PC double bond

Phosphaalkenes (continued) reactions at PC double bond, see PC double bond... 

PC double bond, 33 381 P NMR, 3 317 propargly rearrangement, 33 315 eraction with tetracyanoethylene, 33 320-33321... 

VI. Cumulated Bond Systems with Participation of the PC Double Bond... 

As shown in Fig. 1 there are two chances. First, the stability can be reached thermodynamically by lowering the energy of the PC double bond via mesomeric 7t-electron delocalization. Second, the stability can be kinetically obtained by introducing voluminous and bulky bridgehead atoms to shield the reactive center. Substituents for the latter process are 2-butyl, mesityl, or especially tri-2-butylphenyl, the so-called supermesityl group. 

In order to lower the energy of the PC double bond for stabilization, ring systems are rather suitable indeed, for the first time, Dimroth and Hoffmann (4), with the phosphamethine cyanin cation, and later on Markl (5) and Ashe (6), with phosphabenzene, succeeded in synthesizing such molecules as stable species. 

The kinetic stabilization of the reactive PC double bond by using bulky substituents to shield the bond originates from Becker (7), who in 1976 noticed the formation of a PC double bond during the reaction of disilylphosphanes with pivaloylchloride via a silatropic movement from the phosphorus to the oxygen [Eq (1)]. 

It is possible in many cases to create PC double bonds via condensation in separations of halosilane, siloxane (22), or even water (23a-c). Likewise, formaldehyde or benzaldehyde reacts with 2,4,6-fri-t-butyl-phenylphosphane, forming the phosphaalkene, supported by dehydration agents such as P4Ol0 or CaO/CaCl2 [Eq. (3)]. This method is excep-... 

Another well-established process available for the preparation of PC double bonds is related to the easy migration of phosphorus-bonded silyl functions toward an a-positioned, doubly bonded element such as N, O, or S [Eq. (4)]. By this silatropic movement the double bond is... 

The deprotonation of amino-bis(methylene)phosphoranes leads to imino-bis(methylene) phosphate anions, which show one PN and two PC double bonds [Eq. (11)] (50a). 

Alkyl substituents at the phosphorus and 7t-donor substituents at the carbon atom (such as O—SiMe3) of the PC double bond generate... 

More information regarding cycloadditions, especially on cyclic compounds containing PC double bonds, is given in a review by Arbuzov and Dianova (84). 

Phosphacarbapolyenes will be dealt with in this section. Here we have compounds that can be deducted from small polyenes by means of substitution of individual CH moieties with phosphorus atoms, creating PC double bonds in conjugation with another PC or CC sequence. The reactivity most impressively demonstrates the amazing affinity for pure carbon systems. 

Finally, the introduction of the sterically pretentious 2,4,6-tri-t-butylphenyl group in le made it possible to protect the PC double bond from further reactions of the diene, which was assumed to be an intermediate in all other cases presented herein (compound 4k). 

In the reaction with cyclopentadiene, the PC double bond in a,b acts as the ene component (62). A 1-diamino-substituted 2-phospha-l,3-butadiene could be synthesized by reaction of the ketene with the phos-phaalkene [Eq. (36)] (87). 

One may therefore tentatively conclude from this, in the case of a [2 + 2] cycloaddition or reversion, that a diradical transition state is most likely, if it is stabilized by substituents in the 2,5-position by means of a mesomeric interaction. Otherwise the equilibrium is shifted to the tetraphosphahexadiene. This explains the observations with respect to the reaction proceeding and hence the influence of the substituents at the two carbon atoms. If, in the case of a hindered orbital overlap between the substituents and the PC double bonds, a quasiaromatic interaction is blocked, the equilibrium is shifted to the tetraphosphahexadiene. On the other hand, if substituents in the 2,5-position are able to interact with the PC double bond, a mesomeric charge transfer into the side chain takes place, obstructing the aromatic transition state and so favoring the 1,4-cyclohexadiyl radical, which recombines to the bicyclic compound (Fig. 8). The carbon atom and its substituents... 

The ability of the PC double bond within tetraphosphahexadienes to participate in pericyclic reactions initiated interest in the synthesis of additional phosphahexadienes and studies of their properties. Attempts to synthesize 1,6-diphosphahexadiene a via a [1,1 ] Cope rearrangement out of 3,4-diphosphahexa-l,5-diene, derived from l,2-dipotassium-l,2-diphenyl diphosphide and vinylbromide, were not successful when carried out up to the temperature of decomposition (above 120°C). 

A number of heteroallenes incorporating the X3 phosphorus demonstrate the amazing relationship between the PC double bond and the CC double bond. Up to now compounds have been synthesized showing cumulated P=C and C=X bond systems (X = PR, NR, O, S, CR2, C=CR2). Only those having bulky groups such as 2,4,6-tri-f-butyl-phenyl (or in a few examples the f-butyl substituent), at the phosphorus could be identified and to some extent be isolated. 

Only a minor bond shortening of the PC double bond (163 pm) occurred with respect to the isolated PC double bond (167 pm), but an important stereochemical criterion, the orthogonal orientation of the aryl substituents relative to the nearly linear P—C—P axis (172.6°), can be elucidated from X-ray structure determinations. Like all allenes... 

Phosphaallenes exhibit three reactive centers, the PC double bond, the CC double bond, and the phosphorus atom. Here the PC double bond is significantly more reactive than the neighboring second double bond. The reactivity pattern is very similar to the phospha-alkenes, as far as the addition, cycloaddition, and coordination reactions are concerned. The cumulated CC double bond therefore has to be treated as an isolated group that is not greatly influenced by reactions at the PC n system. 

Thus phosphaallenes react with HC1 or methanolate, respectively, adding to the PC double bond and forming the phosphinous acid chloride or the corresponding methyl ester. In these cases the phosphorus atom acts as an electrophilic center. During the turnover of la,c together with H202/H20 the phosphorus atom again is attacked in a nucleophilic way and is oxidized to phosphinic acid 4a,c (Scheme 15). 

Among the cycloadditions, the addition to the PC double bond prevails. 2,4-Bisimino-l,3-diphosphetane is formed in a [2 + 2] cycloaddition of the unstable monomer l-phospha-3-azaallene (54, 60). It can be decomposed into the monomer compound in a flashlight vacuum pyrolysis (135). Nevertheless, this method cannot be used to produce the monomer compound on a preparative scale [Eq. (72)]. 

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