7r-Donor substituents

The kinetic and thermodynamic properties of Fischer-type carbene complexes have also been addressed by Bernasconi, who relates the strength of the 7r-donor substituent to the thermodynamic acidity [95-101] and the kinetics and mechanism of hydrolysis and reversible cyclization to differences in the ligand X [96,102]. 

It is not possible to describe the interactions of 1 with 7r-donor substituents (let alone tr-donor/(j-acceptor substituents) on the basis of Walsh orbitals. For this purpose one has to retreat to the full set of SCF orbitals ... 

Conventionally, carbene complexes are grouped into two classes Fischer [2] and Schrock [3] carbenes. Fischer carbenes may be identified by their 7r-donor substituents on the carbene carbon and having 7r-acceptor ligands bonded to a low-oxidation state, late-transition metal. These ligands may be thought of as singlet carbenes that donate a... 

Alkyl groups which are para to strong 7r-donor substituents such as hydroxy or methoxy can be removed from aromatic rings under acidic conditions, if the alkyl group is capable of forming a stable carbocation ... 

TT MO energy levels for ethylene with a 7r-donor substituent. 

Fischer-type carbene complexes, generally characterized by the formula (CO)5M=C(X)R (M=Cr, Mo, W X=7r-donor substitutent, R=alkyl, aryl or unsaturated alkenyl and alkynyl), have been known now for about 40 years. They have been widely used in synthetic reactions [37,51-58] and show a very good reactivity especially in cycloaddition reactions [59-64]. As described above, Fischer-type carbene complexes are characterized by a formal metal-carbon double bond to a low-valent transition metal which is usually stabilized by 7r-acceptor substituents such as CO, PPh3 or Cp. The electronic structure of the metal-carbene bond is of great interest because it determines the reactivity of the complex [65-68]. Several theoretical studies have addressed this problem by means of semiempirical [69-73], Hartree-Fock (HF) [74-79] and post-HF [80-83] calculations and lately also by density functional theory (DFT) calculations [67, 84-94]. Often these studies also compared Fischer-type and... 

An EPR study of the monomeric 02 adducts of the Schiff base complexes of Co(bzacen)(py) (71a) and the thiobenzoyl analog Co(Sbzacen)(py) (71b) characterized the five-coordinate mono (pyridine) precursors and the six-coordinate 02 adducts.327 Increased covalency in the Co—S bonds was seen in the EPR parameters, indicative of 7r-backbonding. Substituent effects on the aromatic rings had no effect on the EPR spectra, but these were reflected in the observed redox potentials. Furthermore, the S-donors stabilize the Co ion in lower oxidation states, which was consistent with destabilization of the 02 adducts. 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

Diorganoboryloxides, R2BO, bearing bulky organic substituents are quasi-alkoxide ligands for main group and transition metals. Because of the 7r-acceptor properties of boron, these anions are weaker 7r-donors than alkoxides, and this is reflected in their structural chemistry. Diethylzinc reacted with the borinic anhydride (9-BBN)20, Scheme 82, to afford the alkylzincboryloxide [EtZnO(9-BBN)]4 129, which also crystallizes in heterocubic form.187... 

Conjugating groups tend to favor attack at C , but the ratio of Ca Ce attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good 7r-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

The proposed exciplex orientation, suggested to dictate the regioselectivity of the photoaddition, is consistent with the orientation of the dipolar attraction between the electronically n,7r excited triplet enone and the ground state alkene. Generally, electron acceptor substituents on the alkene provide preferential formation of the head-to-head (H,H) products, whereas electron donor substituents provide preferential formation of the head-to-tail (H,T) photoproducts55 (Scheme 16). 

The efficiency of the E/Z photoreaction - and especially the speed of the thermal Z/E reverse reaction - is strongly dependent upon the electronic and steric influence of the substituents on the arene units. 7r-Donors in para position accelerate it 7r-acceplors inhibit it push-pull substituents are particularly effective in lowering the barrier. 

Fortunately, the correct explanation of which of these effects is dominant is subject to experimental test. If a donor substituent possessing no low-lying vacant orbital of 7r symmetry, such as R2N or RO is placed on an electron-deficient alkene, then the reversal of polarization cited in the last section cannot occur. However, the secondary orbital interactions described here will still be present. Thus, cycloadditions... 

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