Doubly activating system

For instance, nitration of 2-pyrimidone 10.20 affords nitropyrimidone 10.21. With doubly-activated systems such as 10.22, nitration to give 10.23 can occur without heating. 

A new concept was introduced in which the bidentate coordination of type E2 facilitated the chemoselective conversion of carbonyl functions [67], Distinct from the above results obtained with the typical tin-Lewis acids, this by 8 doubly activating system is suited even for the selective aldolization of aldehydes with enol silyl ethers. KSA and allylstannane reagents showed similar bias of preferential transformation of aldehydic carbonyls (Table 2-8). 

Little has investigated monoactivated and doubly activated alkenes tethered to butenolide with respect to electroreductive cyclization [202]. The geminally activated systems 227 undergo cyclization to diastereomeric products 228 and 229 in an 1 1 mixture, whereas both the a,j8-unsaturated monoester and a,/ -unsaturated mononitrile fail to cyclize. Only saturation of the C-C double bond of butenolide is observed. The author explains these results by distinct reactivity and lifetime of the intermediate radical anions. The radical anions derived from the monoactivated olefins are less delocalized than those of 227 and therefore should be shorter lived and more reactive. In this case preferential saturation occurs. The radical anions derived from the doubly activated alkene 227 are comparatively long-lived and less basic and thus capable of attacking the C-C double bond of the butenolide moiety. A decrease in saturation, accompanied by a marked increase in cyclization, is observed. 

The procedure described here for compound 1 is a scaleup of a published method.6 Phase-transfer catalysis7 and concentrated alkali are used to effect a one-pot conversion of diethyl malonate to the cyclopropane diacid, which is easily obtained by crystallization. Apparently alkylation of the malonate system occurs either at the diester or monocarboxylate, monoester stage since the method fails when malonic acid itself is used as the starting material. This method of synthesizing doubly activated cyclopropanes has been extended to the preparation of 1-cyanocyclopropanecar-boxylic acid (86%) by the use of ethyl cyanoacetate and 1-acetyl-cyclopropanecarboxylic acid (69%) by use of ethyl acetoacetate.6... 

Activity in this area has remained at a similar level to that reported for 2007. Well-established topics such as the chemistry of diphosphenes, phos-phaalkenes and phosphaalkynes have continued to attract attention, as also has work on the less-developed classes of low coordination number phosphorus compounds, in particular phosphenium ions and phosphinidenes, and their metal complexes. Recent work on kinetically-stabilised doubly bonded systems involving two of the heavier Group 15 elements has been... 

For example, in a 4-electron, 6-orbital CAS—specified as CASSCF 4,6)—performed on a singlet system, the active space would consist of the two highest occupied molecular orbitals (where the four electrons reside) and the four lowest virtual orbitals. Similarly, for a 6-electron, 5-orbital CAS on a triplet system, the active space would consist of the four highest occupied MO s— two of which are doubly-occupied and two are singly-occupied—and the LUMO (the keyword is CASSCF(6,5)). 

More recently, the C02 insertion into the Rh(I)-H bond of the T-shape RhH(PH3)2 system has been theoretically investigated with MP2//MP2 and QCISD(T)//MP2 methods [38], As can be seen in Table 2, this insertion reaction requires a much smaller activation barrier (4.3 kcal/mol with MP2//MP2 and -4.3 kcal/mol with QCISD(T)//MP2) than the insertion reaction in RhH(PH3)3. The reason is in the coordination number of rhodium. In RhH(PH3)2, the d8 configuration of Rh(I) means that the dapproaches this empty coordination site, it does not give rise to exchange repulsion with a doubly occupied d orbital. As a result, the Rh-H-0 angle is much smaller than that of the transition state of the insertion reaction of RhH(PH3)3 and the Rh-0 bond is almost formed in the transition state. 

For our first ever fourteen-electron example, we chose a dimethylenecyclobutadiene chain with three rings, as shown in Figure 2. Spin-coupled (SC) calculations have been reported for different spin multiplicities of this system by Raos et al [71], who treated only ten n electrons as active, optimizing two doubly-occupied orthogonal orbitals for the four n electrons of the terminal C-C linkages, and freezing all of the o orbitals at... 

Ooi has recently reported application of chiral P-spiro tetraaminophosphonium salt 37 as a catalyst for the highly enantio- and diasterioselective direct Henry reaction of a variety of aliphatic and aromatic aldehydes with nitroalkanes (Scheme 5.51) [92]. Addihon of the strong base KO Bu generates in situ the corresponding catalyhcally active triaminoiminophosphorane base A. Ensuing formation of a doubly hydrogen-bonded ion pair B positions the nitronate for stereoselective addition to the aldehyde. This catalyst system bears many similarities to guanidine base catalysis. 

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