Rhodacyclobutane complexes

Metallacyclobutane complexes can undergo net reductive rearrangement to form alkene complexes or undergo decomposition with the liberation of free alkene. The rhodacyclobutane complex 112, for example, rearranges thermally to give propene complex 113 (Equation 33) <19980M4484>. 

Following Bergman s initial report <1984JA7272, 1986JA7346>, cyclopropane C-H activation and subsequent isomerization of the intermediate tr-cyclopropylrhodium hydride 169 to the rhodacyclobutane complex 112 has been demonstrated for a tris(pyrazolyl)borate analogue (Scheme 39), confirming that net cyclopropane oxidative addition to rhodium follows a rather unexpected indirect mechanistic course <19980M4484>. 

Tp Rh(j/ -H2C = CHMe)(CNCH2 Bu) (153 ) has also been prepared, but by thermolysis of the rhodacyclobutane complex 154, which is derived from C-C bond activation of a d-cyclopropyl moiety (Section II-C.l). 

A curious reactivity of the cyclopropyl complex 397 is its tendency, under ambient conditions, to rearrange intramolecularly to afford the rhodacyclobutane complex 154 (ri/2 = 95min). In turn, 154 rearranges thermally to the f/ -propene complex 153, which upon prolonged heating in loses propene to afford... 

The rearrangement of Tp RhH(CH2CH = CH2)(CO) to Tp Rh( / -MeCH = CH2)(CO), and the formation and chemistry of the rhodacyclobutane complex Tp Rh(o--l,3-CH2CH2CH2)(CO) have also been documented in Ghosh, C. K. PhD Thesis, University of Alberta, Edmonton, AB, Canada, 1988 though details have not appeared in the primary literature. 

The cyclopropyl-hydrido complex 415 exhibits appreciable thermal instability, such that it rearranges quantitatively in ambient temperature benzene solutions to the rhodacyclobutane complex Tp Rh(cr,cr -CH2CH2CH2)(CNCH2 Bu) (154, Scheme 34, Section II-C.l). Thermal isomerisation of 154 (65 °C, 2.5 h) affords a 7i-propene complex (153) that is able to activate benzene. Alternatively, in the presence of excess CNCH2 Bu, 154 thermally inserts two equivalents of the isonitrile to ultimately afford the rhodacyclohexane complex 427 (Scheme 34). 

A mechanistic study of the rearrangement of [Rh(CgMeg)(L)(cyclo-propyDH] (L PMe3) to the rhodacyclobutane [ (CgMe ) L)R hCH CH H2], and a new method of synthesis of the rhodacyclobutane complex, has been reported.Isotope effects in the activation of arene C-H bonds by the rhodium intermediate [Rh(C Mec)(PMe,)] have been... 

Generation of the 16-electron fragment Tp Rh(CNCH2CMe3) (Tp =[HB(3,5-dimethylpyrazolyl)3]) t> the presence of cyclopropane resulted in C-H activation of the hydrocarbon. The cyclopropyl hydride complex rearranged in benzene solvent to the metallacyclobutane complex Tp Rh(CNCH2CMe3)(CH2CH2CH2). Thermolysis of the rhodacyclobutane complex produced an 77 -propylene complex (Scheme 38). ... 

It should also be mentioned that very recently, a new cycloisomerization of enynes has been shown to proceed via a rhodium-vinylidene complex,187 which, after [2 + 2]-cycloaddition and ring opening of a rhodacyclobutane, furnishes versatile cyclic dienes (Scheme 47).188 Not only does this constitute a fifth mechanistic pathway, but it also opens new opportunites for C-C bond constructions. 

Equation 7.39 describes a transformation with first the C-H bond of cyclopropane adding to the Rh complex, followed by RE of H2, and then rearrangement to give the rhodacyclobutane.82 Metallacyclobutanes are thought to be intermediates in some alkene polymerization and metathesis reactions these compounds will appear again in Chapter 11. 

Bergman et al. presented an important mechanistic study of the oxidative addition of cyclopropane [14]. The reaction of cyclopropane with coordinatively unsaturated rhodium complex 3 at -60°C results in C-H insertion. No C-C bond cleavage was observed at that temperature. Upon raising the temperature to -20°C, (cyclopropyl)(hydride)rhodium complex 4 undergoes direct rearrangement to rhodacyclobutane 5. The C-H insertion product is kinetically favored, and the C-C insertion product is thermodynamically favored. The kinetic preference for C-H insertion clearly demonstrates the greater steric accessibility of the C-H bond compared with the C-C bond, as mentioned above. Evidence sug-... 

In another early example Periana and Bergman made an important mechanistic observation by reacting the unsaturated Rh complex with cyclopropane [Eq. (6.108)]. At -60°C C-H insertion resulted in complex 127, which is the kinetically favored product. This underwent direct rearrangement to the thermodynamically favored 128 rhodacyclobutane. 

Periana RA, Bergman RG (1984) Rapid intramolecular rearrangement of a hydridocyclopro-pylrhodium complex to a rhodacyclobutane. Independent synthesis of the metallacycle by addition of hydride to the central carbon atom of a cationic rhodium Tr-allyl complex. J Am Chem Soc 106 7272-7273... 

It is often observed that C-H activation precedes C-C activation. For instance, photoirradiation of Cp Rh(PMe3)(H2) generated coordinatively unsaturated Cp Rh(PMeg) with liberation of dihydrogen (Scheme 1.3) [4]. The rhodium complex reacted with cyclopropane at -60°C to furnish a C-H oxidative addition product. No cleavage of a C-C bond was observed at this low temperature. Upon raising the temperature to 0-10 C, the cyclopropylrhodium rearranged to a rhodacyclobutane. This result indicates that oxidative addition of a C-H bond is kinetically favored and oxidative addition of a C-C bond is... 

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