Complexes four-centre

A perfectly ordered polymer composite by four-centre-type photopolymerization of a molecular complex 166... 

A PERFECTLY ORDERED POLYMER COMPOSITE BY FOUR-CENTRE-TYPE PHOTOPOLYMERIZATION OF A MOLECULAR COMPLEX... 

Ferrocenyltriphenylphosphonium perchlorate (84) has been synthesized from iodoferrocene, tetrakis(acetonitrile)copper(i), and triphenylphosphine in nitromethane. The authors suggest that iodoferrocene first forms the complex (85) which then breaks down via a four-centred transition state to (84). 

The investigation of factors affecting facial selectivity in the hydroboration of steroidal -alkenes revealed the facial (a vs /3) stereoselectivities of hydroboration of androst-5-enes (69) and B-norandrost-5-enes (70) do not parallel the difference between the calculated force-field energies for a- and jS-cyclobutane models (71)-(74). This finding appears to suggest that the facial selectivity is not determined by the four-centre transition state but by the relative ease of formation of the initial tt-complex. ... 

Co (Con) exists in equilibrium with the hydrido form, (HCo(Con) + (pXa x 1.0), which gradually decomposes to Co (Con) and H2. The hydrido complexes are more reactive nucleophiles than are Co1 (Con), and their addition to unsaturated bonds gives cis products probably via an unsym-metric four-centre mechanism (Scheme 100, iii, iv). The Co—C bond is also formed between Con(Con) + and alkyl radicals (Scheme 100, v), and Com(Con) 2+ and carbanions (Scheme 100, vi). 

Reaction (166) is also catalyzed by acidic rhodium chloride solutions under similar conditions to those used for ruthenium. Aquachloro complexes were again implicated and the most active species was [RhCl5(H20)]3. The hydration step was thought to involve a four-centred transition state (134).616 Here also, inhibition at low and high chloride ion concentration occurred. 

The k -)/hr KIE equals 2.27 (CdS), 1.84 (ZnO) and 2.07 (Ti02) for ketone 539 in labelled methanol indicated that H- transfer is involved in the RDS. No H/D exchange in CD3OH was observed. This excludes the participation of free radicals in the hydrogen exchange. The possibility of II/I I exchange in the four-centre complex 541 has not been included in the reaction scheme shown in equation 317. 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

Figure 3.32 Supposed four-centred activated complexes for secondary insertion of a propylene molecule...

These experimental measurements have prompted ab initio MO calculations of model additions of lithium hydride (Houk et al., 1985). With ethylene, monomeric lithium hydride initially forms a stable rc-complex [7], which passes through a four-centre cyclic transition structure [8] to yield the ethyllithium [9], with energies relative to reactants of-50.0, 28.5, and... 

The course of these additions of lithium hydride resembles that found for the addition of borane (Nagase et al., 1980 Graham et al., 1981). With ethylene, the initial step is exothermic formation of a Jt-complex without barrier, then rate-determining transformation to the borane via a four-centre transition structure. In both the borane and lithium hydride additions, there is relatively little development of the new C—H bond with distances of 1.692 and 1.736 A respectively in the transition structures. When a carbanionic product is not formed, for example in the reaction of lithium hydride with cyclopropenyl cation yielding cyclopropene and lithium cation (Tapia et al., 1985), reaction again occurs via a hydride-bridged complex, but the C- H- -Li array remains nearly linear throughout the reaction. 

The mechanism (Following fig.) involves the alkene n bond interacting with the empty p orbital of boron to form a n complex. One of BH3 s hydrogen atom is then transferred to one end of the alkene as boron itself forms a o bond to the other end. This takes place through a four-centred transition state where the alkene s n bond and the B-H bond are partially broken, and the eventual C-H and C-B bonds are partially formed. There is an imbalance... 

Primary process III is more likely to involve the formation of a six-centred transition complex than of a four-centred one suggested by McDowell and Sifniades (ref. 86). The suggestion of the occurrence of processes IV and V was based on the identification of methane and crotonyl radical, respectively. It should be pointed out, however, that reliable data here too are very scanty. 

Bamford and Norrish observed that the free radical formation is the sole primary process in the photolysis of cyclohexanone, while step II is the major reaction occurring in the photolysis of 1-menthone. These results are rather difficult to interpret if reaction II occurs through a four-centred ring complex however, if a six-centred complex is involved, the consideration of the steric factors leads to a conclusion which is reconcilable with the results of Bamford andNorrish. The significance of steric factors (stereoelectronic requirements) appears from the fact that type II elimination is the major intramolecular path in the photolysis of ciy-2- -propyl-4-t-butyl cyclohexanone, while the photolysis of the tram compound yields the cis isomer as the major product The difference has been explained... 

The simple ketone molecule is directly derivable from the four-centred complex, while it is the enol form that is expected to be first formed from the six-centred complex, and then converted into the final keto form. Since we know that enol H atoms are capable of hydrogen-deuterium exchange, we have another possibility to decide between the two mechanisms. 

Several theoretical studies have considered LiH addition as a model for the more computationally difficult L1A1H4 and NaBH4 [45]. However, reaction of aldehydes and ketones with LiH seldom if ever leads to reduction [46]. AIH3, while less commonly used than the complex boron and aluminium hydrides, is useful for reducing carbonyls [47] and therefore is a suitable model for computational study. Calculations [2, 5] show that gas-phase reduction of formaldehyde by AIH3 occurs by formation of complex 1, which rearranges via a four-centre transition state to form an aluminium methoxide product. Two conformational isomers of... 

Hydroboration is usually carried out in the ether tetrahydrofuran, in which borane exists as a complex 50, from which BH3 is added to an alkene, e.g. 2-methylpropene (44) in Scheme 4.11. Addition takes place at a face of the alkene by means of a four-centre transition state, as shown in 51. The partial bonds in 51 represent progressive formation of bonds between C and H, and between C and B, together with simultaneous weakening of the 7i bond and the B-H bond. In Scheme 4.11 the reaction of borane 52 is detailed this borane has two remaining B-H bonds, and a similar reaction of these two bonds with two further molecules of alkene results in exhaustive alkylation, with formation of the trialkylborane 53. The nature of the transition state 51 implies that H and B are delivered syn (to the same face), and simultaneously, to the double bond. 

The alkene is coordinated to vacant site and an insertion reaction occurs. After promotion of an electron from the Tialkyl bond to a molecular orbital of the complex, a four centre transition state is produced which enables an alkyl group to transfer to coordinated alkene. 

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