P-doubling

Much of the chemistry of oxygen can be rationalized in terms of its electronic structure (2s 2p ), high electronegativity (3.5) and small size. Thus, oxygen shows many similarities to nitrogen (p. 412) in its covalent chemistry, and its propensity to form H bonds (p. 52) and p double bonds (p. 416), though the anionic chemistry of 0 and OH is much more extensive than for the isoelectronic ions N , and NH2. Simi-... 

M — External corner N — Internal corner P — Double corners... 

Del2 Delahay, P. Double Layer and Electrode Kinetics, New York John Wiley Sons, 1965,... 

Compounds of multiple bond systems involving heavier main group elements were long considered to be unstable and synthetically inaccessible. In particular, the so-called double bond rule, which forbade the formation of (pn-pn) multiple bonds between silicon and other elements, hindered the development of the chemistry of low-coordinate silicon compounds containing Si=X (X = C, N, Si, P) double bonds for some years. 

The molecular structures of germaphosphenes (entries 10 and 11, Table I) show much the same characteristics as germanimines.19,20 They are planar about the germanium atom and the Ge=P double bond is about 8% shorter than its single-bond analog. The angle at phosphorus is smaller... 

For example, stilbene oxygenases, which cleave the interphenyl a,P-double bond of stilbene derivatives, are known to be members of the CCO family of enzymes that act on substrates other than... 

Delahay, P. Double-Laver and Electrode Kinetics. Interscience Publishers, New York, 1965. 

Although no En cluster molecules with n > 4 are known in the condensed phase, it has been shown [27] that neutral P6 can be generated in the gas phase by neutralization-reionization mass spectrometry with cp 2 6 as a precursor [cp = C5(CH3)5] [28]. Since the cp 2P6 precursor molecule already has a hetero-benzvalene P6 skeleton, it was inferred that the neutral P6 molecule should have the benzvalene structure as shown in Figure 2.6-5. This is in agreement with a multitude of quantum chemical calculations [8, 11, 27, 29-32]. Note that the handle of the basket of P6 includes two dicoordinate P atoms linked by a P=P double bond (cf. R-P=P-R 203 pm [33]). 

Refer to the table showing dice results. Out of 36 possible outcomes, there are six possibilities that are doubles P(doubles) = ... 

Schemes 9 and 10 show the synthesis of triphosphanes with functional substituents in the 1,2-position. Additional reaction proceeds with elimination of MejSiCl and the formation of the corresponding cyclotetraphosphane in which the P(CMe3)2 groups are adjacent. The reaction progresses with the formation of the P=P double bond, which can be proved by the addition of cyclopentadiene (Scheme 10). The...

Triphenylstannane reduced the double bond in dehydro-)J-ionone in 84% yield [872], Complex copper hydrides prepared in situ from lithium aluminum hydride and cuprous iodide in tetrahydrofuran at 0° [873], or from lithium trimethoxyaluminum hydride or sodium bis(methoxy-ethoxy)aluminum hydride and cuprous bromide [874] in tetrahydrofuran at 0° reduced the a,p double bonds selectively in yields from 40 to 100%. Similar selectivity was found with a complex sodium bis(iron tetracarbonyl)hydride NaHFe2(CO)g [875]. 

Alanylphenylalanine, leucylphenylalanine p Double reciprocal 58 addition of urea, 17-correction, pH-in-dependent KB... 

Dansyl-amino acids P Double reciprocal 60 nonaqueous CE (A-methylformamide), 17-correction... 

NMR spectroscopy of phosphaalkenes and other compounds containing C—P double bonds has been reviewed309. Considerable differences in the 31P chemical shifts have been found for diastereomeric pairs of phosphaalkenes, for example310,311 ... 

Various a,P y,5-unsaturated 1,3-dipoles are known to undergo 1,7-cyclization by a Bir-electrocyclization process (329,330), and the corresponding diazo compounds behave similarly. 5-Diazopenta-1,3-diene derivatives such as 285 (Scheme 8.70), generated in situ by thermolysis of the corresponding tosylhydrazone sodium salts, cyclize to form 1,2-diazepines (286) (331). Sharp and co-workers studied the mechanism, scope, and limitations of this transformation. It was found that cis-substitution about the y,8-double bond prevents the 1,7-cyclization and directs the system toward 1,5-cyclization (332,333) (i.e., formation of a 3/7-pyrazole), and that the cc,p-double bond can be part of a phenyl ring (334). In special cases, the y,5-double bond can be incorporated as part of an aromatic [287 —> 288 (335)] or 2- or 3-thienyl ring as well (336). 

The isolation of compounds containing simple C=P double bonds parallels the triple-bond work. The first stable acyclic phosphaalkene was synthesized over fifteen years ago.17 Again, base-induced dehydrohalogenation and stabilization by bulky groups is important ... 

Lewis acids inhibit the photochemical deconjugation of a,p- to p,y-unsaturated butenoic acids and promote rearrangement of (E)- to (Z)-isomers. Lewis acids also promote selective (E)- to (Z)-isomerization of the a,p-double bond of methyl 2,4-hexadienoate.6... 

The underlying reason for the investigation of dehydrocorrins by the synthetic techniques of pyrrole and porphyrin chemistry was that subsequent hydrogenation might lead to the corrin nucleus of vitamin B12. This objective was achieved by Johnson s group with the selective hydrogenation of the P-double bonds of the complex (99) (equation 52).262... 

Whereas carbon is able to form stable (p-p) double and triple bonds with carbon itself, nitrogen and oxygen, leading to coordination numbers 2 and 3 at the carbon atom, silicon cannot build up such bonds (compare Table 2). Although the existence of monomeric species such as SiO and SiNH has been established under specific conditions, and (p-p)ff bonded intermediates have been postulated in a number of... 

Colagiuri S, Miller JJ, Petocz P. Double-blind crossover comparison of human and porcine insulins in patients reporting lack of hypoglycaemia awareness. Lancet 1992 339(8807) 1432-5. 

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