G ti followed by migration of the attached alkyl

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Unformatted text preview: ulation is another major application. Stereoregular (isotactic) polypropene and many copolymers of ethene are also manufactured. Much work has been done in an attempt to elucidate the chemical nature of the catalysts and the mechanism of their action; the active site may differ in detail from system to system but there is now general agreement that polymerization is initiated by v 2 coordination of ethene to the partly alkylated lower-valent transition-metal atom (e.g. Ti"') followed by migration of the attached alkyl group from transition-metal to carbon (the Cossee mechanism, see Scheme below). An alternative suggestion involves a metal-carbene species generated by a-hydrogen transfer from carbon to the transition Coordination of the ethene or propene to Ti"' polarizes the C-C bond and allows ready migration of the alkyl group with its bonding electron-pair. This occurs as a concerted process, and transforms the y2-alkene into a 0bonded alkyl group. As much as 1 tonne of polypropylene can be obtained from as little as 5 g Ti in the catalyst. Finally, in this subsection, we mention a few recent examples of the use of specific ligands to stabilize particular coordination geometries about the organoaluminium atom (see also p. 256). Trigonal planar stereochemistry has been achieved in R2AlCH2AlR2 { R = (Me3Si),CH-}, which was prepared as colourless crystals by reacting CH2(AlC12)2 with 4 moles of LiCH(SiMe3)2 in pentane.(77) It is also noteworthy that the bulky R groups permit the isolation for the first time of a molecule having the AlCHzAl grouping, by preventing the dismutation which spontaneously occurs with the Me an Et derivatives. The linear cation [A1Me21f has been stabilized by use of crown ethers (p. 96).(78) For example, 15-crown-5 gives overall pentagonal bipyramidal 7-fold coordination around A1 with axial Me groups having A1-C 200pm and angle Me-Al-Me 178" (see Fig. 7.19a). With the larger ligand 18-crown-6, the A1 atom is bonded to only three of the six 0 atoms to give unsymmetrical 5-fold coordination with A1-C 193pm and angle Me-Al-Me 141". Symmetrical (square-pyramidal) 5-coordinate A1 is found 7 6M. L. H. GREEN,Pure A ppl. Chem. 50, 2 7-35 (1978). K . J . IVIN, J . J . ROONEY,C. D. STEWART, . L. H. GREEN M and R . MAHTAB, . Chem. SOC., Chem. Commun., 604-6 J (1978). 7'M. LAYH nd W. UHL,Polyhedron 9, 2 77-82 (1990). a 78S. G . BOT, A . ALVANIPOUR, . D. MORLEY, D. A . S ATWOOD, . M. MEANS, W. COLEMAN J. L. ATWOOD, C A. and Angew. Chem. I nt. Edn. Engl. 26, 4 85-6 (1987). 262 Aluminium, Gallium, Indium and Thallium Ch. 7 Figure 7.19 (a) Structure of the cation in [AlMe2(15-crown-5)]+[AlMe2C12]- showing pentagonal bipyramidal coordination of A1 with axial Me groups. (b) Structure of [AlEtL] where L is the bis(deprotonated) form of the macrocycle H ~ [ C ~ ~ H Z Z N ~in (c). shown ] in the complex [AlEt.L] (Fig. 7.19b) formed a general decrease of chemical reactivity of the by reacting A12Ek in hexane solution with M-C bond in the sequence A1 > Ga % In > T1, Hz[C~ZHZZN~],H2L, shown in Fig. 7 . 1 9 ~ 5 ~ ~ )and this is particularly noticeable for compounds i.e. The average AI-N distance is 19...
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