Apical substitution

A theoretical study of model apically-substituted and equatorial ly-substituted acyclic phosphoranes, H PX, revealed a ligand apicophilicity in the order, Ct > CN > F > CeCH > H > CHg > OH >... 

Orbital symmetry arguments and EHT calculations have also provided a way of discriminating between axial and apical substitution in the above mentioned case of pentacoordinate phosphorus. This analysis leads the way to more complex problems of coordination around transition metal atoms. 

The preparation of 1-IB5H8 in high yield has been accomplished by reacting pentaborane(9) with iodine 183.184>, iodine monochloride 185> or iodine trichloride 183>. This compound is a thermally stable crystalline solid which decomposes slowly on contact with the atmosphere 183>. The apically substituted compound is apparently much more stable... 

The structure of decaborane(16) 200> reveals that it should be considered as an apically substituted derivative of pentaborane(9). The bond energy of the unique boron-boron bond which connects the two B5H8 cages has been determined to be 3.2 0.2 e.v. from mass spectrometry 201> the standard heat of formation is 34.8 kcal. per mole 202>. An analysis of the vibrational spectrum of decaborane(16) has been made by Pinson and Lin 203> who note the absence of the strong absorption at 901 cm-1 which was previously reported 204>. 

The apically substituted monomethyl derivative of B5H9 is a product of dimethyl ether cleavage by l-BrBjHg,... 

Electron diffraction measurements on 1- and 2-silylpentaboranes confirmed the conclusion of an EHMO calculation that the former should be more stable. Thus, r(B—Si) in the former is F981 0.005A, in the latter 2.006 0.004 A.Thus there appears to be greater 7r-interaction for the apically substituted isomer. The analogous methyl derivatives (where a similar, but much smaller stability difference was calculated) give r(B—CH3) values which are very similar (1.592 0.005, 1.595 0.005 A). 

Table 21 gives the alkyl substitution energies for the diamantane skeleton. The order of preference of alkyl substitution in apical > medial > secondary. In agreement with these predictions the C15 and C16 C H2n-g saturated stabi-lomers are known to be methyl (58) and dimethyl (59) apically substituted di-amantanes. 

The dominance of the apical ligands in the LCAO description of the semi-occupied orbital of these radicals is emphasised not only by the invariance of aj q gqj to apical substitution, but also by the relatively minor changes ina.3] and ajq p) on equatorial substitution. Alkoxy groups, as opposed to fluoro alkoxy groups, appear preferentially in equatorial positions in phosphoranylradicals. A comparison of the data for PF4 with those for its ethoxy-substituted derivatives (Table IV) shows that apical substitution of OEt for F has a far greater effect on the Pand ypej-fineinterac-... 

Other known acid-catalyzed pentaborane substitution reactions also result in apically substituted pentaboranes (251, 261-263). This is in agreement with the predicted site of electrophilic substitution based on charge distributions, as determined from valence bond and molecular orbital treatments (17,264,265). Furthermore, the position of observed attachment of boron on the carbon skeleton (i.e., the most substituted carbon when unsymmetrical alkenes are used) is consistent with this predicated mechanism (249). The position of boron attachment is kinetically rather than thermodynamically controlled, for basal substituted alkylpentaboranes are found to be about 3 kcal/mole more stable than the corresponding apically substituted pentaboranes (266). 

---