Removable derivative coupling

Eq. (26). All the removable derivative coupling is transformed away, which is probably the most systematic way to arrive at optimum (quasi )diabatic states in general. Of course, in a given example the smallness of the residual couphng in the (quasi)diabatic basis remains to be verified and other methods could be superior when matrix elements of the derivative couplings with nuclear wavefunctions are considered. 

From Eqs. (30a)-(30c), the singularity in as the conical intersection is approached, is of order 1/p. Only /7, (n= 0, derivative coupling can be used to consfruct a local diabatic representation that removes the singularity [10]. 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

To decide whether to use Ddz or Trt protection, the following considerations apply In general, Ddz-protected derivatives couple more efficiently that the corresponding Trt derivatives. Thus, Trt-Gly-OH and Trt-Ala-OH couple very well, but more sterically crowded amino acids with Trt protection couple slowly and Ddz is preferred. However, because Ddz removal conditions require a somewhat higher acid concentration, low-level premature cleavage (1-3%) of dipeptide from the resin can occur as a side reaction. 

The pinacol coupling reaction can be applied to most ketones and aldehydes when two identical molecules are being coupled. An improvement in the reaction involves trapping the initial coupling product as the bis(trimethylsilyl) derivative. Coupling of 4 -methylacetophenone (250) with zinc in the presence of chlorotri-methylsilane gave 76% of the bis-O-silylether, 251.200 Treatment with tetrabutylammonium fluoride removed... 

Owing to Eq. (35), there is no reason to expect that a strictly diabatic basis exists. Nevertheless, one can construct quasidiabatic states which are extremely useful in solving and understanding many relevant problems abundantly discussed in the literature. With their help it is possible to remove a substantial part of the derivative couplings and make the group-Born-Oppenheimer Eq. (26) more transparent and better amenable to explicit numerical calculations. That part of the derivative couplings which can be removed by an unitary transformation U( (R) is called... 

We now address the removability of the singular derivative couplings from the potential energy surfaces alone. This important aspect has been pointed out in the literature ° and is reiterated here for the sake of clarity. We start from a diabatic basis in which the singular terms have been eliminated (see Sec. 2.2 above) and confine ourselves to two relevant degrees of freedom, x and y. The potential matrix is expanded in a Taylor... 

The two variants differ in the treatment of the non-singular derivative couplings but both remove the singular part fuUy. °... 

While AE/j is quite small, less than 0.5 cm , for the points of conical intersection given in Table I, a numerical procedure can never produce an exact crossing point. To prove the existence of a point of conical intersection, the circulation of f fR) can be evaluated in the g-h plane. Consider again Rmex- The circulation of along Ci is 0.9995 r, proving the existence of a conical intersection at Rmex- Since A(Ci) = O.OOOSn the derivative coupling is removable to an excellent approximation inside this loop. 

Pyrrole derivatives are prepared by the coupling and annulation of o-iodoa-nilines with internal alkynes[291]. The 4-amino-5-iodopyrimidine 428 reacts with the TMS-substituted propargyl alcohol 429 to form the heterocondensed pyrrole 430, and the TMS is removed[292]. Similarly, the tryptophane 434 is obtained by the reaction of o-iodoaniline (431) with the internal alkyne 432 and deprotection of the coupled product 433(293]. As an alternative method, the 2,3-disubstituted indole 436 is obtained directly by the coupling of the o-alky-nyltrifluoroacetanilide 435 with aryl and alkenyl halides or triflates(294]. 

Hydroxyisoquinolines. Hydroxy groups in the 5-, 6-, 7-, and 8-position show phenoHc reactions for example, the Bucherer reaction leads to the corresponding anainoisoquinolines. Other typical reactions include the Mannich condensation, azo-coupling reactions, and nitrosation. Both 0-methyl and /V-methyl derivatives are obtained from the methylation of 1-hydroxyisoquinoline, indicating that both tautomeric forms are present. Distillation of various hydroxy compounds, eg, 1- and 4-hydroxyisoquinoline, with zinc dust removes the oxygen. Treatment of 1-isoquinolinol with phosphoms tribromide yields 1-bromoisoquinoline [1532-71 -4] (178). 

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