HUDSON & BROWN
204
amines, and ethers appear to be the only classes of volatile compounds nonreactive with the alkalis. Crosley carried out some preliminary experiments on radical detection in some simple hydrocarbons. The photochemistry of the simple hydrocarbons, methane, ethane, propane, etc., has been studied in detail by a number of workers. I n each case there is a mixture of final products whose yield varies with wavelength. To explain the products a variety of initial modes of dissociation are postulated. Some of these modes involve the formation of radical fragments, others of molecular fragments, still others of biradicals. If one could measure the concentration of the intermediate radicals, some of the indefiniteness of the photochemistry would be removed. To be specific, consider the photolysis of C2Hs which produces Hz, CH,, C2H4, C3H8,and C4Hlo. S o less than four modes of dissociation have been proposed to explain the end products, ViZ.
+ hv
4 AEe, and these esters hydrolyze rapidly'j with retention of configuration in spite of the unfavorable stereoelectronic arrangement.
I
10
=
k
OH 15
Acyclic phosphinates are known to hydrolyze with inversion of configuration,13and since this reaction does (10) E. A. Dennis and F. H . Restheimer, J . Amer. Chem. Soc., 88, 3432 (1966). (11) E. L. Muetterties. W. Mahler, and R . Schmutzler, I m r g . Chem., 2 , 613 (1963); R. Schmutzler, Angew. Chem., Int. Ed. Engl., 4, 496 (1965). (12) (a) F. Ramirez, Accounts Chem. Res., 1, 168 (1968); (b) D. G. Gorenstein and F. H . Westheimer, J . Amer. Chem. SOC., 89,2762 (1967). (13) 11.1. Green and R . F. Hudson, J . Chem. Soc., 540 (1963).
H
CH3
H31g,y:
P=O \OCH3 OH
OCHB 17
18
This is also the case with cyclic phosphonium salts. AksnesI6 has found that the five-membered cyclic phosphonium salt, 19, undergoes alkaline hydrolysis ca. l o 3 times faster than the six-membered analog 20. Acyclic salts are known to react with inversion of configuration,'7 and 20 could react with this stereochemistry.
OH 20
In the case of 19, however, configuration 21 would lead to a considerable increase in ring strain. The reaction thus proceeds through the alternative form 22 ill which the ring strain is released (- AEB),but the unfavorable stereoelectronic rearrangement increases the energy (by AEe). The net result in this case is an increased reactivity of the cyclic salt (i.e., AE, > AE,). (14) G. Aksnes and K . Bergesen, Acta Chem. Scand., 2 0 , 2508 (1966). (15) W. Hawes and S. Trippett, Chem. Commun., 577 (1968). (16) G. Aksnes and K . Bergesen, Acta Chem. Scand, 19, 931 (1965). (17) M . Zanger. C. 9. S'ander Werf, and IS'.E. McEwen, J . Amer. Chem. SOC.,81, 3806 (1959); K. F. Kumli, C. A. Vander Werf, and W. E. MoEwen, tbid., 81,3805 (1959).
REACTIVITY OF HETEROCYCLIC PHOSPHORUS COMPOUNDS
Vol. 5, 1972
22
The alkaline hydrolysis of phospholanium salts involves predominant retention of configuration, as shown by Marsi,18who followed the change in stereochemistry of 23. However, with a highly electronegative leaving group, as in 24, the increase in ring strain,
H3cy$ /\
H3C
H3cQ
/\
CH&&
Assuming that 28 hydrolyzes by the “normal” mechanism, with inversion of configuration, and 27 and 29 both react with retention of configuration, the rate factor of 50 gives a measure of the stereoelectronic effect ‘v 2.4 kcal/mole). If the release in steric strain on formation of intermediate 22 is equated to that reflected in the reactivity of the cyclic phosphonate ester (Table I), the relative reactivity of 19 and its acyclic analog is given (according to eq 1) by 105/50 = 2 X lo3. This is close to the observed rate enhancement.16 There is therefore a considerable measure of internal consistency in the application of the stereoelectronic principle and ring strain hypothesis to the reactivity and stereochemistry of these cyclic phosphorus compounds.
(a,
L1
1IoH-
207
C6H5
23
osic1,
24
AE,, on formation of an intermediate of structure 25 is less than the difference in stereoelectronic energies AE, of structures 25 and 26. In this case the reaction proceeds with predominant inversion of configuration. l9 H&..
Tervalent Compounds The natural angle of phosphines and phosphites is ca. loo”, and when phosphorus acts as a nucleophile this should increase as a result of rehybridization to ca. 109”. In the case of the cyclic compound, the ring strain should be increased and hence the cyclic compound should be less reactive than the acyclic analog. On the other hand, when the reaction involves nucleophilic attack on phorphorus to give a ten-electron system, the ring angle 0-P-0 should decrease, leading to an enhanced reactivity for the cyclic compound, although this will depend on the transition-state configuration (vide infra). This is shown schematically below.
pcH3
‘“P
H3C-P OSiCl, 25
SiCI, 26
An estimate of AE, for the hydrolysis of 19 may be made by analogy with the hydrolysis of certain acyclic salts. TrippettZ0has found that the lert-butylphosphonium salt 27 hydrolyzes ca. 50 times more slowly than 28. Moreover, 29 hydrolyzes with retention of
27
28
30
b~
e 0
- 60
a. Phosphorus(II1) as a Nucleophile. This hypothesis’ has been investigated by following the iirbusov reactions of the amidite 32 and the corresponding cyclic compound 33 with methyl iodide. The reaction of the
29
configuration. This presumably occurs as a result of the initial formation of the intermediate 30, with the tert-butyl group in an apical site. Deprotonation of this specieszl and pseudorotation would give 31, from which the benzyl anion may be expelled apically to give the product phosphine oxide with retention of configuration.
