Gem-dichlorides

Aliphatic aldehydes and ketones can be converted to gem-dichlorides by treatment with PCI5. The reaction fails for perhalo ketones. " If the aldehyde or ketone has an a hydrogen, elimination of HCl may follow and a vinylic chloride is a frequent side product ... 

Treatment with hydrazine to give the hydrazone and then CuBr2/f-BuOLi generated the gem-dibromide. " Oximes gives gem-dichlorides upon treatment with chlorine and BF3-OEt2, and then HCl. Some dithianes can be converted to gem-... 

Ketones can be converted to gem-dichloride through their reaction with phosphorus pentachloride which can be used to synthesize alkynes. 

Problem 7.1 Write structural formulas and lUPAC names for all isomers of (a) C HuBr. Classify the isomers as to whether they are tertiary (3°), secondary (2°), or primary (1°). (b) C H Clj. Classify the isomers which are gem-dichlorides and vic-dichlorides. ... 

For the dichlorobutanes the two Cl s are first placed on one C of the straight chain. These are geminal or gem-dichlorides. 

The mechanism of gem-dichloride formation involves initial attack of PCI4 + (which is present in solid PC15) at the oxygen, followed by addition of Cl to the carbon 2314... 

Alkynes. Alkynes are obtained by bisdehydrohalogenation of n c-1,2-dlhromides or gem-dichlorides with solid potassium r-butoxide in petroleum ether in the presence of 0.1 mole % of 18-crown-6. Dichlorides obtained from methyl ketones are converted into I-alkynes only if the 3-position is blocked. Alkynes arc also formed from (F.)-haloalkenes bv. wt-elimination. Yields of alkynes are >80% for the most part. 

Treatment of chroman-4-ones with PCI5 yields 3,4-dichlorochromenes. It is postulated that the reaction proceeds by initial a-chlorination, followed by conversion to the gem-dichloride. Dehydrohalogenation leads to the observed product. The chloroketone has been isolated from 2,2-dimethylchroman-4-one and phosphorus pentachloride (79TL3901). 

A review covering the use of titanium carbene complexes in organic synthesis has appeared.8 Special emphasis is placed on titanium carbene generated from titanocene bis(triethylphosphate) by action on thioacetals or gem-dichlorides. 

For the preparation of diaryl thioketones, the corresponding ketones may be converted into the gem-dichlorides, for instance by S0C12, which then are sulfurized by thioacetic acid60 [Eq. (5)]. 2,6-Diaryl-... 

If the reaction is carried out with anhydrous hydrogen fluoride replacing the Lewis acid, a mixture of geminally dihalogenatcd compounds is obtained gem-dichlorides 7, gpm-chlorofluo-rides 8 and getn-diiluorides 3 (Table 7). ... 

Although mixtures of halogenated compounds are obtained, and good procedures for the synthesis of gem-dichlorides and difluorides are known, this reaction can be useful for the preparation of gem-chlorofluoroalkanes. Only very few examples of synthetic procedures for these particular mixed geminally dihalogenated compounds are known. 

Direct conversion of alcohols to phenylselenides is also possible. For example, treatment of 13 with Bu3P/PhSeCN affords the selenide 14 in 82% yield. Irradiation with a sun lamp in the presence of PhjSnH gives the monocyclic dichloride 15 [Eq. (2)] [7]. This example illustrates also the high chemoselectivity for the homolysis of the carbon-selenium bond relative to the carbon-chlorine bond of a gem-dichloride. 

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