H
SeC(NH^),
(271)Br
(242)Se4-
H
Se
B
A
(272)— 103 —
(271) was treated with triethylainine, the free base (272) was
liberated. This compound can be formulated either as a selenouronium fluorenylide (272B) or as a Se-fluorenyliso- selenourea (272A). The available evidence from spectra and chemical reactivity suggests that the ylide form (272B) makes a negligible contribution, if any. The ultra-violet spectrum of (272) shows the absence of a long wavelength absorption such as was found in the thiouronium analogue (sect. 2), and in fact the u.v. spectrum of (272) is unchanged from that of the pre-
1
cursor salt (271). The H n.m.r. spectrum, cannot be interpreted unequivocally, owing to the difficulty in distinguishing the
signals due to the N-H protons from that due to the C-9 proton 13
on the fluorene nucleus. The C n.m.r. spectrum will be
discussed more fully later, but was not found to give conclusive evidence for one structure or the other. However, the unreact ivity of (272) towards nitrosobenzene suggested that there must be a negligible contribution from the ylide form (272B), because none of the anil oxide (242) (or fluorenone anil) could be
detected even after a prolonged reaction period. This supposition can be justified on the grounds that it has been shown (see
introduction, sect. 4) that selenonium ylides are generally more reactive than the corresponding sulphonium compounds, and further more ]N,N'-diphenylthiouronium fluorenylide (191) is known to react readily with nitrosobenzene to afford the anil oxide (242).
Hence the free base (272) must exist as a Se-fluorenyli so - selenourea (272A), and in some respects the situation is analogous
— 104 —
to that found for N-fluorenyl-N',N"-diphenylguanidine (see sect 8), As a continuation of this work, it would be interesting to see if
an ylide could be obtained from an alkyl substituted selenourea. This might be more likely because of the lower acidity of the N-H protons, which might lead to the removal of the proton a to the selenium atom by base rather than the N-H proton, as is
found to prevail in the aryl substituted case. § § 6 - 8 GUANIDINES
In view of the fact that stable thiouronium ylides (273) can
be prepared, as described in § 3 of this discussion, it was of
interest to investigate the possibility that nitrogen analogues (274) might be capable of preparation and isolation. Stabilisation
R R V + /NHg C — S = = (
NH,
(273) RNHm
I: ^
'/
NH.
R
N H g ^ ; : H - N = : (NH,
(274) (275)NH,
C'
HN
NH,
(276) (277)in such an ylide (274), a guanidinium ylide, would arise from extensive delocalisation of the positive charge within the hetero group, as occurs in the stable guanidinium cation (276)
— 105 • —
The result of this stabilisation in guanidine itself (277) is that it is one of the strongest organic bases and is comparable in strength to the hydroxide ion. An ylide such as (274)
might be expected to show lower stability than its sulphur analogue (273) owing to the inability of the nitrogen atom to undergo valence shell expansion, a feature which has already been noted in nitrogen ylides (introduction, sect. 5). The ylide (274) may also undergo a proton shift to give a non- dipolar tautomer (275), the contribution of which will be
discussed more fully in the light of the results to be described. It was decided to use fluorene once again as the stabilising group for the carbanionic moiety in order to suppress unwanted cyclisation reactions and also in view of the ready availability of the starting materials.
The line of attack adopted in the first instance was to prepare variously substituted fluorenyl guanidinium salts (278)
(§6), and then to use base to generate the corresponding free base (279) (§7), in a manner similar to that employed for the
H fN R
4-NR
N R g (278) . (279)
thiouronium ylides. However, at an early stage it became apparent that several of the more highly substituted salts were not available by any simple known procedure and accordingly a new route involving the thermal decomposition of diazofluorene with guanidines was
106
§6. PREPARATION AMD ATTEMPTED PREPARATION OF GUANIDINIUM SALTS 1) By nucleophilic substitution of halide or tosylate
Owing to the low nucleophilicity and high basicity of guanidine, reaction of guanidine and N-diphenylguanidine with 9-bromofluorene led instead to the isolation of proto- debromination■products when the reactants were heated under reflux or kept at room temperature in various solvents, as shown below. The formation of bifluorenyl (265) on reaction of