Steric factors electronic and

Cyanohydrin formation is reversible and the position of equilibrium depends on the steric and electronic factors governing nucleophilic addition to carbonyl groups described m the preceding section Aldehydes and unhindered ketones give good yields of cyanohydrins... 

In addition to reaction sequences of type (66) -> (67), electrophilic reagents can attack at either one of the ring nitrogen atoms in the mesomeric anions formed by proton loss e.g. 70 71 or 72 see Section 4.02.1.3.6). Here we have an ambident anion, and for unsymmetrical cases the composition of the reaction product (71) + (72) is dictated by steric and electronic factors. 

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. 

Benzylidene acetals have the useful property that one of the two C—O bonds can be selectively cleaved. The direction of cleavage is dependent on steric and electronic factors as well as on the nature of the cleavage reagent. 

When both the 1,3-dipoIe and the dipolarophile are unsymmetrical, there are two possible orientations for addition. Both steric and electronic factors play a role in determining the regioselectivity of the addition. The most generally satisfactory interpretation of the regiochemistry of dipolar cycloadditions is based on frontier orbital concepts. As with the Diels-Alder reaction, the most favorable orientation is that which involves complementary interaction between the frontier orbitals of the 1,3-dipole and the dipolarophile. Although most dipolar cycloadditions are of the type in which the LUMO of the dipolarophile interacts with the HOMO of the 1,3-dipole, there are a significant number of systems in which the relationship is reversed. There are also some in which the two possible HOMO-LUMO interactions are of comparable magnitude. 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

Protecting groups are generally formed by nucleophilic attack on the carbonyl group and the rate of this process is determined by steric and electronic factors associated with the ketone. In steroid ketones steric effects are usually more important due to the rigid tetracychc skeleton. 

The increase in the proportion of the tetrasubstituted isomer in the cases of the morpholine and piperidine enamines of 2-methylcyelohexanone has been ascribed to both steric and electronic factors. The authors propose that the overlap of the electron pair on the nitrogen atom and the v electrons of the double bond is much more important in the case of the pyrrolidine enamines and much less with the others. Support for this postulate was provided by the NMR spectra of these enamines, wherein the chemical shifts of the vinylic protons of the pyrrolidine enamines were at a higher field than those of the corresponding morpholine and piperidine enamines by 20-27 Hz. The greater amount of overlap or electron delocalization, in the case of pyrrolidine enamine, is in accord with the postulate of Brown et al. (7- ) that the double bond exo to the five-membered ring is more favored than the double bond exo to the six-membered ring. 

Phosphinamides are stable to catalytic hydrogenation, used to cleave benzyl-derived protective groups, and to hydrazine. The rate of hydrolysis of phosphinamides is a function of the steric and electronic factors around the phosphorus. This derivative has largely been used for the protection of amino acids and occurs few, if any, times in the general synthetic literature. 

The conformation of alkylcyclohexanes is determined largely by steric repulsion (see Chapter 5, Problems 6 and 7). More polar substituents may show different conformational preferences due to a combination of steric and electronic factors. 

In contrast to the facile condensation of o-nitrotoluene with diethyl oxalate, other a-alky] nitrobenzenes are sluggish to react with diethyl oxalate or fail to react at all. It has been suggested that this is due both to steric and electronic factors effected by the alky] group, which destabilizes the methylene group in regard to formation of the carbanion. ... 

Both steric and electronic factors have been claimed to control the selectivity in the cyclization step. Not only the control of the selectivity on the ring closure but also the lack of activity toward cyclization was observed. In one example of this, methyl substituted aminoindole 96 provided cyclization product 99 while attempted cyclization of methyl ether 98 led to decomposition. ... 

However, the 0-alkyl derivatives are potentially unstable with respect to thermal elimination of a carbonyl compound and consequent reduction to the corresponding lactam. A combination of steric and electronic factors may permit this decomposition, i.e., 133 -a- 134, to occur at quite moderate temperatures. The 0-methyl derivative of the benzalphthalimidine (132) undergoes slow loss of formaldehyde at 177° (Ti/a in dimethyl sulfoxide 40 minutes), but this elimination is much faster in certain thiohydroxamic acid derivatives, e.g., 135, which lose benzaldehyde readily at 139° in dimethyl sulfoxide (T1/2 6 minutes). The outstanding example of this decomposition, however,... 

The rate of aromatic hydrogenation is influenced by both steric and electronic factors (20,25,53). In general, rates decrease as substitution by alkyl groups increases (47), unless the substituents introduce exceptional strain. Strained aromatic systems will undergo facile saturation even over palladium under mild conditions (3JJ2,33). 

Steric and electronic factors are both important in determining reactivity. Sterically, we find within a series of similar acid derivatives that unhindered,... 

The phosphine and arsine complexes of gold(I) have been intensively studied since the early 1970s. The possibilities of coordination numbers between 2 and 4 have been explored, though the use of bulky ligands is less essential than with the isoelectronic M(PR3)2 (M = Pd, Pt) compounds and the coordination numbers depend on both steric and electronic factors [71]. 

