C-attack

A = no appreciable corrosion B = some attack, but not enough to preclude use C = attacked enough to preclude use and D = rapid attack. 

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

Electrostatic potential map for cyanide + methyl iodide C attack shows most negatively-charged regions (in red) and less negatively-charged regions (in blue). 

Halogens Although tantalum is severely attacked by flourine at room temperature it does not react with liquid chlorine, bromine and iodine up to 150°C and the metal suffers no appreciable attack in wet or dry bromine, chlorine and iodine below 250°C. It is virtually uncorroded by hydrogen bromide and hydrogen chloride below 370°C, attack starting at about 375 and 410°C respectively. 

Rohrig, van Duzer and Fellows exposed samples in an experimental superheater fed with steam at 2-6MN/m from a power plant. Some 42 materials were tested for periods of up to 16(XX)h, attack being estimated after test by weight loss following descaling. It was concluded that at 593°C attack continues at a high rate on carbon steel, whereas the rate for most alloy steels decreases with time (Table 7.10). 

Wagner and co-workers explored the different selectivity of 1,3-dipolar cyclo additions of nitrones 140 and cinnamonitrile 139 leading to oxadia-zolines 141 derived from an exclusive CN attack instead of a C = C attack (Scheme 50). This behavior was observed when cinnamonitrile was coordinated to a transition metal like Ft or Pd [89]. A similar approach to platimun-promoted nitrile-nitrone cyclo additions was reported using cychc nitrones. In this case, the authors reported a higher stereoselectivity of cychc nitrones with respect to the acyclic nitrones, due to a rigid E conformation adopted by cyclic nitrones [90]. 

Next, option B was examined. Oxidation of 14 with mCPBA proceeded well, leading to epoxide 22. However, cyclizahon of the enolate of epoxide 22 did not provide the desired five-membered product 23. The only isolated product was a tetrahydrofuran derivative 24, which resulted from the O-attack of the enolate to the epoxide instead of the desired C-attack. Therefore, development of this route was also terminated. 

The influence of the temperature. It has been established that the temperature has a dramatic effect on the occurrence of the Sn(ANRORC) process. Whereas participation of the Sn(ANRORC) mechanism in the amino-debromination of 4-t-butyl-6-bromopyrimidine at -75°C was found to occur for 77% according to the Sn(ANRORC) mechanism (79RTC5), it decreased to 33% when the amination was carried out at -33°C. Apparently at -75°C attack of the amide ion on C-2 is clearly favored over attack on C-6 (kinetic vs thermodynamic control) (78TL3841). Notice that 4-t-butyl-6-chloropyrimidine, when aminated at -33°C, reacts for nearly the... 

At ordinary temperatures magnesium is stable in alkalies, both dilute and concentrated. However, hot solutions of alkalies above 60°C attack the metal. 

The mechanism is similar to that of the Skraup synthesis (Problem 20.37) in that carbonyl O is protonated and the electrophilic C attacks the benzene ring in cyclization, to be followed by dehydration and oxidation. 

Electrophilic substitution hexahydrophenanthrenes. Electrophilic attack could occur at three sites in 1, but no products of a-substitution have been reported. Benzyl bromide reacts exclusively by y-attack. Aldehydes and ketones react usually by e- and/or y-attack, but with variahlc regioselectivity. c-Attack is useful because products can be obtained that undergo an intramolecular Diels-Alder reaction. An example is the synthesis of the two isomeric hexahydrophenanthrenes (3, equation... 

Further work has shown that the most obvious alternative, metal-carbon bond homolysis to form free radicals, occurs with Os(CO)4(CH3)2. The methyl radicals that were produced (slowly even at 162.5°C) attack a variety of solvents. Further, quantitative verification of their intermediacy was obtained by measuring kn/ko in mixtures of n-Ci2H26 and n-Ci2D26 The ratio, 5.3, is characteristic of the known selectivity for methyl radicals at that temperature (16). 

Similarly, treatment of methyl 2,3-anhy dro-/J-D-riboeide with hydrogen halides has been shown by two groups of workers 4 921 to Involve chiefly C attack, yielding a mixture of isomer derived from b Xyloee and D-araUnose (Eq. 732). 

In the field of sugar chemistry Foster and co-workers5 0 have found that diethylmngneeiiim adds to Cia) of methyl 4,6-0-ben2ylidene-2,3-anhydro-a-o-mannoside (Eq. 797), and Richards145 has observed C attack with methyl 4,6-0-benzylidene-2r3-anhydro- -l>-aUo6jde and diphenylmagncsi um (Eq. 798). 

Examine the highest-occupied molecular orbital (HOMO) of cyanide anion. Is the larger lobe on carbon or nitrogen Would you expect cyanide to act as a carbon or nitrogen nucleophile Does this lead to the lower energy transition state (compare the energy of cyanide+methyl iodide C attack and cyanide+methyl iodide N attack) ... 

When the electrophile E+ is a proton, the equilibrium A lies to the left, and the routes B (addition of nucleophile X at carbon to give overall syn addition, followed by anti elimination of McsSiX in D) and C (attack of nucleophile X at silicon) are followed. Both these routes give overall retention of configuration. 

Firstly, Aharoni et al. [128] described a process of ozonization of polymers or copolymers containing unsaturations in a mixture of two solvents. One of them is inert to ozone and the other one is less reactive than polymer double bonds but more reactive than the single C-C bonds of this polymer. A typical solvent mixture is composed of toluene and 1,1,2,2,-tetra-chloroethane, or xylene and decaline. This mixture permits one to control the attack of polymer only onto unsaturations and not to produce unstable sites in the polymer backbone (as peroxides or hydroperoxides coming from single bond C-C attack) which could decompose in a second step producing undesirable by-products. In this way, only unsaturations are reacted and... 

Using a physical-chemical protocol, Cymerman-Craig et al. (1966) employed phospholipase C attack on the naturally occurring alkenylacylglycero-phosphocholine to yield alkenylacylglycerol. The latter derivative was subjected to base-catalyzed methanolysis to produce alkenylglycerol. The latter... 

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