolecular aminopalladation proceeds more easily[13,14,166], Methylindole (164) is obtained by the intramolecular exo amination of 2-allylaniline (163). If there is another olefinic bond in the same molecule, the aminopalladation product 165 undergoes intramolecular alkene insertion to give the tricyclic compound 166[178]. 2,2-Dimethyl-l,2-dihydroquinoline (168) is obtained by endo cyclization of 2-(3,3-dimethyiallyl)aniline (167). The oxidative amination proceeds smoothly... 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

In the prostaglandin synthesis shown, silyl enol ether 216, after transmetaJ-lation with Pd(II), undergoes tandem intramolecular and intermolecular alkene insertions to yield 217[205], It should be noted that a different mechanism (palladation of the alkene, rather than palladium enolate formation) has been proposed for this reaction, because the corresponding alkyl enol ethers, instead of the silyl ethers, undergo a similar cyclization[20I],... 

As a unique reaction of Pd(II), the oxidative carbonylation of alkenes is possible with Pd(ll) salts. Oxidative carbonylation is mechanistically different from the hydrocarboxylation of alkenes catalyzed by Pd(0), which is treated in Chapter 4, Section 7.1. The oxidative carbonylation in alcohol can be understood in the following way. The reaction starts by the formation of the alkoxy-carbonylpalladium 218. Carbopalladation of alkene (alkene insertion) with 218 gives 219. Then elimination of /3-hydrogen of this intermediate 219 proceeds to... 

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

A cr-aryl-Pd bond is formed by the transfer of an aryl group even from arylphosphines to Pd and alkene insertion takes placefl 17-119], This reaction is slow and it is not a serious problem when triarylphosphine is used as a ligand. The cinnamate 149 is obtained by the reaction of PhsP with acrylate in the presence of Pd(OAc)2 in AcOH. 

In these cydizations, the reaction can be terminated in other ways than elimination of /3-hydrogen. Typically the reaction ends by an anion capture process[154]. The following anion transfer agents are known H, OAc , CN, S02Ph, CH(C02R)2, NHRj, CO/ROH, and RM [M = Sn(IV), B(lll), Zn(II)]. Trapping with an amine after alkene insertion to give 189 and 190 is an example. A-Acetyl protection is important in this reaction[155]. 

The alkylpalladium intermediate 198 cyclizes on to an aromatic ring, rather than forming a three-membered ring by alkene insertion[161], Spirocyclic compounds are easily prepared[l62]. Various spiroindolines such as 200 were prepared. In this synthesis, the second ring formation involves attack of an alkylpalladium species 199 on an aromatic ring, including electron-rich or -poor heteroaromatic rings[l6.5]. 

A interesting and useful reaetion is the intramolecular polycyclization reaction of polyalkenes by tandem or domino insertions of alkenes to give polycyclic compounds[l 38]. In the tandem cyclization. an intermediate in many cases is a neopentylpalladium formed by the insertion of 1,1-disubstituted alkenes, which has no possibility of /3-elimination. The key step in the total synthesis of scopadulcic acid is the Pd-catalyzed construction of the tricyclic system 202 containing the bicyclo[3.2. Ijoctane substructure. The single tricyclic product 202 was obtained in 82% yield from 201 [20,164). The benzyl chloride 203 undergoes oxidative addition and alkene insertion. Formation of the spiro compound 204 by the intramolecular double insertion of alkenes is an exam-ple[165]. 

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

Intramolecular reaction can be used for polycyclization reaction[275]. In the so-called Pd-catalyzed cascade carbopalladation of the polyalkenyne 392, the first step is the oxidative addition to alkenyl iodide. Then the intramolecular alkyne insertion takes place twice, followed by the alkene insertion twice. The last step is the elimination of/3-hydrogen. In this way, the steroid skeleton 393 is constructed from the linear diynetriene 392(276]. 

In the presence of a double bond at a suitable position, the CO insertion is followed by alkene insertion. In the intramolecular reaction of 552, different products, 553 and 554, are obtained by the use of diflerent catalytic spe-cies[408,409]. Pd(dba)2 in the absence of Ph,P affords 554. PdCl2(Ph3P)3 affords the spiro p-keto ester 553. The carbonylation of o-methallylbenzyl chloride (555) produced the benzoannulated enol lactone 556 by CO, alkene. and CO insertions. In addition, the cyclobutanone derivative 558 was obtained as a byproduct via the cycloaddition of the ketene intermediate 557[4I0]. Another type of intramolecular enone formation is used for the formation of the heterocyclic compounds 559[4l I]. The carbonylation of the I-iodo-1,4-diene 560 produces the cyclopentenone 561 by CO. alkene. and CO insertions[409,4l2]. 