ADMET of av j-dicncs has been a focus of research in the Wagener laboratories for many years now, where we have studied this chemistry to explore its viability in synthesizing polymers possessing both precisely designed microstructures as well as a variety of functionalities. The requirements for this reaction, such as steric and electronic factors, functionalities allowed, appropriate choice of catalyst, and necessary length or structure of the diene, have been examined.3,12-14 A detailed discussion will be presented later in this chapter with a brief synopsis of general rules for successful ADMET polymerization presented here. 

Kinetic studies using 1,9-decadiene and 1,5-hexadiene in comparison widi catalyst 14 and catalyst 12 demonstrate an order-of-magnitude difference in their rates of polymerization, widi 14 being the faster of the two.12 Furdier, this study shows diat different products are produced when die two catalysts are reacted widi 1,5-hexadiene. Catalyst 14 generates principally lineal" polymer with the small amount of cyclics normally observed in step condensation chemistry, while 12 produces only small amounts of linear oligomers widi die major product being cyclics such as 1,5-cyclooctadiene.12 Catalyst 12, a late transition metal benzylidene (carbene), has vastly different steric and electronic factors compared to catalyst 14, an early transition metal alkylidene. Since die results were observed after extended reaction time periods and no catalyst quenching or kinetic product isolation was performed, this anomaly is attributed to mechanistic differences between diese two catalysts under identical reaction conditions. 

Two papers have appeared on the reactions of halogenophosphines with tervalent phosphorus compounds. In a detailed study of the reactions at 20 °C of a range of tertiary phosphines with phosphorus trichloride, dichlorophenylphosphine, and chlorodiphenylphosphine, it has been shown that, in general, 1 1 adducts are formed, provided that the tertiary phosphine is a good nucleophile. With diphenylchlorophosphine, for example, an adduct (18) is formed with dimethylphenylphosphine, but not with diphenylmethylphosphine, although the relative importance of steric and electronic factors remains to be established. The related reactions of phosphorus trichloride and of dichlorophenylphosphine are much more complex, and the initial crystalline products are not amenable to analysis. The reactions at 280 °C of a similar system have been shown to lead to halogen exchange, e.g. the conversion of (19) to (20). 

B. Reactions.—This year has seen the publication of a number of papers on the reactions of olefins and acetylenes with phosphorus pentachloride, to produce new phosphorus-carbon bonds. An investigation into the structural requirements of trisubstituted olefins (40) undergoing the above reaction has shown that both steric and electronic factors are important, e.g. an adduct forms with (40 X = CH3) but no reaction occurs for (40 X = Ph). Further examples of the reactions of unsaturated ethers include the formation and decomposition of adducts from a-methoxystyrene... 

The reactivity of different alkenes toward mercuration spans a considerable range and is governed by a combination of steric and electronic factors.24 Terminal double bonds are more reactive than internal ones. Disubstituted terminal alkenes, however, are more reactive than monosubstituted cases, as would be expected for electrophilic attack. (See Part A, Table 5.6 for comparative rate data.) The differences in relative reactivities are large enough that selectivity can be achieved with certain dienes. 

Intermolecular recognition and self-assembly processes both in the solid, liquid, and gas phases are the result of the balanced action of steric and electronic factors related to shape complementarity, size compatibility, and specific anisotropic interactions. Rather than pursuing specific and definitive rules for recognition and self-assembly processes, we will afford some heuristic principles that can be used as guidelines in XB-based supramolecular chemistry. 

Note that any attempt to differentiate carboxylic acids from phenols on the basis of how broad their respective signals are is to be discouraged in the strongest possible terms Whilst carboxylic acids tend to be broader than phenols, it is by no means guaranteed that this is always the case. Steric and electronic factors and hydrogen bonding can reverse this in certain situations. 

In molecules containing a 1,3-diene unit and an isolated double bond, the diene is cyelopropanated preferentially (Scheme 7) 72,82). What has been said about the influence of steric and electronic factors as well as the nature of the catalyst (see above), can also be applied to explain the product distribution in these cases. The inertness of a trisubstituted double bond and the low reactivity of an E-disubstituted olefinic bond are quite obvious in these intramolecular competitions. 

Overall, steric and electronic factors, which are seen to be small, are found to work in opposite directions and, to some degree, cancel each other out. Consequently, the intrinsic free activation barriers and reaction free energies (AG nt, AG nt), respectively, span a small range for catalysts I-IV and differ by less than l.Okcalmol-1. Thus, oxidative coupling represents the one process (beside allylic isomerization, cf. Section 5.3) among all the critical elementary steps of the C8-cyclodimer channel, that is least influenced by electronic and steric factors. 

Reductive decoupling, 181, 187 Reductive elimination, 171-174, 190-197 refined catalytic cycle, 206-211 and selectivity, 212-214 steric and electronic factors, 202-205, 217 Retro-Wittig decomposition, 37, 78, 79-83 thiobetaines, 57-58, 60, 65 Ring cleavage, 105, 276, 281 Ring closure in oligomerization, 172, 174, 190-197... 

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