Electrophilic Cleavage

With Electrophiles.—Electrophilic Cleavage. The vapour-phase isomerization of cyclopropane to propene has been studied and it is found that boron tribromide is by far the most effective catalyst (A ,, HBr 1, BBr3 140). Acid-catalysed cleavage of cis-l,2,3-trimethylcyclopropane has been shown previously to involve retention 

The stereoselective rearrangement of medium-ring bicyclo[n,l,0]alkan-2-ols has been observed for the first time in a large-ring counterpart. Thus treatment of the trans, anti-alcohol (66) with acid affords the trans-cycloalkenol (167) in a highly 

The alkylation of benzene and toluene by cyclopropane and a number of its alkylated derivatives has been subjected to further scrutiny and the acylation of cyclopropanes has been shown to proceed by way of a cyclic carboxonium ion salt.  

Apart from its use as a synthetic method,the silver-ion-assisted solvolysis of gfcm-dihalogenocyclopropanes has received further detailed scrutiny. The solvolyses of 9,9-dibromobicyclo[6,l,0]-nonane and -non-4-ene both show both first- and second-order dependence on silver(i) ion. That part of the reaction first-order in 

Fujisaki, A. Nagashima, S. Murata, and S. Kajigaeshi, Nippon Kagaku Kaishi, 1976,477 Chem. Abs., 1976,85,45730) A. Kumar and D. Devaprabhakara, Synthesis, 1976,461. 

With Electrophiles.—Electrophilic Cleavage. Acid-catalysed hydration of 17-substituted 3a,4a-cycloandrost-6-enes (166) results in unexpected deuterium incorporation at C-7, with no preference for a- or 3-addition. The failure to observe 

Electrophilic cleavage of the three-membered ring of (167) is best rationalized by initial endo-attack to give (ultimately) (168) and (169). Despite endo-protonation of 

Surprisingly, the omission of water from the reaction medium only slightly reduces the preference for the retention pathway, in complete contrast to the behaviour of structurally related caged compounds. Quadricyclanes (171) afford (172) and (173) in a 1 1 ratio with arylsulphenyl chlorides. The appearance of (173) 

Ketone (174) acts as a precursor to 7,7-disubstituted bic clo[2,2,l]heptanes, the initial step in the sequence involving cleavage of the cyclopropane ring to give (175). Analogous processes have been used in the conversion of 8,9-dehydro-adamantan-2-ones to 2-exo-substituted protoadamantanones.  

Friedel-Crafts alkylations of chlorobenzene, chloropropylbenzenes, and p-xylene by cyclopropane have been examined, employing sulphuric acid and aluminium trichloride as catalysts. The alkylation of benzene, toluene, and anisole by 1-bromo-1-fluorocyclopropane results in indanes, indenes, and alkenylbenzenes. Intramolecular acylation of (183) affords products of attack at C-1 and C-2 of the cyclo- 

4 Reactions of Three- and Four-membered Rings With Electrophiles.—Electrophilic Cleavage. A re-investigation of the acid-catalysed 

Berridge, D. Bryce-Smith, and A. Gilbert, J.C.S. Chem. Comm., 1974, 964. 

R = R = Me) has established that cleava of the three-membered ring occurs from the more substituted side, affording (372). This contradicts a recent report by Russian workers.  

The stereochemistry of some tricyclo[5,4,0,0 ]undecane derivatives has also been corrected. These were previously formulated with a /ra s-decalin fusion and have now been shown to be cis by n.m.r., c.d., and i.r. spectra, and acid-catalysed cleavage to cis-decalins. In the case of (373), ring-opening with HBr-HOAc followed by 

The vinylmethylenecyclopropane (375) undergoes acidic cleavage to methylcyclo-heptatrienes with = 23kcalmol and AS = — 5.6 1.3 e.u. The first step is evidently protonation of the exo-methylene group, followed by internal cleavage in the ring-fused cyclopropylcarbonium ion and finally deprotonation.  

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Hydrometallation and carbometallation of alkynylsilanes proceeds regio-and stereospecifically, the metal becoming attached to the silicon-bearing carbon atom in what is normally a co-addition process (hydrostannylation, however, shows the opposite regioselectivity). Electrophilic cleavage, with retention, of the carbon-metal bond then leads to vinylsilanes of various types. 

Furthermore, gallium compounds can serve as model systems for aluminum congeners. Cationic gallium alkyls are of interest in synthesis and catalytic applications involving polar substituents because of the relative stability of the Ga—R bond toward hydrolysis and electrophilic cleavage compared to the otherwise superior Al-R species [11]. 

Metal alkyl reagents react with the acidic OH groups of silica, probably by the electrophilic cleavage of the metal-carbon bond. For example, the electrophilic cleavage of the metal-carbon bond occurs when organometallic reagents react with the electrophilic OH groups of the silica surface (Scheme... 

