Substituted metallacyclopentanes

Substituted metallacyclopentanes have also been prepared by this route, e.g., for the bicycles 5744 and cis- or trans-58 [Eq. (22)], the cis isomer having been structurally characterised.61... 

Metallacyclopentane complexes can form through the addition of an olefin to an olefin complex. Unsubstituted metallacyclopentane complexes have been observed in solutions of biphenolate complexes under ethylene [47]. One biphenolate complex, W(NArci)(Biphen)(C4Hg) [48], and one disiloxide, Mo(NAr)(C4Hg)(OSiPh3)2 [43], have been structurally characterized. Substituted metallacyclopentanes have not yet been identified. 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Many cyclization reactions via formation of metallacycles from alkynes and alkenes are known. Formally these reactions can be considered as oxidative cyclization (coupling) involving oxidation of the central metals. Although confusing, they are also called the reductive cyclization, because alkynes and alkenes are reduced to alkenes and alkanes by the metallacycle formation. Three basic patterns for the intermolecular oxidative coupling to give the metallacyclopentane 94, metallacyclopentene 95 and metallacyclopentadiene 96 are known. (For simplicity only ethylene and acetylene are used. The reaction can be extended to substituted alkenes and alkynes too). Formation of these metallacycles is not a one-step process, and is understood by initial formation of an tj2 complex, or metallacyclopropene 99, followed by insertion of the alkyne or alkene to generate the metallacycles 94-96, 100 and 101-103 (Scheme 7.1). 

Semiempirical molecular orbital calculations on this model [309] suggest that, in the case of propylene polymerisation, equatorial 2,4-substitution of the metallacyclopentane ring is the most stable form this would lead to regiose-lective head-to-tail propagation during the polymerisation of propylene and, moreover, to the formation of isotactic polypropylene [51]. Such calculations concern a case, however, that has not been confirmed by experiments a coordination of propylene at Ti(II) species and subsequent reaction according to the above scheme is not as obvious as that of ethylene. 

Many crystal structures of nickel-alkene complexes have been reported. As demonstrated in Scheme 55, bis(alkene) complexes may exist in equilibrium with the corresponding metallacyclopentane complex. However, several alkene complexes which have the potential to undergo oxidative cyclization to a metallacycle have been fuUy characterized. The substitution chemistry of bis(iY -cycloocta-l,5-diene)nickel(0) (2) is representative of most nickel(0)-alkene complexes, which are readily substituted by a variety of ligands. Bis(q -ethene)(tricyclohexylphosphine)nickel(0) has been prepared and fully character-ized,l " l and a variety of complexes of electron-deficient alkenes such as 69 have been prepared which tend to be more stable than the complexes of ethene (Scheme 56).l" " The alkene complexes may be prepared directly from bis(q -cycloocta-l,5-diene)nickel(0) (2)l" l or from nickel(ll) chloride " that is reduced by zinc metal. 

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