Rhodium alkyne insertion into

The catalytic cycle was established by the outcome of these deuterium labeling studies. Thus, after transmetallation from boron to give the phenylrhodium species, the alkyne inserts into the phenyl-rhodium bond to form the alkenylrhodium intermediate (Scheme 4.29). It follows that rhodium moved from the vinylic position of the alkenylrhodium intermediate to the ortho position of the phenyl ring [42]. The so-formed 2-(alkenyl)phenylrhodium intermediate then undergoes hydrolysis with D2O to give the deuterio phenylation product and the active hydroxorhodium intermediate. 

Direct alkyne insertion into a Rh—Si bond has been observed for the intermediate rhodium silyl complex (dtbpm) Rh[Si(OEt)3] (PMe3) [dtbpm = di(ferf-butyl)phosphino methane] in the hydrosilylation of 2-butyne with triethoxysilane catalyzed by the rhodium alkyl complex (dtbpm)RhMe(PMc3). The crystal structure of (dtbpm)Rh[Si(OEt)3j (PMes) shows that the coordination around the Rh metal is planar with a Rh—Si bond length [2.325(2) A] similar to that found for the complex (Me3P)3RhH(C6F5) Si(OEt)3 (Table ll) . The proposed mechanism for the hydrosilylation reaction of 2-butyne with HSi(OEt)3 yielding mainly the E-isomer of MeCH=C(Me)Si(OEt)3 is outlined in Scheme 36. 

Alkyne insertion into the cyclopropenone C-C bond was catalyzed by [RhCl(CO)2]2 to give cyclopentadienones (Scheme 2.109) [166]. Rhodium(I)-NHC complexes also exhibited catalytic activity [167]. Benzyne can also participate in the reaction to provide indenone. 

The addition of Rh-Csp a-bonds to enones is a well-studied process. Lee s reaction is predicated on the idea that rhodium-catalyzed conjugate addition of boronic acids to enones can be interrupted by 1,1-insertion into an alkyne. Thanks to the high reactivity of rhodium toward alkynes and the effects of tethering, a partly intramo-... 

In the presence of a ruthenium catalyst, 3-diazochroman-2,4-dione 716 undergoes insertion into the O-H bond of alcohols to yield 3-alkyloxy-4-hydroxycoumarins 717 (Equation 285) <2002TL3637>. In the presence of a rhodium catalyst, 3-diazochroman-2,4-dione 716 can undergo insertion into the C-H bond of arenes to yield 3-aryl-4-hydroxy-coumarins (Equation 286) <2005SL927>. In the presence of [Rh(OAc)2]2, 3-diazochroman-2,4-dione 716 can react with acyl or benzyl halides to afford to 3-halo-4-substituted coumarins (Equation 287) <2003T9333> and also with terminal alkynes to give a mixture of 477-furo[3,2-f]chromen-4-ones and 4/7-furo[2,3-3]chromen-4-ones (Equation 288) <2001S735>. 

The reaction of acetylenic alcohols with ethyl diazoacetate and a rhodium acetate catalyst resulted in competitive addition to the alkyne and insertion into the H-0 bond in ratios varying from 15 to 0.6 depending on the substrate and the catalyst. ... 

C-C cleavage of strained rings and ketones has been used to develop useful catalytic reactions. For example, vinylcyclopropanes and vinylcyclobutanes react with alkynes (Equation 6.66) to generate products from 5+2 and 6+2 addition processes that form seven- and eight-membered ring products by overall transformations that are homologs of the Diels-Alder reaction. " The mechanism of these catalytic reactions continues to be studied, but these reactions most likely occur by coordination of the olefin to rhodium and insertion of the metal into the cyclopropene or cyclobutane. Decarbonylation of dialkyl ketones, including relatively unstrained cyclic ketones, has been reported and most likely occurs by oxidative addition into the acyl-alkyl C-C bond, subsequent de-insertion of CO, and C-C reductive elimination. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

Rhodium complexes provide some of the most attractive catalysts for carbon manipulation with high reactivity, regioselectivity, scope, and functional group tolerance. In particular, rhodium complexes have displayed potential for the synthesis of various heterocyclic and carbocyclic compounds through the C—H bond activation reactions. Rhodium complexes have been shown to catalyze sp C—H bond insertion into several pi bonds including alkenes, alkynes, aldehydes, and imines. ... 

