Substitution structural isomerisations

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

As shown in the synthetic sequence, to induce isomerisation of the c -B/C junction to a trans configuration, a double bond between C(6) and C(7), that can transmit the electronic effects of carbonyl group at C(5) must be created ex profeso. Yhis relatively simple structural modification requires no less than five different synthetic operations i) bromination at C(6) ii) substitution of the bromine atom by an OH group iii) oxidation iv) 0-methylation with dimethyl sulphate, and... 

It is well established that vinyllithiums of the general structure 313, where R aryl or silyl, are configurationally stable in solution.145 146 Aryl- and silyl-substituted vinyllithiums may however isomerise in solution in the presence of coordinating solvents such as THF. For example, 314a and 314b are configurationally stable at room temperature in hexane in the presence of less than 1 equiv. THF, but equilibrate at -30 °C in the presence of more than 5 equivalents of THF.147-148 A silyl substituent at R1 of 313 also induces configurational instability.149... 

The alkylation of toluene with methanol over HZSM-5 proceeds at low temperatures via a protonated methanol species in the transition state [107] and weakly coadsoibed toluene as classically predicted for Friedel Crafts alkylation. The reaction rate is directly proportional to the concentration of the chemisorbed methanol (in the presence of excess toluene) as shown in Figure 6 [108]. Alkylation leads preferentially to ortho- and para- substituted products which rapidly isomerise in the zeolite pores. Specific reaction conditions and tailoring of the catalyst pore structure can be employed so that para- substituted products are preferentially... 

Some l,3,2-oxazaphospholidin-4-ones (28) were prepared as shown. The diastereomeric compounds (R = H) were formed as predominantly the cis isomers which isomerised to the more stable trans isomers at room temperature. Diphenylchlorophosphine with 5-fluorouracil gave a mixture of the di-O-substi-tuted compound 29 and the di-A-substituted compound 30, from which 30 could be isolated by precipitation with pentane. The structures follow from NMR and an X-ray crystal structure of 30. 

Unstabilised a-iodoalkyl ylides (1) have been prepared and used in Wittig reactions to give 1-iodoalkenes with (Z)-stereoselectivity. The P-fluoro-substituted ylides (3), containing a chiral alkoxy ligand, are formed as 1 1 mixtures of diastereoisomers from the corresponding alkoxydifluorophosphoranes (2). However, (3) can be epimerised to predominately (>95% in one case) one diastereomer by treatment with lithium fluoride. The first carbodiphosphoranes (4) and (6) containing a P-H bond have been reported. Compound (4) is thermally stable in the solid state but in solution at room temperature both (4) and (6) slowly isomerise to the ylides (5) and (7), respectively. The structures of all these products are supported by n.m.r. data. 

The processes considered in this section - hydrogenation, disproportionation, and isomerisation - are frequently encountered in the chemistry of the terpenes and steroids, but even with the simpler cycloalkenes many problems remain to be solved. It is unfortunate that no LEED or vibrational spectroscopy seems to have been performed on substituted cycloalkenes, so that stereochemical preferences cannot be related to adsorbed structure as defined for example by the na factor (Section 4.42). 

The ring-chain isomerisation of phosphino-substituted triazolo-pyridines was found to depend on the nature of the phosphine substituent. Conversion of the phosphines to their selenides resulted in a complete shift of the equilibrium towards electron-acceptor structure D (Scheme 10). Acenaphthene and acenaphthylene were converted to polycyclic phosphole derivatives via Ti(II)-mediated cyclization of the corresponding dialkynylated arenas. The related phosphole oxides were stable species (Scheme 11). Asymmetric lithiation of dimethyl-tert-butylphosphine sulfide and trimethylsilylation gave an intermediate that was subjected to a five-step reaction sequence including metallation, P-functionalization (in three steps) and removal of the silyl group to yield the precursor of Mini-PHOS (Scheme 12). ... 

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