Metallacycle cycloadditions

As complex 40 proved to be active in cycloaddition reactions and is isoelectronic to Rh(+1), which is a potent catalyst for [2 + 2 + 2] cycloadditions [20, 21], it was expected that 40 might also be active in those reactions, which is indeed the case. Triyne 54 could be converted to the [2 + 2 + 2]-cycloaddition product 55 in good yield (eq. 3 in Scheme 11). Mechanistically, this reaction is also assumed to proceed via a metallacyclic intermediate. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

Strained ring compounds undergo insertion of a low-valence metal complex to give metallacycles and the cycloaddition of metallacycles has a potential in synthesis, as described above. This method is useful in ring transformations of cyclobutenediones and cyclobutenones. 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

The first metal-catalyzed [4 +2]-reaction of tethered dienes with 7r-systems was reported by Wender and Jenkins using alkynes initially as the two-carbon component.21 This study was based on the earlier observation by Wender and Ihle that in the [4 + 4]-cycloaddition of bis-dienes a competing side-reaction is the [4 + 2]-cycloaddition of the diene with a mono-ene portion of a second diene. The extension of this reaction to the synthesis of seven-membered rings by trapping the metallacycloheptadiene with CO, a formal [4 + 2 + l]-cycloaddition, has been shown in preliminary studies to be feasible. For example, tethered diene-yne 160 can be converted to cycloheptadienone 163 in an Rh(l)-catalyzed [4 + 2 + l]-reaction with CO, albeit the [4 + 2]- and [2 + 2 + l]-reaction products dominate (Equation (29)). The mechanistic scheme (Scheme 44) illustrates the possible metallacyclic intermediates leading to the observed products and provided the conceptual basis for the realization of three novel reaction types ([4 + 2], [2 + 2 + 1], and [4 + 2 + 1 ]).1... 

Coordination of Ni(0) to the alkyne gives a n complex, which can be written in its Ni(II) resonance form. Coordination and insertion of another alkyne forms the new C6-C7 bond and gives a nickelacyclopenta-diene. Maleimide may react with the metallacycle by coordination, insertion, and reductive elimination to give a cyclohexadiene. Alternatively, [4+2] cycloaddition to the metallacycle followed by retro [4+1] cycloaddtion to expel Ni(0) gives the same cyclohexadiene. The cyclohexadiene can undergo Diels-Alder reaction with another equivalent of maleimide to give the observed product. 

Cycloaddition refers to a process of unsaturated moieties forming a metallacyclic compound. It is sometimes categorised under oxidative additions, but we prefer this separate listing. Examples of the process are presented in Figure 2.22. Metal complexes which actually have revealed these reactions are M = L2Ni for reaction a, M = Cp2Ti for reactions b and c, M = Ta for d, and M = (RO)3W for e. The latter examples involving metal-to-carbon multiple bonds have only been observed for early transition metal complexes, the same ones mentioned under a-elimination, 2.20. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

Cycloreversion of four-membered metallacycles is the most common method for the preparation of high-valent titanium [26,27,31,407,599-606] and zirconium [599,601] carbene complexes. These are usually very reactive, nucleophilic carbene complexes, with a strong tendency to undergo C-H insertion reactions or [2 -F 2] cycloadditions to alkenes or carbonyl compounds (see Section 3.2.3). Figure 3.31 shows examples of the generation of titanium and zirconium carbene complexes by [2 + 2] cycloreversion. 

In the previous sections, a number of Ir(I)-catalyzed cycloadditions were described where the oxidative coupling of unsaturated motifs (e.g. alkynes and aUcenes) provides metallacycles as common intermediates. At this point, mention will be made of three examples of Ir(III)-catalyzed cyclizations. 

Transformations to the cyclotrimeric boiazines and cyclotetrameric tetraza-2,4,6,8,l,3,5,7-tetraboracanes also occur. The rate of dimerization for amino iminoboranes has been shown to be stabilized by bulky substituents (76,79,83). This stabilization through dimerization is essentially a [2 + 2] cycloaddition. There are a number of examples of these compounds forming cycloadducts with other unsaturated polar molecules (78). Iminoboranes can add to electron-deficient carbene complexes of titanium such as (C5H5)2Ti(CH2) [84601-70-7] by [2 + 2] cyclo addition, yielding the metallacycle shown in equation 26 (84). 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

Dihydroazepines have been synthesized by the first rhodium-catalyzed hetero-[5+2] cycloaddition of cyclopropylimines and alkynes (Scheme 8.62) [138]. The reaction proceeds via formation of metallacycle 147 which undergoes migratory insertion of dimethyl acetylenedicarboxylate (DMAD) to form 148. Finally, dihydroa-zepine 149 is obtained via reductive elimination. 

The Ni-catalyzed cyclizations of butadiene and acetylene opened a fruitful field of cycloaddition of various unsaturated compounds to afford various cyclic compounds. These cyclizations are now understood by the formation of metallacycles as intermediates (eq. 1.8). 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

Cycloadditions are useful for the preparation of cyclic ompounds. Several thermal and photoactivated cycloadditions, typically [4+2] (Diels-Alder reaction), are known. They proceed with functionalized electronically activated dienes and monenes. However, various cycloaddition reactions of alkenes and alkynes without their electronical activation, either mediated or catalysed by transition metal complexes under milder conditions, are known, offering a useful synthetic route to various cyclic compounds in one step. Transition metal complexes are regarded as templates and the reactions proceed with or without forming metallacycles [49]. 

The key step is the cycloaddition of 0s04 to the olefin. There has been some speculation regarding the actual addition step, for which experimental data suggest the possible involvement of two separate steps. Thus, the question arises during these discussions of whether the key step takes place via an initial (3+2)-addition (1,3-dipolar cycloaddition), or by a (2+2)-addition followed by expansion of the metallacycle. 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

The molecular orbital analysis of the nucleophilic addition at the carbyne C atom infers the orbital control of the reaction since the C atom undergoing attack is the most negative one in the carbyne complex. [2 + 2] cycloadditions of [ReCp(CO)2(CPh)]+ with MeN=C(Ph)H, t-BuN=0, and ArN=NAr (Ar = aryl) but not with aUcenes or aUcynes, give the metallacycles. These reactions are driven by the nucleophilic attack of the lone pairs of the N atom at the electrophilic carbyne carbon atom. These metallacycles are... 

The )] -l-azaallyl complex (51) is formed by [2- -2] cycloaddition of (50) with alkyne. The metallacycle intermediate is considered to be involved in this transformation (Scheme 27). ... 

Imido zirconocenes see Imide Complexes) are synthesized from thermolysis of Cp2Zr(Me)NHR. They react with a wide variety of organic substrates (alkynes, imines, azides, carbodiimides, ketones) to yield [2 + 2] cycloaddition products (Scheme 4) see Metallacycle) ... 

Mechanistically, the term cycloaddition applies in only a formal sense to most of the reactions in the section virtually none of these reactions involves true single-step cycloaddition of all three components that appear in the final product. More typically, cycloaddition involving a transition metal fragment and one or two other components leads to a metallacyclic intermediate whose reaction with the remaining... 

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