Metallacycles seven-member

R=Me the connectivity is TiCp2(/U-S3)(/U-S4)NMe. This different behavior between the SyNH and SyNMe systems cannot be attributed to either recognizable steric or electronic reasons. By contrast, the reaction with RN(/t-S2)2NR (R=Me, n-Oct) in hexane at 20 °C gave not the expected seven-membered ring compound but a six-membered metallacycle, TiCpyl/f-SylNR [36] (Scheme 5). 

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

Scheme 55 Cyclization of alkynes and alkenes via three-, five-, and seven-membered metallacycles containing Mo or W.

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... 

Metallacyclic complexes containing two molecules of diyne per MCp2 group have also been isolated, that with titanium containing 2,4-alkynyl substituents (225) while with zirconium, the unusual seven-membered metallacumulene structure 226 is adopted, which has only one alkynyl substituent. The bi- and tricyclic 2/2 complexes 227 and 228 have so far been obtained only from reactions of TiCp2 with PhC=CC=CPh.30 ... 

The seven-membered metallacycles 89 (formed by ketone addition to [(butadiene)ZrCp2] cleanly add a nitrile molecule at elevated temperatures to yield the respective nine-membered metallacyclic products 96. Their hydrolysis then yields the 6-hydroxy-substituted non-conjugated unsaturated imines 97 under kinetic control. Within a few hours at room temperature these rearrange to the thermodynamically favored primary dienamine products 98.107 In this case the thermochemical diene conjugational energy makes the primary dienamines more stable than their conjugated imine tautomers (Scheme 32). 

Three metal stabilized seven-membered rings containing 1,2,3-butatriene units have been prepared [Eq. (62)). In the metallacycle 367, independently synthesized by Hsu et al.153 and Burlakov et a/.114 by essentially the same method, zirconium serves as both a member of the seven-membered ring and as a cumulene-stabilizing metal. An X-ray crystal structure excluded the alternative structure 368, which was proposed as a possible intermediate in the reaction. The X-ray of 367 is consistent with the cumulene formulation in that bond lengths for C2-C3 [1.337(6) A], C3-C4 [1.298(6) A], and C4-C5 [1.337(6) A] indicate bonds of roughly similar bond order. The bond angles for C2-C3-C4 [148.8(5)°] and C3-C4-C5 [160.1(5)°] are also consistent with a severely distorted (from linearity) cumulene. 

Figure 50 Trigonal bipyramidal geometry possibly accommodated by the strain due to steric bulk of the highly oxygenated seven-membered metallacycle.

The reaction of dilithium dialkyl bipyridines 44 derived from dilithiation of 3,3 -dimethyl-2,2 -bipyridine with (R EClg (R1 = Me or Ph E = Si, Ge, or Sn) afforded the axially symmetric seven-membered metallacycles 45, which were purified by column chromatography and characterized by their MS and NMR spectra and for 45c and 45e the X-ray structure has been determined. Subsequent reaction of 45d-f with W(CO>6 in toluene gave the corresponding bimetallic complexes 46a-c (Scheme 8) <19970M4839>. 

Either SO2 or, with the exception of rhenium and nickel, CO can be inserted in the same way into metal-carbon a bonds of the four- to seven-membered metallacycles 38 -44. - - - In each case the ring-expanded... 

The relative ease of the cyclization step from A to C may also be linked to the nucleophilic or coordinative ability of the heteroatom bound to the metal. The reaction of 7 with diphenylacetylene (Ph2C2) leads to the seven-membered derivatives 68 and 69 after prior isolation of the monoinsertion product 24, treatment with a silver salt, followed by the usual thermolytic conditions. This is another rare example of an intramolecular formation of a C-S bond within the coordination sphere of a transition metal and a novel, albeit limited to one alkyne, route to the rare family of dibenzo[bd] thiepins. With the closely related 8, which differs from 7 only by the tertiary amine unit in the metallacyclic framework instead of a thioether function, a carbocyclic product 71 is obtained (see under carbocycle reactions, next section). The formation of the seven-membered S-heterocycles is attributed to the good coordinative ability of the thioether group in 7. The S-atom remains close to the vinylic carbon function before the cyclization. With the poorly coordinating, readily displaced amine function in 8, the N-atom is detached from the metal and ultimately affords a spirocyclic product (see Scheme 18). 

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