Direct path b

Direct path (b) is operative when the reductive elimination is easier than the ligand dissociation. Owing to the simple mechanistic feahire, detailed information 

---

Many early reports of condensations between 1,4-dicarbonyl compounds (61) and 1,2-diamines (62) to give 1,4-diazocines (63) must be considered suspect, since it has been subsequently demonstrated that in many cases the pyrroloimidazole system (64) is formed either directly (path b) or indirectly (paths a and c) (Scheme 4). [In fact, routes b and d have been used as an entry into the 1,4-diazocines by Sulkowski and co-workers and other groups... 

Cyclopropenes are readily cleaved by acids. The reactions are rationalized either by initial protonation of the double bond and subsequent electrocyclic ring-opening of the cyclopropyl cation thus formed (path a, equation 82), or by protonation of a cyclopropene o bond to give the allylic cation more directly (path b, equation 82) The final products of reaction are derived from nucleophilic capture of the allyl ion. 

The formation of a significant amount of the C-substituted product with 2, 4-dinitro-N-methyl aniline is more unexpected (Table I, exp. C) and the possible reaction paths are best discussed in terms of the network in Scheme 1. The C-substituted product could be derived either directly (path b) or via the nitramine (path a and c). 

There are assnmed to be three n bonds. A, B, and C, in benzene. Here we consider the electron delocalization from A to C. The electron delocalization via B is the same as that in the linear conjngate hexatriene (Schemes 2 and 3) used as a model of non-cyclic conjngate systems. The cyclic orbital interaction has been shown to be favored by the phase continnity (Scheme 5a). There is an additional path for the delocalization in cyclic geometry, which is the direct path from A to C or from a to c. The path gives rise to the cyclic a-b-c and a-b -c interactions. The cyclic orbital interactions satisfy the orbital phase continnity conditions... 

The constraint transformation algorithm accepts a network of goals partially ordered by constraints, and generates a constraint network of primitive actions, such that, if there exists a directed path from goal A to goal B (i.e., A must be achieved before B) in the first network, and if OP-A is the primitive action that achieves goal A, and OP-B the action that accomplishes B, then OP-A and OP-B are labels on nodes in the generated network, and there exists a directed path from the node labeled with OP-A to the node labeled with OP-B. 

In the case when the pendulum path is an ellipse the effect of the earth s rotation is to cause the ellipse to rotate with an angular velocity equal to (—co sin X). As we know, at the poles it is approximately 7.3 x 10 s. In accordance with Equation (3.94), for an observer located at the z-axis above the earth the rotation is clockwise in the northern hemisphere, (A>0), and counterclockwise in the southern hemisphere, (A<0). For example, at the North Pole the direction of the pendulum rotation and the earth are opposite to each other. During swinging the pendulum moves from the point a to the opposite point of the path b, which is shifted at some small distance because of the earth s rotation. Fig. 3.5c. Suppose that the radius of the circle is sq, then the displacement bd during a half period of swinging, Tjl, is... 

Although the path (a), which is the initiation stage of the catalytic reaction, was actually confirmed by a stoichiometric reaction, no direct evidence has been provided about paths (b)-(d). When the reaction was carried out by using PdCl2(PhCN)2 as a catalyst precursor, the Markovnikov adducts formed in situ isomerized into internal vinyl sulfide 14 (Eq. 7.10) [22]. 

In the original Medicinal Chemistry route, protection of the phenolic OH with a benzoate was carried out prior to N-methylation. In order to simplify the process, the direct N-methylation of hydroxypyrimidinone 3 was investigated. To our delight, methylation of 3 gave a mixture of the desired N-methyl product 31 and the undesired O-methyl by-product 32 as a 70 30 mixture (Scheme 6.7 path b). Surprisingly, methyl ethers 28-30 were not observed at all (Scheme 6.7 path a). 

The alkylation of 15 (path b, Scheme 15)40 has been performed according to conventional procedures using lithium or Grignard reagents. The preliminary isolation of 15 is not essential thus, the synthesis of the zirconium alkyl or aryl derivatives can be carried out in situ directly from 3 (path a, Scheme 15). The h NMR spectra of 71-74 all imply C2V symmetry, as revealed by the CH2 (one pair of doublets), Bu (two singlets), and MeO (one singlet) patterns. All the alkyl derivatives 71-73 are thermally stable up to 80°C in CgDg. The aryl derivative... 

As discussed above, olefins were initially proposed as approaching the oxo-Mn center along path a, with an orientation directing their bulkier substituent away from the C3(C3 )-substituent (see Scheme 14). It is noteworthy that path a runs over the downwardly bent benzene ring. Later, Jacobsen et al. proposed path b for the epoxidation with (26) possessing /-butyl groups at C5 and... 

Isoxazolines 38 and 39 were obtained in different ratios by direct cycloaddition of 4-t-butylbenzonitrile oxide with acids 35 (R = H, path B) and by the intermediate formation of cyclodextrin derivatives 36 and 37 followed by basic hydrolysis and acidification (path A). The reversed regioselectivity as well as an increased rate of the cycloaddition step could be explained through the temporary association of the nitrile oxide with the cyclodextrin to give the inclusion complex 40 <06CEJ8571>. 

Fig. 3 Transient spectra obtained upon the application of a 200-fs laser pulse to a solution of stilbene (S) and chloranil (Q) in dioxane. (a) The fast decay ( 20 ps) of the contact ion-radical pair S+ , Q generated by direct charge-transfer excitation (CT path), (b) The slow growth ( 1.6 ns) of the ion pair S+ Q due to the diffusional quenching of triplet chloranil (A path) as described in Scheme 13. Reproduced with permission from Ref. 55.

Brown s result was supported by later experiments in which bromonium ions were generated by bubbling gaseous hydrobromic acid through a solution of bromohydrins in halogenated solvents. Under these conditions, bromine is eliminated as it is formed, so that the resulting alkene is observed directly (Scheme 15). This method has been applied to the bromohydrins derived from cis- and trans-stilbenes (Scheme 16) and from 5//-dibenzo[a,d]-cycloheptene and -azepine systems ([29a] and [29b] respectively Scheme 17), in which steric constraints should favour elimination (path a) as against substitution (path b). 

On the other hand, when the oxidative carbonylation of a ,a -disubstituted propynylamines was carried out in the presence of an excess of CO2, the intermediate carbamate species could undergo cyclization with incorporation of CO2 into the five-membered cycle, either by direct nucleophilic attack of the carbamate oxygen to the triple bond coordinated to Pd(II) (Scheme 33, path a) or through the intermediate formation of a palladium carbamate complex followed by triple bond insertion (Scheme 33, path b). Carbon monoxide insertion into the Pd - C bond of the resulting stereoisomeric vinylpalladium intermediates then led to the final oxazolidi-none derivatives. 

Considering that /Tarninornercury(II) tetrafluoroborates are polar enough to undergo nucleophilic attack by the lone electron pair of an amine, ether or alcohol in the case of the 1,3-cyclooctadiene, 179, it has been assumed that the first formed 1,4-adduct can give the reaction product by displacement of mercury by amine with direct participation of the nucleophile in an assisted breakage of the anti C—Hg bond (path a) or by spontaneous reduction of mercury in the intermediate allylic organomercurial (path b) (equation 157). 

---