Metallacycles, reviews

From the results discussed so far, it is evident that only CH2 groups have been observed in the very early stages of the ethylene polymerization reaction. Of course, this could be due to formation of metallacycles, but can be also a consequence of the high TOP which makes the observation of the first species troublesome. To better focalize the problem it is useful to present a concise review of the models proposed in the literature for ethylene coordination, initiation, and propagation reactions. 

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

The currently known carbometallation chemistry of the group 6 metals is dominated by the reactions of metal-carbene and metal-carbyne complexes with alkenes and alkynes leading to the formation of four-membered metallacycles, shown in Scheme 1. Many different fates of such species have been reported, and the readers are referred to reviews discussing these reactions.253 An especially noteworthy reaction of this class is the Dotz reaction,254 which is stoichiometric in Cr in essentially all cases. Beyond the formation of the four-membered metallacycles via carbometallation, metathesis and other processes that may not involve carbometallation appear to dominate. It is, however, of interest to note that metallacyclobutadienes containing group 6 metals can undergo the second carbometallation with alkynes to produce metallabenzenes, as shown in Scheme 53.255 As the observed conversion of metallacyclobutadienes to metallabenzenes can also proceed via a Diels-Alder-like... 

Comprehensive coverage has not been attempted, but rather the intention has been to provide an introduction for the non-specialist which illustrates the scope of the field and highlights the main features of interest to organic chemists. Citation of all relevant publications is impracticable because of the large volume of literature. However, it is hoped that those key papers and specialist review articles which are included will enable the reader to locate related literature of interest. Advances in the chemistry of metallacycles and metal-hydrocarbon ir-complexes are thoroughly reviewed in surveys of organometallic chemistry published annually since 1972 (B-72MI12200). 

Owing to the growing importance of metallacycles, and since the subject has not been reviewed for some time, we embarked on this review. Previous reviews are in 1982 by Puddephatt9 and Chappel and Cole-Hamilton.10 This topic has also been highlighted in the book by Collman et al.n... 

There has been a recent comprehensive review written on Group 10 metallacycles12 and only the more recent and relevant papers in this group will be covered. In this present review, we will discuss particularly the synthesis, structure, reactivity and applications of metallacycles. 

Theoretical studies published since 1993 reporting computationally optimized structures for four-membered boracycles and metallacycles are listed in Table 1. Many of these investigations, however, maintain a specific focus on molecular transformations (i.e., reaction mechanisms) and no longer explicitly consider the details of metallacyclobutane structure. The most significant theoretical investigations of boracyclobutene derivatives were conducted sufficiently long ago to have been reviewed in CHEC-II(1996) <1996CHEC-II(lb)887> and are not discussed further. 

The series [ReCp ( 0)2)2 (/r-E) (96) (E = O, S, Se, Te) and related complexes have also been prepared from the elements or from EH2 and (92). Slow decomposition of solid (92) in a glovebox produces the dimer Cp Re(CO)2=Re(CO)2Cp (see Section 11). Reaction of (91) with PhC=CH gives the vinylidene ReCp(CO)2(=C=CHPh), but the structure of the coordinated PhC=CPh in ReCp(CO)2( -PhC=CPh), in spite of being bent, has an almost unperturbed C=C bond with a rather short length. In contrast, the azobenzene derivative ReCp(CO)2( 7 -PhN=NPh) has an almost pure single N N bond and is best described as a metallacyclic structure. The extensive research on the coordination of unsaturated N N bonds to the fragment [Re] = ReCp(CO)2 as related to N2 reduction involving species sketched in Scheme 22 has been reviewed. ... 

Only a few syntheses of cobalt-containing metallacycles have been carried out over the period under review. In 1998, Klein and co-workers reported the synthesis of the 18-electron cobalt(i) complex 83, smoothly formed by an aldehyde reaction with a suitable methylcobalt(l) according to Equation (24) <19980M4196>. 

Another type of reactivity typical of zirconium and hafnium alkyne complexes is ring expansion to yield five-membered and larger metallacycles. A variety of unsaturated organic reagents can be added including alkenes, alkynes, and ketones. This chemistry has been reviewed in detail. Heteroatom-stabilized zirconocene alkyne... 

These heterocycles contain at least one transition metal as part of a strained three-membered ring. Their structures therefore are partially described as heterocycles and partially as transition metal n complexes of heteroalkenes (Equation (1)). Evidence for metallacyclic structures is therefore be highlighted. A wide variety of transition metal complexes could be covered but this chapter is restricted to compounds where the ring carbon is tetravalent, bonded to hydrogen or carbon, and bonded to not more that two metal atoms. Related areas which have been reviewed include CO bridged bimetallics <84AOC(23)2i9> and C02 metal complexes <83AOC(22)l29>. 

The application of PNS and PNSe linear systems for the formation of main group metallacycles has been reviewed. Cyclic metal complexes have been prepared from [SP(Ph2)NP(Ph2)S] and Pt(II) compounds.The X-ray structure of Pt(PPh3)[N(SPPh2)2] Cr (10) has been reported. ... 

Numerous cyclopropane products can be obtained using these methods in stoichiometric or catalytic versions. The metallacyclic intermediates were isolated and characterized in some cases. A concise review on these results is available. Some examples, particularly with decomplexation of cyclopropane products from isolable metallacycles, are discussed here. For a more detailed description, see Section l.B.2.2.2. 

Phosphoies and other reiated heterocycies are an important class of main group compounds. The chemistry of phosphoies and their preparation has been reviewed extensiveiy by Mathey.3 We provide details here for a simple, one-pot procedure for the preparation of 1-phenyl-2,3,4,5-tetramethylphosphole applying zirconocene chemistry.4 The procedure involves reduction of (ri-C5H5)2ZrCl2 with butyllithium in the presence of 2-butyne which (as reported initially by Negishi, et al. ) forms a zirconium metallacycle. Addition of dichiorophenylphosphine to this reaction mixture produces the phosphole. One other procedure for the preparation of 1-phenyl-... 

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