The first transformation is the electrophilic cleavage of the carbon-metal bonds, which allows the functionalization of the substrate. This is more frequently observed with the group 4 metals (Ti, Zr, see Chapter 10.06). Carbonylation... 

The intermediate formation of the ferrocenyl-substituted silylium ion 16 by protonation of the ansa-ferrocenyl silane 17 can be regarded as a special case of electrophilic cleavage of an activated C-Si bond (see Scheme 7). The driving force for this reaction is the release of a strain by formation of the silyl cation. In a... 

Similarly, [(Bu CH2)2Mo(=NAr)(=CHCMe2R)] reacts with the silanols group of a silica or a molecular silanol to yield [(=SiO)Mo(=NAr)(=CHCMe2R)(CH2Bu )] via direct electrophilic cleavage of the Mo-C bond, according to mass balance analysis, NMR and calculations (Scheme 2.27) [72, 73]. 

Table 7. Regioselective Electrophilic Cleavage of Annelated Cycloproparenes...

Cyclopropylmethyl cations 1-5 may be generated from hydrocarbons through electrophilic cleavage of strained r-bonds or electrophilic attack on appropriately located ji-bonds and rearranged to cyclobutanes or cyclobutenes. 

Lewis acids have also been found to promote the electrophilic cleavage of cyclobutanes. The key feature of this reaction can be depicted by the conversion of the chiral 4-(2-hydroxy-6-methoxyphenyl)-6,6-dimethylbicyclo[3.1.1]heptan-2-ones 5 and 4-(2-methoxy-6-hydroxyphenyl)-6,6-dimethylbicyclo[3.1.1]hept-3-en-2-ones 7 to (6a5, 10a/ )-(-)-l-methoxy-6,6a,7,8,10,10a-hexahydro-6,6-dimethyl-9//-dibenzo[b,c/]pyran-9-ones 6 and (6aS)-( + )-methoxy-6,6a,7,8-tetrahydro-6,6-dimethyl-9//-dibcnzo[b//]pyran-9-ones 8, respectively, on treatment with either tin(IV) chloride in chloroform or with aluminum trichloride in dichloromethane.52... 

In the search for a reactive functional group which could be substituted on the acetylacetonate ring, chloromethylation of these chelates was attempted. The initially formed products were too reactive to be characterized directly. Treatment of rhodium acetylacetonate with chloromethyl methyl ether in the presence of boron trifluoride etherate afforded a solution of a very reactive species, apparently the chloromethyl chelate (XXX) (26). Hydrolytic workup of this intermediate yielded a polymeric mixture of rhodium chelates, but these did not contain chlorine On the basis of evidence discussed later on electrophilic cleavage of carbon from metal chelate rings and on the basis of their NMR spectra, these polymers may be of the type shown below. Reaction of the intermediate with dry ethanol afforded an impure chelate which is apparently the trisethyl ether (XXXI). Treatment of the reactive intermediate with other nucleophiles gave intractable mixtures. 

A more satisfactory solution to the mechanism of these substitutions now seems experimentally feasible. It is likely that the trisamino chelate (XXXIII) could be completely resolved by salt formation with a suitable optically active acid. The optically pure amine could then be converted by electrophilic cleavage into optically active bromo-, chloro-, and thiocyanate-substituted chelates. It would thus be a simple matter to determine whether these substitutions proceed with complete retention of asymmetry. Further, the question of a symmetrical five-coordinate intermediate in racemization of such compounds could probably be elucidated by a study of solvent polarity or salt effects on the kinetics of the racemization of these chelates. 

An interesting variant of electrophilic cleavage is intramolecular attack at the linking carboxylic acid derivative by a substrate-bound electrophile (Figure 3.23). This cleavage strategy generally produces lactones. 

Figure 3.23. Intramolecular electrophilic cleavage of carboxylic acid derivatives from polymeric supports. X leaving group Y NR, O.

THE EFFECT OF /7-SUBSTITUENTS ON THE ELECTROPHILIC CLEAVAGE OF BENZYL-METAL AND (X-CARBETHOXYBENZYL-METAL BONDS... 

RELATIVE RATES OF ELECTROPHILIC CLEAVAGE OF THE CARBON-METAL BOND IN SOME /7-SUBSTITUTED BENZYL-METAL AND... 

A detailed investigation showed that PhB(OH)2 does not interfere with the chiral ligand, leaving the enantioselective step of alkene oxidation intact. Its main role-apart from protecting the diol products against potential over-oxidation - is to remove the diol via an electrophilic cleavage, which is in contrast to the conventional hydrolytic cleavage of the AD protocols.126... 

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