The acylpalladium complex formed from acyl halides undergoes intramolecular alkene insertion. 2,5-Hexadienoyl chloride (894) is converted into phenol in its attempted Rosenmund reduction[759]. The reaction is explained by the oxidative addition, intramolecular alkene insertion to generate 895, and / -elimination. Chloroformate will be a useful compound for the preparation of a, /3-unsaturated esters if its oxidative addition and alkene insertion are possible. An intramolecular version is known, namely homoallylic chloroformates are converted into a-methylene-7-butyrolactones in moderate yields[760]. As another example, the homoallylic chloroformamide 896 is converted into the q-methylene- -butyrolactams 897 and 898[761]. An intermolecular version of alkene insertion into acyl chlorides is known only with bridgehead acid chlorides. Adamantanecarbonyl chloride (899) reacts with acrylonitrile to give the unsaturated ketone 900[762],... 

The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

The alka-l,2,4-trienes (ailenylaikenes) 12 are prepared by the reaction of methyl propargyl carbonates with alkenes. Alkene insertion takes place into the Pd—C bond of the ailenyipailadium methoxide 4 as an intermediate and subsequent elimination of/3-hydrogen affords the 1,2,4-triene 12. The reaction proceeds rapidly under mild conditions in the presence of KBr. No reaction takes place in the absence of an alkali metal salt[4j. 

Butyrolactones are prepared by intramolecular reaction of haloallylic 2-alkynoates. The a-chloromethylenebutyrolactone 301 is prepared by the intramolecular reaction of300[150,151]. 4 -Hydroxy-2 -alkenyl 2-alkynoates can be used instead of haloallylic 2-alkynoates, and in this reaction, Pd(II) is regenerated by elimination of the hydroxy group[152]. As a related reaction, the q-(chloromethylene)-7-butyrolactone 304 is obtained from the cinnamyl 2-alkynoate 302 in the presence of LiCl and CuCbflSS]. Isohinokinin (305) has been synthesized by this reaction[l 54]. The reaction is explained by chloro-palladation of the triple bond, followed by intramolecular alkene insertion to generate the alkylpalladium chloride 303. Then PdCb is regenerated by attack of CuCb on the alkylpalladium bond as a key step in the catalytic reaction. 

Pd(II)-catalyzed cyclization of the siloxyhexatriene 34 offers a cyclohexe-none annulation method. The Pd enolate 35, formed by transraetallation of the silyl enol ether with Pd(II), is an intermediate which undergoes intramolecular eWo-alkene insertion. Then Pd(II) is regenerated to give 36, and finally cyclohexenone is formed[38]. 

In the first process alkenes insert into the Al-C bonds of monomeric AIR3 at 150° and 100 atm to give long-chain derivatives who.se composition can be clo.sely controlled by the temperature, pressure and contact time ... 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

The phosphine-based platinum(O) catalysts do not catalyze the diboration of alkenes because of the high coordination ability of phosphine over the alkene double bond, but platinum(O) complexes without a phosphine ligand such as Pt(dba)2 [128] and Pt(cod)2 [129] are an excellent catalyst allowing the alkene insertion into the B-Pt bond under mild conditions (Scheme 1-30). The diboration of aliphatic and aromatic terminal alkenes takes place smoothly at 50°C or even at room temperature. The reaction is significantly slow for disubstituted alkenes and cyclic alkenes, but cyclic alkenes having an internal strain afford ds-diboration products in high... 

A chelation-assisted Pd-catalyzed Cope rearrangement was proposed in the reaction of phenanthroline to generate isoquinolinone derivatives (Eq. 12.78).177 The use of aqueous media and ligands enables a double-Heck reaction on a substrate favoring alkene insertion over (J>-hydride elimination. 

Scheme 13 Generalized mechanism for the alkene insertion and cr-bond metathesis reactions catalyzed by lanthanocenes...

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

The moderate level of regioselectivity seen in the alkyne insertion is dependent on added PPI13, but the alkene insertion occurs with excellent regioselectively. This is the only catalytic, late transition metal system shown to intermolecularly couple alkenes with alkynes. 

The Ir11 dimer [Ir(oep)]2 (oep = octaethylporphyrin) has been prepared in low yield by photolysis of (oep)IrCH3 in C6D6.473 This preparation has been improved by Chan et al.474, as shown in Reaction Scheme 24, where TEMPO = 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical. The dimer undergoes several organometallic reactions, including oxidative addition of alkyl C 11 bonds and alkene insertions.475... 

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