Metallacyclopropane

Attempts to exploit the reaction of the dianion with alkyl halides to produce a c/.v-dialkyl complex by using 1,2- or 1,3-dihaloalkanes did not indeed give this result. The reaction of Ru(Por) " with 1,2-dibromoethane was sucessful, but the resulting metallacyclopropane product is better formulated as a /r-complex of ethene, and will be discussed below in the section on alkenc and alkyne complexes. The corresponding reaction of the diiinion with 1,3-dichloropropane gave no evidence for a metallacyclobutane. but instead free cyclopropane was detected by GC analysis and the porphyrin product was Ru(TTP)(THF)2. ... 

Similarly to the Ti atom, the singlet state of HfH2 undergoes barrierless coordination of ethylene to make a metallacyclopropane dihydride. Both structural and NBO metrics strongly suggest that we should describe this complex as a simple metallacycle. The characteristics of this class seem to be well represented by the TiC2H4 prototype. 

We now consider some simple examples of the more common alkene coordination mode that is intermediate between the limiting extremes of weak dative coordination and strong metallacyclopropane insertion. Our model systems are the simple ethylene adducts of the group 10 metals Ni, Pd, and Pt. Because these metals exhibit... 

The question as to whether a transition metal complex of type 4 is best described as an alkene 7T-complex 4A or as a metallacyclopropane 4B, which is of practical importance, has been addressed in several computational studies on the relationship between alkene 7T-complexes and three-membered rings [48—52]. It has been concluded that the titanium complexes of type 4 are best represented as titanacydopropanes, i.e. by resonance structure 4B, if one is willing to accept the notion that 4A and 4B are limiting resonance forms [52],... 

The first isolable alkenetitanium complex, the bis(pentamethylcyclopentadienyl)-titanium—ethylene complex 5, was prepared by Bercaw et al. by reduction of bis(penta-methylcyclopentadienyl)titanium dichloride in toluene with sodium amalgam under an atmosphere of ethylene (ca. 700 Torr) or from ( (n-C5Mc5)2Ti 2(fJ-N2)2 by treatment with ethylene [42], X-ray crystal structure analyses of 5 and of the ethylenebis(aryloxy)trimethyl-phosphanyltitanium complex 6 [53] revealed that the coordination of ethylene causes a substantial increase in the carbon—carbon double bond length from 1.337(2) A in free ethylene to 1.438(5) A and 1.425(3) A, respectively. Considerable bending of the hydrogen atoms out of the plane of the ethylene molecule is also observed. By comparison with structural data for other ethylene complexes and three-membered heterocyclic compounds, the structures of 5 and 6 would appear to be intermediate along the continuum between a Ti(11)-ethylene (4A) and a Ti(IV)-metallacyclopropane (4B) (Scheme 11.1) as... 

The classical tin-lithium exchange reaction was first applied to cyclopropanes in the stereospecific transformation of 7-ewrfo-bromo-7-ex o-trimethyltinnorcarane to the corresponding 7-exo-lithio compound with BuLi in THF, at -95 °C (vide supra)10. The retention of configuration in cyclopropyl tin-lithium exchange reactions has also been confirmed in monocyclic 2-metallacyclopropane carboxamides60. More recently, this metal exchange reaction was used for the preparation of sensitive methylenecyclopropanes (equation 21)61. 

Figure 3 Two of the possible bonding modes of tetracyanoethylene, TCNE (a) metallacyclopropane and (b) end-on...

NMR spectroscopic studies of two rhodium complexes 18 and 19 also indicate that the binding of the C2F4 group to rhodium is better pictured as a conformationally locked metallacyclopropane (59). The activation barrier for rotation of the C2H4 ligands around the Rh-olefin (centroid) vector in 19 was determined to be 15.0 0.2 kcal/mol. In contrast, the rotational barrier for the C2H4 ligand in 18 was demonstrated to be 13.6 0.6 kcal/mol... 

The binding of alkenes to transition metals, to form n complexes or r 2 complexes or metallacyclopropanes, is a Lewis acid-Lewis base interaction that is made more elaborate by back bonding from the metal to the alkene. There are many examples of deuterium IEs on complexation. One that was studied extensively is the binding of ethylene, propylene, and 2-butene to Ag+, where the deuterated alkene binds more strongly.88 For example, Acd,=cd,/Ac[ i, [Pg.145]

Metallacyclopentenes, in enyne carbometallation, 10, 324 Metallacyclopropanes, with Ti(IV), 4, 359 Metalladiboranes, with Groups 8 and 9, 3, 157—158 2-Metalla-l,3-dichalcogena-[3]ferrocennophanes, Rh complexes, electrochemistry, 7, 149 Metallaoxiranes, preparation, 4, 917 Metallasilazanes, preparation and characteristics, 3, 450 Metallasiloxanes, preparation and characteristics, 3, 458 Metallate(III) compounds, isolated, preparation, 4, 751 ortho-Metallated complexes... 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

Smith and co-workers reported the dehydrogenative borylation of ethylene with catecholborane in the presence of decamethyltitanocene Cp2 Ti catalyst to produce 5-vinyl catechol boronate 137. Reaction of Cp2 Ti with ethylene leads to the formation of metallacyclopropane species 135 that undergoes reaction with catecholborane 38a to... 

Finally, a word about notation. The relative contributions of classical 7r-complex and metallacyclopropane resonance forms in coordinated al-kenes and alkynes varies widely with changes in both the metal and its ligands.9 For the sake of simplicity, we use the classical jr-complex notation throughout, with the caveat that this notation implies nothing about the importance or lack thereof of the metallacyclopropane form. 

In contrast to the chemical shift, the values for V(CH) alter little upon complexation. It has, however, been pointed out (41) that since the coupling in free ethylene (157 Hz) and in cyclopropane (161 Hz) is very similar, it is to be expected that this coupling will be relatively insensitive to any contribution from a metallacyclopropane form and hence provide little insight into the nature of the metal-carbon bond. 

In a simplified picture, the mechanism of the Zr-catalyzed ethylmagnesation can be rationalized as shown in Scheme 1 [8]. At first, the zirconocene-ethene complex 12 is generated from the catalyst precursor Cp2ZrCl2. Complex 12 can also be regarded as a metallacyclopropane 16. After coordination and insertion of the alkene 10, a metalla-cyclopentane 13 is formed, which subsequently reacts with the Grignard reagent regioselectively to the open-chain intermediate... 

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