Tetrafluoroethylene reacts with Fe(CO)3(butadiene) by a two-step oxidative addition process to give (43). The reaction of the rhodium(i) complex Rh(acac)(cod) with hexafluorobut-2-yne results both in trimer-ization of this alkyne, and either its insertion into a y-bonded acetyl-acetonate intermediate or a concerted addition to the acetylacetonate ring, to produce (44), RhCl3,3H20 reacts with 2-methylallyl alcohol, in the presence of 4-methylpyridine (pic), probably via two insertion reactions, to... 

Aromatization of enediynes with catalytic insertion of C-H bond—In the case of the enediynes 3.718 bearing long alkyl substituents terminating one alkyne branch, the cycloaromatization occurs with radical insertion into a C-H bond of the alkyl group (Scheme 3.80) [257, 263]. The route taken by catalysis with ruthenium differs from that with rhodium [257]. In the rhodium system, cyclization is initiated by a rhodium-vinylidene intermediate which forms OTe i2-diradical naphthalene intermediate A. In the case of ruthenium, the cyclization comprises primary of the formation of a ruthenium-Ti-alkyne, which forms para-dirsidicsil B that converts to the product 3.719. 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

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 the example shown in Figure 4.4 either of these mechanisms leads to insertion of the alkyne into the C-Rh double bond of the initially formed acylcarbene rhodium complex. The resulting vinylcarbene complex undergoes intramolecular cyclopropanation of the 1-cyclohexenyl group to yield a highly reactive cyclopropene, which is trapped by diphenylisobenzofuran. 

Hydroformylation reactions that are mediated by rhodium catalysts can also be incorporated into cascade sequences. The zwitterionic rhodium complex 694 promotes a tandem cyclohydrocarbonylation/CO insertion reaction producing pyrroli-none derivatives that contain an aldehyde functional group in good yields (01JA10214). In one example, exposure of a-imino alkyne 693 to catalytic quantities of 694 and (PhO)3P under an atmosphere of CO and H2 at 100 °C produced pyrrolinone 695 in 82% yield (Scheme 113). A variety of alkyl substitutents can be tolerated in this reaction. 

Rhodium complexes generated from A-functionalized (S)-proline 3.60 [933, 934, 935] or from methyl 2-pyrrolidone-5-carboxylates 3.61 [936, 937, 938] catalyze the cyclopropanation of alkenes by diazoesters or -ketones. Diastereoisomeric mixtures of Z- and E-cydopropylesters or -ketones are usually formed, but only the Z-esters exhibit an interesting enantioselectivity. However, if intramolecular cyclopropanation of allyl diazoacetates is performed with ligand 3.61, a single isomer is formed with an excellent enantiomeric excess [936,937], The same catalyst also provides satisfactory results in the cyclopropanation of alkynes by menthyl diazoacetate [937, 939] or in the intramolecular insertion of diazoesters into C-H bonds [940]. 

The hydrosilation of terminal alkynes involves the insertion of the alkyne into the M-SiRs bond, as has been determined for rhodium and iridium [M(triso)L2] complexes (triso = CH(P(0)Ph2)3, L = CO, C2H4, cyclooctene) [202]. The yd-vinylsilanes obtained are a mixture of cis and trans products and an isomerization of the initiaiiy formed cis siiylvinyl metai complex, through an form, accounts for the sterochemistry found (Scheme 6.64). Competition of both Chalk-Harrod... 

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