Vinylidene intermediates

Electrocyclizations of arenes and alkynes lead to interesting fused systems via vinylidene intermediates (Equation (178)).144... 

Akiyama et al. extended this reaction to alkynylimines for the preparation of quinoline derivatives [28]. Treatment of N-aryl(alkynyl)imines 99 with 20 mol% W (CO)5(thf) in THF at reflux followed by oxidative work-up using NMO gave 2-arylquinolines 100 in reasonable yield through electrocydization of the vinylidene intermediate (Scheme 5.31). 

Ohe, Uemura et al. further developed this electrocyclization into the [3,3]-sigmatropy of a cyclopropane system substituted with ethynyl and an acyl or an alkenyl group [33]. Thus, treatment of cis-l-acyl-2-ethynylcyclopropanes 125 with 5 mol% Cr CO)5(thf) in the presence of EtgN at rt induced isomerization to phenol derivatives through the [3,3]-sigmatropy of the vinylidene intermediate 126 to give... 

Finn et al. reported the first instance of a metal-catalyzed aromatization of enediynes via vinylidene intermediates [7]. Aromatization of unstrained enediynes is knovm as Bergman cyclization and occurs at 200-250 °C via diradical intermediates [8]. Ruthenium-vinylidene complex 7 was formed when 1,2-benzodiyne 6 was treated with RuCp(PMe3)2Cl and NH4PF6 at 100 °C, ultimately giving good naphthalene product 8 ingood yields (Scheme 6.4). This process mimics Myers-Saitocyclizationof5-allene-3-... 

Catalytic Carbocyclization via Electrocyclization of Ruthenium-Vinylidene Intermediates 195... 

Various cycdization products have been observed in the cycloisomerization of 3,5-dien-l-ynes using [Ru(Tp)(PPh3)(CFl3CN)2]PF6 catalyst the cyclization chemos-eledivity is strongly dependent on the type of substrate structures, which alters the cycdization pathway according to its preferred carbocation intermediate. The reaction protocols are summarized below ruthenium vinylidene intermediates are responsible for these cyclizations (Scheme 6.10). 

Movassaghi et al. [21[ reported the synthesis of substituted pyridine derivatives via ruthenium-catalyzed cycloisomerization of 3-azadienynes. To avoid the isolation of the chemically active alkynyl imines, trimethysilyl alkynyl amines served as initial substrates, as shown in Scheme 6.19. The formation of ruthenium vinylidene intermediates is accompanied by a 1,2-silyl migration according to controlled... 

Scheme 6.20. This ruthenium catalyst (10 mol%) was active for the cydization of ds-1 -ethynyl-2-vinyloxiranes to afford various 2,6-disubstituted phenols in reasonable yields. Under similar conditions, 1,1,2,2,-tetrasubstituted oxiranes gave the 2,3,6-trisubstituted phenols with a skeleton reorganization [22]. The 1,2-deuterium shift of the alkynyl deuterium of d-Sle was indicative of mthenium vinylidene intermediates (Scheme 6.20).

Based on the labeling experiments, a plausible mechanism involving mthenium vinylidene intermediates SS is proposed in Scheme 6.21. Cydization of this vinylidene intermediate leads to the formation of the epoxy carbenium 56, which then undergoes an epoxide opening to form l,4-dien-3-ol 57. A subsequent pinacol rearrangement of this alcohol furnishes ketone 58, providing the required skeleton for the observed phenol product 54. 

Catalytic Carbocyclization via Cycloaddition of Ruthenium Vinylidene Intermediates... 

Catalytic Carbocyclization via Cycloadclition of Ruthenium Vinylidene Intermediates 209... 

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). 

Ruthenium-catalyzed 1,1-difunctionalization of alkynes can be achieved through ruthenium vinylidene intermediates. In this context, Lee s group reported the... 

The stoichiometric interaction of an enyne and [RuCl(PCy3)(pcymene)]B(Ar )4 XVIIIa containing a bulky non-coordinating anion B(ArF)4 showed by NMR at —30 ° C the formation of the alkenyl alkylidene ruthenium complex and acrolein. This formation could be understood by the initial formation of a vinylidene intermediate and transfer of a hydride from the oxygen a-carbon atom to the electrophilic vinylidene carbon, as a retroene reaction step (Scheme 8.13) [54]. 

None of the methods described in this chapter utilize a pre-formed metal vinylidene as an active catalyst precursor. The occurrence of metal vinylidene intermediates is instead inferred on the basis of product structure, isotopic labeling experiments, and computational studies. 

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). 

Lee and coworkers envisaged engaging metal vinylidene intermediates derived from simple enynes (16) in formal intramolecular 47t-cycloaddition (Scheme 9.4) [8]. 

Scheme 9.4 Cycloisomerization of 1,6-enynes via rhodium vinylidene intermediates.

Tungsten- and molybdenum-catalyzed methods involving vinylidene intermediates have been described for this transformation (Chapter 5). The use of rhodium provides some advantages in terms of catalyst turnover and selectivity. The catalyst formed in situ from a RhCl source and a fiuormated triarylphosphine promotes the cyclization of a variety of alkynols (Table 9.6). 

According to a deuterium-labeling experiment, Miyuara s hydroboration is actually a 1,1-addition process with concomitant 1,2-H-shift, rather than a true transaddition. The olefin geometry of the product boronate is presumably determined during an insertion of boron or hydrogen into the a-position of rhodium vinylidene intermediate 43 (Table 9.8). 

Many Rh(I)-complexes are capable of dimerizing or oligomerizing alkynes to some degree. Seemingly small changes in reaction conditions can affect the stereo- and regio-selectivity of dimerization. The formation of (Z)-head-to-head dimers, however, can be indicative of metal vinylidene intermediates. This correlation was observed for Ru( 11)-catalyzed dimerizations (Chapter 10) and also holds true for the Rh(I)- and Ir (I)-catalyzed processes described herein. 

---