Insertion, alkenes/alkynes selectivity

A related unprecedented double insertion of electron-deficient alkynes has also been reported in the reactions of the linear Pt2Pd heterotrimetallic complex 64 with 65 (RO2CCSCR) (Scheme 24) [95,96]. A series of unsymmetri-cal A-frame clusters 68 has thus been obtained in which a first insertion of the alkyne takes place site-selectively into the Pt-Pd bond vs the Pt-Pt bond (66). After a zwitter-ionic polar activation of the resulting inserted alkene (67), a subsequent reaction with the phosphine unit of the dpmp allows one to obtain the products 68 via the nucleophilic migration of the terminal P atom from the Pd center to the CH terminal carbon (formation of the P-C bond). 

C-H Bond Functionalization. The title corrpound is usually combined with metal acetates, such as NaOAc, to generate Cp Rh(OAc) species that is selective for ortho C-H bond activation with the aid of an intramolecular directing group, such as the imino group, to afford rhodacycle corr5>lexes (eq 1). The reactivity of these rhodacycle complexes and the reaction mechanism for the cyclometallation via C-H bond activation have been investigated. Recently, this method has been applied in the synthesis of various kinds of carho- and heterocycles via insertion into alkynes and alkenes. 

Keywords Alkenes Alkynes Catalysis Insertion Mechanism Selectivity... 

Acylzirconocene chlorides 78, which are easily available through the hydrozirco-nation of alkenes or alkynes with Cp2Zr(H)Cl and subsequent CO insertion, can be used as acyl anion equivalents Cu(I)-catalyzed reactions with propargyl compounds 77 afford allenyl ketones 79 (Scheme 3.40) [86]. The use of an excess of 77 (2 equiv. to 78) is important for the selective preparation of 79, which prevents an undesirable side reaction of the allenic products 79 with 78. 

Alkynes show the same reaction and again the product obtained is the anti isomer. After a suitable elimination from the metal the alkene obtained is the product of the anti addition. Earlier we have seen that insertion into a metal hydride bond and subsequent hydrogenation will afford the syn product. If we use BH4 as the nucleophile we can accomplish anti addition of a hydride. Thus, with the borohydride methodology and the hydrogenation route either isomer can be prepared selectively. 

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

Two different mechanisms have been proposed for this dehydrogenative silylation process. The first mechanism proposed by Oro, Esteruelas and coworkers includes the oxidative addition of 1-alkyne to the Ir—Si bond, followed by the reductive elimination of 151 (equation 61)117,118. The proposed mechanism is supported by the identification of [IrH(C=CPh)( j2-( -Pr)2PCH2CH20Me)]BF4 in stoichiometric as well as catalytic conditions by 31P 1H NMR analyses118. The other mechanism proposed by Jun and Crabtree includes the insertion of 1-alkyne into the Ir—Si bond, followed by isomerization and /J-hydride elimination (equation 62)113, which is consistent with the mechanism proposed for the highly selective formation of (Z)-l-silyl-l-alkenes (see Section IILB)115. 

Under mild experimental conditions, compounds containing B-H bond add to the alkenes or alkynes to form anti-Markovnikov hydroboration products via -insertion of alkenes. A variety of borane reagents are available for selective hydroboration (Scheme 6). 

Table 3 Products, yield, and selectivity for alkyne/alkene insertion into EBTHI zirconaaziridines...

A method for the selective hydrogenation of alkynes to (Z) alkenes has been described using a Pd(0) complex as catalyst (Scheme 6.15) [65]. The mechanism has not been elucidated, but the catalytic cycle probably involves the formation of an alkyne Pd(0) complex, oxidative addition of H2, insertion, and reductive elimination of the hydride-alkenyl. 

Scheme 25) was observed when cyclic alkenes (e.g., 214) were treated with ruthenium carbene complex 18 in the presence of terminal alkynes (e.g., 215). A mechanism involving initial ROM, followed by alkyne insertion of the intermediate carbene complex, followed by ROM from intermediate 217, was proposed. In order to account for the unexpectedly high yield (the yield is higher than the anticipated E Z selectivity in the formation of 217) of the process, a second source of the observed product involving metathesis of an additional mole of cyclopentene from intermediate 217 was suggested. 

Alkynes are more reactive than alkenes in carbopalladation. Facile insertion of internal alkynes to some Pd—C bonds (carbopalladation of alkynes) generates the alkenylpalladiums 2 and 7 by mainly selective syn addition of organopalladium species 1 and 6 to alkynes. Formally the species 2 can be generated by oxidative addition of appropriately substituted alkenyl halides 3 to Pd(0). 

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