Rhodium vinylidene complexes

It should also be mentioned that very recently, a new cycloisomerization of enynes has been shown to proceed via a rhodium-vinylidene complex,187 which, after [2 + 2]-cycloaddition and ring opening of a rhodacyclobutane, furnishes versatile cyclic dienes (Scheme 47).188 Not only does this constitute a fifth mechanistic pathway, but it also opens new opportunites for C-C bond constructions. 

Not only ruthenium but also rhodium was used in the transition metal-catalyzed cycloaromatization via transition metal-vinylidene complexes. For example, the reaction of an acyclic (Z)-3-ene-l,5-diyne in the presence of RhCl(P(/-Pr)j)2 (5mol%) afforded the corresponding allyUc benzene presumably through the rhodium-vinylidene complex (Scheme 21.56) [63]. 

In 2005, Elakkari et al. reported a unique cyclodimerization of arylacetylenes 59 catalyzed by a rhodium-porphyrin complex into the corresponding 1-arylnaphthalenes 60 together with triarylbenzenes as side products (Scheme 21.26) [35]. The intermolecular cycloaddition between a rhodium-vinylidene complex and an arylacetylene was proposed to form such naphthalene. A mthenium-porphyrin complex also worked as a good catalyst toward the same cyclodimerization. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

Uemura and coworkers discovered another unique rhodium vinylidene-mediated cycloisomerization reaction [11]. They found that in the presence of an electron-rich Rh(I)-complex, [ RhCl(iPr3P)2]2, (Z)-hexa-3-en-l,5-diynes bearing an alkyl substituent at one terminus undergo cycloisomerization to give allylbenzenes (Equation 9.3). 

Rh(I)/R3P complexes also catalyze (Z)-selective hydrosilylation of alkynes (Equation 9.6) [19]. Although Miyaura s hydroboration and this reaction bear superficial similarities to one another, rhodium vinylidenes are not part of the generally accepted mechanism in the latter case. 

Optimized reaction conditions call for the use of Wilkinson s catalyst in conjunction with the organocatalyst 2-amino-3-picoline (60) and a Br0nsted add. Jun and coworkers have demonstrated the effectiveness of this catalyst mixture for a number of reactions induding hydroacylation and C—H bond fundionalization [25]. Whereas, in most cases, the Lewis basic pyridyl nitrogen of the cocatalyst ads to dired the insertion of rhodium into a bond of interest, in this case the opposite is true - the pyridyl nitrogen direds the attack of cocatalyst onto an organorhodium spedes (Scheme 9.11). Hydroamination of the vinylidene complex 61 by 3-amino-2-picoline gives the chelated amino-carbene complex 62, which is in equilibrium with a-bound hydrido-rhodium tautomers 63 and 64. 

Chatani s proposed mechanism bears some similarity to that of Jun s reaction (Scheme 9.12). They both begin with hydroamination of the C=C 7t-bond of a rhodium vinylidene. The resultant aminocarbene complexes (71 and 62) are each in equilibrium with two tautomers. The conversion of 71 to imidoyl-alkyne complex 74 involves an intramolecular olefin hydroalkynylation. Intramolecular syn-carbome-tallation of intermediate 74 is thought to be responsible for ring closure and the apparent stereospecificity of the overall reaction. In the light of the complexity of Chatani and coworkers mechanism, the levels of chemoselectivity that they achieved should be considered remarkable. For example, 5 -endo-cyclization of intermediate 72 was not observed, though it has been for more stabilized rhodium aminocarbenes bearing pendant olefins [27]. 

The concept behind this reaction can be traced to the work of Werner and coworkers who demonstrated the clean a-insertion of phenyl and other organic groups into rhodium vinylidenes to give vinyl-rhodium complexes (Scheme 9.14) [29]. 

The proposed reaction mechanism is shown in Scheme 9.15. Starting from the phenyl-rhodium complex 87, alkyne rearrangement is expected to furnish the phenyl-vinylidene complex 88. Migration of a phenyl ligand onto the vinylidene moiety of 88 must occur such that the vinyl Rh-C bond and the enone tether of the resultant complex (89) attain a cis-relationship to one another. Intramolecular conjugate... 

The reaction between acetylene and RhfCOXi CjH i -QH,) [which acts as a source of the Rh(COXf/5-C9H7) fragment] affords 33 in 50% yield (61). The reaction is supposed to proceed via oxidative addition of the alkyne to the rhodium fragment, followed by isomerization to the vinylidene complex which then interacts with a second rhodium fragment ... 

Shortly after Finn s work came to light a catalytic rhodium(I) system was reported. An acyclic enediyne 40 was heated to 50 °C in the presence of just 0.05 equiv of RhCl(/-Pr2P)2 and EtjN in benzene to provide substituted arene 41 in 58% yield. The latter reaction is presumed to involve Myers-Saito cyclization of an in situ formed vinylidene complex. A catalytic cycle becomes possible due to steps involving /3-hydride elimination and reductive elimination. ... 

DFT calculation revealed the origin of the (Z)-selectivity of the anft -Markovnikov hydroalkoxylation of terminal alkynes (122), catalysed by the rhodium(I) 8-quinolinolato carbonyl chelate (123). The reaction is likely to commence by the formation of the // -complex PhC=CH[Rh], which tautomerizes via a 1,2-hydrogen shift to generate the Rh(I) vinylidene complex PhCH=C=[Rh]. Methanol, as an oxygen nucleophile, then attacks the Ca, and via the transition state (124), which is 1.2kcalmol lower in energy than its stereoisomer, thus giving the (Z)-vinyl ether (125). An improvement in the (Z)-selectivity in the related Rh(I)-catalysed 0... 

Some of the vinylidene complexes include cobalt, rhodium and rhenium in halfsandwich complexes, which are synthesized from acetylene complexes". This reaction involves an intermediate alkinyl(hydrido) complex, which can sometimes be isolated. The bonding between the metal and the a-carbon atom in vinylidene rhodium complexes is shorter than in carbene rhodium complexes, which indicates a high electron density on the center atom. 

The reaction of rhodium vinylidene metal complexes 3 with sulfur, selenium and tellurium affords complexes of metal substituted thio-, seleno- and telluroketenes 4. ... 

The chemistry and reactivity of iridium vinylidene complexes has progressed during the last decade. Particularly, Werner and co-workers have continued studying unsaturated carbenes of rhodium and iridium and have developed an original route to prepare iridium disubstituted vinylidene complexes, /ra 3-[IrCl(C=CRR)(PPry2] (R = Me, Ph 561), which are accessible in moderate yield by using vinyl chlorides and sodium (see Scheme 79). " Analogous compounds with SbPr 3 were also described. 

Rhodium catalysts also give cyclization products via vinylidene complexes generated in situ (Scheme 3.25) [256, 257]. So enediyne 3.505... 

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