Internal attack

Mass loss (S ) = ML(i) - Mt.(0/ML(i) x 100 Internal attack exiusivc of corrosion mass losses... 

Pd(H20)4] at 40°C [73]. A kinetic study indicated that internal attack on the Pd-co-ordinated nitrile ligand by the aqua (not hydroxide) ligand and external attack on the nitrile ligand by solvent water occur at a similar rate. 

In cases where surface decarburization predominates over internal attack, the actual values of pressure-temperature combinations have not been extensively studied but the limits deAned by Naumann8 probably give the most accurate trends. Naumann s work, which is based on 100-hour tests, indicates decarburization tendencies however, long-time exposures have indicated lower operating limits. 

The addition of carbide stabilizers to steel reduces the tendency toward internal Assuring. Elements such as chromium, molybdenum, tungsten, vanadium, titanium, and niobium reduce the number of nucleation sites by forming more stable alloy carbides which resist breakdown by hydrogen and, therefore, decrease the propensity to form methane.9 The solid-line curves in Figure 1 reflect the increased resistance to internal attack when molybdenum and chromium are present. 

Initial attack by base on (34) yields the alkoxide anion (36), internal attack by this ROe then yields the epoxide (37) with inversion of configuration at C (these cyclic intermediates can actually be isolated in many cases) this carbon atomf, in turn, undergoes ordinary SN2 attack by eOH, with a second inversion of configuration at C. Finally, this second alkoxide anion (38) abstracts a proton from the solvent to yield the product 1,2-diol (35) with the same configuration as the starting material (34). This apparent retention of configuration has, however, been brought about by two successive inversions. 

The involvement of ion pairs in the addition process has also been related to the stereochemical behavior. The remarkable difference in configuration between the rearranged chlorides and acetates has been rationalized, as shown in equation 113, on the basis of a syn internal attack of Cl- on ion c and anti external attack of AcOH from the solvent pool. 

Considerable attention has been paid to this transformation (which is sometimes referred to as hydration ) in the past 15 years. 2. early example of the effect was the marked acceleration of the base hydrolysis of 2-cyanophenanthroline by Ni +, Cu + and Zn " " ions. The second-order rate constant is lO -fold higher for the Ni complex than for the free ligand, residing mainly in a more positive AS An external OH attack on the chelate was favored but an internal attack by Ni(II) coordinated OH cannot be ruled out. Nickel-ion catalysis of the hydrolysis of the phenanthroline-2-amide product is much less effective, being only about 4 x 10 times the rate for spontaneous hydrolysis. ... 

A pretty good diastereocontrol was achieved in the six-membered ring cyclization of 47, where diethylaluminium chloride and a catalytic amount of CuBr2 SMe2 were used to promote the internal attack of the Reformatsky centre to the sterically hindered carbonyl unit to give 48a and 48b (equation 32)108. 

Consideration of the anionic nature of the propagating species shows that the catalyst, too anionic for isotactic polymerizations, reacts internally more readily to produce the inactive species which GOODE and co-workers proposed. The less anionic isotactic producing species, is less prone for this internal attack. 

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

There are various types of classification of attacks. For example, division into passive and active, external and internal attacks, deliberate and unintentional. It should be mentioned that many models of attacks are currently well known one-to-one or one-to-many, i.e., attack proceeds from one point many-to-one and many-to-many, i.e., distributed or coordinated attacks hybrid attacks also named the blended threat [12]. 

Cyclopenta[l,2-6 3,4-6 ]dithiophenes were formed by the internal attack of dithienyl carbenes <84ACS(B)533, 84JCS(P1)571>. 

Tlie internal attack by a palladium coordinated alkoxide is supported by some experimental observations ... 

The last part of this reaction comprises the attack by the oxygen anion on the 8+ carbon that bears the bromine atom, which in turn leaves as the bromide anion so that the carbon never has more than four covalent bonds around its nucleus at any one time. This internal attack results in the formation of the three-membered epoxide ring. 

In this case there is an initial acid/base reaction in which the hydroxyl oxygen is protonated, and this in turn activates the carbon to which the oxygen is attached. Then, there is an internal attack by the bromine atom, which forms a bridged bromonium ion as an intermediate. This may then be attacked by the incoming bromide ion at either end to give the desired products. It is the formation of the bridged bromonium ion, and the stereochemical demands thereby placed on any further attack, that explains the stereochemistry of the products. 

The alkoxide anion performs an internal attack upon the C-Cl bond. This proceeds in a similar manner to a normal SN2 reaction. Thus, there is an attack from the opposite side to the C-Cl bond, with the required inversion of configuration at that carbon, to produce the epoxide. Suggest what may follow in order to produce the 1,2-diol with overall retention at the carbon centre. 

You will recall that thionyl chloride usually reacts via an SNi mechanism. In this case, the intermediate is attacked by the displaced chloride ion at the 3-position, i.e. in an SN2 manner, before the sulphur complex has had a chance to fragment in order to allow internal attack by the second chloride ion. 

Two mechanistically plausible scenarios for nucleophilic attack on the q benzyl palladium species seem feasible. Formation of the C N bond could occur either via external attack of the amine through inversion of configuration at the carbon stereocenter, or alternatively the amine could coordinate to palladium followed by an internal attack on the q benzyl ligand. Mechanistic investigations [15] using stoi chiometric amounts ofthe enantio and diastereomerically pure q benzyl palladium complex [ (R) Tol BINAP [q 1 (2 naphthyl)ethyl Pd](OTf) (8) revealed that the re action with aniline produced predominantly (R) N1 (2 naphthyl)ethylaniline R) 9), consistent with external nucleophilic attack (Scheme 11.3) [15]. However, it was noted that the catalytic reaction of [ (1 ) Tol BINAP Pd(OTf)2] with vinyl arenes and amines produced preferentially the opposite enantiomeric (S) amine hydroamination... 

Nucleophilic substitution of the halo sugars occurs with such strong nucleophiles as potassium hydrogen sulfide, potassium hydrogen sele-nide, sodium thiosulfate, potassium thiolacetate, potassium methane-thioxide, and sodium azide. If internal attack is not possible, and in favorable cases, substitution occurs with such weaker nucleophiles as ammonia, hydrazine, nitrate ion, and fluoride ion. ... 

Reaction with sodium methoxide in methanol is highly selective for the 4-chlorosnbstitutent, whereas lithinm 2-(trimethylsilyl)ethoxide is eqnaUy selective, bnt for the 2-chloro snbstitnent. The former is the normal situation for nucleophilic displacements - 4-chloro > 2-chloro - the second case is the exception, where strong co-ordination of lithinm in a non-polar solvent to the more basic nitrogen, N-1, leads to activation, and possibly also internal attack, at C-2. Under acidic conditions, an approximately 1 1 mixture of the two methoxy products is formed. Here, hydrogen bonding to the proton on N-1 provides the mechanism for encouraging attack at C-2. Selectivity with other nucleophiles is dependent on the nature of the nucleophile and on reaction conditions. 

Vomicidine is phenolic, soluble in alkali, and capable of the ready oxidative destruction typical of o-aminophenols the fact that vomicine itself appears not to be phenolic, on the other hand (not acylatable or soluble in alkali no ferric chloride test), has been ascribed to a masking of the hydroxyl by internal attack on the lactam carbonyl (CIX) (336). Another explanation is that the hydroxyl is just very strongly... 

---