Intramolecular using

The free-radical construction of C—C bonds either inter- or intramolecularly using a hydride as mediator is of great importance in chemical synthesis. The propagation steps for the intermolecular version are shown in Scheme 2. For a successful outcome, it is important (i) that the R sSi radical reacts faster with RZ (the precursor of radical R ) than with the alkene and (ii) that the alkyl radical reacts faster with alkene (to form the adduct radical) than with the silane. In other words, for a synthetically useful radical chain reaction, the intermediates must be disciplined. Therefore, in a synthetic plan one is faced with the task of considering kinetic data or substituent influence on the selectivity of radicals. The reader should note that the hydrogen donation step controls the radical sequence and, often, the concentration of silane provides the variable by which the products distribution can be influenced. 

Treatment of acylhydrazones of o-hydroxyaryl ketones with LTA in THF at room temperature results in smooth, very high yield conversion into 1,2-diacylbenzenes, e.g. 1 — 2. The mechanism of this very unusual replacement of a phenolic OH group by an acyl group has been studied in detail, and standard crossover experiments using, for example, a mixture of 1 and its 3,5-dibromo derivative established that the process was intramolecular. Use of 1 in which the oxygen atom of the benzoyl group was labelled as 180 gave exclusively the labelled product 3. 

In more recent applications, Takajo and Kambe have reported a new synthesis of perhydropyrimid-ines (90) by a double Mannich reaction (one step is intramolecular) using hydrobenzamide (87) and methyl cyanoacetate (89 equation 15). The reaction is general for other highly acidic methylene compounds including malononitrile, dimethyl malonate and nitroethane. In some cases, the intramolecular Mannich step is slow and side products arising from decomposition of the initial adduct are formed. This phenomenon is temperature dependent, indicating that intermediates in the reaction are formed reversibly. 

Reactions.—An alternative route to /3-keto-amides is by condensations between enolates of N,N-dimethylamides and acyl carbonates." For the three examples quoted, yields are 70—85%. N,N-Dimethyl-w-chloro-pentanamide and -hexanamide can be cyclized intramolecularly using LiNEt2 yields are greatly lowered when the alkane chains are branched." ... 

Mukamel S and Shan K 1985 On the selective elimination of intramolecular vibrational redistribution using strong resonant laser fields Chem. Rhys. Lett. 5 489-94... 

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

For the case of intramolecular energy transfer from excited vibrational states, a mixed quantum-classical treatment was given by Gerber et al. already in 1982 [101]. These authors used a time-dependent self-consistent field (TDSCF) approximation. In the classical limit of TDSCF averages over wave functions are replaced by averages over bundles of trajectories, each obtained by SCF methods. 

Z-matriccs arc commonly used as input to quantum mechanical ab initio and serai-empirical) calculations as they properly describe the spatial arrangement of the atoms of a molecule. Note that there is no explicit information on the connectivity present in the Z-matrix, as there is, c.g., in a connection table, but quantum mechanics derives the bonding and non-bonding intramolecular interactions from the molecular electronic wavefunction, starting from atomic wavefiinctions and a crude 3D structure. In contrast to that, most of the molecular mechanics packages require the initial molecular geometry as 3D Cartesian coordinates plus the connection table, as they have to assign appropriate force constants and potentials to each atom and each bond in order to relax and optimi-/e the molecular structure. Furthermore, Cartesian coordinates are preferable to internal coordinates if the spatial situations of ensembles of different molecules have to be compared. Of course, both representations are interconvertible. 

Suppose we are using an empirical energy function such as the following to describe the inter- and intramolecular interactions in our ethanol/ethane thiol system ... 

---