OH
e’+
I
C6H5
31
(18) K . L.Marsi, J . Amer. Chem. Soc., 91, 4724 (1969). (19) W. Egan, G. Chauvihre, K. Mislow, R. T. Clark, and K. L. Marsi, Chem. Commun., 733 (1970). (20) N. J. De’ath and S. Trippett, ibid., 172 (1969). (21) W. E. McEwen, G. Axelrad, M. Zanger, and C. A. Vander Werf, J. Amer. Chem. SOC., 87, 3948 (1965).
33
cyclic compound 33 is more complex, as 1 mole of ethylene is liberated after the initial Arbusov reaction, but this does not affect the kinetics. The reactions in nitrobenzene were followed by nmr. The rate constants (Table 11) show the cyclic compound to be less reactive than the acyclic analog, in agreement with the above hypothesis. Similar small rate reductions have been observed by Aksnes22in the reactions of ethyl iodide with cyclic
(22) G. Aksnes and R. Eriksen, Acta Chem. Scand., 20, 2463 (1966).
HUDSON & BROWN
208
Table I1 Rate Data for Reaction of Phosphites and Related Compounds with Alkyl Halides'sza Alkyl halide
mole-1 sec-1
CHaI
0.097"
CzHsI C2HaI
4.80 7.96
CzH61
2.7
C2HJ
0.7
CzHjI
1.1
106k2, 1.
Pseudo-first-order rate constants, sec-1.
esters in acetonitrile (Table 11). I n considering the magnitude of these rate decreases, it should be noted that reduced steric hindrance should lead to greater reactivities for the cyclic compounds, as in the quaternization of certain cyclic amines, e . g . , pyrrolidine is slightly more reactive than piperidine toward methyl iodide and acylating agents,23and aziridines and azetidines show even greater increasesS2* The angles in amines are close t o those in the quaternary salts, and the reactions of the cyclic compounds probably lead to negligible increases in steric strain. I n other r e a c t i ~ n s ,we ~ have observed much larger rate decreases for the cyclic phosphite. Thus in acylationgb (eq 2 ) rate ratios of the order of lo2, depending on the nature of the acyl halide used, are ~ b t a i n e d . ~ (R0)3P
+ C6HsCOC1
(RO)a&-COC6H6----f c10
I/
(RO)2P-COC6Ht
f RC1
(2)
Moreover the phosphite-catalyzed trimerization of isocyanates, under pseudo-first-order conditions, shows the cyclic phosphite t o be ca. 30 times less reactive than the acyclic analog.9b The mechanism of dimerization, catalyzed by tertiary phosphines, has been investigated in detail,26and by analogy a similar mechanism can be adopted for the trimerization 0
+
+ C6HsNCO JT (RO)aP-C-hC&
(RO)BP
ki
II
;
i"2
0
(RO)IP-&-NC~HB
+ 2CsHjNCO
ba --f
(C&aNC0)3
C0l2, which would follow from the nucleophilic reactivity of amide ions. Even greater rate differences are indicated by reactions involving acid catalysis. Thus AIichalskiZ6has recently shown that the mixed anhydride 35 is unreactive toward dialkyl phosphates whereas the acyclic analog 36 (X = lone pair) gives pyrophosphate 36 (X = 0) readily.
35
CH, Q
Accounts of Chemical Research
+ (R0)sP
The reaction is pseudo first order in isocyanate, at least in the early stages, provided that ICz < kQ[C6H5r\l-
36
This large difference in reactivity can be understood if the initial step involves protonation of the phosphitc t o give a good leaving group, a process which would be energetically less favorable in the case of the cyclic compound, followed by nucleophilic attack a t phosphoryl phosphorus by the dialkyl phosphate anion. A similar explanation may be put forward to account for the stability of 2-chloro-1,3,2-dioxaphospholane in thc presence of free hydrogen c h l ~ r i d e . ~ ' An estimate of the difference in basicity of acyclic and cyclic phosphites may be made from the ratcs of their acid hydrolysis. According to the data of Covitz and '1Vestheimer,28the cyclic ester 37 hydrolyzes ea. six times less rapidly than trimethyl phosphite. This can be understood in terms of the following mechanism.
OCH,
The relative reactivity of cyclic t o acyclic ester is then given by K,k,/K,k, N l / 6 . If the recond stage is compared with the acid hydrolysis of phosphate esters,2i.e. 0
OH
+ OH
I1
HOCH2CH20P-OCH3
I
OH
then lcc'/lca' N 106, and hence if ko'/k,' 'v 1cc,/ka, it follows that K J K , N 6 X lo6. According to t'he results discussed in this section, it appears that the influence of strain in the cyclic compound increases with the extent, of bond formation with the electrophilic reagent. The Brgnsted coefficient, P, is a convenient' measure of t,his interaction, and from the data so far available it is found to increase regularly wit,h the relat,ive reactivit,y of cyclic and acyclic compounds (Table 111). (26) J. Mikolajcsyk, J. Michalski, and A. Zwieraak, Chem. Commun., 1257 (1971).
(23) H. I
